[PDF] Catalytic pervaporation membranes for close integration of reaction and separation

1 Catalytic pervaporation membranes for close integration of reaction and separation PROEFSCHRIFT ter verkrijging van de...

Catalytic pervaporation membranes for close integration of reaction and separation PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op woensdag 29 maart 2006 om 16.00 uur

door

Thijs Andries Peters

geboren te Breda

Dit proefschrift is goedgekeurd door de promotor:

prof.dr.ir. J.T.F. Keurentjes

Copromotor: dr.ir. N.E. Benes

© 2006, Thijs Peters A catalogue record is available from the Library Eindhoven University of Technology Peters, Thijs A. Catalytic pervaporation membranes for close integration of reaction and separation / door Thijs Andries Peters. – Eindhoven: Technische Universiteit Eindhoven, 2006. ISBN-10: 90-386-3057-3 ISBN-13: 978-90-386-3057-1 Subject headings: esterification / pervaporation / catalytic membrane / dehydration / membrane technology / catalysis / modelling / structured catalyst / condensation reaction. Trefwoorden: verestering / pervaporatie / katalytisch membraan / ontwatering / membraantechnologie / katalyse / modelleren / gestructureerde katalysator / kondenseringsreactie. An electronic copy of this thesis is available from the site of the Eindhoven University Library in PDF format (http://www.tue.nl/bib).

Summary Esterifications represent a significant group of reactions commonly found in industry. These are typical examples of reactions controlled by thermodynamic equilibrium and are commonly catalysed using liquid mineral acids. As a result, conventional esterification processes are faced with problems in product purification. Pervaporation is an ideal candidate to enhance conversion in reversible condensation reactions in which water is generated as a by-product. In pervaporation-esterification coupling, one side of a membrane is exposed to the reaction mixture, while at the other side of the membrane vacuum is applied. Water permeates preferentially through the membrane, evaporates, and is then removed on the low pressure permeate side. The removal of water drives the esterification reaction to the ester-side. Compared to distillation, pervaporation offers a number of advantages, e.g., the energy consumption is low, the reaction temperature is independent from the reflux temperature, and the separation efficiency is not solely determined by the relative volatility. Liquid mineral acid catalysts, generally used for esterification reactions, suffer from several drawbacks such as a corrosive nature, the occurence of side reactions, and the fact that the catalyst can not easily be separated from the reaction mixture. Heterogenous acid catalysts offer an alternative since they are not corrosive and, coated onto a support, they can easily be reused. The present study aims to develop pervaporation membranes showing catalytic activity. These membranes enable close and efficient integration of reaction and separation, while allowing independent optimisation of the catalytic and separation functions. Using reaction kinetics and membrane parameters, model simulations show the potential added value of a composite membrane system and indicate the existence of an optimum catalytic layer thickness, within practically reachable dimensions. At the optimum catalytic layer thickness, the performance of a catalytic membrane reactor can exceed the performance of a membrane reactor in which the same amount of catalyst is dispersed in the bulk liquid.

Amorphous silica and cross-linked PVA water selective membrane layers have been prepared on the outer surface of hollow fibre ceramic substrates. These substrates consist of an α-Al2O3 hollow fibre coated with an intermediate γ-Al2O3 layer. This intermediate layer provides a sufficiently smooth surface for the deposition of the ultra-thin water selective layers. High helium permeance (1.1-2.9·10-6 mol⋅m-2⋅s-1⋅Pa-1), high He/N2 permselectivity (~ 100-1000), and Arrhenius type temperature dependence of gas permeance indicate that the silica membranes are microporous and possess a low number of defects. In the dehydration of 1-butanol (80°C, 5 wt.% water), initially a high water flux and selectivity (2.9 kg·m-2·h-1 and 1200, respectively) are observed. The composite ceramic/PVA membranes have been characterised in the dehydration of different alcohol/water mixtures. In the dehydration of 2-propanol and 1-butanol, a simultaneous increase in both water flux and separation factor is observed with increasing temperature or water concentration. This remarkable behaviour is in contrast to the trade-off generally observed for polymer membranes, i.e., an increase in flux is typically combined with a decrease in separation factor. A possible explanation for this behaviour is a low degree of three dimensional swelling in the vicinity of the γ-Al2O3 – PVA interface due to an enhanced structural stability. In other words, the expansion of the polymer chains and the increase in free volume, permits an increase in water transport with an increase in temperature, whereas the transport of the larger 2-propanol and 1-butanol molecules is still severely hindered. For the dehydration of smaller alcohols, i.e., ethanol and 1-propanol, the traditional trade-off between an increase in flux and a decrease in selectivity is observed with an increase in temperature. Catalytic pervaporation membranes have been developed by deposition of H-USY zeolite and Amberlyst 15 layers on silica and PVA membranes, respectively. Membrane pre-treatment and the addition of binder to the dip-coat suspension appear to be crucial in the process. Tuning of the catalytic layer thickness is possible by varying the number of dip-coat steps. This procedure avoids failure of the coating due to the high stresses, which can occur in thicker coatings during firing. In the pervaporationassisted esterification reaction the catalyst-coated pervaporation

membranes are able to couple catalytic activity and selective water removal. Mass transfer resistances in the catalytic later are found to be negligible. For both coated catalysts the catalytic activity is comparable to the activity of the bulk catalyst. The performance of H-USY-coated membranes is mainly limited by reaction kinetics. This in contrast to Amberlyst-coated membranes, for which the performance is limited by the water removal rate due to their higher activity. It is conceivable that a further improvement in reactor performance can be realised by optimisation of the catalytic layer in terms of thickness and, for example, porosity. A preliminary large scale composite membrane reactor evaluation is carried out based on the obtained experimental data, e.g., membrane permeability and catalyst activity. The process evaluation suggests that membranes coated with a thin Amberlyst layer can be interesting candidates to be applied in pervaporation-assisted esterifications. A more efficient coupling between water production and removal can be obtained by varying the catalytic layer thickness over the length of the module. To achieve the same effect, a sequence of modules having a different catalytic layer thickness can also be used. In this thesis two new membrane concepts are introduced; ceramic supported polymer membranes that show an improved performance compared to existing membranes, and catalytic membranes for which close integration of reaction and molecular separation offers advantages over conventional processes. Academic studies of these new concepts can be extended towards future applications, such as ceramic-supported hydrophobic polymers for organic removal, and other equilibrium reaction systems such as the direct formation of DMC from methanol and carbon dioxide. At this point, the challenge is to design and demonstrate reliable pilot scale membrane processes, based on the new concepts presented in this thesis.

Samenvatting Veresteringen zijn reacties waarbij een organisch zuur en een alcohol met elkaar reageren tot een ester. Veresteringsreacties behoren tot de belangrijkste types reacties in de chemische industrie. Er worden meer dan 500 verschillende esters gemaakt, die bijvoorbeeld worden gebruikt in geur- en smaakstoffen, pesticiden, oplosmiddelen, medicijnen, oppervlakte actieve stoffen, chemische tussenproducten, en monomeren voor hoogmoleculaire polymeren. In het algemeen worden veresteringen hom*ogeen gekatalyseerd, waarbij de conversie gelimiteerd wordt door het thermodynamisch evenwicht. Voor conventionele productieprocessen is daarom het verkrijgen van de gewenste uiteindelijke productzuiverheid dan ook niet eenvoudig. De evenwichtsligging van een verestering kan naar de productzijde worden verschoven door water aan het reactiemengsel te onttrekken. Pervaporatie is een geschikte methode om dit te verwezenlijken. In pervaporatie-gekoppelde veresteringen wordt water selectief door een membraan verwijderd en aan de permeaatzijde verdampt. Vergeleken met andere scheidingsprocessen, zoals destillatie, biedt pervaporatie een aantal voordelen. Zo is het energieverbruik laag, de reactietemperatuur onafhankelijk van het kookpunt, en wordt de scheiding niet enkel bepaald door het verschil in relatieve vluchtigheden van de te scheiden componenten. In het algemeen worden voor veresteringen zure hom*ogene katalysatoren gebruikt. Het gebruik van deze katalysatoren brengt verschillende nadelen met zich mee. Zo zijn ze moeilijk van het reactiemengsel te scheiden en daardoor niet eenvoudig opnieuw te gebruiken. Bovendien zijn de hom*ogene zuren erg corrosief. Heterogene zure katalysatoren daarentegen zijn minder corrosief, en gecoat op een support zijn ze wel eenvoudig te hergebruiken. Dit proefschrift beschrijft het onderzoek naar pervaporatiemembranen met een katalytische functionaliteit, en de toepassing van deze membranen in veresteringen. Dergelijke membranen zouden een effectieve koppeling van reactie en scheiding mogelijk moeten maken. Door de twee verschillende functies, scheiding en katalytische activiteit, in afzonderlijke lagen onder te brengen kunnen deze onafhankelijk van elkaar worden geoptimaliseerd. Met behulp van modelberekeningen is de toegevoegde

waarde van de te ontwikkelen composietmembranen onderzocht. Deze berekeningen voorspellen een optimum in de dikte van de katalytische laag, welke praktisch realiseerbaar is. Voor lagen die dunner zijn dan de optimum dikte is waterverwijdering erg efficiënt, en is de bereikte conversie hoger dan wanneer eenzelfde hoeveelheid katalysator zou worden gedispergeerd in het reactiemengsel. Voor lagen die dikker zijn dan de optimum dikte worden massatransportlimiteringen belangrijk en neemt de behaalde conversie af. Waterselectieve lagen van zowel amorf silica als vernet PVA zijn aangebracht op keramische holle vezels. Deze vezels zijn opgebouwd uit een α-Al2O3 holle vezelsubstraat, waarop een γ-Al2O3 tussenlaag is aangebracht. De aanwezigheid van de tussenlaag zorgt ervoor dat een voldoende glad oppervlak wordt verkregen voor het defectvrij aanbrengen van de uitermate dunne waterselectieve lagen. In de ontwatering van 1-butanol (80°C, 5 wt.% water) laten de amorfe silica membranen een hoge waterflux en een hoge selectiviteit (2.9 kg·m-2·h-1 and 1200) zien. Daarnaast zijn gaspermeatie metingen indicatief voor een microporeus karakter van deze membranen; een hoge He/N2 permselectiviteit (~ 100-1000), gecombineerd met een hoge He permeatie (1.1-2.9·10-6 mol⋅m-2⋅s-1⋅Pa-1), en een Arrhenius type temperatuursafhankelijkheid. De composiet keramisch/PVA membranen zijn getest in de ontwatering van verschillende alcohol/water mengsels. In de ontwatering van 2-propanol and 1-butanol, is een gelijktijdige toename van zowel de waterflux als de selectiviteit gemeten, bij zowel een toename van de temperatuur als een toename van de waterconcentratie in de voeding. Dit uitzonderlijke gedrag is in tegenstelling tot wat normaal wordt geobserveerd voor polymeer membranen; een toename van de flux resulteert dan in een afname van de selectiviteit. Een mogelijke verklaring voor dit onverwachte gedrag is een gelimiteerde zwelling van de selectieve laag, met name in de nabijheid van het γ-Al2O3 - PVA grensvlak. Door een dergelijke gelimiteerde zwelling zal vooral de bewegelijkheid van grotere moleculen, zoals 2-propanol en 1-butanol, minder toenemen bij een temperatuurstijging of bij een toename in waterconcentratie in de voeding. Voor de kleinere alcoholen, ethanol en 1-propanol, is wel de traditionele wisselwerking tussen flux en selectiviteit gemeten.

Met behulp van de dip-coat techniek zijn katalysatorlagen van zeoliet H-USY en Amberlyst 15 aangebracht op respectievelijk silica en PVA membranen. Hierbij is een juiste voorbehandeling van het membraanoppervlak en de toevoeging van een binder aan de dip-coat oplossing noodzakelijk. Door het aantal dip-coat stappen te variëren zijn zowel de dikte als de morfologie van de katalytische laag te sturen. Door sequentieel dunne katalysatorlagen aan te brengen wordt de ontwikkeling van spanningen voorkomen die tijdens het drogen van één dikke laag tot materiaalbreuk kunnen leiden. De composietmembranen zijn in staat om op effectieve wijze katalytische activiteit en waterverwijdering te combineren. Uit pervaporatiemetingen blijkt dat transportlimiteringen in de katalytische laag kunnen worden verwaarloosd. Tevens is de katalytische activiteit voor de gecoate katalysatoren vergelijkbaar met die van de katalysatoren die zijn gedispergeerd in het reactiemengsel. De behaalde conversie van H-USY-gecoat membranen wordt vooral gelimiteerd door de reactiekinetiek vanwege de lage activiteit van de zeolietkatalysator. Bij Amberlyst-gecoate membranen wordt de behaalde conversie juist gelimiteerd door de hoeveelheid water die door het membraan kan worden verwijderd. Een evaluatie van een industriële membraanreactor is uitgevoerd gebruikmakend van experimentele gegevens, zoals katalytische activiteit en membraanpermeabiliteit. Uit deze evaluatie blijkt dat vooral membranen gecoat met een relatief dunne Amberlyst katalysatorlaag interessant kunnen zijn voor toepassing in pervaporatie-gekoppelde veresteringen. Door de dikte van de katalytische laag te variëren over de lengte van de membraanreactor kan een efficiëntere koppeling tussen productie en verwijdering van water worden bewerkstelligd. Eenzelfde effect kan worden verkregen met behulp van een aaneenschakeling van modules met verschillende katalytische laagdiktes. Samengevat zijn in dit proefschrift twee nieuwe membraanconcepten geïntroduceerd: composiet keramisch/PVA membranen met uitzonderlijke eigenschappen, en katalytische membranen die een efficiënte integratie van reactie en scheiding mogelijk maken. De resultaten van het onderzoek beschreven in dit proefschrift kunnen mogelijk worden uitgebreid naar andere toepassingen. Hierbij kan bijvoorbeeld worden gedacht aan de

ontwikkeling van hydrofobe keramiek/polymeer membranen voor het scheiden van organische mengsels. De katalytische membranen zouden kunnen worden gebruikt voor andere evenwichtsreacties, zoals de directe vorming van dimethylcarbonaat uit koolstofdioxide en methanol. Voor de acceptatie en toepassing op industriële schaal van dergelijke nieuwe ontwikkelingen zal robuustheid in het bijzonder cruciaal zijn. Het zal daarom noodzakelijk zijn om de betrouwbaarheid en de toegevoegde waarde van de nieuwe ontwikkelingen op pilot-schaal aan te tonen.

Contents Summary ...........................................................................................................2 Samenvatting....................................................................................................7 Contents...........................................................................................................11 1. Introduction ................................................................................................15 1.1 Introduction...........................................................................................16 1.2 Pervaporation ........................................................................................16 1.2.1 Polymer pervaporation membranes......................................................................18 1.2.2 Inorganic pervaporation membranes....................................................................18

1.3 Esterification..........................................................................................19 1.4 Pervaporation-esterification coupling ...............................................20 1.5 Aim and outline of this thesis .............................................................21 2. Design directions for composite catalytic membranes.......................27 2.1 Introduction...........................................................................................28 2.2 Pervaporation-coupled esterification model ....................................31 2.2.1 Reaction kinetics ......................................................................................................31 2.2.2 Reactor model ..........................................................................................................31

2.3 Results and Discussion ........................................................................36 2.3.1 Geometric properties...............................................................................................36 2.3.2 Effect of catalyst position and catalytic layer thickness......................................36 2.3.3 Effect of kinetic and membrane parameters.........................................................40

2.4 Conclusions ...........................................................................................42 3. Comparison of solid acid catalysts for the esterification of acetic acid with butanol ................................................................................................49 3.1 Introduction...........................................................................................50 3.2 Theory ....................................................................................................51 3.2.1 Esterification reactions ............................................................................................51

3.3 Experimental .........................................................................................52

3.3.1 Catalysts....................................................................................................................52 3.3.2 Characterisation techniques ...................................................................................53 3.3.3 Esterification procedure..........................................................................................55

3.4 Results and discussion .........................................................................55 3.4.1 Catalyst performance ..............................................................................................55 3.4.2 Amberlyst 15 ............................................................................................................64

3.5 Conclusions ...........................................................................................67 4. Synthesis of hollow fibre microporous silica membranes ................75 4.1 Introduction...........................................................................................76 4.2 Theory ....................................................................................................77 4.2.1 Gas permeation ........................................................................................................77 4.2.2 Pervaporation...........................................................................................................77

4.3 Experimental .........................................................................................78 4.3.1 Membrane preparation ...........................................................................................78 4.3.2 Membrane characterisation ....................................................................................79 4.3.3 Gas permeation ........................................................................................................79 4.3.4 Pervaporation...........................................................................................................80

4.4 Results ....................................................................................................81 4.4.1 Membrane characterisation ....................................................................................81 4.4.2 Gas permeation ........................................................................................................83 4.4.3 Pervaporation...........................................................................................................85

4.5 Conclusions ...........................................................................................89 5. Thin high flux ceramic-supported PVA membranes..........................93 5.1 Introduction...........................................................................................94 5.2 Experimental .........................................................................................94 5.2.1 Membrane preparation ...........................................................................................94 5.2.2 Membrane characterisation ....................................................................................95

5.3 Results ....................................................................................................97 5.3.1 Membrane characterisation ....................................................................................97

5.3.2 Pervaporation...........................................................................................................98 5.3.3 Membrane stability................................................................................................112

5.4 Conclusions .........................................................................................116 6. Preparation of zeolite-coated pervaporation membranes ................121 6.1 Introduction.........................................................................................122 6.2 Experimental .......................................................................................124 6.2.1 Activity of catalysts ...............................................................................................124 6.2.2 Coating and testing of the pervaporation membranes .....................................125

6.3 Results ..................................................................................................127 6.3.1 Activity of heterogeneous catalysts.....................................................................127 6.3.2 Coating of the ceramic pervaporation membranes ...........................................129 6.3.3 Pervaporation-esterification coupling.................................................................132

6.4 Conclusions .........................................................................................135 7. Preparation of Amberlyst-coated pervaporation membranes.........139 7.1 Introduction.........................................................................................140 7.2 Experimental .......................................................................................141 7.2.1 Activity of catalysts ...............................................................................................141 7.2.2 Coating and testing of the pervaporation membranes .....................................141

7.3 Results ..................................................................................................145 7.3.1 Amberlyst 15 activity ............................................................................................145 7.3.2 Amberlyst coating .................................................................................................146 7.3.3 Effect of the Amberlyst coating on the pervaporation performance...............147 7.3.4 Pervaporation-esterification coupling.................................................................149

7.4 Conclusions .........................................................................................155 8. Preliminary process design for catalytic pervaporation membrane-assisted esterification reactions.................................................159 8.1 Introduction.........................................................................................160 8.1.1 Conventional processes ........................................................................................160 8.1.2 Pervaporation.........................................................................................................162

8.2 Pervaporation-esterification processes ............................................163 8.2.1 Process configurations ..........................................................................................163 8.2.2 Operating parameters ...........................................................................................165

8.3 Reactor evaluation ..............................................................................167 8.4 Conclusions .........................................................................................170 9. Conclusions, recommendations and perspectives.............................175 9.1 Conclusions and recommendations .................................................175 9.2 Future perspectives ............................................................................181 9.3 Concluding remarks...........................................................................183 Dankwoord ...................................................................................................187 Personal .........................................................................................................189 List of publications......................................................................................191

Chapter 1 Introduction

Esterifications represent a significant group of reactions commonly found in the chemical industry. Esterifications are typical examples of reactions controlled by thermodynamic equilibrium and are commonly catalysed using liquid mineral acids. As a result, conventional esterification processes are faced with problems in product purification. In this perspective pervaporation is an alternative. Compared to distillation, the energy consumption is low, the reaction temperature is independent from the reflux temperature, and the separation efficiency is not solely determined by the relative volatility. Liquid mineral acid catalysts suffer from several drawbacks, such as a corrosive nature, the existence of side reactions, and the fact that the catalyst can not easily be separated from the reaction mixture. Solid acid catalysts offer an alternative since they are not corrosive and, coated onto a support, they can easily be reused. The present study aims to develop composite catalytic pervaporation membranes. These membranes enable close, and hence efficient, integration of reaction and separation, while allowing independent optimisation of the catalytic and separation function. The present chapter gives a general introduction on membrane technology, pervaporation processes, and the various types of pervaporation membranes. The beneficial aspects of pervaporationassisted esterifications using composite catalytic membranes are discussed. Finally, an outline of the contents of this thesis is given.

Chapter 1

1.1 Introduction Membrane technology has received considerable attention in the chemical industry during the last decades [Feng and Huang, 1997]. A membrane is a semi-permeable active or passive barrier, which selectively permits passage of one or more components of a mixture. Membranes are able to separate mixtures due to their ability to influence the transport of one or more components of the mixture to a larger extent, because of differences in physical and/or chemical interaction between the membrane and the permeating components [Ho and Sirkar, 1992]. Transport of a component through a membrane requires a driving force, for instance a pressure or a concentration (activity) difference across the membrane. Membranes are mainly used in purification processes, i.e., gas and liquid purification. Membranes can, however, also be used to enhance the conversion of a thermodynamically limited reaction by selectively removing one or more product species from the reacting mixture. The application of this principle has been mainly restricted to gas phase applications like dehydrogenations and synthesis gas production [Coronas and Santamaría, 1999]. However, there are many equilibrium limited liquid phase systems where the same principle can be applied. In this respect, pervaporation-esterification coupling has received the most attention [Liu et al., 2001], and will also be the main topic of this thesis.

1.2 Pervaporation Pervaporation is the separation of liquid mixtures by selective partial vaporization through a membrane. The word pervaporation is derived from the two basic steps of the process; permeation and evaporation. Transport of a specific component through the membrane is induced by the partial vapour pressure difference between the feed solution and the permeate [Baker, 2004]. Commonly, this difference is achieved by employing a sweep gas or using a vacuum at the permeate side. Figure 1.1 gives a schematic representation of pervaporation. The steps included in the separation are the sorption of the components in the membrane, diffusion across the membrane, and finally desorption into a vapour phase at the permeate side of the membrane. The selectivity

Introduction

is related to the sorption selectivity of the components in the membrane material and the relative mobility of the components through the membrane [Feng and Huang, 1997]. Membrane module Permeate (vapour)

Feed (liquid)

Retentate (liquid)

Figure 1.1 Basic schematic diagram of pervaporation.

The applications of pervaporation can be classified into three categories: (i) dehydration of organic solvents, (ii) removal of organic compounds from aqueous solutions, and (iii) separation of anhydrous organic mixtures. Pervaporation has been commercialized for two types of applications: one is the dehydration of alcohols and other solvents, and the other is the removal of small amounts of organic compounds from contaminated waters [Feng and Huang, 1997]. Currently, about one hundred pervaporation units are operating worldwide, most of them dehydrating solvents, such as ethanol and iso-propanol [Wynn, 2001]. In the pervaporation-coupled esterification process, the water removal capability of hydrophilic pervaporation membranes is utilised to selectively remove water from the reaction mixture. Polymeric materials are commonly used for this purpose. However, their temperature and solvent stability is rather limited. Compared to polymer membranes, inorganic membranes generally possess superior chemical and thermal stability [Verkerk et al., 2001]. Nevertheless, only a very few actual industrial processes based on inorganic membranes can be found, e.g., drinking and waste water treatment, separation in organic media, and purification of used oil [Sekulic, 2004a].

Chapter 1 1.2.1 Polymer pervaporation membranes

Polymer membranes can be divided into three classes: glassy, rubbery and ionic polymer membranes. Generally, for pervaporative dehydration processes hydrophilic rubbery polymer membranes, like poly(vinyl)alcohol and poly(acrylic)acid, are suitable due to their affinity for water [Feng and Huang, 1997]. The performance of these membranes, however, is greatly affected by changes in process conditions, like temperature, feed water concentration and pressure [Waldburger and Widmer, 1996; Verkerk et al., 2001]. Swelling of the membrane due to sorption of water may cause an increase in solubility and diffusivity of the unwanted component and consequently a decrease in membrane selectivity [Yeom et al., 2001; Guo et al., 2004]. In order to decrease the extent of swelling and improve membrane stability, a common approach proposed in the literature is to utilise cross-linked and/or blended polymeric membrane materials, that are optimised for specific chemical mixtures and operating conditions [Verkerk et al., 2001]. An alternative approach is to use ceramic materials as stable support structures for the polymeric membrane, thereby sufficiently increasing chemical and thermal stability in order to withstand anhydrous solvents and higher temperatures [Yoshida and Cohen, 2003]. 1.2.2 Inorganic pervaporation membranes The majority of inorganic membranes are porous and their selective features are often closely related to their pore size. Ceramic membranes obtained by the combination of a metal with a non-metal in the form of an oxide, nitride, or carbide form the main class of inorganic membranes. At present, ceramic membranes based on amorphous silica and zeolite membranes are mainly investigated for pervaporation purposes. Amorphous silica membranes for pervaporation purposes are prepared using the sol-gel technique in which a ceramic support is modified by dip-coating with a polymeric silica sol [de Lange et al., 1995]. Generally, membranes based on amorphous silica have an asymmetric structure with the actual selective microporous silica positioned on a macro/mesoporous support structure comprising several α- and γ-alumina layers. Silica membranes were discovered more than a decade ago and are still subject of

Introduction

extensive study. Especially the replacement of both γ-alumina and silica by a more chemical resistant material, like zirconia and titania is frequently being researched [van Gestel et al., 2002; Sekulic et al., 2004b]. Zeolite membranes, like ZSM-5, A, and T are good candidates for dehydration purposes due to their uniform pore size distribution and high hydrophilicity [Cui et al., 2004]. One of the promising applications of zeolite membranes is the acid-catalyzed esterification in which the membrane acts as both separator and catalyst [Li et al., 2003]. The reproducible preparation of defect-free membranes, however, is still challenging although different synthesis methods, like secondary growth, have been developed to improve membrane quality [Lovallo et al., 1996]. Additionally, zeolite membrane preparation methods are still carried out batch-wise whereas continuous methods are needed in order to scale-up production [Pina et al., 2004].

1.3 Esterification Esterification reactions are reversible reactions in which a carboxylic acid reacts with an alcohol to form an ester and water. Esterifications are typical examples of reactions controlled by thermodynamic equilibrium and are commonly catalysed using liquid mineral acids. As a result, conventional esterification processes are faced with problems in product purification. A large excess of one of the reactants or reactive distillation to accomplish in-situ removal of one of the products is typically used to drive the equilibrium to the ester side [Reid, 1952]. While employing a large excess of one of the reactants may lead to an inefficient use of the available reactor volume and a higher cost of subsequent separations to recover the unused reactant, reactive distillation is only effective when the difference between the volatility of product species and reactant species is sufficiently large [Feng and Huang, 1996]. Additionally, the preferred reaction temperature range should match the temperature for distillation and energy consumption can be significant due to large reflux ratios. Moreover, if the reaction mixture forms an azeotrope, a simple reactive distillation configuration is inadequate [Feng and Huang, 1996]. In this perspective, pervaporation has attracted attention as an alternative to conventional esterification processes.

Chapter 1

1.4 Pervaporation-esterification coupling Pervaporation is an ideal candidate to enhance conversion in reversible condensation reactions in which water is generated as a by-product. Water permeates preferentially through the membrane and evaporates at the permeate side, which drives the esterification reaction to the ester side. Pervaporation reactors are attractive because their energy consumption is low, since only heat is required for the evaporation of the permeating water. Additionally, the reaction can be carried out at a temperature independent from the reflux temperature, and the separation efficiency in pervaporation is not solely determined by the relative volatility as in reactive distillation [Benedict et al., 2003]. In most pervaporation-coupled esterification studies presented so far, the membranes used are catalytically inactive [Benedict et al., 2003; David et al., 1991; Domingues et al., 1999; Feng and Huang, 1996; Keurentjes et al., 1994; Krupiczka and Koszorz, 1999; Li and Lefu, 2001; Lim et al., 2002; Liu et al., 2001, Liu and Chen, 2002; Tanaka et al., 2001; Waldburger and Widmer, 1996; Zhu et al., 1996]. Several authors have used membranes in which the selective layer itself is catalytically active, and have shown that equilibrium displacement can be enhanced by close integration of production and removal of water [David et al., 1992; Bagnell et al., 1993; Bernal et al., 2002; Nguyen et al., 2003]. Additionally, solid acid catalysts offer an interesting alternative to liquid mineral catalysts since they are not corrosive and, coated onto a support, they can easily be reused. Moreover, this approach does not need an additional catalyst neutralization step, which is normally needed if a hom*ogeneous catalyst is used. The integration of the selective and catalytic function into one single layer, however, demands contradicting material properties. For example, to achieve a high water selectivity the diffusion of all components except water inside the material should be low, whereas efficient use of the catalytic properties requires the diffusion of the reactants to be high. The conflicting demands on materials properties can be avoided by accommodating the selective and catalytic features in two different distinct layers [Nguyen et al., 2003], which are in close physical contact, as it can be seen in Figure 1.2. This approach allows independent optimisation of the selective and the catalytic properties.

Introduction

Catalytic layer Selective layer Support layer

Figure 1.2 Esterification reaction in a composite catalytic membrane reactor.

1.5 Aim and outline of this thesis The present thesis aims to develop composite catalytic pervaporation membranes as the integration of reaction and separation offers advantages in terms of process efficiency. In Chapter 2, the technical viability of composite catalytic hollow fibre membranes for condensation reactions is examined. A parametric pervaporation-esterification model study was carried out to provide a fundamental understanding of the catalytic membrane reactor behaviour as a function of the operating conditions and design parameters. Chapter 3 deals with the heterogeneous catalysis of esterification reactions. Various acid catalysts are tested for their activity in the esterification between acetic acid and butanol. Chapter 4 deals with the preparation and separation properties of hollow fibre ceramic silica membranes. In Chapter 5, the preparation of composite ceramic/polymer pervaporation membranes and their pervaporation characteristics in the dehydration of various alcohols is described. Subsequently, the preparation of catalyst-coated ceramic silica and composite PVA membranes for the integration of reaction and separation is described in Chapters 6 and 7. This approach allows independent optimisation of the selective and the catalytic properties. A preliminary large-scale composite membrane reactor evaluation is carried out in Chapter 8, based on the obtained experimental data such as membrane permeability and catalyst activity. Finally, conclusions, recommendations and perspectives are given in Chapter 9.

Chapter 1

References Bagnall, L., Cavell, K., Hodges, A.M., Mau, A.W., Seen, A.J., The use of catalytically active pervaporation membranes in esterification reactions to simultaneously increase product yield and permselectivity flux, J. Membr. Sci., 85 (1993) 291-299. Baker, R.W., Membrane Technology and Applications, 2th edition, John Wiley & Sons, Ltd, Chichester, England, 2004. Benedict, D.J., Parulekar, S.J., Tsai, S-P., Esterification of Lactic Acid and Ethanol with/without pervaporation, Ind. Eng. Chem. Res., 42 (2003) 2282-2291. Bernal, M.P., Coronas, J., Menéndez, M., Santamaría, J., Coupling of reaction and separation at the microscopic level: esterification processes in a H-ZSM-5 membrane reactor, Chem. Eng. Sci., 57 (2002) 1557-1562. Coronas, J., Santamaría, J., Catalytic reactors based on porous ceramic membranes, Catal. Today, 51 (1999) 377-389. Cui, Y., Kita, H., Okamoto, K-I., Zeolite T membrane: preparation, characterization, pervaporation of water/organic liquid mixtures and acid stability. J. Membr. Sci, 236 (2004) 17-27. David, M.O., Gref. R., Nguyen, T.Q., Neel, J., Pervaporationesterification coupling. I. Basic kinetic model. Trans. IChemE, Part A, Chem. Eng. Res. Des., 69 (1991) 335-340. David, M.O., Nguyen, T.Q., Neel, J., Pervaporation membranes endowed with catalytic properties, based on polymer blends. J. Membr. Sci, 73 (1992) 129-141. Domingues, L., Recasens, F., Larrayoz, M., Studies of a pervaporation reactor: kinetics and equilibrium shift in benzyl alcohol acetylation, Chem. Eng. Sci., 54 (1999) 1461-1465. Feng, X., Huang, R.Y.M., Studies of a membrane reactor: esterification facilitated by pervaporation, Chem. Eng. Sci., 51 (1996) 4673-4679. Feng, X., Huang, R.Y.M., Liquid separation by membrane pervaporation: a review, Ind. Eng. Chem. Res., 36 (1997) 1048-1066.

Introduction

van Gestel, T., Vandecasteele, C., Buekenhoudt, A., Dotremont, C., Luyten, J., Leysen, R., van der Bruggen, B., Maes, G., Alumina and titania multilayer membranes for nanofiltration: preparation, characterization and chemical stability, J. Membr. Sci., 207 (2002) 73-89. Guo, W.F., Chung, T-S., Matsuura, T., Pervaporation study on the dehydration of aqueous butanol solutions: a comparison of flux vs. permeance, separation factor vs. selectivity, J. Membr. Sci., 245 (2004) 199-210. Ho, W. S. W., Sirkar, K.K., Membrane Handbook, Chapman & Hall, New York, 1992. Keurentjes, J.T.F., Janssen, G.H.R., Gorissen, J.J., The esterification of tartaric acid with ethanol: kinetics and shifting the equilibrium by means of pervaporation. Chem. Eng. Sci., 49 (1994) 4681-4689. Krupiczka, R., Koszorz, Z., Activity-based model of the hybrid process of an esterification reaction coupled with pervaporation, Sep. Pur. Technol., 16 (1999) 55-59. de Lange, R.S.A., Hekkink, J.H.A., Keizer, K., Burggraaf, A.J, Formation and characterization of supported microporous ceramic membranes by sol-gel modification techniques. J. Membr. Sci., 99 (1995) 57-75. Li, X., Lefu, W., Kinetic model for an esterification process coupled by pervaporation, J. Membr. Sci., 186 (2001) 19-24. Lim, S.Y., Park, B., Hung, F., Sahimi, M., Tsotsis, T.T., Design issues of pervaporation membrane reactors for esterification, Chem. Eng. Sci., 57 (2002) 4933-4946. Li, G., Kikuchi, E., Matsukata, M., Separation of water-acetic acid mixtures by pervaporation using a thin mordenite membrane, Sep. Pur. Technol., 32 (2003) 199-206. Liu, Q.L., Zhang, Z., Chen, H.F., Study on the coupling of esterification with pervaporation, J. Membr. Sci., 182 (2001) 173-181. Liu, Q.L., Chen, H.F., Coupling of esterification of acetic acid with n-butanol in the presence of Zr(SO4)2.4H2O coupled pervaporation, J. Membr. Sci., 196 (2002) 171-178.

Chapter 1

Lovallo, M.C., Tsapatsis, M., Okubo, T., Preparation of an asymmetric zeolite L film, Chem. Mater., 8 (1996) 1579-1583. Nguyen Q.T., M’Bareck C.O., David, M.O., Métayer, M., Alexandre, S., Ion-exchange membranes made of semi-interpenetrating polymer networks, used for pervaporation-assisted esterification and ion transport, Mat. Res. Innovat., 7 (2003) 212-219. Pina, M.P., Arruebo, M., Felipe, M., Fleta, F., Bernal, M.P., Coronas, J., Menéndez M., Santamaría J., A semi-continuous method for the synthesis of NaA zeolite membranes on tubular supports, J. Membr. Sci., 244 (2004) 141-150. Reid, E.E., Esterification, in Unit Processes in Organic Synthesis, 4th edition Groggins, P.H. (ed), McGraw Hill, New York, USA, 1952, pp 596-642. Sekulic, J., Mesoporous and microporous titania membranes, Ph.D. thesis, University of Twente, Fedodruk BV, Enschede, The Netherlands, 2004a. Sekulić, J., ten Elshof, J. E., Blank, D.H.A., A microporous titania membrane for nanofiltration and pervaporation, Adv. Mater., 16 (2004b) 1546-1550. Tanaka, K., Yoshikawa, R., Ying, C., Kita, H., Okamota, K., Application of zeolite membranes to esterification reactions, Cat. Today, 67 (2001) 121-125. Verkerk, A.W., van Male, P., Vorstman, M.A.G., Keurentjes, J.T.F., Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures, Sep. Purif. Technol., 22 and 23 (2001) 689-695. Waldburger, R.M., Widmer, F., Membrane reactors in chemical production processes and the application to the pervaporation-assisted esterification, Chem. Eng. Technol., 19 (1996) 117-126. Wynn, N., Pervaporation comes of age, CEP magazine, 97 (2001) 66-72. Yeom, C.K., Lee, S.H., Lee, J.M., Pervaporative permeations of hom*ologous series of alcohol aqueous mixtures through a hydrophilic membrane, J. Appl. Pol. Sci., 79 (2001) 703-713.

Introduction

Yoshida, W., Cohen, Y., Ceramic-supported polymer membranes for pervaporation of binary organic/organic mixtures, J. Membr. Sci., 213 (2003) 145-157. Zhu, Y., Minet, R.G., Tsotsis, T.T., A continuous pervaporation membrane reactor for the study of esterification reactions using a composite polymeric/ceramic membrane, Chem. Eng. Sci., 51 (1996) 4103-4113.

Chapter 2 Design directions for composite catalytic hollow fibre membranes

A parametric model study was carried out to provide a fundamental understanding of the composite catalytic membrane reactor behavior in pervaporation assisted esterifications. With increasing catalytic layer thickness, the conversion becomes no longer limited by the amount of catalyst present in the reactor but by diffusion in the catalytic layer. An optimum catalytic layer thickness was found to be around 100 µm under the prevailing conditions, which is within practically reachable dimensions. At this optimum catalytic layer thickness, the performance of a catalytic membrane reactor exceeds the performance of an inert membrane reactor due to the close integration of reaction and separation. This shows the potential added value of such a membrane system compared to more usual reactor designs.

This chapter is based on: Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Keurentjes, J.T.F., Design directions for composite catalytic hollow fibre membranes for condensation reactions, Trans. IChemE, Part A, Chem. Eng. Res. Des., 82 (2004) 220-228.

Chapter 2

2.1 Introduction The use of membranes in the chemical industry has received considerable attention during the last decades. Membranes can be used to enhance the conversion of a thermodynamically limited reaction by selectively removing one or more product species from the reacting mixture. The application of this principle has, in practice, been mainly restricted to gas phase applications like dehydrogenations and synthesis gas production. However, there are many equilibrium-limited liquid phase systems where the application of the same concept is obvious. In this respect, pervaporation-esterification coupling has received the most attention [Liu et al., 2001]. Esterification reactions are limited by a thermodynamic equilibrium. To drive the equilibrium to the ester side, it is typical to use a large excess of one of the reactants, usually the alcohol, or to use reactive distillation to accomplish in-situ removal of products [Reid, 1952]. While employing a large excess of one of the reactants may lead to an inefficient use of the available reactor volume and a higher cost of subsequent separations to recover the unused reactant, reactive distillation is only effective when the difference between the volatility of product species and reactant species is sufficiently large [Feng and Huang, 1996]. Additionally, the preferred reaction temperature range should match that for the distillation, energy consumption can be significant due to the large reflux ratios and, in the case where the reaction mixture forms an azeotrope, a simple reactive distillation configuration is inadequate [Feng and Huang, 1996]. Pervaporation is an ideal candidate to enhance conversion in reversible condensation reactions, generating water as a by-product. In pervaporation-esterification coupling, one side of a membrane is exposed to the liquid phase, while at the other side of the membrane vacuum is maintained. Water permeates preferentially through the membrane and evaporates. The water is then removed on the low pressure permeate side driving the esterification reaction to the ester-side. Pervaporation reactors are attractive because the energy consumption is low, the reaction can be carried out at the optimal temperature, and the separation efficiency in pervaporation is not determined by the relative volatility as in reactive distillation [Benedict et al., 2003].

Design directions for composite catalytic hollow fibre membranes

In most pervaporation-coupled esterification studies presented so far, the membranes used are catalytically inactive, and are operated batch wise [Li and Lefu, 2001; Tanaka et al., 2001; Liu et al., 2001; Liu and Chen, 2002] or placed in a recycle loop [David et al., 1991; Keurentjes et al., 1994; Feng and Huang, 1996; Domingues et al., 1999; Krupiczka and Koszorz, 1999; Benedict et al., 2003]. Several authors investigated the coupled pervaporation-esterification process in a continuous-flow inert membrane reactor [Zhu et al., 1996; Waldburger and Widmer, 1996; Lim et al., 2002; Bernal et al., 2002] to obtain advantages in terms of compactness and process efficiency by combining the membrane and reactor into the same unit. In inert membrane reactors, the membrane does not participate in the reaction directly: it is used to add reactants to, or to remove products from the reactor. David et al., [1992], Bagnell et al., [1993], Bernal et al., [2002] and Nguyen et al. [2003], however, used catalytically active membranes to carry out esterifications of different alcohols. They showed that the equilibrium displacement could be enhanced through reaction coupling if the membrane itself provides the catalytic function. In this case, however, the membranes turn out to be not very selective to water in the esterification mixtures [Nguyen et al., 2003]. Therefore, it was suggested to make use of a composite catalytic membrane to avoid reactant and ester loss. In this way, both the selective layer and the catalytic layer of the composite catalytic membrane can be optimized separately. The principle of an esterification reaction in a composite catalytic membrane reactor is schematically shown in Figure 2.1.

Catalytic layer Selective layer Support layer

Figure 2.1 Esterification reaction in a catalytic membrane reactor.

Chapter 2

The acid and alcohol diffuse into the catalytic layer. In the catalytic layer, the acid and alcohol are converted to the ester and water. The formed ester diffuses back towards the bulk liquid whereas water is removed insitu through the membrane due to the close integration of reaction and separation. Consequently, the hydrolysis of the formed ester is reduced and the attained conversion will be increased compared to the inert membrane reactor [David et al., 1992; Bagnall et al., 1993; Bernal et al., 2002]. Additionally, heterogeneous catalysts might be used more efficiently due to the thinness of the catalytic layer present on the membrane support [Vergunst et al., 2001]. Other advantages are that no catalyst neutralisation or recovery is needed. Our research aims to develop a continuous catalytic pervaporation membrane reactor as both integration of reaction and separation and continuous operation may offer advantages in terms of process efficiency and compactness. The present chapter examines the viability of composite catalytic hollow fibre membranes for condensation reactions. The esterification reaction between acetic acid and butanol has been taken as a model reaction, for which a parametric pervaporation-esterification model study was carried out to provide a fundamental understanding of the catalytic membrane reactor behaviour. The presented parametric study helps the identification of critical (process) parameters and aids in an optimal design of the envisioned catalytic membrane. The influence of catalyst position, catalytic layer thickness, reaction kinetics and the membrane permeability on the performance of the catalytic membrane reactor are investigated.

Design directions for composite catalytic hollow fibre membranes

2.2 Pervaporation-coupled esterification model 2.2.1 Reaction kinetics Esterification is a reversible reaction in which a carboxylic acid (A) reacts with an alcohol (B) in the presence of an acid catalyst to form an ester (E) and water (W). This type of reaction can be written as

R − COOH + R ' − OH ( A)

( B)

R − COO − R ' + H 2O (E)

(W )

(2.1)

Assuming first order reaction kinetics for all the components, the rate of esterification can be represented by kfw [A][B] whereas the rate of hydrolysis can be represented by kbw [E][W], where the quantities between brackets represent the molar concentration of the reacting species. Thus, for concentrations at chemical equilibrium, then

k fw kbw

= Ki =

[ E ] ⋅ [W ] [ A] ⋅ [ B]

(2.2)

The constant K is called the equilibrium constant of the reaction. The rate of ester production can be written as

[ E ] ⋅ [W ]   RE = k fw ⋅  [ A] ⋅ [ B] −  K  

(2.3)

2.2.2 Reactor model The hollow fibre module considered in the modelling in this paper is a single-pass reactor without recirculation of reactants. The reactor consists of tubular catalytic hollow fibres mounted in a module. The structure of the modules is depicted in Figure 2.2 and the geometric properties are summarized in Table 2.2.

Chapter 2

Table 2.2

Geometric parameters for the catalytic membrane reactor.

hydraulic diameter

[m]

dh =

d 2 m ,i − N f ⋅ d 2 f , o

(2.4)

d m ,i + N f ⋅ d f , o  d f ,o + 2 ⋅ δ cat    d m ,i  

module void fraction

εm

[-]

ε m = 1 − N f ⋅ 

initial module void fraction

εm,in

[-]

ε m ,in = 1 − N f ⋅  f ,o   d m ,i 

geometric surface area

[m-1]

am =

volume fraction of catalyst

φcat

[-]

φcat =

d



(2.5)

(2.6)

4 ⋅ d f ,o ⋅ N f L ⋅ ( d 2 m ,i − N f ⋅ ( d f ,o + 2 ⋅ δ cat ) 2 )

[

Nf ⋅ ( d f , o + 2 ⋅ δ cat ) 2 − d 2 f , o (d

m ,i

− Nf ⋅ d

f ,o

]

(2.7)

(2.8)

)

df,o = 2 mm

dm,i

δcat

df,o

Figure 2.2 Definition of geometric properties for a composite catalytic membrane module.

One of the important issues in the development of membrane reactors is to obtain a sufficient high ratio of membrane surface over reactor volume. In the modelling in this paper, hollow fibres are used, leading to a high membrane area over volume ratio. Since ceramic hollow fibre membranes are commercially available with the water selective layer on the outside of the support we have chosen to have the reacting liquid at the shell-side of the membrane. The composite catalytic membranes consist of a support

Design directions for composite catalytic hollow fibre membranes

coated with a water selective layer and on top of that a porous catalytic layer. In the development of the pervaporation-coupled esterification model, several simplifying assumptions have been made. I. The membrane reactor behaves as an ideal isothermal plug-flow reactor; axial diffusion in the catalytic layer and in the liquid is not taken into account; II. External transport limitations on the permeate-side are negligible under the pressure conditions at the permeate side. Concentration polarization effects in the shell-side, however, are taken into account; III. The membrane is completely water selective, so no other species than water will permeate through the membrane; IV. The catalyst particles are assumed to be non-porous and the catalytic activity is uniformly distributed over the catalyst coating layer. The catalytic membrane reactor is modelled using a one-dimensional catalyst model with mass transfer to and from the catalytic layer and diffusion and reaction occurring in the catalytic layer. The mathematical description of the catalytic membrane reactor is depicted in Figure 2.3 and the governing equations are summarized in Table 2.3.

δz r

Fout

Fin Cin

Cout

δr

bulk fluid polarisation layer catalytic layer membrane

Nw Figure 2.3 Coordinate system for the modelling of a catalytic membrane reactor and proposed concentration profiles.

Chapter 2 Table 2.3

Model equations for the catalytic membrane reactor.

Mass balances • Liquid bulk

d ( F ⋅ Ci )  ∂C  2 = − 2 ⋅ π ⋅ rcat ⋅ N f ⋅ De ,i ⋅  i  + Rb ,i ⋅ ε m ⋅ π ⋅ rm ∂ dz r   rcat with De ,i =

• Catalytic layer

∂ 2Ci ∂r

ε cat ⋅D ζ cat i

(2.10)

1 ∂C i ε cat ⋅ R cat , i ⋅ + =0 r ∂r D e ,i with Rcat ,i = k r ⋅

(2.9)

(2.11)

(1 − ε cat ) ⋅ ρ cat

ε cat

⋅ (a A ⋅ a B −

a E ⋅ aW ) K

(2.12)

Boundary conditions • Membrane-catalytic layer

 ∂C  * Pi ⋅ (γ i ⋅ x i ⋅ p i ) − ( y i ⋅ p p ) = De ,i ⋅  i   ∂r  rmem

(2.13)

• Catalytic layer-bulk interface

 ∂C  De ,i ⋅  i  = k F ,i ⋅ (C b ,i − C mem ,i )  ∂r  rcat

(2.14)

Mass transfer polarisation layer

[

Sh =

]

k F ,i ⋅ d h Di

= 3.66 + 1.2 ⋅

(

(1 − ε m ) )

− 0 .8

(2.15)

The change in bulk concentration of the reactants and products is calculated as the sum of the mass transfer from the bulk liquid to the catalytic layer and the reaction rate in the bulk liquid (equation 2.9). The catalytic layer surface concentration is required for this calculation. This concentration is calculated from a mass balance over a slice of the catalytic layer at axial position, z, and a mass balance across the catalytic layer surface. Under steady state conditions, the net rate of water diffusion through a concentric slice of width dr into the water selective membrane at the radial position rmem must equal the water flux through the membrane water selective layer (equation 2.13). The water flux through the membrane has been calculated using the partial water vapour pressure difference between the retentate and the permeate side of the membrane multiplied by the permeance of the membrane [Verkerk et al., 2001]. Verkerk et al. [2001] calculated a water permeance value, Pw, of 2·10-6 mol·m-2·s-1·Pa-1 for a typical ceramic pervaporation membrane for the dehydration of 1-butanol

Design directions for composite catalytic hollow fibre membranes

water mixtures. In pervaporation-assisted esterification reactions, however, water fluxes are strongly reduced [Verkerk, 2003]. This reduction in water flux is most likely a result of the adsorption of the alcohol onto the selective silica layer and the supporting layers whereby the alcohol imparts hydrophobic characteristics to the membrane surface [Font et al., 1996; Dafinov et al., 2002]. Therefore, a water permeance value, Pw, of 2·10-7 mol·m-2·s-1·Pa-1 is used in the simulations. Equation 2.14 defines the requirement that the transport from the bulk liquid to the catalytic surface by mass transfer equals the transport into the catalytic layer. The mass transfer coefficient in the concentration polarisation layer is calculated using general empirical Sherwood correlations. For laminar flow in direction parallel to the fibres, for a fully developed hydrodynamic and concentration profile, the Sherwood number is expressed by equation 2.15 [Lipnizki and Field, 2001]. The model is solved in Matlab using a Runge-Kutta procedure in the axial direction coupled with a differential element method for the mass transfer and reaction in the radial direction. During each iteration, all activity coefficients, calculated by means of the UNIFAC method [Fredenslund et al., 1975], all diffusion coefficients [Wilke and Chang, 1955; Leffler and Cullinan, 1970], and the viscosity [Poling et al., 2001] and density [Perry et al., 1997] of the mixture are calculated. Table 2.4 shows the feed composition and operating conditions used in the calculations of the catalytic membrane reactor for condensation reactions.

Table 2.4 Feed composition and operating conditions used in the modelling of the catalytic membrane reactor. Operating conditions T 348 p 100 TP 348 pP 6 τ 1 Feed composition initial equimolar concentration

K kPa K kPa hr

Membrane properties PW 2·10-7 mol·m-2·s-1·Pa-1 εcat 0.5 ζcat 2 Catalytic properties ρcat 0.9 g·cm-3 -8 kobs 1.8·10 m3·mol-1·gcat-1·s-1

Chapter 2

The initial module void fraction, εm,in (equation 2.6), and the catalytic layer thickness, δcat, were varied. The coating thickness 2was varied between 1 and 400 µm, whereas the initial module void fraction was varied between 0.2 and 0.5. For the performance of a pervaporation-coupled esterification process, the ratio of water removal to water production is found to be the key factor, for which the ratio of membrane area to reactor volume, amount of available catalyst and residence time in the membrane reactor are the major influencing parameters [Liu et al. 2001]. In this modelling, the ratio of feed flow rate to membrane surface area and the amount of catalyst present in the reactor are used to represent the ratio of water removal to water production.

2.3 Results and Discussion 2.3.1 Geometric properties The geometric properties of the catalytic membrane reactor will change if the catalytic layer thickness and the initial module void fraction are changed during the modelling. To minimise the influence of this effect on the modelling results, the required diameter of the membrane module is chosen to increase with increasing catalytic layer thickness to maintain a constant residence time in the module. This implies that the module void fraction decreases and thus, the mass transfer coefficient in the concentration polarisation layer will change according to equation 2.15. However, this proves to have a minor influence on the attained performance compared to the influence of the ratio of feed flow rate to membrane surface area and the amount of catalyst present in the reactor. 2.3.2 Effect of catalyst position and catalytic layer thickness The viability of a composite catalytic membrane reactor can be examined by comparing the performance of the composite catalytic membrane reactor with the performance of an inert membrane reactor. Figure 2.4 shows the effect of catalyst position on the performance of the catalytic membrane reactor. Two simulations are performed with the parameters shown in Table 2.4, the first with the composite catalytic membrane reactor (CMR) and the second one with an inert membrane reactor (IMR) with the same amount of catalyst but now dispersed in the liquid bulk of the reactor.

Design directions for composite catalytic hollow fibre membranes

CMR

IMR

Figure 2.4 Effect of catalyst position and catalytic layer thickness on module performance; F/Amem = 3.5*10-4 m s-1.

For the inert membrane reactor, the conversion increases monotonically with an increase in the amount of heterogeneous catalyst, since complete catalyst effectiveness is assumed. For the catalytic membrane reactor, however, a maximum in reached conversion is observed. It can also be seen that the performance of the catalytic membrane reactor exceeds the performance of the inert membrane reactor at relatively thin catalytic layers. This shows the potential added value of a composite catalytic membrane reactor compared to more usual reactor designs. This is similar to the results obtained by Bernal et al. [2002]. Based on experiments, it was concluded that an integration of reaction and separation at the microscopic level has numerous potential applications. The esterification reaction between acetic acid and ethanol was studied, for which the performance of a catalytic membrane reactor exceeded the performance of an inert membrane reactor with the same amount of heterogeneous catalyst located in the bulk liquid. In a heterogeneous catalytic reaction employing porous catalysts, the reaction rate of an individual reaction is influenced by the following processes occurring in series and parallel: external transport from the bulk liquid to the catalyst surface, intraparticle transport (porous diffusion) and

Chapter 2

the kinetics of the reactions at the catalyst surface (adsorption/desorption and surface reaction steps). Two parameters that are used to indicate if mass transfer limitations are pronounced are the internal and external effectiveness factor. These are shown in Figure 2.5a and b, respectively. Figure 2.5a shows the external effectiveness factor, defined as the concentration of component i on the surface of the porous catalytic layer divided by that in the bulk liquid, over the length of the membrane reactor.

Figure 2.5 (a) External effectiveness factor over the length of the membrane reactor; δcat = 100 µm, F/Amem = 3.5*10-4 m s-1; (b) Internal effectiveness factor as a function of the catalytic layer thickness.

The external effectiveness factor indicates that external mass transfer limitations can be neglected. The internal effectiveness factor is defined as the actual rate of reaction in the whole catalytic layer divided by the reaction rate evaluated at outer surface conditions [Xu and Chuang, 1997]. For this, the concentration profile within the catalytic layer is needed. This concentration profile is calculated using equation 2.11. Figure 2.5b demonstrates that the internal effectiveness factor of the catalytic layer decreases with increasing catalytic layer thickness, indicating substantial internal mass transfer limitations at increasing catalytic layer thickness. At catalytic layers thicker than 50 µm, the diffusion of reactants inside the catalytic layer cannot keep up with the reaction rate and mass transfer limitations become pronounced, whereas at a thin catalytic layer, the amount of available catalyst limits the conversion rate. Additionally, the

Design directions for composite catalytic hollow fibre membranes

water removal capacity of the catalytic membrane becomes lower with increasing catalytic layer thickness, as water has to diffuse through a longer path before it is removed through the membrane. Figures 2.6a and b show typical two-dimensional water concentration profiles in the catalytic layer for a relatively thin and a thick catalytic layer, respectively.

Figure 2.6 Water concentration profiles in the catalytic layer for different catalytic layer thickness; (a) δcat = 25 µm; (b) δcat = 200 µm; F/Amem = 3.5*10-4 m s-1.

It can be seen that the water concentration in the radial direction of the catalytic layer becomes higher with increasing catalytic layer thickness. Due to the higher catalyst loading, water production is higher as reaction rates are enhanced and consequently, water build-up in the bulk liquid is increased since not all the water formed is removed through the membrane but instead diffuses towards the bulk liquid. The internal mass transfer limitations at thicker catalytic layers and deficiency of catalyst at thin catalytic layers lead to an optimum catalytic layer thickness. From Figure 2.4 it can be seen that this optimum in catalytic layer thickness is observed at a thickness of 100 µm under the prevailing conditions. The concentration profiles along the length of the reactor for an initial module void fraction of 0.2 and a catalytic layer thickness of 100 µm are presented in Figure 2.7.

Chapter 2

Figure 2.7 Bulk concentration of the components and reached conversion in the membrane reactor; δcat = 100 µm, F/Amem = 3.5*10-4 m s-1

It can be seen that acetic acid and butanol are converted rapidly in the beginning of the reactor, whereas the reaction rate is lower at the end of the reactor. At the entrance of the membrane reactor, water concentration in the bulk liquid increases as a part of the water formed in the catalytic layer diffuses towards the bulk liquid. Gradually, water concentration in the bulk liquid decreases as more water is removed at the low pressure permeate side of the membrane than is formed by reaction. Consequently, the water concentration in the membrane reactor is reduced because water is removed in-situ after formation. Thus, hydrolysis of the formed ester can be neglected and, compared to the inert membrane reactor, reaction rates will be enhanced. 2.3.3 Effect of kinetic and membrane parameters The optimum catalytic layer thickness is a function of the reaction rate constant and the membrane permeability. From Figures 2.8 and 2.9 it can be seen that an increase in catalyst activity, which may be realised by increasing the temperature, suggests a lower optimum catalytic layer thickness whereas an increase in membrane permeability gives a thicker optimum catalytic layer thickness.

Design directions for composite catalytic hollow fibre membranes

Figure 2.8 Effect of catalyst activity on module performance; δcat = 100 µm, F/Amem = 3.5*10-4 m s-1.

Figure 2.9 Effect of membrane permeability on module performance; δcat = 100 µm, F/Amem = 3.5*10-4 m s-1.

Due to the higher reaction rates with increasing catalyst activity, the diffusion of reactants inside the catalytic layer cannot keep up with the reaction rate and mass transfer limitations become pronounced. Additionally, a faster water build-up in the reactor due to the higher reaction rates causes the optimum catalytic layer thickness to decrease as

Chapter 2

the water removal from the membrane reactor is increased if the catalytic layer thickness decreases. The attained conversion increases with an increased catalyst activity. This occurs only up to a certain level, as shown in Figure 2.8 since at a certain catalyst activity mass transfer limitations become pronounced. A higher membrane permeability, however, increases the water removal capacity resulting in a thicker optimum catalytic layer as can be seen by Figure 2.9. It is shown that the reactor performance is hardly affected by the membrane if the membrane permeability is really low. On the other hand, no clear optimum in catalytic layer thickness is observed if the membrane permeability has a value of 2·10-6 mol·m-2·s-1·Pa-1.

2.4 Conclusions The developed mathematical model can be used to provide design rules for composite catalytic hollow fibre membranes for condensation reactions. The composite catalytic membranes used in the model consist of a support coated with a water selective layer and on top of that a porous catalytic layer. Equilibrium displacement can be enhanced through integration of reaction and separation by means of composite catalytic membranes. An optimum catalytic layer thickness was found to be around 100 µm under the prevailing conditions, which is within practically reachable dimensions. The exact position of this optimum is a function of the reaction rate constant and the permeability of the membrane used. At this optimum catalytic layer thickness, the performance of a catalytic membrane reactor exceeds the performance of an inert membrane reactor due to the close integration of reaction and separation, which shows the potential added value of such a membrane system compared to more usual reactor designs. With increasing catalytic layer thickness, the conversion becomes no longer limited by the amount of catalyst present in the reactor but by diffusion in the catalytic layer. External mass transfer was never found to be rate-limiting. Based on the solution method developed, catalytic membrane modules can be designed conveniently if both the reaction kinetic parameters and the membrane parameters are known.

Design directions for composite catalytic hollow fibre membranes

Acknowledgements This work was performed in a cooperative project of the Centre for Separation Technology and was financially supported by TNO and NOVEM.

Nomenclature Ci

concentration

[mol·m liq -3]

diameter

[m]

De,i

effective diffusion coefficient

[mliq2·s-1]

diffusion coefficient

[mliq2·s-1]

flow rate

[mliq3·s-1]

reaction rate constant

[mliq3·mol-1·s-1]

Chen, Z., Iizuka, T., Tanabe, K., Niobic acid as an efficient catalyst for vapor phase esterification of ethyl alcohol with acetic acid, Chem. Lett., (1984) 1085-1088. Chen, X., Xu, Z., Okuhara, T., Liquid phase esterification of acrylic acid with 1-butanol catalyzed by solid acid catalysts, Appl. Catal. A: General, 180 (1999) 261-269. Corma, A., Garcia, H., Iborra, S., Primo, J., Modified faujasite zeolites as catalysts in organic reactions: esterification of carboxylic acids in the presence of HY zeolites, J. Catal., 120 (1989) 78-87. Gangadwala, L., Mankar, S., Mahajani, S., Esterification of acetic acid with butanol in the presence of ion-exchange resins as catalysts, Ind. Eng. Chem. Res., 42 (2003) 2146-2155. Guo, C., Qian, Z., Acidic and catalytic properties of niobic acid crystallized at low temperature, Catal. Today, 16 (1993) 379-385. Hino, M., Arata, K., Synthesis of esters from acetic acid with methanol, ethanol, propanol, butanol, and isobutyl alcohol catalyzed by solid superacid, Chem. Lett., (1981) 1671-1672. Hino, M., Arata, K., Solid catalysts treated with anions. 13. Synthesis of esters from terephthalic and phthalic acids with n-octyl and 2-ethylhexyl alcohol, acrylic acid with ethanol and salicylic acid with methanol catalyzed by solid superacid, Appl. Catal., 18 (1985) 401-404. Hiyoshi, M.¸ Lee, B., Lu, D., Hara, M., Kondo, J.N., Domen, K., Supermicroporous niobium oxide as an acid catalyst, Catal. Lett., 98 (2004) 181-186. Hoek, I., Nijhuis, T.A., Stankiewicz, A.I., Moulijn, J.A., Kinetics of solid acid catalyzed etherification of symmetrical primary alcohols: Zeolite BEA catalyzed etherification of 1-octanol, Appl. Catal. A: General, 266 (2004) 109-116. Hu, J., Wei, C., Qiu, F., Xu, Z., Chen, Q., Preparation of a new type solid acid SO42-/Fe2O3-ZrO2-SiO2 and its application in the esterification of acetic acid and butanol, J. Nat. Gas Chem., 9 (2000) 212-216.

Solid acid catalysis for the esterification of acetic acid with butanol

Iizuka, T., Fujie, S., Ushikubo, T., Chen, Z.H., Tanabe, K., Esterification of acrylic acid with methanol over niobic acid catalyst, Appl. Catal., 28 (1986) 1-5. Kirumakki, S.R., Nagaraju, N., Chary, K.V.R., Narayanan, S., Kinetics of esterification of aromatic carboxylic acids over zeolites Hb and HZSM5 using dimethyl carbonate, Appl. Catal. A: General, 248 (2003) 161-167. Kirumakki, S.R., Nagaraju, N., Narayanan, S., Kinetics study of esterification of acetic acid with isobutanol in the presence of amberlite catalyst, Appl. Catal. A: General, 273 (2004) 1-9. Kuriakose, G., Nagaraju, N., Selective synthesis of phenyl salicylate (salol) by esterification reaction over solid acid catalysts, J. Mol. Cat. A: Chem., 223 (2004) 155-159. Lee, M.J., Chiu, J.Y., Lin, H.M., Kinetics of catalytic esterification of propionic acid and n-butanol over Amberlyst 35, Ind. Eng. Chem. Res., 41 (2002) 2882-2887. Lilja, J., Murzin, D. Yu, Salmi, T., Aumo, J., Mäki-Arvela, P., Sundell, M., Esterification of different acids over heteregenous and hom*ogeneous catalysts and correlation with the Taft equation, J. Mol. Cat. A: Chem., 182-183 (2002) 555-563. Liu, W.T., Tan, C.S., Liquid-Phase Esterification of propionic acid with n-butanol, Ind. Eng. Chem. Res., 40 (2001) 3281-3286. Ma, Y., Wang, Q.L., Yan, H., Ji, X., Qiu, Q., Zeolite-catalyzed esterification. I. Synthesis of acetates, benzoates and phthalates, Appl. Catal. A: General , 139 (1996) 51-57. Machado, M. da S., Pérez-Pariente, J., Sastre, E., Cardoso, D., de Guereñu, A.M., Selective synthesis of glycerol monolaurate with zeolitic molecular sieves, Appl. Catal. A: General , 203 (2000) 321-328. Manohar, B., Reddy, V.R., Reddy, B.M., Esterification by ZrO2 and Mo-ZrO2 eco-friendly solid acid catalysts, Synth. Commun., 28 (1998) 3183-3187.

Chapter 3

Mazzotti, M., Neri, B., Gelosa, D., Kruglov, A., Morbidelli, M., Kinetics of liquid-phase esterification catalyzed by acidic resins, Ind. Eng. Chem. Res., 36 (1997) 3-10. Namba, S., Wakushima, Y., Shimizu, T., Yashima, T., Catalytic application of hydrophobic properties of high-silica zeolites. II. Esterification of acetic acid with butanols, Stud. Surf. Sci. Cat., 20 (1985) 205-211. Nowak, I., Ziolek, M., Niobium compounds: preparation, characterization, and application in heterogeneous catalysis, Chem. Rev., 99 (1999) 3603-3624. Srinivas, N., Singh, A.P., Ramaswamy, A.V., Finiels, A., Moreau, P., Shape-selective aklylation of 2-methoxynaphtalene with tert-butanol over large-pore zeolites, Cat. Letters, 80 (2002) 181-186. Tanabe, K., Niobic acid as an unusual acidic solid material, Mat. Chem. and Phys., 17 (1987) 217-225. Thorat, T.S., Yadav, V.M., Yadav, G.D., Esterification of phthalic anhydride with 2-ethylhexanol by solid superacidic catalysts, Appl. Catal. A: General, 90 (1992) 73-96. Wang B., Lee, C.W., Cai, T-X., Park, S-E., Identification and influence of acidity on alkylation of phenol with propylene over ZSM-5, Cat. Letters, 76 (2001) 219-224. Wilke, C.R., Chang, P., Correlation of Diffusion Coefficients in Dilute Solutions, AIChE J., 1 (1955) 264-270. Xu, Z.P., Chuang, K.T., Kinetics of acetic acid esterification over ion exchange catalysts, Can. J. Chem. Eng., 74 (1996) 493-500. Xu, Z.P., Chuang, X.T., Effect of internal diffusion on heterogeneous catalytic esterification of acetic acid, Chem. Eng. Sci., 52 (1997) 3011-3017. Yadav, G.D., Mehta, P.H., Heterogeneous catalysis in esterification reactions: preparation of phenethyl acetate and cyclohexyl acetate by using a variety of solid acidic catalysts, Ind. Eng. Chem. Res., 33 (1994) 2198-2208.

Solid acid catalysis for the esterification of acetic acid with butanol

Yadav, G.D., Nair, J.J., Sulfated zirconia and its modified versions as promising catalysts for industrial processes, Micr. Meso. Mater., 33 (1999) 1-48. Yadav, G.D., Thathagar, M.B., Esterification of maleic acid with ethanol over cation-exchange resin catalysts, React. Funct. Polym., 52 (2002) 99-100. Zhang, H.B., Zhang, K., Yuan, Z.Y., Zhao, W., Li, H.X., Surface acidity of zeolite HZSM-5 and its catalytic properties in esterification of acetic acid with alcohols, J. Nat. Gas Chem., 6 (1997) 228-236. Zhang, H.B., Zhang, K., Yuan, Z.Y., Zhao, W., Li, H.X., Esterification of acetic acid with n-amyl alcohol over zeolite beta, J. Nat. Gas Chem., 7 (1998) 336-345.

Chapter 4 Synthesis of hollow fibre microporous silica membranes Thin microporous silica membranes were prepared on the outer surface of hollow fibre ceramic substrates. In principle this enables relatively fast and inexpensive production of large membrane surface area, combined with a low support resistance and a high membrane surface area / module volume ratio (> 1000 m2·m-3) compared to tubular membranes. Membranes were analyzed using SEM, SNMS, single gas permeance and pervaporation. High He permeance (1.1-2.9·10-6 mol⋅m-2⋅s-1⋅Pa-1), high He/N2 permselectivity (~ 100-1000) and Arrhenius type temperature dependence of gas permeance indicate that the membranes are microporous and possess a low number of defects. The contribution of the hollow fibre substrate to the overall mass transport resistance is found to be small, even for fast permeating gases like helium. In the dehydration of 1-butanol (80°C, 5 wt.% water) initially a high flux and selectivity were observed (2.9 kg·m-2·h-1 and 1200, respectively).

This chapter is based on: Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Benes, N.E., van Dam, R.A., Vroon, Z.A.E.P., van Soest-Vercammen E.L.J., Keurentjes, J.T.F., Hollow fibre microporous silica membranes for gas separation and pervaporation; synthesis, performance and stability, J. Membr. Sci., 248 (2005) 73-80.

Chapter 4

4.1 Introduction Compared to their organic counterparts inorganic membrane materials generally possess superior structural stability, e.g., no swelling and compaction, even in harsh chemical environments and at high temperatures [van Gemert and Petrus-Cuperus, 1995; Verkerk et al., 2001; Wynn, 2001]. The majority of inorganic membranes are porous and their selective features are often closely related to their pore size. Amorphous silica is an inorganic material containing exceptionally small pores. Membranes based on this material have an asymmetric structure with the actual selective microporous silica positioned on a support structure comprising several α- and γ- alumina layers. Silica membranes were discovered more than a decade ago [de Lange et al., 1995; Uhlhorn et al., 1992] and are still subject of extensive study. Silica membranes reported in literature have either a flat plate or tubular geometry. The flat plate geometry is advantageous from an academic point of view, but it usually has a small surface area (typically ~ 10-2 m2) due to limitations imposed on the dimensions by the dip-coating technique. The surface area of tubular silica membranes is larger and their geometry is also more compatible with the technology developed in organic membrane science. Consequently, commercially available membranes for pervaporation have a tubular geometry [Wynn, 2001]. Drawbacks of this geometry include a relatively low surface area-to-volume ratio (typically < 500 m2·m-3) and high costs associated with tubular ceramic membrane supports. In this work silica layers are positioned on top of ceramic hollow fibres. In principle this enables the relatively rapid and inexpensive preparation of a large membrane surface area, combined with a high membrane surfacearea-to-volume ratio (> 1000 m2·m-3). Additionally, pervaporation modules built with externally coated hollow fibres allow for high mass and heat transfer coefficients combined with low liquid pressure drops. The membranes were characterised using SEM, SNMS, single gas permeance and pervaporation.

Synthesis of hollow fibre microporous silica membranes

4.2 Theory 4.2.1 Gas permeation Numerous theories for describing transport in microporous media have been presented in literature [Barrer, 1939; van den Broeke et al., 1999; Kaerger and Ruthven, 1992]. These theories become increasingly complex when the microporous medium is less uniform and when more mobile species are present. For our purpose, i.e., the assessment of membrane quality, a simple phenomenological approach is sufficient. For single gas permeation of permanent gases through amorphous microporous silica membranes, at sufficiently high temperatures and low pressures, transport is activated and permeance is independent of pressure [de Lange et al., 1995; Uhlhorn et al., 1992; de Vos and Verweij, 1998a]. Hence, permeance is described by:

P≡

N = ( H 0 D0 ) exp((Q − ED ) / RT ) ∆p

(4.1)

where N is the molar flux, H0 and D0 are pre-exponential factors related to the Henry and diffusion coefficients, respectively, and R and T have their usual meaning. The overall thermally activated nature of transport arises from the simultaneous occurrence of diffusion (ED) and sorption (Q). 4.2.2 Pervaporation For dehydration of solvents by pervaporation the performance of a membrane is usually expressed in terms of water flux, or permeance, and separation factor α. The latter is defined as:

α=

y H2O xH2O yj xj

(4.2)

where y and x are the molar fractions in the permeate and retentate, respectively. Permeance is defined as the flux divided by the partial pressure difference over the membrane. The equilibrium vapour pressure of component i at the retentate side is related to the mole fraction x and activity coefficient γ in the liquid mixture

pi* = γ i ⋅ xi ⋅ pi0

(4.3)

Chapter 4

where and pi0 is the vapour pressure of the pure component i. When the pressure at the permeate side is small compared to the equilibrium vapour pressure at the retentate side, permeance can be expressed as [Verkerk et al., 2001]:

Pi =

Ni γ i ⋅ xi ⋅ pi 0

(4.4)

4.3 Experimental 4.3.1 Membrane preparation Support The ceramic hollow fibres membrane supports (CEPAration, The Netherlands) have a porosity of ~30%, a pore diameter of either 150 or 300 nm, a length in the range of 20-30 cm, and an inner and outer diameter of 2.0 and 3.0 mm, respectively. γ-Al2O3 intermediate support preparation On the outer side of the hollow fibre substrates intermediate mesoporous γ-Al2O3 layers were prepared by sequential dip-coating with a boehmite coating solution. The boehmite solution was made by adding aluminium-tri-sec-butoxide (Aldrich) dropwise to water at 90°C under vigorous stirring, and subsequent boiling for 90 minutes to remove the 2-butanol produced during the hydrolysis. A white solution was obtained, which was peptised with 1 mol·l-1 HNO3 (water/alkoxide/acid ratio: 70/1/0.07). The peptisation was accompanied by a change in colour from white to “nano” blue. After refluxing for 16 hours the resulting solution had a pH of 3.8. Finally, 120 ml polyvinyl alcohol (PVA) solution was added to 180 ml boehmite solution, followed by stirring at room temperature for 30 minutes and subsequently stirring at 90°C for 150 minutes. The PVA solution was prepared by dissolving 8.75 gram PVA (Aldrich, PVA Powder, average Mw 89-98 kD, hydrolysis grade 98%) in 250 ml of 0.05 M HNO3. The dip-coat process was performed at room temperature in a laminar flow cupboard (Interflow, quality class 100) to minimize dust contamination. The substrate speed was 10 mm·s-1 and the dip-time was 25 seconds. The membranes were dried in a climate chamber

Synthesis of hollow fibre microporous silica membranes

(Espec 100) at 40ºC and 60 RH% for at least 120 minutes. After drying, the membranes were sintered at 600ºC for 180 minutes (heating rate 1ºC·min-1). The procedure for dipping, drying and sintering was repeated three times in order to obtain defect-free intermediate γ-Al2O3 membranes. Silica separation layer preparation The intermediate γ-Al2O3 layers were modified by dip-coating with a polymeric silica sol, prepared via acid catalysed hydrolysis and subsequent polycondensation of tetraethylorthosilicate (TEOS) (Aldrich, >99 %). The polymeric silica solution was prepared by drop-wise addition of water and HNO3 to a TEOS/ethanol solution under vigorous stirring (water/TEOS/acid/ethanol: 6.4/1/0.085/3.8) and refluxing at 60°C for 180 minutes. The resulting solution was diluted 18-fold with ethanol. Dip-coating was performed in a laminar flow cupboard (Interflow, quality class 100). After dip-coating (substrate speed 10 mm·s-1, dip-time 5 seconds) the membranes were dried for 30 minutes at 40°C and 60 RH%. After drying, the membranes were sintered at 350-600°C for 180 minutes (heating rate 0.5ºC·min-1). 4.3.2 Membrane characterisation The thickness and morphology of the different layers were studied by Scanning Electron Microscopy (SEM) using a JEOL 840 microscope. Samples were sputtered with a thin layer of gold. Independently, the thickness of the silica layers was determined by Secondary Neutral Mass Spectrometry (SNMS). 4.3.3 Gas permeation Single gas permeance of one single fibre was measured in a pressure controlled dead-end set-up. Viton O-rings were used for sealing, limiting the temperature to 210°C. The effective membrane length was 17 cm leading to an effective membrane area of 0.0017 m2. Prior to measurements, the membranes were pre-treated by permeating He at 180°C for two days. Measurements were performed with He and N2 (>99% pure) at temperatures from 25°C to 200°C. The pressure at the permeate side was kept slightly above ambient pressure, while the pressure at the feed side was varied in the range 150 - 350 kPa. The flow required to maintain the pressure drop over the membrane was measured by a mass flow indicator.

Chapter 4 4.3.4 Pervaporation

The organic solvent used in dehydration experiments was 1-butanol (pro-analyse, Merck). Activity coefficients of water and 1-butanol were approximated using the Wilson equation [Gmehling and Onken, 1977]. Vapour pressures were calculated using the Antoine equation [Gmehling and Onken, 1977]. The recommended values from the Dechema Data Series were used. A common set-up was used for pervaporation [Verkerk et al., 2001]. On the permeate side a vacuum was maintained (10 mbar) by a cascade of a liquid nitrogen cold trap and a vacuum pump. In the dehydration of 1-butanol the feed was supplied to the membrane by simply submerging the membrane in a large vessel of 1-butanol/water. Vigorous stirring (3 pitched-blade stirrers mounted on a single axis, Ø=5 cm, 1300 rpm) of the bulk mixture was used to avoid polarisation effects on the outside of the membrane. Due to safety regulations the experiments were stopped overnight, during which time the membranes remained in the feed liquid without vacuum applied on the permeate side (1-butanol at room temperature). Subsequent start of an experiment was preceded by a stabilisation period of one hour, during which vacuum was applied on the permeate side. The retentate was analysed using Karl-Fischer (Mettler Toledo DL50 Graphix), while the permeate composition was analysed using gas chromatography.

Synthesis of hollow fibre microporous silica membranes

4.4 Results 4.4.1 Membrane characterisation The SNMS depth profile of the silica membrane (Figure 4.1) shows a monotonous decrease in Si concentration from the membrane surface (0 nm) towards the intermediate γ-Al2O3 layer. Correspondingly, the Al concentration increases with depth and at approximately 20 nm reaches a practically constant value. The crossover point is at 5 nm. The SNMS profiles indicate that the silica layer on top of the γ-Al2O3 layer has a thickness of at least 20 nm, which is in reasonable agreement with the thickness determined by SEM (20-60 nm). The gradual decrease in Si concentration with depth may be due to the surface roughness of the γ-Al2O3 and partial penetration of silica into this layer. In Figure 4.2 SEM pictures of a hollow fibre substrate (Figure 4.2a), coated with intermediate γ-Al2O3 layers (Figure 4.2b), and coated with silica (Figure 4.2c) are shown. Figure 4.2b clearly shows that the four γ-Al2O3 layers form a single 3-4 µm thick hom*ogeneous layer on the substrate, providing a sufficiently smooth surface for silica to be deposited on. For thinner intermediate layers the preparation of defect free silica layer appeared unsuccessful. The particle size in the intermediate layer is in the range 30-80 nm (Figure 4.2c), which is comparable to the thickness of the silica layer (20-60 nm).

Figure 4.1 SNMS depth profile of a silica membrane from batch TNO-3.

Chapter 4

a) Substrate cross-sections (magnification 100x).

b) Cross-section intermediate γ-Al2O3 layers on top of substrate (magnification 3.300x).

γ-Al2O3 intermediate layer

SiO2 layer

c) Cross-section silica membrane on the intermediate γ-Al2O3 layers (magnification 75.000x)

Figure 4.2 SEM micrographs of the various layers constituting the membrane.

Synthesis of hollow fibre microporous silica membranes

4.4.2 Gas permeation Single gas permeation has been performed using helium and nitrogen at different trans-membrane pressure and temperatures. For both gases the permeance appears to be independent of pressure. At a temperature of 200°C the helium and nitrogen flux are 2.2·10-6 mol·m-2·s-1·Pa-1 and 4.0·10-9 mol·m-2·s-1·Pa-1, respectively. The high helium permeance and the high permselectivity of this gas with respect to the larger nitrogen are typical for microporous silica membranes. The low permeance of nitrogen suggests that the pore size of the silica is similar to the molecular dimensions of N2 (kinetic diameter 3.64 Å). Under the conditions applied, the contribution of the hollow fibre support to the total resistance for mass transport is smaller than 5%. Hence, the small dimensions of the substrate enable further improvement of the silica layer without the support resistance becoming significant [de Vos and Verweij, 1998b]. For both helium and nitrogen excellent linear fits of the Arrhenius plots are obtained (Figure 4.3), confirming that transport is thermally activated. The increase in helium permeance with temperature (apparent energy of activation -6.6 kJ·mol-1) indicates that for this inert gas the temperature dependence of diffusion predominates that of sorption. For nitrogen a decrease in permeance is observed with temperature (apparent energy of activation 3.3 kJ·mol-1). The less pronounced change with temperature indicates that for nitrogen the temperature dependence of sorption predominates that of diffusion, albeit only slightly. Table 4.1 contains permeance data for several batches silica membranes. Every batch of membranes consists of 20 membranes from which at least 5 membranes were tested. In total 42 membranes were tested out of which 35 show a helium/nitrogen permselectivity exceeding 150. High values are observed for the permeance of helium (1.1 - 2.9·10-6 mol·m-2·s-1·Pa-1, 200°C). Differences in helium/nitrogen permselectivity have both been found between the 7 batches likely due to differences in silica dip-coat solution and in one batch probably due to differences in substrate quality. The performance of the hollow fibres prepared in our study is comparable to that of the flat plate membranes described by De Vos and Verweij [1998b]. Nair et al. [2000] measured a helium permeance of approximately one order of magnitude lower (2·10-7 mol m-2 s-1 Pa-1, 135°C), with a permselectivity of

Chapter 4

1230, comparable to our results. The high permeance value can partly be attributed to the small contribution of support resistance.

Figure 4.3 Arrhenius plots of helium and nitrogen permeance, transmembrane pressure difference 100 kPa, membrane TNO-8a.

Table 4.1 Helium single gas permeance and He/N2 permselectivities for several silica membranes, 200°C. Membrane series code

P(He) [10-6 mol·m-2·s-1·Pa-1]

P(N2) [10-6 mol·m-2·s-1·Pa-1]

Permselectivity He/N2

TNO-3

0.85

0.003

290

TNO-4

1.51

0.002

760

TNO-5

2.07

0.022

100

TNO-6

1.51

0.003

510

TNO-7

1.86

0.002

940

TNO-8

2.20

0.004

560

TNO-9

1.86

0.010

190

Synthesis of hollow fibre microporous silica membranes 4.4.3 Pervaporation Dehydration of 1-butanol

Dehydration of 1-butanol has been carried out using several silica membranes and their performance is summarized in Table 4.2. Typical values for the initial water flux and selectivity are in the range of 2-3 kg·m-2·h-1 and 500-1200, respectively.

Table 4.2 Pervaporation performance of various membranes in the dehydration of 1-butanol, 80°C, 5 wt.% water. α0

αt

Membrane code

TNO-3a

2.92

1.31

11.1

5.28

1200

900

TNO-3b

2.53

1.82

9.94

7.32

500

TNO-5a

2.05

0.82

8.20

3.22

200

150

TNO-5b

3.04

1.11

12.1

4.54

1000

500

TNO-7a

2.23

0.76

9.17

3.03

900

300

[kg·m-2·h-1]

[kg·m-2·h-1·bar-1]

[-]

tpr [h]

Figure 4.4 shows typical data for the water permeance and selectivity as a function of time (80ºC, 5 wt.% water). Clearly, the water permeance decreases with time. The selectivity also appears to decrease with time, although this effect is less apparent and notably influenced by the overnight stops of the measurement.

Chapter 4

Figure 4.4 Water permeance and selectivity in the dehydration of 1-butanol as a function of time, T = 80°C, 5 wt.% water, membrane TNO-3a.

The observed decline in permeance with time is likely due to interactions of water and alcohol with the silica material, i.e., adsorption on and reaction with hydroxyl groups on the silica surface. Reaction of the 1-butanol with these hydroxyl groups would render the silica more hydrophobic. This would be advantageous for the solvent flux, while it would induce a decrease in water flux. Reactions of 1-butanol with the supporting alumina layers may enhance this effect [Dafinov et al., 2002]. Possibly, reactions between water and silica could lead to a reduction in the surface energy associated with the excess surface area of the pores, resulting in a densification of the material accompanied by an increase of the pore size. Again, the result would be a decrease in performance. A decline in flux with time has also been observed by others. Van Veen et al. [van Veen et al., 2001] measured the performance of a silica membrane in the dehydration of IPA over a period of 73 days (80°C, 4.5 wt.% water). They found that after an initial decrease of both the water and the alcohol flux the process stabilized (water flux 1.9 kg·m-2·h-1, selectivity 1100). Similar behaviour was found by Sommer [Sommer, 2003]. Asaeda et al. [Asaeda et al., 2002] investigated the performance of a SiO2-ZrO2 membrane in the dehydration of isopropyl alcohol (75°C, 10 wt.% water) and observed

Synthesis of hollow fibre microporous silica membranes

a constant water permeance in time, accompanied by an increase in selectivity. They claim that the inherent hydrothermal stability of their composite material circumvents a substantial decline in the water flux and attribute the increase in selectivity to pore blocking by the alcohol. Comparison with literature data Table 4.3 gives an overview of pervaporation data presented in open literature, including the results of our study. Clearly, the performance of our hollow fibre silica ceramic membranes is comparable to that of tubular ceramic silica membranes. For the dehydration of 1-butanol system the selectivity of the hollow fibre membranes is significantly higher compared to commercially available membranes [Gallego-Lizon et al., 2002a; Gallego-Lizon et al., 2002b; Sommer et al., 2002] and comparable to that of membranes prepared by ECN [van Veen et al., 2001; Verkerk et al., 2001].

Table 4.3 Literature data for pervaporation performance of various silica membranes in the dehydration of various solvents. Organic compound

Water Permeate Feed pressure concentration temperature [mbar] [wt.%] [°C]

Water flux

Process selectivity -2 -1 [kg·m ·h ] [-]

Type

Ref.

Ethanol

0.15

400

Home

Van Gemert, 1995

2-propanol

0.16

600

“

1-Butanol

2.3

680

ECN

Verkerk, 2001

IPA

2.1

600

“

1-Butanol

4.5

600

ECN

Van Veen, 2001

IPA

4.5

1.9

1150

“

Methanol

8-10

1.85*

PervaTech

ten Elshof, 2003

t-Butanol

8-10

3.5

144

SMS

Gallego-Lizon, 2002a

IPA

8-10

50-100

SMS

Gallego-Lizon, 2002b

IPA

1.4

SMS

Sommer, 2002

Butanol

1000 m2·m-3). Combined with the good performance of the hollow fibre membranes and the low support resistance this makes them interesting candidates for a variety of applications.

Nomenclature Ci

concentration

[mol·m-3]

total flux

[kg·m-2·h-1]

flux of component i

[kg·m-2·h-1]

pressure

[Pa]

pi*

partial equilibrium vapour pressure

[Pa]

pi0

vapour pressure of pure component i

[Pa]

permeance

temperature

TNO-Yx

[mol·m-2·s-1·Pa-1] or [kg·m-2·h-1·bar-1]

membrane x from membrane series Y

[K] [-]

molar fraction retentate

[mol·mol-1]

molar fraction permeate

[mol·mol-1]

Chapter 4

Greek letters

separation factor

[-]

γi

activity coefficient

[-]

Subscripts c

calcination

component i

component j

initial

permeate

process

retentate

final

References Asaeda, M., Sakou, Y., Yang, J., Shimasaki, K., Stability and performance of porous silica-zirconia composite membranes for pervaporation of aqueous organic solutions, J. Membr. Sci., 209 (2002) 163-175. Barrer, R.M., Activated diffusion in membranes, Trans. Faraday Soc., 35 (1939) 644-656. van den Broeke, L.J.P., Bakker, W.J.W., Kapteijn, F., Moulijn, J.A., Binary permeation through a silicalite-1 membrane, AIChE J., 45 (1999) 976-985. Dafinov, A., Garcia-Valls, R., Font, J., Modification of ceramic membranes by alcohol adsorption, J. Membr. Sci., 196 (2002) 69-77. ten Elshof, J.E., Abadal, C.R., Sekulic, J., Chowdhury, S.R., Blank, D.H.A., Transport mechanisms of water and organic solvents through microporous silica in the pervaporation of binary liquids, Micr. Meso. Mater., 65 (2003) 197-208.

Synthesis of hollow fibre microporous silica membranes

Gallego-Lizon, T., Edwards, E., Lobiundo, G., Freitas dos Santos, L., Dehydration of water/t-butanol mixtures by pervaporation: comparative study of commercially available polymeric, microporous silica and zeolite membranes, J. Membr. Sci., 197 (2002) 309-319. Gallego-Lizon, T., Ho, Y.S., Freitas dos Santos, L., Comparative study of commercially available polymeric and microporous silica membranes for the dehydration of IPA/water mixtures by pervaporation/vapour permeation, Desalination, 149 (2002) 3-8. van Gemert, R.W., Petrus-Cuperus, F., Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci., 105 (1995) 287-291. Gmehling, J., Onken, U., DeChema Chemistry Data Series. Vapor-Liquid Equilibrium Data Collection, Vol. 1, Pt. 1: Aqueous-Organic Systems, 1977. Kaerger, J., Ruthven, D.M., Editors., Diffusion in zeolites and other microporous solids, Wiley, New York, 1992. de Lange, R.S.A., Hekkink, J.H.A., Keizer, K., Burggraaf, A.J., Formation and characterization of supported microporous ceramic membranes prepared by sol-gel modification techniques, J. Membr. Sci., 99 (1995) 57-75. Nair, B.N., Keizer, K., Suematsu, H., Suma, Y., Kaneko, N., Ono, S., Okubo, T., Nakao, S.I., Synthesis of gas and vapor molecular sieving silica membranes and analysis of pore size and connectivity, Langmuir, 16 (2000) 4558-4562. Petrus-Cuperus, F., van Gemert, R.W., Dehydration using ceramic silica pervaporation membranes - the influence of hydrodynamic conditions, Sep. Purif. Technol., 27 (2002) 225-229. Sekulic, J., Luiten, M.W.J., ten Elshof, J.E., Benes, N.E., Keizer, K., Microporous silica and doped silica membrane for alcohol dehydration by pervaporation, Desalination, 148 (2002) 19-23. Sommer, S., Klinkhammer, B., Melin, T., Integrated system design for dewatering of solvents with microporous silica membranes, Desalination, 149 (2002) 15-21.

Chapter 4

Sommer, S., Pervaporation and vapor permeation with microporous inorganic membranes, Thesis, RWTH, Aachen, 2003. Uhlhorn, R.J.R., Keizer, K., Burggraaf, A.J., Gas transport and separation with ceramic membranes. Part II. Synthesis and separation properties of microporous membranes, J. Membr. Sci., 66 (1992) 271-287. van Veen, H.M., van Delft, Y.C., Engelen, C.W.R., Pex, P.P.A.C., Dewatering of organics by pervaporation with silica membranes, Sep. Purif. Technol., 22 and 23 (2001) 361-366. Verkerk, A.W., van Male, P., Vorstman, M.A.G., Keurentjes, J.T.F., Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures, Sep. Purif. Technol., 22 and 23 (2001) 689-695. de Vos, R.M., Verweij, H., Improved performance of silica membranes for gas separation, J. Membr. Sci., 143 (1998) 37-51. de Vos, R.M., Verweij, H., High-selectivity, high-flux silica membranes for gas separation, Science, 279 (1998) 1710-1711. Wynn, N., Dehydration with silica pervaporation membranes, Mem. Techn., 2001 (2001) 10-11.

Chapter 5 Thin high flux ceramic-supported PVA membranes Thin, high-flux and highly selective cross-linked poly(vinyl)alcohol water selective layers have been prepared on top of hollow fibre ceramic supports. The supports consist of an α-Al2O3 hollow fibre substrate and an intermediate γ-Al2O3 layer, which provides a sufficiently smooth surface for the deposition of ultra-thin PVA layers. Membranes have been characterised by SEM and pervaporation experiments. The thickness of the PVA layer formed on top of the γ-Al2O3 intermediate layer is in the order of 0.3-0.8 µm. In the dehydration of 1-butanol (80°C, 5 wt.% water) the membranes exhibit a high water flux (0.8 - 2.6 kg·m-2·h-1), combined with a high separation factor (500 - 10.000). In the dehydration of 2-propanol and 1-butanol, a simultaneous increase in both water flux and separation factor is observed with increasing temperature or water concentration. This remarkable behaviour is in contrast to the trade-off generally observed for polymer membranes, i.e., an increase in flux is typically combined with a decrease in separation factor. A possible explanation for this behaviour is a low degree of three dimensional swelling in the vicinity of the γ-Al2O3 - PVA interface due to an enhanced structural stability.

This chapter is partially based on: Peters, T.A., Poeth, C.H.S., Benes, N.E., Buijs, H.C.W.M., Vercauteren F.F., Keurentjes, J.T.F., Ceramic-supported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm, J. Membr. Sci., (2006) accepted.

Chapter 5

5.1 Introduction Pervaporation is a promising process for separating azeotropic mixtures, close-boiling mixtures, and for the dehydration of temperaturesensitive products. In the development of high-flux membranes, much effort has been made to change the membrane structure from a dense thick film to an asymmetric or composite structure, in order to reduce the effective thickness of the membrane. However, the reduction in thickness is typically accompanied by a decrease in selectivity, due to a higher number of defects. Hence, to what extent the layer thickness can be decreased is limited by various aspects, including the pore size, porosity and surface roughness of the membrane support [Rezac and Koros, 1992]. In this article we report the synthesis of thin high-flux and highly selective cross-linked poly(vinyl)alcohol (PVA) water selective layers on top of an α-Al2O3 hollow fibre substrate. Intermediate γ-Al2O3 layers have been prepared on top of the substrates in order to provide a sufficiently smooth surface for the ultra-thin PVA layer. Compared to polymeric supports, ceramic supports exhibit a high surface porosity, and superior structural stability, e.g., no swelling and compaction, while introducing negligible transport resistance [Rezac and Koros, 1992; Song and Hong, 1997; Yoshida and Cohen, 1997]. In principle the preparation method described in this paper enables relatively fast and inexpensive production of large membrane surface areas, combined with a high membrane surface area / module volume ratio (> 1000 m2·m-3). The membranes have been analysed by SEM and the pervaporation performance has been determined for the dehydration of various aqueous alcohol mixtures as a function of both temperature and feed water concentration.

5.2 Experimental 5.2.1 Membrane preparation Ceramic hollow fibre membrane supports (α-Al2O3) were used (CEPAration, the Netherlands) with a porosity of ~30%, a pore diameter of 300 nm, a length in the range of 20-30 cm, and an inner and outer diameter of 2.0 and 3.0 mm, respectively. On the outside of these fibres, mesoporous γ-Al2O3 layers were prepared by four times sequential dip-coating with a

Thin high flux ceramic-supported PVA membranes

boehmite coating solution [Peters et al., 2005]. In order to minimise the amount of imperfections in the intermediate γ-Al2O3 layer, the layer is prepared in a clean-room environment. The intermediate γ-Al2O3 layers were modified by dip-coating in a 0.75 wt.% PVA (Mowiol 56/98, Clariant) solution. The weight average molar mass of the PVA used is 195 kg·mol-1, corresponding to a weight average degree of polymerisation of 4300. The degree of hydrolysis is 98.4%. Maleic anhydride (MA) was used as cross-linking agent in a concentration of 0.05 mol MA per mol of PVA. The membranes were dried at 55°C for 30 min and cured at 130°C for 1 hour. 5.2.2 Membrane characterisation Scanning Electron Microscopy (SEM) The thickness of the various layers and the nature of the surface were determined by Scanning Electron Microscopy (SEM) using a JEOL 5600 apparatus. The samples were sputtered with a thin layer of gold prior to analysis. Post-process analysis was performed with a SEM (Philips XL30 FEG-ESEM) equipped with an Energy Dispersive X-ray Analysis device. The EDX-samples were sputtered with a thin layer of carbon (Emitech K575X). Pervaporation Dehydration experiments of ethanol, 1-propanol, 2-propanol and 1-butanol (pro-analyse, Merck, Darmstadt, Germany) were carried out in a common pervaporation set-up [Verkerk et al., 2001] at temperatures between 50 and 90°C, and a feed water concentration between 2.5 and 19 wt.%. At the permeate side a vacuum was maintained (10 mbar) by a cascade of a liquid nitrogen cold trap and a vacuum pump. The feed was supplied to the membrane by simply submerging the membrane in a large vessel containing an alcohol/water mixture. Sufficient stirring (3 pitched-blade stirrers mounted on a single axis, 1300 rpm) of the bulk mixture was used to avoid polarisation effects at the outside of the membrane. A further increase in stirrer speed did not result in an altered membrane performance. The retentate composition was analysed using Karl-Fischer titration, while the permeate composition was analysed using gas chromatography. The performance of a membrane is usually expressed in terms of the water flux and separation factor α. The latter is defined as:

Chapter 5

α=

y H2O xH2O yj xj

(5.1)

pi* = γ i ⋅ xi ⋅ pi0

(5.2)

where pi0 is the vapour pressure of the pure component i. When the pressure at the permeate side is small compared to the equilibrium vapour pressure at the retentate side, permeance can be expressed as [Verkerk et al., 2001]:

Pi =

Ni γ i ⋅ xi ⋅ pi 0

(5.3)

Activity coefficients of water and alcohol were approximated using the Wilson equation. Vapour pressures were calculated using the Antoine equation. Recommended values from the Dechema Data Series were used [Gmehling and Onken, 1977].

Thin high flux ceramic-supported PVA membranes

5.3 Results 5.3.1 Membrane characterisation A cross-section of the composite ceramic-supported PVA membrane is shown in Figure 5.1. The membrane consists of three layers; the hydrophilic PVA layer, intermediate γ-Al2O3 layers and an α-Al2O3 hollow fibre substrate.

PVA layer

γ-Al2O3 intermediate layer

α-Al2O3 support

Figure 5.1 SEM-picture of cross-section of the PVA water selective layer on the intermediate γ-Al2O3 layers (magnification 10.000x).

From Figure 5.1 it can be seen that the four γ-Al2O3 intermediate layers form a single 3-4 µm thick layer on the substrate providing a smooth surface for the PVA layer. A 0.3 - 0.8 µm thick PVA layer is formed on top of the γ-Al2O3 intermediate layer. Clearly, the presence of the intermediate layer enables the formation of a defect-free thin selective layer. Furthermore, the small pore-size of the intermediate layer circumvents significant infiltration of PVA into the ceramic support, as can be expected from the large hydrodynamic radius of the PVA. An estimation of the hydrodynamic radius of polymer coils is given by the Flory radius, RF = N0.6·a [Yoshida and Cohen, 1997], where N is the number of monomer units per chain, and a is the monomer size. Using an estimated monomer size of 0.37 nm, the Flory radius of the PVA used is calculated to be 56 nm, which is clearly larger than the 4 nm intermediate layer pore size.

Chapter 5 5.3.2 Pervaporation 5.3.2.1

Membrane separation capability

The pervaporation performance of the membranes has been determined for the dehydration of 1-butanol. Figure 5.2 shows a typical behaviour of the water flux and selectivity in time observed for a fresh membrane. From this figure it is apparent that after a period of three days, in which the water flux decreases, both water flux and selectivity are stable.

Figure 5.2 Dehydration of 94.5 (± 0.5) wt.% 1-butanol, influence of process time, T = 80°C, Membrane TNO-PVA-4a. The performance of various membranes prepared using the same procedure is summarized in Table 5.1. The data given in Table 5.1 indicate that all composite membranes prepared show a high flux and a high separation factor indicating that the preparation method is reproducible. The best performance was observed for membrane TNO-PVA-3, having a water flux and separation factor of 2.39 kg·m-2·h-1 and 6300, respectively (80°C, 5 wt.% water).

Thin high flux ceramic-supported PVA membranes

Table 5.1 Separation performance of various membranes measured. Dehydration of 95 (± 0.5) wt.% 1-butanol, T = 80°C. α0

αt

Membrane code

PVA-1a

1.78

1.4

1050

1040

PVA-2a

1.43

1.29

>10.000

PVA-2b

0.79

0.74

1500

1400

PVA-2c

1.92

1.29

4000

3700

PVA-3a

2.66

2.39

8600

6300

PVA-4a

1.76

0.97

500

460

200

[kg·m-2·h-1]

[-]

tpr [h]

The flux values obtained in this study for the dehydration of 1-butanol are substantially higher compared with values presented in literature in the dehydration of similar compounds by polymer-supported PVA membranes, see Table 5.2. It should be noted that comparison of the various selectivity data is complicated by the fact that different alcohols are used. However, for the dehydration of alcohols using cross-linked PVA membranes, generally an increase in separation factor in the following order is found: MeOH 98%) was mixed with ethanol and water. A small amount of nitric acid (65 wt.% in water) as a silicate oligomerisation catalyst was added and the resulting mixture was heated at 60°C for 3h under continuous stirring. The reaction mixture had a molar ratio TEOS/ethanol/water/HNO3 of 1/3.8/6.4/0.085. Commercial colloidal silica (Ludox AS-40, Aldrich) has also been used as a binder solution. Zeolite dip-coat suspensions. The dip-coat solutions were prepared by mixing H-USY crystals and ethanol. Dealuminated zeolite H-USY with a Si/Al ratio of 40 was selected for the coating experiments. The Na2O content was 0.05 wt.% and the surface area 650 m2·g-1. The amount of zeolite crystals was varied between 5 wt.% and 26 wt.%. The suspensions were sonicated at room temperature for 1h prior to use. 10 wt.% of standard binder solution was added to the zeolite dip-coat solution to enhance particle deposition and layer strength. Dip-coat process. Membranes were first immersed (16 mm·s-1) in the binder solution for 2 min and removed from the binder solution at the same speed. Next, membranes were dipped into a zeolite suspension for 2 min (longer times did not significantly change the final loading) at a speed of 16 mm·s-1. Typically, the zeolite dip-coat procedure started 1 min after the pre-treatment. Subsequently, the membranes were dried in air at 60°C for 2h and calcined for 16h in air at 500°C with a heating rate of 100 K·h-1.

126

Chapter 6

Characterization. The loading of zeolite crystals on the membrane surface was determined by the weight increase during the dip-coat process. The adhesion of zeolite crystals was tested by an ultrasonic bath treatment for 1h followed by scanning electron microscopy (SEM) to evaluate the amount of crystals remaining at the surface. SEM pictures were taken using a JEOL JSM-5600 with an acceleration voltage of 15 kV. Dry coated membrane samples were gold-coated prior to scanning. Catalytic test The activity of the zeolite-coated pervaporation membrane was measured in the esterification reaction between acetic acid and butanol under the same reaction conditions as mentioned above in the catalyst screening experiments. The membrane was placed in a stainless steel module module using Kalrez O-rings. Effective membrane area used was 17 cm2. The set-up used is depicted in Figure 6.2. Vacuum system

Membrane module

Supply vessel

Pump

Figure 6.2 Set-up used in the catalytic tests and the pervaporationesterification coupling.

The supply vessel was charged with a certain amount of acetic acid. Then, the system was heated up to the reaction temperature (T=75°C) after which the pre-heated equimolar amount of alcohol was added. The

127

Preparation of zeolite-coated pervaporation membranes

reaction temperature was maintained by means of a thermostatic water bath in which the system was immersed. The liquid reaction mixture was recirculated through the membrane module by means of the pump (valves are in position 1) at a liquid flow rate of 40 l·h-1, which corresponds to a liquid superficial velocity exceeding 2 m·s-1 in order to eliminate polarization effects. On the permeate side a vacuum was maintained (10 mbar) by a cascade of a liquid nitrogen cold trap and a vacuum pump. After recirculation through the membrane module, the flow towards the supply vessel was stopped, decreasing the reactor volume to 30 ml, and the liquid was only flowing through the membrane module containing the catalytic membrane (valves are in position 2).

6.3 Results 6.3.1 Activity of heterogeneous catalysts All experiments showed first-order behaviour in acetic acid and butanol concentration. The activity of the catalysts is expressed in terms of the second-order reaction rate constant normalised for the catalyst weight. As the reaction also proceeds without adding a catalyst, the first-order reaction rate constant of this reaction is also given. The results are shown in Table 6.1. Table 6.1 Activity of the different catalysts in the esterification reaction between acetic acid and butanol, T=75°C. catalyst

Tc [°C]

k [m3·mol-1·s-1]

k [m3·mol-1·gcat-1·s-1]

No catalyst

1.33·10-9

H-USY-20

9.82·10-10

500

H-ZSM5-12.5

5.91·10-11

500

ZrO2

8.74·10-9

550

Large-pore zeolites, like X and Y-type zeolites have proven to be efficient catalysts for esterification reactions [Corma et al., 1989; Namba et al., 1985]. Zeolite catalytic properties like pore size, acid strength distribution and hydrophobicity can be designed by preparing zeolites with different crystalline structure, acid exchange level and framework

128

Chapter 6

Si/Al ratio. For example, with increasing Si/Al ratios, the zeolite surface becomes more hydrophobic and, therefore, has more affinity for the reactants, whereas the number of acid sites decreases. Hence, an optimum Si/Al ratio may exist. Figure 6.3 shows the influence of the zeolite Si/Al ratio for H-USY on the zeolite activity in the esterification reaction between acetic acid and butanol. The highest activity for the H-USY zeolite activity was found at a Si/Al ratio of around 20. This agrees with the value obtained by Corma et al. [1989] for the esterification of carboxylic acids in the presence of HY zeolites. H-ZSM5 had the lowest activity of the catalysts tested suggesting that the esterification is internally diffusion-limited due to the medium sized H-ZSM5 zeolite pores [Namba et al., 1985]. Thus, most of the conversion occurs on the external surface of the crystallites.

Figure 6.3 Effect of the H-USY Si/Al ratio on the reaction rate constant, T=75°C. The activity and stability of sulphated zirconia towards the esterification reaction highly depends on the catalyst pre-treatment. At increasing calcination temperature, the crystallinity of the sulphated zirconia increases, whereas the surface area (see Table 6.1) and the amount of surface sulphur species decreases [Hu et al., 2000]. Hence, an optimum in reactivity as a function of calcination temperature may exist. This optimum was found at a calcination temperature of 550°C. This agrees with the optimum calcination temperature obtained by Hino et al. [1981] for the

Preparation of zeolite-coated pervaporation membranes

129

esterification reaction of acetic acid and ethanol in the presence of sulphated zirconia. From Table 6.1 it can be seen that the weight based activity of sulphated zirconia exceeded the activity of the two zeolites tested. However, due to water adsorption on the active sites or loss of surface sulphur sites, the activity of sulphated zirconia gradually decreased. Additionally, for the regeneration of this type of catalysts multiple steps are needed whereas for regeneration of zeolites only calcination in air has to be performed [Hino and Arata, 1981]. Therefore, H-USY zeolite is considered to be the most suitable catalyst to be applied as the catalytic layer in a catalytic membrane reactor. 6.3.2 Coating of the ceramic pervaporation membranes 6.3.2.1

Ludox AS-40 as binder

Commercial reactive colloidal silica nanoparticles such as Ludox AS-40 (40 wt.% suspension of colloidal silica in water) are commonly used as binder to enhance zeolite crystal attachment on aluminosilicate substrates [Beers et al., 2001a; Beers et al., 2001b; Jansen et al., 1997; Nijhuis et al., 2002]. From Figure 6.4 it can be seen that after pre-treatment of the membrane surface in a Ludox AS-40 solution, zeolite attachment was increased dramatically.

X10.000

1µm

X10.000

1µm

Figure 6.4 SEM pictures of a H-USY zeolite-coated membrane (top view), prepared with a 20 wt.% H-USY mixture in ethanol. (a) without pre-treatment; (b) with pre-treatment in a Ludox AS-40 binder solution.

130

Chapter 6

Discontinuous zeolite coverage is clearly observed. However, the most interesting feature is that a Ludox layer is required to enable zeolite crystals to be attached to the surface. Figure 6.4b shows a Ludox layer of approximately 0.5 µm between the zeolite crystals and the selective silica layer of the membrane. Where there are no AS-40 particles (or very few) virtually no attachment is observed. This signifies how important pre-treatment of the membrane surface is for zeolite adhesion. The difficult attachment could be ascribed to the fact that the original membrane surface probably does not contain enough reactive sites to induce chemical attachment or does not exhibit enough surface roughness to induce physical attachment. Unfortunately, a full coverage could not be achieved and the zeolite layer was unstable under ultrasonic treatment. Therefore, oligomeric silica particles were considered as binder to enhance zeolite attachment and zeolite layer strength. Such a binder should not form a thick layer unlike the above-mentioned results, which could be detrimental to permeation. Additionally, an improved membrane coverage is expected as the reactivity of TEOS is higher compared to Ludox AS-40. 6.3.2.2 Oligomeric silica as binder Figure 6.5 shows a zeolite-coated membrane obtained from a 20 wt.% zeolite dip-coat solution after pre-treatment in a 10 wt.% TEOS binder solution. It can be seen that a crack-free zeolite layer was obtained. The coverage is improved as compared to the membrane pre-treated in a Ludox AS-40 binder solution.

X100

100µm

X5.000

5µm

Figure 6.5 SEM pictures of a H-USY zeolite-coated membrane, prepared with a 20 wt.% H-USY mixture in ethanol pre-treated in a TEOS solution. (a) top view; (b) side view.

Preparation of zeolite-coated pervaporation membranes

131

Coating a membrane once in a coating mixture of 20 wt.% H-USY resulted in a zeolite layer thickness of around 2 µm after calcination as seen by SEM, later confirmed using a profilometer. The amount of zeolite deposited on the membrane can be predicted from the solution film thickness that should remain on the surface upon withdrawal from the zeolite solution; on the basis of viscosity, surface tension, density, and the speed of withdrawal, the resulting zeolite layer thickness for a single withdrawal should be 3 µm [Lacy et al., 1999]. This prediction agrees well with our experimental results. A significant amount of zeolite remains on the surface after 1 h of sonication, which indicates firm attachment of the zeolite layer. In order to increase zeolite layer strength, 10 wt.% of binder was added to the dip-coat solution. The influence of this binder on the zeolite layer is clearly visible from Figure 6.6. Due to the presence of binder in the dip-coat solution, separate zeolite particles in the zeolite layer are intergrown and the resulting zeolite layer is strengthened.

X5.000

5µm

X5.000

5µm

Figure 6.6 SEM pictures of a H-USY zeolite-coated membrane, prepared with a 20 wt.% H-USY mixture in ethanol pre-treated in a TEOS solution; (a) no binder in dip-coat solution; (b) 10 wt.% binder in dip-coat solution.

Varying the zeolite concentration in the coating mixture between 3 and 20 wt.% did not seem to affect the final layer thickness substantially. In more concentrated mixtures the zeolite layer thickness increased dramatically. In these thick layers, however, cracks appeared during drying and the layer was easily removed during the ultrasonic treatment. Therefore, in order to increase the layer thickness, the dipping procedure

132

Chapter 6

was repeated four times. This deposition method avoids failure of the coating due to the high stresses, which can form in thicker coatings during drying or firing. A calcination step has been applied after every dip-coat experiment. The results are presented in Figure 6.7. From Figure 6.7b it can be observed that the zeolite layer thickness is approximately 10 µm (5 X 2 µm). The coverage is uniform as shown in Figure 6.7a. Hence, tuning of the zeolite coating thickness is possible by varying the number of dip-coat steps.

X500

50µm

X2.000

10µm

Figure 6.7 SEM pictures of a H-USY zeolite-coated membrane, prepared with a 20 wt.% H-USY mixture in ethanol pre-treated in a TEOS solution, procedure repeated four times. (a) top view; (b) side view.

6.3.3 Pervaporation-esterification coupling The catalytic function of the H-USY-coated pervaporation membrane combined with its selective features was tested in the esterification reaction between acetic acid and butanol. For this, a membrane is selected which is dip-coated ten times resulting in a layer thickness of around 40 µm. Zeolite coverages up to 10 g·m-2 of membrane area, which translates to reactor loadings of zeolite up to 80 kg·m-3 of reactor volume were obtained. The activity of the coated membrane as compared to the activity obtained for the bulk zeolite catalyst. In both reactions the (initial) observed rate constants and intrinsic rate constants per gram of zeolite are in the same order of magnitude. The coated membrane exhibits an activity of 9.2·10-10 m3·mol-1·s-1·gcat-1, which is only 7 % lower than the value given in Table 6.1. Most likely, inhibition of external acid sites by the TEOS binder is

Preparation of zeolite-coated pervaporation membranes

133

the explanation for the somewhat lower activity [Kunkeler et al., 1997]. However, due to the relatively low amount of catalyst compared to the area-to-volume ratio of the experimental set-up (25%) since the excess of water is removed. In the module containing the catalyst-coated membranes, the produced water is removed efficiently due to the close integration of reaction and separation. Product Retentate Feed

Permeate

Figure 8.4 Conceptual pervaporation modules.

process

flow

diagram

containing

two

Process design of pervaporation-assisted esterification reactions

167

8.3 Reactor evaluation Due to the relatively low amount of catalyst compared to the area-to-volume ratio of the experimental laboratory set-up used in the experiments described in the previous chapters (2000 m2·m-3 in a practical membrane module) a substantial conversion could not be reached within a reasonable time scale. Therefore, in order to evaluate the performance of a larger scale practical membrane module, model simulations are performed. Additionally, the performance of a catalytic pervaporation membrane module is compared with the performance of a reactor in which the same loading of heterogeneous catalyst is simply dispersed in the bulk liquid. Data used in the simulations, e.g., membrane permeability and catalyst activity, are taken from the experiments described in the preceding chapters. The reactor dimensions are based on an annual production of 10.000 metric tons of butyl acetate. For configuration A, the feed consists of acetic acid and butanol in a stoichiometric ratio. For configuration B, the feed of the catalytic membrane module is an equilibrium mixture from which the majority of the water is removed. Along the length of the reactor water is removed continuously through the pervaporation membranes. The reactor is operated at 75°C and 1 bar. Based on a production rate of 1 m3·h-1 and a residence time of 6 hour, a reactor volume of 6 m3 is required. This reactor volume corresponds to a catalyst loading up to 1300 kg. Using hollow fibre membranes with a diameter of 3 mm and a module void fraction of 0.2, a high membrane surface-area-to-volume ratio is obtained (4700 m2·m-3). The reaction rate constant for the zeolite and Amberlyst-coated pervaporation membranes is estimated to be 9.2·10-10 m3·mol-1·s-1·gcat-1 and 1.8·10-8 m3·mol-1·s-1·gcat-1, respectively. In the calculations, the catalytic activity of the unsupported zeolite catalyst is taken 6.6% higher compared to the activity of the supported catalyst. As discussed in Chapter 7, the activity of the unsupported Amberlyst catalyst is equal to the activity of the supported catalyst. For both membranes, the membrane permeability is approximated to be 5.2·10-7 mol·m-2·s-1·Pa-1.

168

Chapter 8

Plug-flow pervaporation membrane reactor (Fig 8.3) Figure 8.5 shows the conversion as a function of the catalytic layer thickness using the operating conditions previously mentioned obtained for zeolite-coated and Amberlyst-coated pervaporation membranes, respectively. Amberlyst-coating Zeolite-coating

Figure 8.5 Effect of catalytic layer thickness on module performance; closed symbols.

From Figure 8.5 it can be seen that in a practical catalytic pervaporation membrane module, substantial conversions are reached within a reasonable timescale. The conversion obtained shows an optimum in conversion at a certain layer thickness, which value is a function of the reaction kinetics and membrane permeability [Peters et al., 2004]. The increase in activity, by changing from zeolite-coated to Amberlyst-coated membranes, evidently results in an increase in conversion. In the case of the higher activity the diffusion of reactants inside the catalytic layer cannot keep up with the reaction rate, causing a decrease in the optimum catalytic layer thickness. Because of the high catalytic activity of Amberlyst 15 as compared to H-USY zeolite coating, almost 100% conversion could be reached using Amberlyst-coated pervaporation membranes. However, equilibrium conversion is already reached in the initial stage of the process caused by

Process design of pervaporation-assisted esterification reactions

169

the fast reaction kinetics, due to the high catalytic activity and loading. As a result, the available membrane area is not sufficient to remove the water formed, leading to a significant increase in bulk water concentration. Consequently, a large part of the plug-flow membrane module is primarily used to remove the water formed in the initial stage of the reaction. In this perspective the performance of the catalytic membrane module might be enhanced by varying the spatial distribution of the different functionalities, e.g., catalytic layer thickness along the length of the module [Lawrence et al., 2005]. At the entrance of the module enough catalyst needs to be available in order to keep the reaction rate high whereas water removal is of less importance. In contrast, water removal has to be efficient at the exit of the reactor whereas the reaction rate, and thus the amount of catalyst, is less important. Hence, it would be advantageous to decrease the catalytic layer thickness along the length the module. To achieve the same effect, a sequence of modules having a different catalytic layer thickness can also be used. In this way the coupling of water production and removal can be tuned more easily. An extreme case for this approach is the configuration in which one single additional membrane module is implemented to remove the excess of water from an esterification mixture at equilibrium conversion. Consequently, the residence time in the catalytic membrane module can be reduced. In the following paragraph calculations based on this configuration are discussed. Process flow diagram containing two pervaporation modules (Fig 8.4) The second proposed configuration consists of a reactor and two pervaporation modules. In this configuration, the liquid reaction mixture from the esterification process is first dehydrated in a conventional pervaporation membrane unit to remove the excess of water. Subsequently, the mixture is lead through a module containing catalytic pervaporation membranes. Compared to the plug-flow pervaporation membrane reactor this implies that the feed of the catalytic membrane module is an equilibrium mixture from which the majority of the water is removed. Consequently, for configuration B selective water removal from the reaction mixture is critical whereas catalytic activity is of less importance. Figure 8.6 shows the effect of catalytic layer thickness and catalyst position on the performance of the catalytic pervaporation module.

170

Chapter 8

IMR CMR

Figure 8.6 Effect of catalyst position and catalytic layer thickness on module performance; τ = 3 h; Feed: acetic acid; butanol; butyl acetate; water: 0.24; 0.24; 0.5; 0.02.

According to the performed reactor evaluation, Figure 8.6 suggests that the Amberlyst-coated membrane reactor, although only slightly, outperforms the reactor in which the catalyst is dispersed in the bulk liquid. Only for high catalytic loading the inert membrane reactor would perform better. However, from a practical point of view a high catalyst loading corresponds to an almost entirely filled void fraction between the membranes. As a consequence the module will behave as a packed bed membrane reactor, irrespective of the fact whether the catalyst is coated onto the membrane or dispersed in the liquid medium. Since transport limitations in the liquid phase are neglected, the performance of the inert membrane reactor is overestimated at high catalyst loadings. Hence, the process evaluation suggests that membranes coated with a thin Amberlyst layer can be an interesting option.

8.4 Conclusions Composite catalytically active membranes can be used to enhance conversion of esterification reactions, as the reaction and separation functions are coupled very efficiently. An additional advantage of such

Process design of pervaporation-assisted esterification reactions

171

composite membranes is that both the selective layer and the catalytic layer can be optimized independently. Furthermore, the catalyst coating will, to some extent, protect the membrane against mechanical damage. A preliminary reactor evaluation indicates that Amberlyst-coated membranes can be used to achieve high conversions, due to the high catalytic activity of Amberlyst. The reactor performance can be optimised by adjusting the catalytic layer thickness. Moreover, further optimisation can be achieved by varying the catalytic layer thickness over the length of the module. To achieve the same effect, a sequence of modules having a different catalytic layer thickness can also be used. In this way the coupling of water production and removal can be tuned more easily. A reactor evaluation has been carried out for an extreme situation, in which the feed to the final catalytic membrane reactor consists of a mixture at equilibrium, from which the excess of water is removed.

Acknowledgements This work was performed in a cooperative project of the Centre for Separation Technology and was financially supported by TNO and NOVEM.

172

Chapter 8

Feng, X., Huang, R.Y.M., Studies of a membrane reactor: esterification facilitated by pervaporation, Chem. Eng. Sci., 51 (1996) 4673-4679. Holtmann, T., Membranreaktoren zur Estersynthese, Ph.D. Universität Dortmund, Shaker Verlag, Aachen, Germany, 2003.

thesis,

van Hoof, V., van den Abeele, L., Buekenhoudt, A., Dotremont, C., Leysen, R., Economic comparison between azeotropic distillation and different hybrid systems combining distillation with pervaporation for the dehydration of isopropanol, Sep. Pur. Technol., 37, (2004) 33-49. Kemmere, M.F., Keurentjes, J.T.F., Industrial membrane reactors, in: Nunes, S.P., Peinemann, K.-V., (Eds.), Membrane Technology, Wiley-VCH, Weinheim, Germany (2001) 191-221. Lawrence, P.S., Grünewald, M., Agar, D.W., Spatial distribution of functionalities in an adsorptive reactor at the particle level, Catal. Today, 105 (2005) 582-588. Lim, S.Y., Park, B., Hung, F., Sahimi, M., Tsotsis, T.T., Design issues of pervaporation membrane reactors for esterification, Chem. Eng. Sci., 57, (2002) 4933-4946. Lipnizki, F., Field, R.W., Ten, P-K., Pervaporation-based hybrid process: a review of process design, applications and economics, J. Membr. Sci., 153, (1999) 183-210. Liu, K., Tong, Z., Liu, L., Feng, X., Separation of organic compounds from water by pervaporation in the production of n-butyl acetate via esterification by reactive distillation, J. Membr. Sci., 256, (2005) 193-201. McKetta, J.J., Encyclopedia of chemical processing and design, Marcel Dekker, Vol 19 (1983). Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Keurentjes, J.T.F., Design directions for composite catalytic hollow fibre membranes for condensation reactions, Trans. IChemE, Part A, Chem. Eng. Res. Des., 82 (2004) 220-228. Reid, E.E., 1952, Esterification, in Unit Processes in Organic Synthesis, 4th edition Groggins, P.H. (ed) (McGraw Hill, New York, USA) pp 596-642.

Process design of pervaporation-assisted esterification reactions

173

Sommer, S., Pervaporation and vapour permeation with microporous inorganic membranes, Ph.D. thesis, RWTH Aachen, Verlag Mainz, Germany, 2002. van Veen, H.M., Reduction of energy consumption in the process industry by pervaporation with inorganic membranes: techno-economical feasibility study, ECN-project report (ECN-C--01-073) 2001. Waldburger, R.M., Widmer, F., Membrane reactors in chemical production processes and the application to the pervaporation-assisted esterification, Chem. Eng. Technol., 19 (1996) 117-126. Wynn, N., Pervaporation comes of age, CEP magazine, 97 (2001) 66-72.

Chapter 9 Conclusions, recommendations and perspectives In this thesis the preparation of composite catalytic pervaporation membranes and their application in condensation reactions has been investigated. These membranes enable close and efficient integration of reaction and separation, while allowing independent optimisation of the catalytic and separation functions. The present chapter summarises the conclusions and provides recommendations based on the findings of this work, ordered by chapter. Additionally, two possible future applications of the work presented in this thesis are discussed.

176

Chapter 9

9.1 Conclusions and recommendations In this thesis the preparation of composite catalytic pervaporation membranes for the integrated reaction and separation in condensation reactions has been investigated. First, a mathematical model has shown the added value of the composite membrane system compared to conventional reactor designs. Then, pervaporation membranes have been prepared by deposition of water selective microporous silica and cross-linked poly(vinyl)alcohol layers on the outer surface of hollow fibre ceramic substrates. The pervaporation performance of these membranes is extensively examined in the dehydration of various alcohols. Subsequently, catalytic pervaporation membranes have been developed by deposition of H-USY and Amberlyst 15 layers on silica and PVA membranes, respectively. Finally, the performance of the composite catalytic membranes as both separator and reactor has been examined in the esterification reaction between acetic acid and butanol. Based on the findings of this work, the following conclusions and recommendations for future research can be given. Pervaporation-esterification modelling A mathematical model has shown the added value of the composite membrane/catalyst system compared to more usual reactor designs. Under the prevailing conditions, an optimum catalytic layer thickness has been found to be around 100 µm, which is practically feasible. At this optimum catalytic layer thickness, the performance of a catalytic membrane reactor might exceed the performance of an inert membrane reactor due to the close integration of reaction and separation. Using the model, catalytic membranes can be designed conveniently if both the reaction kinetic parameters and the membrane parameters are known. Currently, several assumptions in the pervaporation-coupled esterification model have been made. For example, the membrane reactor is assumed to behave as an ideal isothermal plug-flow reactor. For a thorough analysis, the temperature drop along the length of the reactor, and axial diffusion in the catalytic layer need to be taken into account. Moreover, it is beneficial to adopt the Maxwell-Stefan approach for the description of diffusion in the catalytic layer, as this approach easily takes

Conclusions, recommendations and perspectives

177

into account drift fluxes caused by the water removal through the membrane. Additionally, the effect of the membrane selectivity on the reactor performance has to be considered. For membranes showing a moderate selectivity, it is more efficient to operate the reactor with a feed composition of the alcohol higher than the stoichiometric value due to the loss of reactant through the membrane [Assabumrungrat et al., 2003]. Solid acid catalysis of esterification reactions Esterification of acetic acid with butanol has been studied in a heterogeneous reaction system, using a variety of solid acid catalysts. The catalysts have been characterised using gas adsorption analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM) and the temperature-programmed decomposition (TPD) of adsorbed isopropylamine. The weight-based activity of the heterogeneous catalysts decreases in the following order: Smopex-101 > Amberlyst 15 > sulphated ZrO2 > H-USY-20 > H-BETA-12.5 > H-MOR-45 > Nb2O5 > H-ZSM-5-12.5. In the heterogeneous catalysis of esterifications using zeolites and sulphated zirconia, no clear relationship has been observed between the total amount of acid sites, measured with the temperature-programmed decomposition of isopropylamine, and the catalytic activity. In order to get more insight it is necessary to discriminate between Brønsted and Lewis acid sites, e.g., using infrared spectroscopy on adsorbed ammonia or pyridine [Meloni et al., 2001], or using solid state MAS NMR spectroscopy [Sutovich et al., 1999]. In this way, it is also possible to obtain more insight in the esterification mechanism using heterogeneous catalysts [Koster et al., 2001]. Moreover, the long-term stability of the heterogeneous catalysts in the esterification reaction needs to be examined further. Hollow fibre silica membranes Thin microporous silica membranes were prepared on the outer surface of hollow fibre ceramic substrates. In principle this enables relatively fast and inexpensive production of a large membrane surface area, combined with a low support resistance and a high membrane surface area / module volume ratio (> 1000 m2·m-3) compared to tubular membranes.

178

Chapter 9

High He permeances (1.1-2.9·10-6 mol⋅m-2⋅s-1⋅Pa-1), high He/N2 permselectivities (~ 100-1000) and an Arrhenius type temperature dependence of gas permeance indicate that the membranes are microporous and have a low number of defects. However, for pervaporation purposes the stability of the γ-alumina intermediate layer and the silica top layer remains an issue. More chemically resistant materials, like zirconia and titania have been applied on flat plate geometries in order to increase the membrane stability [van Gestel et al., 2002]. However, their application on hollow fibre supports is not straightforward and therefore all steps in the preparation method, e.g., sol-gel synthesis, dip-coat procedure and sintering strategy need to be optimised for hollow fibre supports. Composite ceramic-supported PVA pervaporation membranes Thin, high-flux and highly selective cross-linked poly(vinyl)alcohol water selective layers have been prepared on top of hollow fibre ceramic supports. The thickness of the PVA top layer is in the order of 0.3 - 0.8 µm, and no substantial infiltration of PVA into the intermediate γ-Al2O3 layer is observed. In the dehydration of 1-butanol (80°C, 5 wt.% water) the membranes exhibit a high water flux (0.8 - 2.6 kg·m-2·h-1), combined with a high separation factor (500 - 10.000). In the dehydration of 2-propanol and 1-butanol, a simultaneous increase in both water flux and separation factor is observed with increasing temperature or water concentration. A possible explanation for this behaviour is a low degree of three dimensional swelling in the vicinity of the γ-Al2O3 – PVA interface due to an enhanced structural stability. The expensive ceramic support can easily be regenerated, i.e., the organic layer can be removed either by washing the membrane with an appropriate solvent, or by thermal treatment. In contrast, full ceramic membranes have to be replaced and disposed of after contamination or damage. The application window for these newly developed composite polymer/ceramic membranes is limited to 95°C. By optimisation of the parameters in the preparative route, i.e., cross-linking agent and content, annealing temperature and time, and the properties of the ceramic support, the separation performance can be improved. Additionally, blending of poly(vinyl)alcohol with other

Conclusions, recommendations and perspectives

179

polymers, like poly(acrylic)acid [Burshe et al., 1998], or incorporation of inorganic fillers, like tetraethoxysilane [Urigami et al., 2002] or zirconia [Kim et al., 2001] in the PVA selective layer can also enhance membrane performance. In general, ceramic materials as a stable support structure for polymeric membranes could play an important role in enhancing membrane stability in all applications in which swelling of the polymeric selective membrane layer has to be minimised. As an example, the application of ceramic supports in the development of stable membranes for the removal of VOCs from waste water, and for the separation of organic/organic mixtures is demonstrated in paragraph 9.2.1. Zeolite-coated pervaporation membranes Catalytic pervaporation membranes have been developed by deposition of H-USY zeolite on top of ceramic silica membranes. Tuning of the catalytic layer thickness is possible by varying the number of dip-coat steps. In the pervaporation-assisted esterification reaction, the catalyst-coated pervaporation membrane is able to couple catalytic activity and water removal. However, the performance of the H-USY-coated membrane is mainly limited by reaction kinetics. Consequently, this type of catalytic pervaporation membranes might be applicable in the case where water removal from the reaction mixture becomes critical, e.g., esterification mixtures with a high conversion or condensation reactions with a very low equilibrium conversion. In this perspective, the application of catalytic pervaporation membranes in the direct synthesis of dimethyl carbonate starting from methanol and carbon dioxide might greatly benefit from the integration of catalytic reaction and water removal due to the very low equilibrium conversion. This application is discussed more elaborately in paragraph 9.2.2. Amberlyst-coated pervaporation membranes Catalytic Amberlyst 15 layers have been deposited on composite ceramic/PVA membranes by a dip-coat technique using Aculyn as binder material. Due to the high activity of the Amberlyst catalyst, the performance of the Amberlyst-coated membrane is limited by the water

180

Chapter 9

removal rate. This in contrast to zeolite-coated pervaporation membranes, which performance is limited by the catalytic activity. A further improvement in reactor performance can be realised by optimisation of the catalytic layer in terms of thickness and, for example, porosity. In this way, the coupling between the water production and removal can be optimised. Aside from pervaporation-assisted esterifications, Amberlyst-coated pervaporation membranes could also be applied in the synthesis of diethylacetal, which is formed in the liquid phase by a reversible reaction between acetaldehyde and ethanol forming water as a by-product [Silva and Rodriquez, 2001]. In this perspective, a catalytic pervaporation membrane reactor can be an alternative for currently available technologies, e.g., chromatographic and distillation techniques [Silva and Rodriquez, 2002]. Process design for esterification reactions

catalytic

pervaporation

membrane-assisted

For the pervaporation-assisted esterification process, two process configurations based on catalytic pervaporation membrane modules are proposed. Preliminary large scale reactor evaluations are carried out based on the obtained experimental data, e.g., membrane permeability and catalyst activity. A reactor evaluation proved that the outlet conversion for the catalytic pervaporation-assisted esterification reaction exceeds the conversion of a conventional inert pervaporation membrane reactor with the same loading of catalyst dispersed in the liquid bulk. This shows the added value of such a membrane system compared to conventional reactor designs. For a proper comparison with conventional existing production routes, e.g., reactive distillation, more elaborate Aspen and FEMLAB calculations and a costs analysis need to be performed. In the following section, two possible applications of the work presented in this thesis are explored. First, an extension of the application window for composite ceramic/polymer membranes is presented. Secondly, the possible use of water selective ceramic membranes under supercritical conditions is explored. As an example, the direct synthesis of dimethyl carbonate from methanol and carbon dioxide is taken.

Conclusions, recommendations and perspectives

181

9.2 Future perspectives 9.2.1 Composite ceramic/polymer membranes for organic separation Currently, most commercial pervaporation processes involve the dehydration of alcohols and other organic solvents [Wynn, 2001]. In recent years, the potential use of pervaporation technology for the separation of organic mixtures and the removal of VOCs from waste water has attracted significant attention [Feng and Huang, 1997]. However, the use of commercially available polymeric membranes in the above mentioned applications has not attained widespread industrial acceptance, mainly due to degradation of the membrane in organic mixtures caused by swelling and a loss of membrane integrity [Jou et al., 1997]. Many different membrane types, such as poly(dimethylsiloxane) (PDMS) [Ohshimia et al., 2005], polyether-block-polyamides (PEBA) [Sheng, 1991; Ji et al., 1994], poly(vinyl)acetate (PVAc) [Jou et al., 1997], ethene-propene-diene terpolymer (EPDM) [Pereira et al., 1998] and modified poly(phenylene oxide) [Doghieri et al., 1994] have been examined for removal of VOCs from water by pervaporation. PDMS has proven to be the most promising material due to its hydrophobic and rubbery nature. However, it proves to be very difficult to prepare mechanically strong membranes from PDMS, because the mechanical strength of PDMS is notably poor [Ohshimia et al., 2005]. In this perspective, the adaptation of ceramic materials as a stable support structure for polymeric membranes as presented in Chapter 5, could play an important role in enhancing the membrane stability in organic/organic separation by pervaporation. However, additional studies are needed to assess whether the remarkable performance observed in the dehydration of alcohols is also observed in the separation of organic/organic mixtures, and in the removal of VOCs from waste water. Additionally, membrane performance needs to be assessed for other combinations of selective polymer materials and mesoporous intermediate layers. In addition to pervaporation, the polymer/inorganic composite membranes developed here may be used in other applications as well, such as nanofiltration and reverse osmosis. For non-aqueous nanofiltration in

182

Chapter 9

particular, the prevention of membrane swelling is very important. In general, all applications in which swelling of the polymeric selective membrane layer upon contact with the feed becomes a problem can benefit from the adaptation of ceramic materials. 9.2.2 Direct synthesis of DMC from methanol and CO2 The development of environmentally benign processes utilising carbon dioxide, which is a cheap and safe C1 resource as well as a non-toxic reaction medium, has received much interest. One of the most attractive reactions is the direct synthesis of dimethyl carbonate (DMC) starting from carbon dioxide and methanol [Hou et al., 2002]. However, the main problem for this reaction is the low equilibrium conversion. The direct synthesis of dimethyl carbonate starting from methanol and carbon dioxide can greatly benefit from water removal to increase the equilibrium conversion. Several studies have been performed to study the effect of water removal on the obtained conversion using molecular sieves [Choi et al., 2002; Hou et al., 2002], or using a series reaction using trimethyl orthoformate [Tomishige et al., 1999] or dimethoxy-propane [Tomishige and Kunimori, 2002]. However, the use of inorganic dehydration agents proved not to be successful due to the reversibility of adsorption under reaction conditions, while the addition of trimethyl orthoformate or dimethoxy-propane was not suitable due to a decrease in DMC formation rate, and the formation of by-products [Tomishige and Kunimori, 2002]. An alternative for water removal by means of molecular sieves is the use of water selective membranes. Li et al., have studied the membrane-enhanced formation of DMC using three types of mesoporous membranes. It was found that the conversion of methanol improved remarkably whereas the selectivity slightly increased [Li et al., 2003]. However, the membranes showed substantial permeation of the reactants and, as a result, it was proposed to use carbon dioxide as a sweep flow containing methanol to compensate for the loss of methanol. Microporous ceramic membranes are an alternative for the mesoporous membranes used by Li et al. [Li et al., 2003]. Microporous inorganic membranes are envisioned as promising candidates for gas separation under harsh conditions and have been extensively used in gas separation and pervaporation [Peters et al., 2005]. In the dehydration of alcohols, these

Conclusions, recommendations and perspectives

183

membranes appear to combine high selectivities with high permeabilities. Under supercritical conditions, however, silica membranes have only been used to separate small species, such as oils, micelles, and hom*ogeneous catalysts, from carbon dioxide while maintaining supercritical conditions [van den Broeke et al., 2001; Verkerk et al., 2002; Goetheer et al., 2003]. If microporous silica membranes would maintain their water selectivity under supercritical conditions, the direct formation of DMC would greatly benefit from this. This requires the membrane to be stable, permselective and to show a reasonable flux at increased temperatures and pressures. Present membranes, however, do not display enough stability [Verkerk, 2003]. Hence, new membranes have to be developed, with properties tailored for application under supercritical conditions. These materials should have a low adsorption of carbon dioxide and organic compounds, which block the membrane material. Potentially the most interesting membrane materials are sol-gel derived amorphous transition metal oxides containing heterogeneous transition metal bonds [Brinker and Scherer, 1990]. These materials show a high stability and the sol-gel synthesis allows tailoring of their properties.

9.3 Concluding remarks In this thesis two new membrane concepts are introduced; ceramic-supported polymer membranes that show an improved performance compared to existing membranes, and catalytic membranes for which close integration of reaction and molecular separation offers advantages over conventional processes. Academic studies of these new concepts can be extended towards future applications, such as ceramic-supported hydrophobic polymers for organic removal, and other equilibrium reaction systems such as the direct formation of DMC from methanol and carbon dioxide. However, industrial acceptance of membrane processes in general is hampered by concerns about reliability. At this point, the challenge is to design and demonstrate reliable pilot scale membrane processes, based on the new concepts presented in this thesis.

184

Chapter 9

References Assabumrungrat, S., Phongpatthanapanich, J., Praserthdam, P., Tagawa, T., Goto, S., Theoretical study on the synthesis of methyl acetate from methanol and acetic acid in pervaporation membrane reactors: effect of continuous-flow modes, Chem. Eng. J., 95 (2003) 57-65. Brinker, C.J., Scherer, G.W., Sol-gel Science, The physics and chemistry of sol-gel processing, Acedemic Press Inc, San Diego, USA, 1990. van den Broeke, L.J.P., Goetheer, E.L.V., Verkerk, A.W., de Wolf, E., Deelman, B., van Koten, G., Keurentjes, J.T.F., hom*ogeneous reactions in supercritical carbon dioxide using a catalyst immobilized by a microporous silica membrane, Angew. Chem. Int. Ed., 40 (2001) 4473-4474. Burshe, M.C., Sawant, S.B., Joshi, J.B., Pangarkar, V.G., Dehydration of ethylene glycol by pervaporation using hydrophilic IPNs of PVA, PAA and PAAM membranes, Sep. Purif. Technol., 13 (1998) 47-56. Choi, J-C., He, L-N., Yasuda, H., Sakakura, T., Selective and high yield synthesis of dimethyl carbonate directly from carbon dioxide and methanol, Green Chemistry, 4 (2002) 230-234. Doghieri, F., Nardella, A., Sarti, G.C., Valentini, C., Pervaporation of methanol-MTBE mixtures through modified poly(phenylene oxide) membranes, J. Membr. Sci., 91 (1994) 283-291. Feng, X., Huang, R.Y.M., Liquid separation by membrane pervaporation: a review, Ind. Eng. Chem. Res., 36 (1997) 1048-1066. van Gestel, T., Vandecasteele, C., Buekenhoudt, A., Dotremont, C., Luyten, J., Leysen, R., van der Bruggen, B., Maes, G., Alumina and titania multilayer membranes for nanofiltration: preparation, characterization and chemical stability, J. Membr. Sci., 207 (2002) 73-89. Goetheer, E.L.V., Verkerk, A.W., van den Broeke, L.J.P., de Wolf, E., Deelman, B., van Koten, G., Keurentjes, J.T.F., Membrane reactor for hom*ogeneous catalysis in supercritical carbon dioxide, J. Catalysis, 219 (2003) 126-133.

Conclusions, recommendations and perspectives

185

Hou, Z., Han, B., Liu, Z., Jiang, T., Yang, G., Synthesis of dimethyl carbonate using CO2 and methanol: enhancing the conversion by controlling the phase behavior, Green Chemistry, 4 (2002) 467-471. Jou, J-D., Yoshida, W., Cohen, Y., A novel ceramic-supported polymer membrane for pervaporation of dilute volatile organic compounds, J. Membr. Sci., 162 (1999) 269-284. Kim, K-J., Park, S-H., So, W-W., Moon, S-J., Pervaporation separation of aqueous organic mixtures through sulphated zirconia-poly(vinyl alcohol) membrane, J. Appl. Pol. Sci., 79 (2001) 1450-1455. Koster, R., van der Linden, B., Poels, E., Bliek, A., The mechanism of the gas-phase esterification of acetic acid and ethanol over MCM-41, J. Catal., 204 (2001) 333-338. Li, C-F., Zhong, S-H., Study on application of membrane reactor in direct synthesis of DMC from CO2 and CH3OH over Cu-KF/MgSiO catalyst, Cat. Today, 82 (2003) 83-90. Liu, Q-L., Xiao, J., Silicalite-filled poly(siloxane imide) membranes for removal of VOCs from water by pervaporation, J. Membr. Sci., 230 (2004) 121-129. Meloni, D., Laforge, S., Martin, D., Guisnet, M., Rombi, E., Solinas, V., Acidic and catalytic properties of H-MCM-22 zeolites 1. characterization of the acidity by pyridine adsorption, Appl. Catal. A: General, 215 (2001) 55-66. Ohshima, T., Kogami, Y., Miyata, T., Uragami, T., Pervaporation characteristics of cross-linked poly(dimethylsiloxane) membranes for removal of various volatile organic compounds from water, J. Membr. Sci., 260 (2005) 156-163. Pereira, C.C., Habert, A.C., Nobrega, R., Borges, C.P., New insights in the removal of diluted volatile organic compounds from dilute aqueous solution by pervaporation process, J. Membr. Sci., 138 (1998) 227-235. Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Benes, N.E., van Dam, R.A., Vroon, Z.A.E.P., van Soest-Vercammen, E.L.J., Keurentjes, J.T.F., Hollow fibre microporous silica membranes for gas separation and pervaporation; synthesis, performance and stability, J. Membr. Sci., 248 (2005) 73-80.

186

Chapter 9

Sheng, J., Separation of dichloroethane-trichloroethylene mixtures by means of a membrane pervaporation process, Desalination, 80 (1991) 85-95. Silva, V.M.T.M., Rodriquez, A.E., Synthesis of diethylacetal: thermodynamic and kinetic studies, Chem. Eng. Sci., 56 (2001) 1255-1263. Silva, V.M.T.M., Rodriquez, A.E., Dynamics of a fixed-bed adsorptive reactor for synthesis of diethylacetal, AIChE J., 48 (2002) 625-634. Sutovich, K.J., Peters, A.W., Rakiewicz, E.F., Wormsbecher, R.F., Mattingly, S.M., Mueller, K.T., Simultaneous Quantification of Brønstedand Lewis-Acid Sites in a USY Zeolite, J. Catal., 183 (1999) 155-158. Tomishige, K., Sakaihori, T., Ikeda, Y., Fujimoto, K., A novel method of direct synthesis of dimethyl carbonate from methanol and carbon dioxide catalyzed by zirconia, Cat. Letters, 58 (1999) 225-229. Tomishige, K., Kunimori, K., Catalytic and direct synthesis of dimethyl carbonate starting from carbon dioxide using CeO2-ZrO2 solid solution heterogeneous catalyst: effect of H2O removal from the reaction system, Appl. Catal. A: General, 237 (2002) 103-109. Urigami, T., Okazaki, K., Matsugi, H., Miyata, T., Structure and permeation characteristics of an aqueous ethanol solution of organicinorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane, Macromolecules, 35 (2002) 9156-9163. Verkerk, A.W., Goetheer, E.L.V., van den Broeke, L.J.P., Keurentjes, J.T.F., Permeation of carbon dioxide through a microporous silica membrane at subcritical and supercritical conditions, Langmuir, 18 (2002) 6807-6812. Verkerk, A.W., Application of silica membranes in separations and hybrid reactor systems, PhD thesis, Eindhoven University of Technology, Universiteitsdrukkerij, Technische Universiteit Eindhoven, Eindhoven, the Netherlands, 2003. Wu, Y., Wang, S., Jin, M., Yang, X., Poly(acrylate-co-acrylic acid)/polysulfone composite membranes for pervaporation of volatile organic compounds from water, Sep. Sci. Technol., 36 (2001) 3529-3540. Wynn, N., Pervaporation comes of age, CEP magazine, 97 (2001) 66-72.

Dankwoord Allereerst wil ik mijn promotor, Jos Keurentjes, bedanken. Bedankt Jos, voor de mogelijkheden die je me hebt gegeven tijdens mijn promotie. Tijdens mijn afstudeerperiode had je al snel door dat ik wel oren had naar een promotie, maar dan moest er wel de mogelijkheid voor me zijn om een gedeelte van het werk in Trondheim, Noorwegen uit te voeren. En zo geschiedde. Het leek er wel op alsof we er alles zo op hadden gepland, zo goed volgde alles op elkaar. Vijf maanden Eindhoven, negen maanden Trondheim, daarna bijna drie jaar Eindhoven en nu weer Oslo. Als ik vantevoren geweten had dat het zo zou gaan lopen dan had ik er meteen voor getekend. Graag zou ik ook Nieck Benes willen bedanken voor de mogelijkheid die ik altijd had, of dacht te hebben, om zijn kantoor binnen te lopen met onduidelijkheden. Aan het einde van de promotie keek hij er al niet meer van op dat ik ondertussen alweer vijf minuten aan zijn tafel zat voordat hij me had opgemerkt. Ik moet je wel, of liever gezegd, sommige mede-promovendi, nogmaals duidelijk maken dat ik daar niet alleen maar zat met problemen, maar ook soms voor de gezelligheid. De afgelopen vier jaren zouden sowieso niet zo aangenaam zijn geweest zonder de goede sfeer binnen de vakgroep. Het hoogtepunt van deze sfeer moet toch wel gezocht worden in Brazilie waar we op een uitermate gezellige en ontspannende studiereis zijn geweest. De AIChE meeting in San Francisco met uitstap naar Yosemite National Park mag hier natuurlijk ook niet vergeten worden. Bij deze zou ik iedereen, die bij heeft gedragen aan deze aangename werksfeer, willen bedanken. In het bijzonder, Marcus van Schilt, die als kamergenoot de afgelopen vijf jaar tevens mijn gepraat en muziek aan heeft moeten horen. Uiteraard wil ik ook Marius Vorstman bedanken voor het begeleiden van mijn promotie gedurende de eerste anderhalf jaar. Je hebt samen met Javier Fontalvo, richting gegeven aan het onderzoek tijdens het altijd onduidelijke eerste jaar. Mede dankzij deze basis, is de rest van het project zo goed verlopen. Bedankt Marius en Javier. Binnen TNO zou ik graag Esther van Soest-Vercammen, Zeger Vroon, Henk Buijs, Frank Vercauteren, Niki Stroeks, Renz van Ee en Roelant van Dam willen bedanken voor de discussies tijdens de projectbijeenkomsten,

het maken van membranen, maar vooral voor de goede samenwerking in het project. Dank ben ik ook verschuldigd aan Anders Holmen en Edd Blekkan voor de mogelijkheid die ik van jullie heb gehad om negen maanden in Trondheim door te brengen. Tusen takk. Bedankt, Jeroen van der Tuin, Joost van de Venne, Christian Poeth en Rene Vermerris, zonder jullie had ik meer tijd moeten doorbrengen op het lab om alle data te verzamelen. Ik hoop dat jullie de afstudeerperiode ook leuk hebben gevonden. Ook wil ik Christophe Houssin bedanken voor de hulp tijdens het aanbrengen van zeolietlagen op membranen. Tevens wil ik de leescommissie, Jaap Schouten, Freek Kapteijn en Lester Kershenbaum, bedanken voor het becommentariëren van het proefschrift. Verder wil ik mijn familie en vrienden bedanken voor de steun de afgelopen jaren. Speciaal mijn ouders, zonder de mogelijkheid om te gaan studeren had ik hier nu niet gestaan. Aan het einde van dit dankwoord, zou ik graag Ingrid willen bedanken met wie ik nu samen een nieuwe periode in het leven inga. Je bent drie jaar geleden voor mij naar Eindhoven verhuisd. We hebben het daar erg naar ons zin gehad ondanks dat het niet altijd even gemakkelijk voor je is geweest. Ik hoop dat je je er toch thuis hebt gevoeld en dat we het net zo leuk gaan krijgen in ons nieuwe appartement in Oslo. Thijs

Personal Thijs Peters was born on the 6th of May 1978 in Breda in the south of the Netherlands. After finishing the secondary school “Mencia de Mendoza Lyceum” in 1996, he studied Chemical Engineering at the Eindhoven University of Technology. During his education he did a traineeship in Barcelona, Spain on the drying of polymer pellets. He also assisted in the practical course for first-year students at the Eindhoven University of Technology. His graduation project was performed at the Process Development Group, under the supervision of prof. dr. ir. J.T.F. Keurentjes and concerned the micellar catalysis of propylene. During his study he was member of the “Chemiewinkel Eindhoven”, an environmental consultancy agency that advises private persons on questions about environmental pollution and environment related questions. After he graduated in August of 2001, he started as a Ph.D. student in the Process Development of prof. dr. ir. J.T.F. Keurentjes on the project called `development of catalytic pervaporation membranes for the integration of reaction and separation`. During his PhD project he worked for nine months in the Kinetics and Catalysis group of Anders Holmen in Trondheim, Norway. From the 1th of November 2005 he is employed by Sintef Materials and Chemistry in Oslo, Norway.

List of publications Articles Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Keurentjes, J.T.F., Design directions for composite catalytic hollow fibre membranes for condensation reactions, Trans. IChemE, Part A, Chem. Eng. Res. Des., 82 (2004) 220-228. Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Benes, N.E., van Dam, R.A., Vroon, Z.A.E.P., van Soest-Vercammen, E.L.J., Keurentjes, J.T.F., Hollow fibre microporous silica membranes for gas separation and pervaporation: synthesis, performance and stability, J. Membr. Sci., 248 (2005) 73-80. Peters, T.A., van der Tuin, J., Houssin, C., Vorstman, M.A.G., Benes, N.E., Vroon, Z.A.E.P., Holmen, A., Keurentjes, J.T.F., Preparation of zeolite-coated pervaporation membranes for the integration of reaction and separation, Catal. Today, 104 (2005) 288-295. Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Zeolite-coated ceramic pervaporation membranes; pervaporation-esterification coupling and reactor evaluation, Ind. Eng. Chem. Res., 44 (2005) 9490-9496. Peters, T.A., Benes, N.E., Holmen, A., Keurentjes, J.T.F., Comparison of commercial solid acid catalysts for the esterification of acetic acid with butanol, Appl. Catal. A: General, 297 (2006) 182-188. Peters, T.A., Poeth, C.H.S., Benes, N.E., Buijs, H.C.W.M. Buijs, Vercauteren, F.F., Keurentjes, J.T.F., Ceramic-supported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm, J. Membr. Sci., (2005) accepted. Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Ceramic-supported thin PVA pervaporation membranes: long-term and temperature stability in the dehydration of alcohols, submitted. Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Amberlyst-coated pervaporation membranes: preparation and pervaporation-esterification coupling, submitted. Patents Buijs, H.C.W.M., Vercauteren, F.F., Peters, T.A., Benes, N.E., Keurentjes, J.T.F., van Soest-Vercammen, E.L.J., Composite membrane and its use in separation processes, Application No./Patent No. 05077144.3-., filed.

Refereed proceedings Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Keurentjes, J.T.F., Design rules for catalytic hollow fibre membrane modules for condensation reactions, in Proc. 3rd International Symposium on Multifunctional Reactors. (ISMR-3, August 27-30, 2003); Editors: L. Kershenbaum, Bath, United Kingdom, (2003) 16-20. Peters, T.A., Fontalvo, J., Vorstman, M.A.G., Keurentjes, J.T.F., Catalytic pervaporation membrane reactors for the integrated reaction and separation in condensation reactions, in Proc. AIChE Annual Meeting 2003. (November 16-21, 2003); Editors: -, San Francisco, CA, United States of America, (2003). Peters, T.A., Benes, N.E., Vroon, Z.A.E.P., Keurentjes, J.T.F., Development of catalytic pervaporation membranes for integration of reaction and separation, in Proc. 6th International Conference on Catalysis in Membrane Reactors. (ICCMR-6, July 6-9, 2004); Editors: R. Dittmeyer, Lahnstein, Germany, (2004) 201. Peters, T.A., Vorstman, M.A.G., Benes, N.E., Keurentjes, J.T.F., Development of catalytic pervaporation membranes for integration of reaction and separation in condensation reactions, in Proc. Euromembrane 2004 (Sept 28 -Oct 1, 2004); Editors: J. Hapke, Ch. Na Ranong, D. Paul, K.-V. Peinemann, Hamburg, Germany, (2004) 339. Vroon, Z.A.E.P., Dam van, R., Soest-Vercammen van, E.L.J., Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Synthesis, performance and stability of solgel derived silica membranes onto hollow fibre ceramic membranes for pervaporation, in Proc. International Symposium on Inorganic and Environmental Materials 2004. (ISIEM 2004, October 18-21, 2004); Editors: -, Eindhoven, the Netherlands, (2004) 221. Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Development of catalytic pervaporation membranes for the integration of reaction and separation, 4th Netherlands Process technology Symposium (NPS4, October 26-27, 2004), Veldhoven, the Netherands. Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Development of catalytic pervaporation membranes for integration of reaction and separation, in Proc. 5th International Symposium on Catalysis in Multiphase Reactors &

4th International Symposium on Multifunctional Reactors. (Camure-5 & ISMR-4, June 15-18, 2005); Editors: J. Levec, A. Pintar, Portoroz, Slovenia, (2005) 139-140. Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Development of catalytic pervaporation membranes for integration of reaction and separation, in Proc. 2th International Symposium on Structured Catalysts and Reactors. (ISOSCAR-2, October 15-18, 2005), Editors: -, Delft, The Netherlands, (2005). Peters, T.A., Benes, N.E., Buijs, H.C.W.M., Vercauteren, F.F., Keurentjes, J.T.F., Thin high flux ceramic-supported PVA membranes, in Proc. Worskhop ‘’Membranes in Solvent Filtration’’. (March 23-24, 2006), Editors: -, Leuven, Belgium (2006). Peters, T.A., Benes, N.E., Keurentjes, J.T.F., Thin high flux ceramicsupported PVA membranes, in Proc. Euromembrane 2006 (September 24-28, 2006); Editors: -, Taormina (Messina), Italy, (2006).

[PDF] Catalytic pervaporation membranes for close integration of reaction and separation - Free Download PDF (2024)

References