Membrane in a Reactor: A Functional Perspective - Industrial

Ind. Eng. Chem. Res. 1999, 38, 3715-3737

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Membrane in a Reactor: A Functional Perspective Kamalesh K. Sirkar,* Purushottam V. Shanbhag,† and A. Sarma Kovvali Center for Membrane Technologies, Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102

Membrane reactors have found utility in a broad range of applications including biochemical, chemical, environmental, and petrochemical systems. The variety of membrane separation processes, the novel characteristics of membrane structures, and the geometrical advantages offered by the membrane modules have been employed to enhance and assist reaction schemes to attain higher performance levels compared to conventional approaches. In these, membranes perform a wide variety of functions, often more than one function in a given context. An understanding of these various membrane functions will be quite useful in future development and commercialization of membrane reactors. This overview develops a functional perspective for membranes in a variety of reaction processes. Various functions of the membranes in a reactor can be categorized according to the essential role of the membranes. They can be employed to introduce/separate/purify reactant(s) and products, to provide the surface for reactions, to provide a structure for the reaction medium, or to retain specific catalysts. Within these broad contexts, the membranes can be catalytic/noncatalytic, polymeric/inorganic, and ionic/nonionic and have different physical/chemical structures and geometries. The functions of the membrane in a reaction can be enhanced or increased also by the use of multiple membrane-based schemes. This overview develops a perspective of each membrane function in a reactor to facilitate a better appreciation of their role in the improvement of overall process performance. 1. Introduction Membrane reactors have been investigated since the 1970s. The early investigations employed primarily polymeric membranes and enzymatic reactions. Later investigations show an abundance of petrochemically relevant systems and inorganic membranes. Whole cell fermentation-based chemical and biochemical productions as well as degradation of pollutants biologically or otherwise have also been studied in membrane reactors. Polymer membrane-based reactors have been blessed with some commercial success. Much of this research has been discussed in a number of reviews.1-9 In these investigations a variety of membrane separation processes as well as membranes have been used. More importantly, the membrane inside the reactor has served a variety of functions. In some studies, the membrane has a single well-defined function. In others, the membrane allows two or more functions to be carried out. The variety of functions achievable via a membrane in a reactor is very broad. An understanding of the breadth of the roles capable of being performed by a membrane is likely to be quite useful in the future development of membrane reactors. This overview proposes to develop a functional perspective of a membrane(s) in a reactor. This perspective is developed by employing a variety of contexts including different membrane separation processes, different membranes, chemical/electrochemical reactions, enzymatic processes, fermentations, catalyst immobilization/segregation, catalytic membranes, integration of functions, etc. A brief enumeration of different membrane separation processes and different classes of membranes investi* To whom correspondence should be addressed. Tel.: (973) 596-8447. Fax: (973) 642-4854. E-mail: [email protected]. † Current address: Compact Membrane Systems, Inc., Wilmington, DE 19804.

gated in the literature is useful in the Introduction before we present the functional perspective. Of the many types of membrane separation processes and membrane-based equilibrium separation processes available for separation,10 membrane reactors have been studied using the following: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), electrodialysis (ED), liquid membranes (LM), pervaporation (PV), gas permeation, vapor permeation, molecular sieving, Knudsen diffusion (and molecular diffusion), gas membrane, membrane solvent extraction, and membrane gas absorption/stripping. An extraordinary variety of membranes have also been used. Membranes are employed in gross physical forms as flat films, hollow fibers, tubules, and tubes, while their physical structures can be as follows: microporous symmetric and asymmetric membranes, nonporous membranes, and composite membranes. Membranes can be of the polymeric variety or be inorganic in nature, which would include zeolitic, ceramic, and metallic membranes. Membranes can also conduct electrical charges and can be chosen from one of following categories: ion-exchange membranes, bipolar membranes, mixed conducting membranes, proton-conducting membranes, etc. In many cases, the membranes have catalysts incorporated in their porous structure or on the surfaces. The membranes in such cases are termed as catalytic membranes. Of course, the membrane can be catalytic by itself without the addition of any catalyst materials from external sources. The term catalytic membrane reactor sometimes includes the above cases as well as a catalytic reactor enclosed by a membrane, which is noncatalytic.2 In the next section, we will first present a compact list of membrane functions in a reactor. Often, the generic membrane function identified will affect the reaction processes in different ways. Such effects on the

10.1021/ie990069j CCC: $18.00 © 1999 American Chemical Society Published on Web 09/18/1999

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Figure 2. Schematic of a recycle-based configuration of a coupled reactor and membrane separator.

membrane in a given reactor is capable of all functions identified in the figure. However, a given membrane under appropriate circumstances can perform more than one generic function. The introduction of another membrane into the reactor can increase the number of generic membrane functions in the reactor or achieve the same generic membrane function vis-a`-vis some other species. Figure 1 also indicates other activities concurrently taking place in the so-called nonreactor (or permeate) side of the membrane as well as in the reactor side of the membrane. A list of the generic membrane functions performed by a membrane or two in a reactor is provided next: Figure 1. Schematic of possible functions of a membrane in a reactor.

reaction will be identified. Each generic membrane function will then be illustrated in a separate subsection using examples from the literature. This illustrative exposition will employ different membrane separation processes, reaction-phase systems, and other distinguishing features to elaborate briefly on particular membrane reactors. The goal is to develop a perspective on the range and the utility of each membrane function in a membrane reactor rather than a review of all investigations. Frequently, a membrane (or two membranes) incorporated in a reactor serves more than one desired function, only one of which may involve a membrane separation process where membrane flux and selectivity are important. The development of such a multifunctional perspective of membranes in a reactor is an additional objective of this paper. The reaction processes of interest in this paper may involve the production of a particular chemical or a biochemical product. Alternately, it may involve the destruction of some organic species in a phase for the purpose of controlling environmental pollution. Reaction processes are sometimes employed to purify a particular fluid stream without destroying the undesirable species into simple compounds such as CO2, H2O, etc. (e.g., enzymatic resolution processes). Although such processes are within the scope of this paper, our main focus will be on those reactive processes where a particular compound or two are produced by the reactions. We specifically exclude those processes where reactions are used to enhance separation of a mixture. 2. Membrane Functions in a Reactor Figure 1 schematically identifies many of the major generic functions performed by a membrane in a reactor. One should not conclude from the figure that a given

2.1. Separation of products from the reaction mixture 2.2. Separation of a reactant from a mixed stream for introduction into the reactor 2.3. Controlled addition of one reactant or two reactants 2.4. Nondispersive phase contacting (with reaction at the phase interface or in the bulk phases) 2.5. Segregation of a catalyst (and cofactor) in a reactor 2.6. Immobilization of a catalyst in (or on) a membrane 2.7. Membrane is the catalyst 2.8. Membrane is the reactor 2.9. Solid-electrolyte membrane supports the electrodes, conducts ions, and achieves the reactions on its surfaces 2.10. Transfer of heat 2.11. Immobilizing the liquid reaction medium Membranes in a reactor existing as membrane laminates or physically separated membranes with a fluid phase between have also been studied. They can provide particular combinations of the above functions sometimes with added and novel benefits;5,11 these novel benefits include product separation and simultaneous concentration, separation of multiple products, reaction intensification, and physically containing the reaction medium in multiphase reaction systems. Before proceeding further, it is necessary to point out that there are many studies where the membrane is physically located in a device external to the reactor proper (structure). The reaction medium is then circulated over the membrane and back to the reactor in a recycle mode (Figure 2). This configuration is frequently employed in reaction processes based on enzymes and whole cells; it is also being proposed for organic syntheses. The reactor vessel in such case is sometimes operated as a batch reactor or more frequently as a continuous stirred tank reactor (CSTR). In many circumstances, the system behavior here can be considered

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to be de facto equivalent to that with a membrane inside a reactor. Therefore, such recycle membrane reactors will also be included in the following treatment: primary emphasis will however be on systems where a membrane (or two) is physically located in the reactor. One must recognize the major advantages of these different arrangements: (1) The mixing conditions and the flow velocities (and therefore the extent of consequent concentration polarization in membrane devices involving liquid-phase systems) can be maintained at different levels in the reactor and the membrane separator if recycle membrane reactors are employed; conditions can be optimized for each. The reactor may require long residence times whereas the membrane device may need a short residence time. (2) Building a reactor with a membrane in it or using a membrane device as the reactor can sometimes be very demanding on the membrane, especially for higher temperature systems. The recycle membrane reactor allows the reactor and the membrane unit to operate at two different temperatures by using heat exchangers in between. (3) Recycle membrane reactors allow use of existing equipment, namely, a separate reactor and a separate membrane device. (4) For fast reactions, the membrane in a reactor is likely to be a more desirable configuration. 2.1. Separation of Products from the Reaction Mixture. Separation of products from the reaction mixture is one of the most common functions of a membrane in a reactor. The separation may be purification, enrichment, or concentration. Consider the following elementary reversible reaction (see Figure 1):

A+BTC+D

(1)

where D is a product needed to be removed via the membrane to the permeate side. The separation process employed may produce a permeate side stream where the mole fraction of D is much higher than that in the reactor side. In the case of species D being H2 and a palladium membrane, pure H2 is obtained in the permeate side. The H2 partial pressure on the permeate side will be lower than the partial pressure of H2 on the feed side. If species C also permeates to some extent through the membrane, the permeate stream is enriched in D vis-a`-vis the product species C: permeation of D leads to partial purification of the product C in the reactor outlet stream. Removal of D via the separation function of the membrane has the following effects on reaction (1) and the reactor performance: (a) The equilibrium condition indicated in the reversible reaction (1) is shifted to the right, i.e., leading to higher equilibrium conversion of A and B to C and D. (b) If there is an undesirable side reaction as shown below,

B+DTE

(2)

taking place in the reactor (see Figure 1), the separation of product D from the reaction mixture reduces the loss of reactant B to the side reaction, increasing the selectivity of conversion to product C (or D) (modeled by Whu et al.12 for nanofiltration-aided liquid-phase organic synthesis). An experimental study by Raich and Foley13 of ethanol dehydrogenation in a palladium

membrane reactor whereby the product H2 is withdrawn through the palladium membrane to shift the reaction

C2H5OH T CH3CHO + H2

(3a)

to the right showed that the deleterious effect of the side reaction

C2H5OH + CH3CHO T CH3COOC2H5 + H2

(3b)

can be drastically reduced, provided the reaction (3a) is shifted to the right via the Cu/SiO2-aq catalyst and H2 removal by the membrane. (c) In consecutive catalytic reactions,

AfB

(4a)

BfC

(4b)

where B is the desired intermediate product, if the rate constant for reaction 4b is significantly larger than the rate constant for reaction 4a, it is difficult to achieve a high selectivity to B using a conventional packed bed, plug flow reactor. By using an inert sweep gas on the outside of a permeable tube having the catalysts and the reaction taking place inside the tube, the intermediate product B may be selectively removed from the reaction zone, leading to increased selectivity.14 Removal of the intermediate products (methanol/formaldehyde) via a membrane in the partial oxidation of methane is an example; this strategy will prevent further oxidation of these products to CO and CO2. (d) In fermentation processes, one of the products may be inhibitory to the fermentation process. Removal of the product from the fermentation broth via a membrane can substantially reduce product inhibition and increase volumetric productivity of the fermentor.15,16 Further, one can use higher concentrations of the substrate in the feed (e.g., glucose for ethanol fermentation) since the product is being removed as it is being formed.17 The separation of a reaction product(s) (C or D or both) can be implemented using a variety of membrane processes. The nature of the membrane process is obviously influenced by the phase of the reaction medium exposed to the membrane and the desired phase of the permeated product stream. Examples of such processes will be provided under two categories, namely, (1) liquid reaction medium/liquid feed phase and (2) gaseous reactions/gaseous feed phase. 2.1.1. Separation from a Liquid Reaction Mixture. We will briefly mention and/or illustrate the use of the following membrane separation processes for removing products from the liquid reaction medium: reverse osmosis, nanofiltration, ultrafiltration, pervaporation, gas membranes, electrodialysis, and liquid membranes. 2.1.1.1. Reverse Osmosis. Vasudevan et al.18 have described a membrane sandwich reactor in which the Saccharomyces cerevisae (ATCC 4126) cells were effectively placed between an ultrafiltration (UF) membrane and a reverse osmosis (RO) membrane; the reactor was fed with a solution of glucose at a high pressure from the UF membrane side and the product solution was forced out through the RO membrane. The product solution concentration progressively increased in ethanol; the glucose in the feed solution was effectively rejected by the RO membrane. The reactor structure shown in Figure 3 has a microfiltration

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Figure 3. Membrane “sandwich”: (1) UF membrane, (2) coarse filter paper (>10 µm), (3) cell mass, (4) fine filter (0.2 µm, microporous), and (5) RO membrane (reprinted from Vasudevan et al.18 with permission).

Figure 5. Reactor performance as a function of enzyme concentration and distance along the membrane for enzymatic reduction of starch to glucose; u j 0 ) 1 cm/s (reprinted with permission from Closset et al.20 Copyright 1973, John Wiley & Sons, Inc.).

Figure 4. Selectivity with respect to the desired product C (SC) as a function of time. (SC, semibatch-coupled with membrane; SU, semibatch-uncoupled; BU, batch-uncoupled) (reprinted from Whu et al.12 Copyright 1999, with permission from Elsevier Science).

membrane and a coarse filter paper on two sides of the yeast cells between the UF and the RO membrane to immobilize and provide physical protection in a highpressure (up to 400 psig) environment. The membranes in this reactor separate the product ethanol from the reactant glucose and effectively immobilize the biocatalyst (whole cell). 2.1.1.2. Nanofiltration. Whu et al.12 have modeled the performance of a semibatch/batch reactor coupled to an external nanofiltration (NF) unit for the synthesis of the desired product C (a hydroxyester of MW ∼ 400) from reactants A (a diketone of MW ∼ 400) and B (an alkoxide of MW ∼ 40-100) present in an organic synthesis solvent, methanol. The membrane removes the low-molecular-weight product, D (MW ∼ 40-100) and the solvent which significantly improves the selectivity if the reaction system consists of the reactions (1) and A + D T E. Figure 4 illustrates the role of the membrane in improving the selectivity of the reaction for the product C. This figure shows that a much higher

selectivity is achieved when the semibatch reactor is externally coupled with a nanofiltration unit to remove the solvent and the product D (in the manner of Figure 2). Operation as a continuous flow stirred tank reactor as in Figure 2, coupled with a NF unit, could also provide a way to increase the concentration of the reactants in the reactor from a dilute feed if the reactants are rejected by the NF membrane. In such a case, separation of products by the NF unit may facilitate conversion of the reactants. 2.1.1.3. Ultrafiltration. Cheryan and Mehaia6 have provided a comprehensive review of enzyme-based and whole-cell-based membrane bioreactors where ultrafiltration is often the predominant mode of membrane separation. Many systems have been described. Some examples are hydrolysis of proteins leading to modified proteins with smaller molecular weights appearing in the permeate; hydrolysis of carbohydrates, e.g., starch, cellulose to produce lower molecular weight sugars; hydrolysis of sugars, e.g., lactose. Perhaps the earliest experimental study was carried out in a stirred tank reactor coupled to an ultrafiltration membrane cell for the hydrolytic breakdown of cellulose to the membrane permeable product glucose using cellulase enzymes.19 The utility of a thin channel membrane reactor lined with UF membranes for the enzymatic reduction of starch to glucose is shown in Figure 5. The figure illustrates the lengthwise variation of the performance indicator fA of the plug flow membrane reactor defined by

fA ) (inlet flow of reactant ( flow of reactant across the tube wall - flow of reactant at a given axial distance) inlet flow of reactant

(5)

with that for a solid tube reactor; the membrane reactor has a much higher conversion for the cases analyzed where the enzyme is completely rejected.20 2.1.1.4. Pervaporation. The pervaporation (PV) process is used to remove volatile reaction products from the reaction mixture; generally, a vacuum on the


Figure 6. Influence of the variation of the membrane area to the solution volume ratio on the esterification rate of propionic acid with 1-propanol. T ) 50 °C; 1 wt % catalyst; Noac/Noalc ) 1 (with permission21). no. S/V (cm-1)

1 1

2 2

3 4

4 8

permeate side is employed to create the needed partial pressure difference. The most common reaction system studied for the application of pervaporation is an esterification reaction between an alcohol and an acid in the presence of a highly acidic catalyst (e.g., concentrated sulfuric acid): Cat-H+

R1COOH + R2OH {\} R1COOR2 + H2O v ester acid alcohol

(6)

This reversible reaction in industrial processing is driven to high conversion by adding a large excess of alcohol. Adding a poly(vinyl alcohol) (PVA)-based waterselective PV membrane to the esterification reactor allows one to shift the equilibrium in reaction 6 to the right (thus reducing the need for excess alcohol beyond that needed for solubilization of the acid). Figure 6 illustrates how the conversion to the ester is increased in a batch reactor with time as the ratio of the membrane surface area (S) to the reaction volume (V) is increased for the esterification reaction between propionic acid and 1-propanol studied by David et al.21 Note that in this case the equilibrium value of the conversion in the absence of any product removal is 0.7; the membrane allows a much higher conversion to be attained much more rapidly. If the temperature of the esterification reaction is high, it will be necessary to employ vapor permeation membranes to remove H2O vapor. Another application of pervaporation studied22 involved selective removal of alcohol from a fermentation broth in a recycle configuration using silicone capillary membranes. Silicone (PDMS) is a highly biocompatible material. Further, some of the other byproducts of fermentation (which can inhibit the fermentation if their concentrations build up in a recycle system) like acetaldehyde, butanol, etc. are also easily removed. Removal of ethanol decreased product inhibition and increased fermentor productivity. 2.1.1.5. Gas Membranes. A hydrophobic microporous or porous membrane having gas-filled pores and two nonwetting aqueous solutions on two sides will allow spontaneous transfer of volatile species from one solution to the other solution through the pore as long as there is a partial pressure difference.23 Such a “gas

Figure 7. Schematic diagram showing the configuration and the basic function of a bipolar membrane for water splitting (reprinted from Strathmann.25 Copyright 1992, Kluwer Academic Publishers).

membrane”-mediated selective transfer of volatile species has been used to remove volatile product species from a reactor solution to that on the receiving side. Twardowski and McGilvey24 have used a porous poly(tetrafluoroethylene) (PTFE) membrane to transfer product ClO2, from the reactant stream in a reactor to the aqueous solution on the other side of the membrane where it is ultimately used to bleach wood pulp, etc. Removal of the ClO2 from the reactor solution is necessary to reduce the high partial pressure of ClO2 inevitably occurring in the gas space of commercial ClO2 generators which leads to localized decomposition of ClO2. 2.1.1.6. Electrodialysis. In electrodialysis processes using bipolar membranes, a solution of a salt, e.g., NaCl, is converted to a pure solution of NaOH and another pure solution of HCl. This acid and base production is carried out first by the production of H+ and OH- ions from water and their collection into separate aqueous solutions into which are fed the corresponding ions, viz., Cl- and Na+, respectively, via the electrodialysis process. The splitting of water into H+ and OH- ions in separate aqueous solutions is carried out in a bipolar ion-exchange membrane-based reactor shown in Figure 7.25 The thin space between a cation-exchange membrane and an anion-exchange membrane laminated together and placed between a cathode and an anode is filled with water. Any ions, e.g., Na+ and Cl-, in this water are quickly removed through the corresponding ion-exchange membrane. This leads to deionized water in the space between the two laminated ion-exchange membranes. The resistance of the aqueous solution becomes very high, which leads to H+ and OH- ions participating in the transport of electrical charges through the membranes. In the water dissociation equilibrium process,

H2O T H+ + OH-

(7)

As the ions H+ and OH- are removed through the cation-exchange membrane and anion-exchange mem-

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Figure 8. Apparatus for the resolution of (R,S)-Phe-O-iPr-HCl with subtilisin, using a hollow fiber SLM module (reprinted with permission from Ricks et al.26 Copyright 1992, American Chemical Society).

brane, respectively, more water is dissociated. Thus, separation of products, H+ and OH-, through the membranes is essential to continue the water-splitting reaction. 2.1.1.7. Liquid Membranes. Enantioselective hydrolysis of (R,S)-phenylalanine isopropyl esters by the enzyme subtilisin Carlsberg in solution in a reactor was shown by Ricks et al.26 to selectively convert the (S)ester to (S)-Phe-COO- while leaving the (R)-ester essentially unchanged. Part of the enzymatic solution was circulated on the shell side of a hollow fiber module in which a liquid membrane of N,N-diethyldodecanamide in dodecane is immobilized as a supported liquid membrane in the pores of hydrophobic Celgard polypropylene hollow fibers. The (R)-ester is permeable through this liquid membrane into an acidic strip solution where it gets protonated and cannot partition back into the feed phase. The (S)-Phe-COO- is essentially impermeable through the liquid membrane and is recovered from the feed solution. A part of the feed solution is recirculated through the reactor and the membrane separator in the recycle mode. If the (R)-ester accumulates in the reaction media, it can inhibit the rate of conversion of the (S)-ester, which can also permeate through the membrane. The retention time of any ester fed to the reaction vessel is adjusted such that hydrolysis is essentially completed (12 min) before the solution is introduced in the membrane module in the recirculation mode (Figure 8). This is an example of an unconverted reactant being removed from the reaction mass as if it were an inhibitory product. Further, a reaction (in this case protonation) is carried out in the permeate phase (Figure 1),

D+FfG

(8)

to maintain a large driving force for the permeation of D. 2.1.2. Separation from a Gaseous Reaction Medium. The membrane processes employed include those using liquid membranes, Knudsen diffusion, gas permeation, molecular sieving, etc. We will briefly identify some typical examples to illustrate product removal by membranes in each case.

2.1.2.1. Liquid Membrane. One of the earliest studies by Ollis et al.27 involved acetaldehyde synthesis in a multiphase catalytic liquid membrane-based reactor. The feed gaseous mixture of O2 and C2H4 as the inner gas phase surrounded by an aqueous catalytic liquid membrane layer was prepared by dispersing the gas as bubbles into an aqueous liquid membrane reservoir containing the palladium-based catalyst PdCl2 and the oxidizing agent CuCl2. The individual gas bubbles surrounded by the aqueous liquid membrane layer then spontaneously rose through a solvent layer kept above the aqueous catalyst reservoir. Any reaction product was extracted into the outer solvent layer. The overall reaction and the continuous extraction of the product acetaldehyde into a solvent phase (e.g., ethyl acetate, n-butanol, etc.) can be indicated by

O2 (w) + C2H4 (w) f CH3CHO (w) f CH3CHO (s) (9) The product acetaldehyde is recovered from the solvent in a separate flash drum, while the solvent phase is recycled back to the reactor to form the outer continuous solvent phase in the double-emulsion liquid membrane employed in the reactor. The liquid membrane in this case allows not only product separation but also the segregation of the soluble catalyst in the aqueous membrane phase (function 2.6) in addition to being the actual site where the reaction is taking place (function 2.8). The product separation here does not lead to a pure product for which an additional step (flash) is necessary. 2.1.2.2. Knudsen Diffusion. Since the mid 1980s, a large number of studies have been carried out using reactants in the gas phase and an inorganic membrane through which one or more of the gaseous products (usually H2) is withdrawn. The membranes used often were microporous/mesoporous, e.g., γ-alumina, Vycor glass, etc., with or without a catalyst deposited in the pores. Tubular noncatalytic membranes were also used packed with catalyst particles. Sun and Khang28 have compared the performances of a catalytic membrane reactor, an inert membrane reactor, and an ordinary reactor without any membrane


Figure 9. Conversion vs temperature for high space-time operations with a Vycor glass membrane for cyclohexane dehydrogenation (reprinted with permission from Sun and Khang.28 Copyright 1988, American Chemical Society).

for the dehydrogentation of cyclohexane: Pt

C6H12 {\} C6H6 + 3H2

(10)

They employed a porous Vycor glass membrane tube with and without a Pt catalyst in the pores (which exhibited Knudsen flow for H2 and N2 but had surface diffusion and other effects for C6H6 and C6H12). For the noncatalytic membrane, catalyst particles were packed in the tube. Their simulation results shown in Figure 9 indicate that at high space-time operations the catalytic membrane reactor is superior to the inert membrane reactor which performs better than a conventional equilibrium-limited reactor without any selective separation of products. Propane dehydrogenation studies in a packed-bed membrane reactor by Ziaka et al.29 employed a 40 Å γ-Al2O3 membrane in a porous multilayered alumina tube containing Pt/γ-Al2O3 catalyst particles; the membrane reactor at a residence time of 10 s provided 1.8 times higher propylene yield than the corresponding equilibrium conversion. Numerical studies30 of catalytic dehydrogenation of ethylbenzene using microporous ceramic membranes possessing Knudsen diffusion behavior indicate only a g5% increase in styrene yield over the thermodynamic limit by a hybrid system, i.e., a fixed-bed reactor followed by a membrane reactor. Numerical analysis by Mohan and Govind31 of a cocurrent packed bed membrane reactor suggest among others, the use of a membrane with high permselectivity to achieve large values of conversion; the loss of reactants to the permeate side through a membrane with not-too-high a selectivity is a serious concern. Hsieh1 has provided a detailed review of the many investigations based on porous/mesoporous membranes. 2.1.2.3. Gas Permeation/Molecular Sieving. A considerable number of membrane reactor studies with gas-phase reactions have employed denser membranes with selectivities much higher than Knudsen diffusion membranes; these membranes include metallic membranes of Pd, molecular sieve membranes, and dense silica membranes among others.1 Catalytic dehydrogenation of ethanol to acetaldehyde in a palladium membrane reactor with H2 removal through the membrane increased ethanol conversion from 60% to nearly 90% with a commensurate rise in selectivity to acetaldehyde

from 35% to 70%, moving the yield from 21% to 63%.13 In the catalytic dehydrogenation of propane using silica membranes (having H2/N2 selectivity of 10-19, much higher than Knudsen diffusion-based selectivity), a propylene yield of 39.6% was obtained at 823 K compared to a yield of 29.6% in a conventional packed-bed reactor operated at the same flow rate.32 Catalytic isobutane dehydrogenation in a dense silica membrane reactor yielded somewhat higher isobutene yield and selectivity than a conventional reactor.33 Catalytic decomposition of NH3 in a gas feed to N2 and H2 in a composite Pd-ceramic membrane reactor achieved an NH3 conversion of over 94% at 873 K and 1618 kPa compared to 53% in a conventional reactor.34 A water-gas shift reaction carried out at 673 K in a palladium membrane reactor enclosing an iron-chromium oxide catalyst achieved higher CO conversion than the equilibrium level due to selective removal of H2.35 A metal membrane-based catalytic membrane reactor for facilitating the water-gas shift reaction at a temperature of 500 °C was run successfully and continuously for 6 months for the economical production of H2 in a 0.4 ft2 plate-and-frame module.36 Recovery of H2 isotopes (tritium, deuterium) is being studied in Pd/ Ag-based membrane reactors from impurities present in fusion reactor exhaust streams.37 Dixon et al.38 have modeled the performance of membrane reactors having mixed conducting O2 permeable ceramic membranes for the very high temperature reactions:

CO2 T CO + 1/2O2

(11a)

NO2 T NO + 1/2O2

(11b)

Removal of O2 through the membrane resulted in dramatic improvements in conversion over the nonmembrane tubular reactors. Armor8 has provided a critical review of the needs in many dehydrogenation membrane reactor applications. The problem areas are: defects in metallic membranes at higher temperatures, phase changes in metallic membranes causing catastrophic failure, leakage between the membrane and device, low surface area per unit volume of commonly used membranes, severe mass-transfer limitations, very low feed flow rates resulting in high residence times, carbon deposition on membrane pores and surfaces, no method (currently available) for repairing defective membranes in situ, and the low turnover number of commercially available dehydrogenation catalysts. The approach adopted by Rezac et al.39 is of great interest in this respect. They propose existing plug flow reactors and heat-exchange equipment to be used in series with an optimized highertemperature stable polyimide-ceramic composite H2 removal membrane module. Thus, C4H10 dehydrogenation is carried out at 480 °C, the product mixture is cooled to 180 °C, and H2 selectively permeated through the polyimide-ceramic composite membrane (H2/C4H10 selectivity > 75). Unreacted C4H10 is passed on to the second-stage reactor after heat exchange and so on. This resulted in a substantial increase in conversion and a high selectivity for the production of n-C4H8; membrane poisoning is also substantially avoided. 2.2. Separation of a Reactant from a Mixed Stream for Introduction into the Reactor. Figure 1 identified a particular function of the membrane as “purify reactant A from species F before addition” to the

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Figure 10. Two reactions coupled through a gas membrane; membrane-assisted synthesis (reprinted from Van Eikeren et al.44 with permission).

reactor on the left-hand side. The effect of this separation on the reaction system is generally quite different from that of a reaction product from the reaction mixture. The purification may lead to pure A being introduced into the reactor; a direct effect of this is prevention of dilution of the reaction mixture. It can also lead to rejection of a class of compounds by the membrane while reactant species (one or a class) may be introduced by the membrane preferentially into the reactor from the feed stream; the species rejected can inhibit the reaction. An additional possibility involves simultaneous operation of two different reactions on two sides of the membrane wherein the products of one reaction feeds the other and vice versa; the latter could be in a coupled mode as well (to be explained later). 2.2.1. Pure O2 from Air. Mixed conducting dense ceramic membranes (see sections 2.3 and 2.9 as well) allow O2 transport from air to a lower O2 partial pressure side without allowing N2 to be transported through the membrane. When such a membrane is used with air on one side and CH4 on the other side, partial oxidation of CH4 to syngas can be carried out at a high temperature (∼800 °C) without contaminating the reaction gas mixture with N2. Further, the need for an O2 plant is eliminated,40 improving the economics considerably. When a similar dense ceramic membrane in a solid-electrolyte-cell reactor (see section 2.9) is used, HCN is produced in a tubular reactor fed with a NH3 + CH4 mixture as O2 permeates through the membrane41 as an anion O2- into the reaction zone; in this case air is fed on the outside of the tubular reactor to produce oxygen throughout the reactor length (function 2.3). Had air been used directly, the auxiliary byproduct of H2 + CO, a high-quality fuel stream, would have been diluted with N2. 2.2.2. Purify Organic Pollutants from Wastewater for Biodegradation. Point-source industrial wastewaters containing a variety of priority pollutants were often considered recalcitrant from a biodegradation perspective. These industrial wastewaters are frequently released from organic synthesis and contain high salts, extreme pHs, and residual catalysts, all of which are either singly or jointly very harmful to the growth of microbial cultures used for biodegradation. Brookes and Livingston42 have employed a silicone capillary membrane-based device to extract organic priority pollutants from these demanding wastewaters. The biological reaction medium is circulated between a bioreactor and one side of this membrane device. On the other side of the silicone capillary membranes flows the wastewater. Priority organic pollutants, e.g., aniline, 4-chloroaniline, 3,4-dichloroaniline, etc., from the waste-

water (having high pH 9-11) are partitioned through the silicone membrane into the biological reaction medium. Reactors operated over 3000 h show very high reductions of the pollutants without any form of pretreatment, pH adjustment, or dilution of the wastewater. The membrane essentially isolates the bioreactor environment from the vagaries of the industrial wastewater properties as the pollutants get extracted out into the bioreaction medium for degradation by the appropriate microorganisms. Successful pilot plant studies have been conducted using this technique. 2.2.3. Purify Organic Compound from a Synthesis Medium. To prepare a concentrated aqueous solution of diltiazem malate by reacting diltiazem (sparingly soluble in water) with malic acid, a liquid membrane was utilized to recover diltiazem from an aqueous reaction mixture containing diltiazem, NaCl, NaHCO3, etc. A contained liquid membrane of decanol was utilized by Basu and Sirkar43 to extract diltiazem into the reaction zone where it reacted with L-malic acid in water to form diltiazem malate (in solution). When a very high concentration of L-malic acid is maintained, the aqueous concentration of diltiazem malate achieved could be 3 orders of magnitude higher than the very low concentration of diltiazem in the feed solution. The membrane not only facilitated production of a highly concentrated and purified solution of diltiazem malate but also avoided two steps of solvent extraction and back extraction which was very problematic due to the orders of magnitude difference in the two aqueous phase flow rates, namely, the very high flow rate of a very dilute feed solution and the very concentrated reactor effluent having a low flow rate. 2.2.4. Coupling of Two Chemical Reactions. In enzymatic methods for the production of pure enantiomers from achiral precursors using, say, dehydrogenase enzymes, specialized and costly reagents, e.g., nicotinamide cofactors (NAD+ or NADH), are required. A process for efficient and continuous regeneration of these nicotinamide cofactors by a chemical reaction isolated from the main reaction by a membrane partition is illustrated in Figure 10.44 The membrane employed is a gas membrane (see 2.1.1.5) utilizing a microporous hydrophobic polypropylene membrane whose pores (∼0.03 µm) are gas-filled. In the main reaction compartment on the left side, D-lactic acid is produced in an aqueous solution by catalyzing the reduction of pyruvic acid using D-lactic acid dehydrogenase. The NAD+ produced by this reaction reacts with ethanol to regenerate NADH required for the main reaction. In an adjacent compartment, sodium borohydride reduction of acetaldehyde leads to ethanol which, being volatile,


enters the main reaction chamber whereas acetaldehyde produced in the regeneration of NADH in the main reaction chamber enters the adjacent chamber (through the membrane). Thus, the membrane allows separation of the volatile species from the rest of the nonvolatile reaction mixtures in both compartments and feeds each volatile species at a controlled rate into the adjacent reaction chamber (function 2.3). A weaker coupling between two reactions on two sides of a membrane (see Figure 1 for A + B T C + D and D + F f G where the membrane controls the addition of reactant D to the permeate side reaction) was explored numerically by Itoh and Govind.45 On the basis of earlier proposals by Gryaznov and Itoh, they modeled the catalytic dehydrogenation of 1-butene to butadiene in the main packed bed reactor. The permeate side was being continuously fed with O2 in air which oxidizes the permeated H2 to produce water and liberate heat by an exothermic reaction. This reaction is surface-catalyzed by the palladium membrane used to remove hydrogen from the main reactor. The heat so generated is conducted through the palladium membrane to the main endothermic dehydrogenation reaction. The membrane has many functions: separate product of main reaction, H2 (function 2.1); provide purified reactant (H2) to second reaction (function 2.2); add reactant H2 to second reaction in a controlled way (function 2.3); supply heat to main endothermic reaction (function 2.10) and thereby control both reactions; and catalyze the second reaction (function 2.7). 2.3. Controlled Addition of a Reactant or Two Reactants. Control of the reaction pathway is a major concern in reaction engineering. Partial oxidation reactions of hydrocarbons are especially relevant here. In particular cases, possibilities of thermal runaway and catalyst poisoning do exist. In biodegradation processes for toxic organics, microorganism growth may be affected by inhibition from the toxic organics unless their concentrations are controlled. In an aerobic wastewater treatment process, high O2 utilization with minimum waste to the atmosphere requires controlled but efficient introduction of O2 to the system. In processes using reactants having limited half-lives, e.g., ozonation of wastewater or for water purification, efficient and localized introduction of O3 at a controlled rate can lead to higher O3 utilization. Using a membrane to introduce a reactant or two in a controlled fashion in the reactor can facilitate achievement of the desired reaction conditions. A number of examples provided below will illustrate the role of a membrane in such processes. 2.3.1. Gas-Phase Reactions. Gas-phase reactions where reactants are introduced into the reaction zone by membranes at a controlled rate(s) (see Figure 1, purify reactant A from species F before addition) can involve three types of membranes: (1) inert porous membranes which provide no selectivity; (2) microporous membranes with some selectivity; (3) nonporous/dense membranes having much higher selectivity. The first two types of membranes may employ catalysts in the pores; the nonporous membrane can be inherently catalytic. All three types of membranes have been studied with O2 as one of the reactants introduced in a controlled manner for partial oxidation or oxidative dehydrogenation reactions. Tonkovich et al.46,47 employed 50 Å γ-Al2O3 (effective layer) or 200 Å R-Al2O3 membrane tubes packed with a magnesium oxide catalyst doped with samarium oxide

to study the oxidative dehydrogenation of C2H6 to C2H4 at 600 °C. Air was introduced via permeation throughout the length of the reactor from the outside of the tube. Controlling the ratio of C2H6 to O2 was found to be crucial to selectivity (with respect to CO2, CO, etc.) and conversion. At low-to-moderate C2H6 to O2 feed ratios (98%, CO selectivity was 90%, and H2 produced was twice that of CO. Zaspalis et al.49,50 have experimentally studied the dehydrogenation of methanol in a microporous γ-Al2O3 membrane with methanol fed from one side and O2 from the other (Figure 1, reactants A and B introduced into the pore from opposite sides). This arrangement avoided direct contact between the two streams (i.e., an alcohol and an oxidant), thereby minimizing undesirable gasphase reactions. Further, the inner surface areas of the membranes were used. Zaspalis et al.50 used silver particles deposited on the γ-Al2O3 membrane pore surfaces; CH3OH was fed from one side and O2 from the other so that activation of the Ag catalyst occurred simultaneously with the methanol conversion to formaldehyde. This opposed flow mode of feeding two reactants must be carried out in a controlled manner so as not to decrease the selectivity for the desired product. 2.3.2. Gas-Liquid Reactions. Catalytic reactions between gas and liquid phases pumped concurrently down a bed of catalyst particles in conventional reactors encounter mass-transport limitations due to intraparticle mass-transfer resistance, liquid film resistance, liquid maldistribution, channeling, etc., apart from hot spots and undesirable side reactions. To overcome these problems, Cini and Harold51 have studied hydrogenation of R-methylstyrene (diluted in mesitylene) to cumene in a porous (6 µm) tubular γ-Al2O3 membrane impregnated with a Pd catalyst on the pore surface area. H2 was supplied on one side of the porous membrane and the liquid reactant flowed on the other side of the

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membrane (Figure 1). The results demonstrated an efficient supply of the volatile reactant, H2, which was also the limiting reactant when compared with the catalyst pellets conventionally used in a trickle-bed reactor. There were no operational difficulties encountered in the membrane-based operation; further, the rate increased by up to a factor of 20. From a study of hydrogenation of nitrobenzene to produce aniline in a tubular γ-Al2O3 membrane (50 Å pore size) having a Pt catalyst deposited on the pore surfaces, Torres et al.52 have also concluded that catalytic membrane reactors are efficient for three-phase reactions as a result of the easy access of the gas to the catalytically active phase. Biodegradation processes to destroy organic and inorganic contaminants in air or water employ organisms in a biofilm attached to a support. The efficient supply of nutrients, e.g., O2 and pollutants, to the biofilm is demanding without incurring excessive power consumption and with minimum loss of O2 to the atmosphere. Hydrophobic porous hollow fibers membranes are especially useful. For example, Brindle et al.53 have immobilized a biofilm on the fiber outer diameter. The shell side is fed with, say, the wastewater containing NH4+ which is oxidized by the nitrifying biofilm into nitrite and then nitrate; the tube side is fed with pure O2. Exceptionally high O2 utilization efficiencies were achieved via efficient interfacial oxygen transfer to the biocatalyst in the biofilm, even at low O2 supply rates. Parvatiyar et al.54 have demonstrated efficient biodegradation of toluene in a hollow fiber membrane-based biofilter: air containing toluene was fed on the lumen side of porous polysulfone hollow fibers on the outer surface of which the biofilm was immobilized. Nutrients were supplied via an aqueous stream on the shell side. High conversion of toluene was achieved because of efficient contact of the biomass catalyst with O2 and toluene through the membrane pores at controlled rates of supply of the reactants in the gas phase. Noncatalytic gas-liquid reactions are employed using O3 to destroy pollutants and bacteria in water purification and wastewater treatment processes. Since O3 has a very low solubility in water, kla (volumetric masstransfer coefficient) controls the mass-transfer rate of O3 and thereby the reaction rate. Further, O3 has a very limited half-life in a moist gaseous phase. Membranebased nondispersive ozonation studied by Shanbhag et al.55,56 provides a much higher value of kla (g5 times) compared to conventional dispersive bubble-based ozonation, eliminating the possibility of gas-phase degradation of the unstable O3 in bubbles and efficiently bringing O3 in contact with the aqueous pollutants along the length of reactor. 2.3.3. Liquid-Phase Reactions. Lee et al.57 carried out simultaneous biodegradation of the pollutants toluene and p-xylene in a completely mixed and conventionally aerated bioreactor using the microorganism Pseudomonas putida. Under aerobic conditions, the microorganisms utilize toluene and xylene as carbon sources. The pollutant species (toluene and p-xylene) were introduced into the reactor in a controlled manner through silicone capillary membranes. The removal efficiency of these species increased at the beginning with an increase in the transfer rate of the pollutant mixture (increased by, for example, the impeller speed and not by the aeration rate or the circulation rate of the solvents in the capillary); however, beyond a certain

Figure 11. Membrane as a phase separator/contactor for the reaction-extraction processes.

rate, the removal efficiency decreased since the limiting substrate shifted from carbon to O2. At this time, the solvent loss in the exit gas also increased. For given impeller speeds, the membrane can be designed to control the rate of introduction of organic pollutants to the biomass-containing medium. 2.4. Nondispersive Phase Contacting. In many reactions, aqueous and organic phases are frequently used together. One phase is dispersed as drops in the other phase followed by coalescence after the process is over. This can be problematic if there are tendencies for emulsification. Microporous/porous membranes can be particularly useful here since the two immiscible phases can be kept on two sides of the membrane with their phase interfaces immobilized at the membrane pore mouths. Solvent extraction is conventionally used to isolate and concentrate dilute organic products obtained from whole cell-based fermentation processes. If the fermentation suffers from product inhibition, then extraction of the product(s) during fermentation increases the fermentor productivity. However, solvent dispersion can lead to a phase-level toxicity58 problem for the whole cells. Nondispersive phase contacting using microporous/porous membranes can resolve this problem. In nondispersive phase contacting employing microporous/porous hydrophobic membranes, the organic phase wets the membrane pores; the aqueous phase is maintained outside the pores at a pressure equal to or higher than that of the organic phase. As long as this excess pressure does not exceed a breakthrough pressure, the aqueous-organic interface remains immobilized on the aqueous side of the membrane with each phase flowing on a particular side of the membrane.59,60 For hydrophilic microporous/porous membranes, the aqueous phase is inside the pores; the organic phase is kept outside the pores at a pressure higher than that of the aqueous phase (Figure 11). This technique has been employed in four types of reaction systems: fermentor-extractor; enzymatic fat splitting; phase transfer catalysis; extractive membrane bioreactor for enzymatic resolution of isomers. The advantages of these techniques are: no dispersion and therefore no need for coalescence; no need for density difference between the two phases; known interfacial area; modular systems leading to easy scale-up; masstransfer rates independent of interfacial tension; no flooding and no loading, allowing widely different phase flow rate ratios to be used. Further, the membrane may provide a very large interfacial area per unit reactor volume. Nondispersive phase contacting advantages are also present in gas-liquid systems already discussed under


Figure 12. Bifunctional nature of hydrophobic membranes. Shown above is the end view of the hollow fiber membranes with biocatalyst particles surrounding them. (a) The membrane can be used to supply gas throughout the reactor volume while at the same time removing carbon dioxide and hydrogen. (b) When a solvent is passed through the membrane lumen under correct pressure conditions, solvent extraction can provide integrated product recovery while removing carbon dioxide (reprinted with permission from Frank and Sirkar.61 Copyright 1986, John Wiley & Sons).

the category of gas-liquid reactions (section 2.3.2) for the controlled addition of one or two reactants. To achieve nondispersive operation, one has to maintain the proper pressure in each phase. For example, for hydrophobic microporous membranes, the aqueous phase is usually outside the pores (which are gas-filled) and maintained at a pressure23 higher than that of the gas. For hydrophilic membranes, the aqueous or organic phase is usually inside the pores and the outside gas phase is at a higher pressure. 2.4.1. Fermentor-Extractor. In fermentation processes for producing ethanol, acetone-butanol-ethanol (ABE), etc., microporous hydrophobic hollow fiber membranes have been introduced into tubular reactors in which whole cells are immobilized on appropriate supports on the shell side; through the bore of the hydrophobic hollow fibers, O2 is supplied in ethanol fermentation and N2 is supplied in ABE fermentation.15,61,62 The gases supplied help the cells grow, maintain the needed anaerobic condition, and remove gases such as CO2 and H2 produced by fermentation. When a substantial concentration of the desired product has been achieved in the shell-side broth, an organic solvent, passed through the fiber lumen at a pressure lower than that in the broth, extracts the products (e.g., ethanol, ABE, etc.) nondispersively (Figure 12). This reduces product inhibition and can lead to considerably increased volumetric fermentor productivity.16 Other inhibitory side products, e.g., acetic acid, etc., are simultaneously extracted out. Although the fermentationextraction process has not been commercialized yet, the membrane-based solvent extraction technique is already being commercially employed in at least two large installations (one in Europe and the other in Japan). 2.4.2. Enzymatic Fat Splitting. In enzymatic splitting of olive oil using hydrophobic microporous membranes and the enzyme lipase immobilized at the aqueous-organic interface63 (Figure 13), olive oil flows on one side of the membrane and wets the pores; an aqueous buffer solution flows on the other side at a higher pressure which immobilizes the aqueous-organic interface. The enzyme Candida cylindracea lipase, spontaneously adsorbed at the aqueous-organic interface of the microporous membrane, catalyzes the following hydrolysis reaction: enzyme

triglycerides + 3H2O {\} glycerine + 3 fatty acids (12) Glycerine is removed in the aqueous phase; fatty acids

Figure 13. Enzymatic fat splitting in a hollow fiber bioreactor (reprinted with permission from Hoq et al.63 Copyright 1985, American Oil Chemists’ Society).

are removed in the oil phase. The membrane allows aqueous-organic phase immobilization, enzyme immobilization, and localized product separation into the appropriate phase. Molinari et al.64 have shown that this reactor was better than a conventional emulsionbased reactor: the specific enzymatic activity was higher, the specific rate was more constant with time, and the two products were separated after the reaction. 2.4.3. Phase Transfer Catalysis. Stanley and Quinn65 have studied the reaction of bromooctane in the solvent chlorobenzene with aqueous iodide to form the displacement products, iodooctane in the solvent chlorobenzene and aqueous bromide. The phase transfer catalyst (PTC), tetrabutylammonium (TBA) ion, was introduced as the bromide salt in the organic feed. The aqueous feed was passed on one side of the microporous hydrophobic flat membrane of poly(tetrafluoroethylene) (PTFE); the organic phase wetting the membrane pores was passed on the opposite side at a pressure lower than that of the aqueous phase. Conventional emulsification/ coalescence problems were avoided in this PTC-facilitated reaction. Further, since the membrane area (therefore the aqueous-organic interfacial area) is known, operation of the reactor can be carried out with greater flexibility. 2.4.4. Extractive Membrane Bioreactor for Enzymatic Resolution. In a multiphase/extractive enzyme membrane reactor66 used for the industrial production of diltiazem chiral intermediate, an asymmetric water-filled hydrophilic hollow fiber membrane of a copolymer of acrylonitrile having a 30000 MWCO (molecular weight cutoff) skin on the fiber internal diameter is employed. The enzyme is immobilized in the porous substructure from the shell side via ultrafiltration (function 2.6). The organic phase in which the enzyme has limited solubility flows on the shell side at a higher pressure, immobilizing the aqueous-organic interface at the pore mouth, thus containing the enzyme in the

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water-filled substructure bounded by the membrane skin impermeable to the enzymes. If the enzyme activity is reduced substantially, the enzyme can be flushed out easily and fresh enzyme loaded in the absence of the organic phase. The organic flow is then restarted. Stereoselective enzymatic hydrolysis of the undesired isomer from a racemic mixture of glycidic esters and its removal in an aqueous buffer is carried out continuously. Here, the membrane provides reversible immobilization of the hydrolytic enzymes in the pores of the hydrophilic asymmetric hollow fiber, immobilization of the aqueous-organic interface on the fiber outside diameter, reaction product extraction, and a very large aqueous-organic interfacial area. 2.5. Segregation of the Catalyst (and Cofactor) in a Reactor. A membrane incorporated in a catalytic reaction system can perform, among others, a number of functions related to the catalyst. If the catalyst is mobile in the reaction fluid, the membrane can prevent its escape from the system. If the catalyst is to be immobilized with easy access to the reactants and convenient exit for the products, a porous/microporous membrane structure may have the catalyst immobilized on/within its structure (function 2.6). Alternately, the membrane material itself may act as the catalyst (function 2.7). We focus here on cases where the catalyst is mobile in the reaction fluid. Examples of such catalysts are enzymes (and cofactors where applicable), whole cells, and homogeneous catalysts (in organic synthesis). The segregation of particulate heterogeneous catalysts by filters is not under consideration. The production of organic compounds by synthesis in organic solvents or an aqueous-organic biphasic reaction medium is very common. Many use homogeneous catalysts whose molecular weights are considerable (e.g., in the range of, say, 300-800). Very few membranes are capable of retaining such species while allowing the organic solvent to pass at appreciable rates. Nanofiltration membranes, just becoming available, have the necessary solvent resistance and rejection behavior in a few cases. Whu et al.12 have identified some of these capabilities including retaining the homogeneous catalysts while passing the organic solvents. More extensive use of such nanofiltration membranes will allow their use in organic synthesis for, among others, retaining the homogeneous catalysts. The use of membranes to segregate enzymes used as catalysts for biosynthesis or biocatalysis is practiced in a wide scale. Primarily, ultrafiltration membranes having molecular weight cutoffs in the range of 5000100000 are used to retain the enzymes (molecular weight range 10000-100000 Da) in the CSTR reactor as smaller products are removed with water through the membrane (Figure 2). Originally suggested by Michaels67 and implemented as early as 1970,19 this technique is used commercially for the production of amino acids. Jandel et al.68 have illustrated continuous production of L-alanine from fumaric acid in a two-stage membrane reactor using the enzyme aspartase in the first stage, aspartase

fumaric acid + NH3 {\} L-aspartic acid

(13a)

and L-aspartate-β-decarboxylase in the second stage, L-aspartic

L-aspartate-β-decarboxylase

acid 98 L-alanine + CO2 (13b)

Some enzymatic synthesis reactions are carried out in hollow fiber ultrafiltration membrane devices in what is called the “perfusion reactor” mode. The enzymes or the whole cells are packed on the shell side with the shell side ports being closed off; substrate-containing feed is pumped through the tube side. The substrate diffuses through the membrane pores to the shell side and reacts with the enzymes/whole cells, and the smaller molecular weight products diffuse back to the tube side and are carried away. The enzymes or whole cells may also be kept in the tube side with both ends closed off: the substrate-containing solution will then flow into and out of the shell side. This latter mode is not commonly used. A review of membrane bioreactors wherein the membranes segregate enzymes/whole cells is available in Cheryan and Mehaia.6 Some enzyme-based reactions, however, require lowmolecular-weight coenzymes or cofactors in addition to the main enzyme to carry out the overall enzymatic reaction. Typical examples of such coenzymes are nicotinamide adenine dinucleotide (NAD+), the reduced form of NAD+, viz., NADH, NADPH (the reduced form of NAD phosphate), etc. Figure 10 shows one such reaction where the D-lactate dehydrogenase enzyme needs NADH for the conversion of pyruvate to D-lactate. In the process, the oxidized form of NADH, viz., NAD+, is produced. Unless this NADH is regenerated from NAD+, one has to continuously supply NADH from external sources. NADH is costly ($1000/mol). A number of strategies have been explored to solve this problem (see Cheryan and Mehaia6). The strategy shown in Figure 10 is an important one, viz., an additional enzymatic reaction employing a regenerating enzyme, in this case an alcohol dehydrogenase, to regenerate the NADH from NAD+. However, this requires a cosubstrate (e.g., ethanol) and yields a coproduct (e.g., acetaldehyde). To design a continuous process to retain the enzymes as well as the cofactors in the system by a membrane as the substrates come into the reaction chamber and the product and coproduct leave the reaction chamber, one needs a very specific membrane. Figure 10 provides a special example for volatile cosubstrate and coproduct. We consider here a separate situation, but one more commonly encountered; viz., enzymes have molecular weights > 40000 Da and the substrates are 5-12 carbon sugars; note that the cofactor molecular weights are around 700 Da. Thus, if we use too tight of a membrane, the substrate introduction into the reaction zone will encounter considerable resistance, although NAD+ and NADH will be retained by the membrane. To solve the problem, Nidetzky et al.69,70 have selected a charged nanofiltration (NF) membrane having -ve charges on the surface (as shown in Figure 14) that preferably retains the cofactors almost completely without binding the enzymes or cofactors. However, the NF membrane had a size exclusion (∼1 kD) slightly higher than those molecules having the molecular mass of the cofactor; this ensured higher substrate fluxes. The charge of the NF membrane (-ve) in this case allows for cofactor retention since both NAD and NADH carry negative net charge at pH values greater than 3. This


Figure 14. The substrates and products flow through the nanofiltration (NF) membrane, whereas the enzyme and the coenzyme stay within the enzyme reactor. The membrane used is asymmetric with a supporting matrix of polyetherketone and polyethersulfone with a thin-coated layer of sulfonated polysulfone. The fixed charge density is ∼1.5 mequiv g-1. NAD+, nicotinamide adenine dinucleotide; NADH, reduced NAD+; UF, ultrafiltration (reprinted with permission from Nidetzky et al.69 Copyright 1996, American Chemical Society).

process has been demonstrated for the conversion of fructose to mannitol by mannitol dehydrogenase69 (or xylose to xylitol by xylose reductase70) as the cosubstrate glucose was converted by the regenerating enzyme, glucose dehydrogenase, to glucono-δ-lactone (ultimately converted to gluconic acid in a separate device). Pilot scale results show a TTN (total turnover number) of coenzyme to be in the range of 75000-100000. This approach to cofactor retention or segregation by the membrane appears to be considerably simpler than the earlier strategy of binding the cofactors to a macromolecule, e.g., poly(ethylene glycol), dextran, etc., which allowed a regular UF membrane to retain the cofactor within the reactor.71 (A similar strategy72 has been pursued for the enantioselective addition of diethyl zinc to benzaldehyde using a homogeneously soluble catalyst retained by a solvent-stable polyaramide UF membrane within the reaction vessel; the R,R-diphenyl1-prolinol used as the chiral ligand was coupled to a copolymer made from 2-hydroxyethyl methacrylate and octadecyl methacrylate (96000 MW) and the solvent used was hexane.) However, the charged NF membranebased retention of NAD+ is at this time somewhat lower, around 0.65-0.85. Optimization of the NF membrane vis-a`-vis the cofactor retention is still needed unless the product happens to have a very high value so that a lower TTN (1000-5000) may be tolerated. 2.6. Immobilization of a Catalyst in (or on) a Membrane. Four basic types of catalysts are relevant: (a) enzymes and (b) whole cells for biocatalysis; (c) oxides and (d) metals for nonbiological synthesis. Biocatalysts will be considered first since their immobilization in (or on) the membrane was explored much earlier. Five techniques have been studied in varying degrees. They are (1) enzyme contained in the spongy fiber matrix; (2) enzyme immobilized on the membrane surface by gel polarization; (3) enzyme adsorbed on the membrane surface; (4) enzyme immobilized in the membrane pore by covalent bonding; (5) enzyme immobilized in the membrane during membrane formation by the phase inversion process of membrane making. Of these, technique (1) is used commercially. An enzyme solution is ultrafiltered from the spongy side (porous substructure) of an asymmetric ultrafiltration membrane to the skin side; this introduces the enzymes in the spongy matrix. Feed solution is also supplied from

the same side; product solution goes out through the membrane skin (see Jones et al.73 for a brief introduction to the literature of hollow fiber enzymatic reactors employing this method of immobilization). An example of this type of immobilization in the context of aqueousorganic enzymatic processing has been illustrated in section 2.4.4. Technique (2) whereby the enzymes are immobilized on the pressurized ultrafiltration membrane surface where an adherent gel layer of concentrated enzymes are formed was suggested by Drioli and Scardi.74 Lipase enzymes (e.g., C. cylindracea) are spontaneously adsorbed (technique (3)) on a hydrophobic microporous polypropylene membrane surface and are utilized in aqueous-organic enzymatic processing (section 2.4.2). Enzymes can be covalently bound to hydrophilic membranes by the cyanogen bromide procedure. This technique has been known for a long time and has been used, for example, for binding chymotrypsin to a Millipore filter membrane by Matson and Quinn75 in their membrane reactor studies. Site-directed mutagenesis has been introduced into enzymes so that the active site of the enzyme is away from the surfaces of membranes such as hydrophobic poly(ether)sulfone; immobilized mutant enzymes on such membranes have much higher activity than randomly immobilized enzymes for catalytic conversions.76 Finally, it is possible to incorporate particular enzymes into the organic casting solution for polymeric membranes and then prepare a membrane by the phase inversion process wherein the final membrane has the enzyme dispersed throughout the membrane structure. Chopped microporous hydrophobic or hydrophilic hollow fibers have been used to grow whole cells in the fiber lumen and the fiber outside surface.77 Such chopped hollow fibers with immobilized cells were later utilized in a tubular fermentor to carry out yeast-based ethanol fermentation for an extended period. The chopped fibers of appropriate lengths provide adequate nutrients as well as O2 to the immobilized cell mass; such a bioreactor could also have continuous lengths of hydrophobic microporous hollow fibers for gas supply and removal as well as for in situ product extraction by dispersionfree solvent extraction.16 Nonbiological synthesis of most products involve chemical and thermal conditions too harsh for almost all of the current polymeric membranes available; therefore, the membranes investigated are primarily inorganic in nature, either ceramic or metallic. The catalysts are predominantly oxides and/or metals. Of the numerous oxides and metals used as catalysts, membrane reactor studies, where the membrane had immobilized catalysts on/in it, have primarily used those employed for dehydrogenation reactions, viz., platinum, palladium, etc., on metal oxides such as alumina and silica.1 Examples are a Pt-impregnated Vycor glass tube28 for cyclohexane dehydrogenation, Ag-modified 40 Å γ-Al2O3 membrane obtained via wet impregnation of AgNO3, and then calcination under a reduced atmosphere for dehydrogenation of methanol.50 For the Claus reaction,

2H2S + SO2 T 3/8S8 + 2H2O

(14)

350-nm R-Al2O3 membrane pores were impregnated with aluminum nitrate and urea solution in water, dried, and calcined.78 For multiphase hydrogenation

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studies, palladium was deposited on microporous γ-Al2O3 deposited on 6 µm R-Al2O3 by soaking the tube in an aqueous solution of ammonium tetrachloropalladium, drying it, and calcining it.51 For nitrobenzene hydrogenation, the platinum catalyst was deposited in a 50 Å γ-Al2O3 membrane tube by ion exchange with H2PtCl6.52 Although the effect of catalyst distribution in a membrane-enclosed catalytic reactor has been studied,79 no studies have been made with a variation of catalyst distribution in the catalytic membrane itself. Polymeric membrane-based studies involving the distribution of nanosized clusters or micron-sized particles in porous or nonporous polymeric membranes has been carried out at GKSS (Geesthacht, Germany) by Fritsch;80 10% Pd on charcoal powder particles (40 µm) were used as catalysts and dispersed in the casting solution for the preparation of porous polyetherimide (PEI) membranes. Transfer hydrogenation of N-CBZL-phenylalanine to L-phenylalanine was successfully carried out at low temperatures. Hydrogenation of propene to propane was carried out by using a nonporous membrane containing nanoclusters of Pd in a dense poly(amide-imide) membrane. The mechanism of such a membrane function has been described in section 2.8. Bellobono et al.81 have described the performance of microfiltration membranes formed with 30% TiO2 as well as some 6% of particular photocatalysts. The monomer, prepolymer blend with the semiconductor TiO2 and the photcatalysts were photografted onto a perforated polyester support; the pore size of photosynthesized membranes were 2.5-4 µm. The membranes were placed coaxially with a UV lamp in a stainless steel casing having a mirror-like surface for reflection. The solutions processed permeated through the membrane; before entering the reactor, the wastewater solutions were saturated with O2 or O3. 2.7. Membrane Is the Catalyst. Most catalytic membrane reactors for higher temperature operations employ ceramic membranes in the pores/micropores of which catalysts were deposited. The base membranes, e.g., silica and alumina, are generally not catalysts for the reactions studied. There are, however, a number of membranes which are inherently catalytic for particular reactions; no catalyst needs to be deposited on or in the membrane. Particular examples are cation-exchange membranes, Nafion membranes, palladium membranes, and zeolite membranes. Consider a cation-exchange membrane and an esterification reaction (15) in which a carboxylic acid, R1COOH (e.g., oleic acid), reacts with an alcohol, R2OH (e.g., methanol), under the influence of an acidic catalyst providing a proton: Cat-H+

R1COOH + R2OH {\} R1COOR2 + H2O

(15)

In the schematic shown in Figure 15, CH3OH and H2SO4 are on one side of the membrane and the reactants, viz., CH3OH and oleic acid, are on the other side of the membrane.82 In this case protons are the counterions in the cation-exchange membrane introduced from the left-hand side of the membrane; naturally, a layer of protons appears on the other membrane surface exposed to the reaction mixture of the alcohol and the acid. These protons catalyze the esterification reaction. No separate catalyst, e.g., H2SO4, p-toluenesulfonic acid, etc., is required, eliminating the need for catalyst separation after reaction if homogeneous catalysts are employed

Figure 15. Concentration profiles around a cation-exchange membrane. Example at the beginning of run no. 11. Catalyzing solution, 0.9 g of H2SO4 in 180 mL of methanol; reacting mixture, 25 g of oleic acid and 75 g of methanol (reprinted from Chemseddine and Audinos.82 Copyright 1996, with permission from Elsevier Science).

(anions, e.g., HSO-4, do not get transported through the cation-exchange membrane). The cation-exchange membrane also separates a reaction product, viz., water, from the reaction mixture. Water is transported from the reaction mixture to the catalyzing mixture, thus achieving equilibrium shift to the right in reaction 15 (section 2.1). If solid ion-exchange beads were used as the catalyst, such a function would not have been possible in a continuous process. The acid form of a Nafion membrane is also capable of carrying out both functions.83 The acid form of the membrane prepared by boiling tubular Nafion membranes in concentrated HNO3 and then in water was found to catalyze the esterification of methanol or n-butanol to methyl acetate or butyl acetate, respectively, with acetic acid. These membranes were also found to be effective in separating water from n-butanol (water/alcohol selectivity ∼ 8.0; water/acetic acid selectivity ∼ 9.0). The Cs+ form of the Nafion membrane was found to have a much higher selectivity for both water/ alcohol (∼71) and water/acetic acid (∼149). Simultaneous catalysis and separation by a catalytically active membrane can potentially increase the membrane flux for the products produced and removed since the reactions occur within the membrane compared to a passive membrane over which the solution is passed after the reaction is completed. Palladium is known to be a catalyst for hydrogenation and dehydrogenation reactions. A palladium membrane is also infinitely selective for H2. Thus, for dehydrogenation reactions, a palladium membrane simultaneously acts as a catalyst and allows the product H2 to be removed through the membrane and obtained in the pure form (see section 2.1 for references). Zeolites are well-known as catalysts. Thin zeolite membranes are being developed for the selective transmission of species preferentially adsorbed or smaller than the pore size. A number of reactions have been studied.2 2.8. Membrane Is the Reactor. In a membrane reactor, catalysts are used frequently. The membrane may physically segregate the catalyst in the reactor (function 2.5) or have the catalyst immobilized in the porous/microporous structure or on the membrane surface (function 2.6). The membrane having the catalyst immobilized in/on it functions almost in the same way as a catalyst particle in a reactor does except separation of the product(s) (function 2.1) takes place, in addition, through the membrane to the permeate side. All such configurations involve the bulk flow of the reaction mixture along the reactor length while diffusion of the reactants/products takes place generally in a perpendicular direction to/from the porous/microporous catalyst.


When the bulk flow of a reaction mixture takes place through the membrane from one membrane surface to the other, the membrane is the reactor. Generally, the membrane in such a case will be porous/microporous to reduce the pressure drop for practical flow rates. The length of the pores/transport corridors is the reactor length; the reactions may take place on the surfaces of such macropores or there may be radial diffusion of reactants into the micropores and products out of the micropores into the main porous corridors where convective motion occurs. The convective motion of the reaction mixture through the membrane created by an applied pressure drop across the membrane thickness may involve Knudsen diffusion, Poiseuille flow, or a transitional regime for gaseous reaction mixtures. Such a reactor can have an exceptionally high value of reactor L/d for even thin membranes since d values can be very small (from ∼0.2 µm to 4 nm). As a result, extremely high conversions are possible, as shown by recent theoretical studies.84 Such a reactor can be identified as a pore flow through reactor (PFTR) since each macropore that traverses the membrane thickness is a reactor tube. Further, the mass-transfer resistance encountered by the reactants to reach the catalytic sites are significantly reduced because of the bulk flow through the membrane pores.85 Pina et al.85 have experimentally demonstrated extremely efficient removal of toluene and methyl ethyl ketone from air by low-temperature oxidation (100-320 °C) using a Pt/γAl2O3 catalytic membrane operating under the Knudsen diffusion regime, supporting the earlier work by Pina et al.86 The membrane in this case does not possess apparently the separation capability characteristic of a membrane in a tubular reactor enclosed by a membrane. The membrane can still separate, for example, particles from a gas mixture as the gas mixture enters the membrane and is convected through the pores. One can have stacks of disks of such membrane reactors with spaces between the disks being used for product separation or reactant addition or heat exchange. Fritsch80 deposited nanoclusters of catalysts such as Pd, Pt/Ag, and Pd/Co in the size range of 1-3 nm in polymeric membranes such as poly(amide-imide) and poly(dimethylsiloxane) (PDMS). Reactions were carried out in the flow through mode except that the membranes were essentially nonporous. In this case, the membrane treats every gas in the feed mixture according to the normal gas permeation selectivity displayed in the polymeric membrane. For example, PDMS has a selectivity of 8.4 from pure gas measurements for C3H6 (propene) over H2. Therefore, a feed gas ratio of at least 8.4 for H2/C3H6 is required for complete conversion of C3H6 to C3H8. In fact, Fritsch80 has illustrated complete conversion of C3H6 to C3H8 using Pd cluster-containing pore-free membranes of PDMS. The metallic catalytic nanoclusters were prepared during the polymeric membrane formation itself. Such thin membranes acting as the reactor are quite useful for fast exothermic reactions or for reactions where one of the intermediates should be the main product. They possess very high catalyst surface area per unit membrane area. However, the throughput per unit membrane area would be low. For example, Fritsch80 has observed permeance values in the range of 1-3.2 × 10-6 cm3/cm2 s cmHg for PDMS membranes. The reactor, therefore, has to have a large area and will have

Figure 16. Membrane and electrode assembly. The five regions of the model are shown (not to scale) (adopted from refs 88 and 89).

the shape of a thin disk of large diameter. A standard industrial reactor will have the shape of a long narrow tube. In an alternate strategy, Wu et al.87 had used a somewhat similar membrane as a reactor via interphase contacting. They employed PDMS membranes modified appropriately and containing titanium silicalite zeolite as a catalyst. Oxyfunctionalization of n-hexane to a mixture of hexanol and hexanone was carried out by bringing in n-hexane and 30 wt % of an aqueous H2O2 solution: the silicone membrane acted as the reaction medium and the reactor. 2.9. Solid-Electrolyte Membrane Supports the Electrode, Conducts Ions, and Achieves the Reactions on the Surface. Solid electrolytes are solid-state materials possessing ionic conductivity. The two ions of the greatest relevance are H+ and O2-, although other ions, Cl-, F-, Ag+, etc., have been found to be conducted as well. Solid polymer electrolytes such as perfluorinated ionomer membranes (e.g., Nafion) allow transport of H+ ions in the presence of water and are often called proton-exchange membranes. Solid solutions of oxides of di- or trivalent cations (e.g., Y2O3) in oxides of tetravalent metals such as ZrO2 can conduct O2- over a wide temperature range. Nonporous disks of such a solid electrolyte can act as membranes for such ionic species and are quite useful for fuel cells and as O2conductors. Consider a solid-polymer-electrolyte fuel cell: porous graphite gas-diffusion electrodes hot pressed onto both sides of a thin polymer membrane (e.g., Nafion) above its glass transition temperature. This cell is fed with wet air on one side and wet H2 on the other side (derived from the reforming of CH3OH in an adjacent reformer and therefore contains CO2 also). The membrane and the electrode assembly is schematically shown in Figure 16.88 The gas-diffusion electrode is made from porous graphite impregnated with a Pt catalyst. H2 gas diffuses through the porous electrode and is oxidized on Pt catalyst sites at the anode in a three-phase region (Figure 16) containing a polymer electrolyte, gaseous reactants, and a carbon matrix89

H2 T 2H+ + 2e-

(16)

Protons transferred through the membrane react with O2 at similar catalyst sites at the cathode to form water

O2 + 4H+ + 4e- T 2H2O

(17)

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Thus, the membrane supports the electrodes on two sides, transports H+, and achieves reactions on its surfaces. The electrons in the external circuit obtained from the chemical energy of the oxidation of H2 provide the required current. A solid ceramic proton conductor tube from a strontium-ceria-ytterbia (SCY) perovskite of the form SrCe0.95Yb0.05O3 has been employed with two porous polycrystalline palladium films deposited on the two sides of the ceramic tube to carry out NH3 synthesis at atmospheric pressure.90 The two electrodes were connected to an external galavanostat-potentiostat by which the appropriate current was applied. At the anode, gaseous H2 is converted to H+:

3H2 f 6H+ + 6e-

(18)

The protons are transported through the solid electrolyte to the cathode where the half-cell reaction +

-

N2 + 6H + 6e f 2NH3

(19)

takes place to complete the overall reaction

N2 + 3H2 T 2NH3

(20)

The reaction rate in this case was strictly controlled by the rate of H+ supply since H2 was the limiting reactant, N2 being present in abundance. The membrane functions in this case again are quite similar to those in the fuel cell example given earlier. At 570 °C and atmospheric pressure, greater than 78% of the electrochemically supplied H2 was converted into NH3. This rate exceeds that in a conventional catalytic reactor under equivalent conditions of compositions, pressure, and temperature by 3 orders of magnitude. Using a yttria-stabilized zirconia-based (YSZ-based) conducting solid electrolyte which transports O2-, McKenna et al.41 have studied the synthesis of HCN in a solid-electrolyte-cell reactor. At the cathode, an O2containing gas (containing N2) passes; adsorbed O2 gets converted to O2- via 3

/2O2 + 6e- f 3O2-

(21)

which is then conducted to the anode surface. At the anode, a mixture of CH4 and NH3 is supplied, which leads to the following reaction,

CH4 + NH3 + 3O2- f HCN + 3H2O + 6e-

(22)

producing HCN. In such a scheme, N2 in the air on the cathode side is rejected by the membrane. For partial oxidation reactions using, for example, CH4, it is not necessary to apply a voltage. Instead, one employs mixed conducting materials having both ionic and electronic conductivities. A gradient of O2 partial pressure across the ceramic membrane induces a gradient of O2- in the same direction and electrons in the opposite direction. The mixed conducting membrane has a catalyst on the O2 side to facilitate the formation of O2-. The catalytic properties on the other surface of the membrane allows CH4 to react in the manner of

CH4 + O2- f CO + 2H2 + 2e-

(23)

generating the electrons which diffuse to the O2 side to maintain charge neutrality in the ionic lattice structure

Figure 17. Methane conversion to synthesis gas (Eltron Research Inc., with permission91).

of the ceramic membranes. See Figure 17 from Eltron Research for CH4 conversion to synthesis gas.91 The membrane in this case separates O2 from air through the membrane, while supporting the catalyst for reforming CH4 and distributing O2 in a controlled fashion throughout the reactor. The study jointly sponsored by DOE and Argonne National Laboratories40 used a rhodium-based partial oxidation catalyst inside the mixed-conducting ceramic tube with O2 from the air being present on the outside of the tube. Such arrangements also reduce the possibilities of an explosive mixture. 2.10. Transfer of Heat. The most recent studies of membrane reactors have been in the context of the petrochemical industry.8 They take place at higher temperatures (>200 °C) and there likely is a need for considerable heat transfer because the reaction may be exothermic or endothermic. Dehydrogenation reactions studied frequently are endothermic. The membrane, if inert, is in a catalytic reactor, packed bed, or fluidized bed.92 Thus, the membrane may have to participate in heat transfer. Itoh and Govind45 have studied an endothermic dehydrogenation reaction on one side of a palladium membrane coupled with an exothermic hydrogen oxidation on the other side. Heat was transferred from the oxidation reaction side to the dehydrogenation side through the membrane. They have concluded that heat transfer across the membrane leading to an adiabatic reactor resulted in a higher conversion than what was possible under isothermal conditions. In actual practice, there will be one particular reaction going on and heat is going to be supplied from a fired heater, molten salt baths, or thermal fluid jackets. Therefore, the membrane is most likely going to be decoupled from the heat transfer process. A common configuration of some interest in a packed bed membrane reactor consists of multiple membrane tubes inside tubular catalyst beds, placed in turn, in another enclosure for heat exchange.7,92 Thermal expansion properties of the membrane tube, sealing at the header, and protection from abrasional damage from catalyst particles are of much greater importance.8 2.11. Immobilizing the Reaction Medium. Many reactions are carried out in an organic solvent. These include two-phase reactions, e.g., those encountered in phase transfer catalysis, gas-liquid reactions, etc. A porous/microporous membrane can immobilize an appropriate reaction medium in the pores. The two different phases containing reactants can be brought to


Figure 18. Schematic diagrams for four configurations of a reactor where two membranes are separated by the reaction medium.

the two sides of the membrane. As long as the two feed phases are immiscible with the reaction medium, reactants can partition into the reaction medium and react and then the products can partition back into the flowing phases on opposite sides of the membrane. Unfortunately, such a configuration, usually termed as the supported liquid membrane (SLM), has limited stability93 because of a variety of reasons including a finite solubility of the reaction medium in the two different reactant-containing phases. Therefore, as discussed in the next section, Guha and Sirkar,94 Chen et al.,95 and Guha et al.11,96 have employed two different hollow fiber membranes for bringing two reactantcontaining streams into the membrane reactor while the reactants partitioned into the liquid reaction medium contained (confined) in the shell side between the two sets of hollow fibers. Kim and Datta97 have used a porous support disk to immobilize a homogeneous catalyst in a high-boiling organic solvent. However, they did not completely wet the pores so that the gas space was left in the pore. The reaction was hydroformylation of ethylene to give propionaldehyde. With an appropriate support membrane, the capabilities of the reaction medium within the membrane could be substantially enhanced. The only issue is the lifetime of the reaction medium.

3. Functions of Multiple Membranes in a Reactor Although more than two different types of membranes can be accommodated in a reactor, this section will consider primarily the functions of only two membranes (different types or similar) in a reactor. There are cases where two similar membranes carry out somewhat different functions or the same function vis-a`-vis different reactants/products. All such functional variations may be considered under two broad categories: (1) Multiple membranes physically separated by the reaction medium (2) Multiple membranes existing as a laminate We have already seen how many functions can be implemented through a single membrane. In a practical situation, only a few such functions (of all possible ones appearing in Figure 1) are implemented via a given membrane. Obviously, when there is another membrane, the number of possible combinations increase greatly. Totally new functions may also become possible, e.g., containing a reaction medium, separation of two product species, enrichment of product, and intensification of a reaction. The particularity of a given chemical reaction will, however, limit the number of desired functions. Thus, instead of considering all possible

3732


combinations of functions possible in a two-membrane reactor, we consider only particular cases that have been studied experimentally and/or analyzed theoretically. Under appropriate conditions, we briefly identify reactors having three or four membranes. First, we review those systems where the reaction medium exists between two or more membranes. 3.1. Two or More Membranes Physically Separated by the Reaction Medium. Of the many combinations possible, we will consider four types of systems as illustrated in Figure 18. 3.1.1. Homogeneous Catalytic Oxidation of Ethylene. As shown in Figure 18a, the membrane reactor has two sets of microporous hydrophobic hollow fibers, one bringing in C2H4 (species A) while the other brings in O2 (species B). The catalytic reaction medium present on the shell side (in the manner of a hollow fibercontained liquid membrane of Majumdar et al.98) is an aqueous solution of palladium chloride, cupric chloride, and cupric acetate. Chen et al.95 have studied the oxidation of C2H4 to CH3CHO in such a system; the product CH3CHO can be removed through both gas streams or through one of the gas streams. These investigators have shown that controlled and separate introduction of C2H4 and O2 (function 2.3) through two separate membranes led to much better conversion compared to that achievable in a single hollow fiber membrane-based reactor where both C2H4 and O2 were simultaneously fed to the reactor in one gas stream. This two-membrane configuration may be replaced by a single porous membrane immobilizing the aqueous reaction medium in the membrane pores; C2H4 and O2 may be separately introduced on two sides of the membrane. One has to be careful that the reaction medium properties remain unchanged with time. 3.1.2. Multiphase Ozonolysis of Organic Pollutants. Figure 18b has two sets of hollow fibers: one set of nonporous silicone capillaries through which gas B (in this case O3 in O2) is introduced into an organic inert perfluorocarbon (PFC) reaction medium on the shell side and another set of porous Teflon capillaries through which wastewater containing organic pollutants, e.g., species A, flow. Pollutants are partitioned into the contained liquid reaction medium at the aqueousorganic interface. The organic reaction medium concentrates the organic pollutants, especially the more hydrophobic ones, by extraction and allows O3 to react with these pollutants without the O3 being consumed by nontargeted chemicals present in wastewater. When first presented by Guha and Sirkar,94 the system used two sets of microporous hydrophobic hollow fiber membranes instead of nonporous silicone capillaries and porous Teflon capillaries. One could use just one set of microporous fibers and immobilize the reaction medium in the pores. Both would lead to a high loss, primarily by vaporization and also by solubilization of the perfluorocarbon (PFC) liquid which has a low but finite vapor pressure. The silicone membrane reduced the vaporization-based loss of PFC liquid drastically. Guha et al.11,96 have studied the system of Figure 18b using silicone capillaries and have demonstrated its high efficiency in destroying most pollutants. The reaction products partition back into the water phase and the gas phase on the two sides. This system avoids direct ozonation of pollutants in wastewater where most of the O3 is wasted. Further, O3 has a very low solubility in water, whereas it has 14 times higher solubility in the

PFC medium. When one considers hydrophobic pollutants which are concentrated in the reaction medium by extraction (section 2.2), one has now considerable process intensification. The thickness of the contained PFC liquid layer is also important. By providing sufficient thickness, one prevents O3 loss into water or the loss of a volatile organic pollutant into the gas phase. It would be difficult to achieve it by immobilizing the reaction medium in the pores of a single membrane, since the required membrane thickness cannot be made to order. Meanwhile, an extremely efficient pollutant destruction environment is created in the reaction medium. This basic reactor configuration has been adopted by Shanbhag et al.99 to destroy volatile organic compounds (VOCs) from a gas phase via ozonation in a PFC liquid contained between three membranes in a reactor. One set of silicone capillaries in the reactor bring in a gaseous mixture of O3 in O2 which permeate into the PFC phase on the shell side. Another set of silicone capillaries bring in the VOCs in air; the VOCs permeate through the silicone membrane and are dissolved in the PFC phase where they react with O3 and are destroyed. The nonvolatile reaction products are extracted into water which flows through the bore of a third set of porous Teflon capillaries in the reactor shell. The volatile reaction products are removable through each membrane. Use of silicone capillaries dramatically reduces the vapor-phase loss of PFC. 3.1.3. Efficient Mixing Membrane Reactor. In some biochemical processes, two reactants present in two separate aqueous (say) solutions are to be mixed together. Direct mixing leads to further dilution of each reactant by the solvent present in the other incoming stream. However, if the scheme of Figure 18c suggested by Stanley and Quinn100 is adopted, then each membrane is used to introduce its own reactant, A and B, respectively, into the reaction liquid between the two membranes. Thus, the extent of dilution of species A as well as B is considerably reduced. One has to make sure, however, that the membrane for introduction of A rejects species B and vice versa. The product P may be permeable through both membranes. Here, each membrane carries out its own function of controlled introduction of one reactant and rejection of the other. However, a combination of the two membranes achieves a novel and important function, among others, i.e., prevention of dilution of the two reactants. 3.1.4. Partial Oxidation Process for Methane to C2 Compounds. Oxidative coupling of methane in membrane reactors is of considerable interest. Lu et al.48 have made a numerical study for a cross-flow reactor with a distributed feed of O2 and product removal. They have modeled a number of reactor configurations. Their results show that distributed feed O2 could give rise to much higher C2 yields than the cofeed reactor as long as the ratio of catalyst loading to initial methane flow rate was sufficiently high. On one hand, their calculations have further shown that although reactors with optimally distributed O2 feed give much higher yield than evenly distributed ones, the improvement is not significant. On the other hand, if a two-membrane reactor is used (Figure 18d) where one membrane is used for the O2 feed and the other for C2 product removal, higher C2 yields could be obtained than that in a single membrane reactor with distributed O2 feed. However, if the membrane for C2 product removal is also


Figure 19. Different membrane laminate-based reactors.

permeable to other species as well as C2 products, low permeability of methane is critical to the achievement of high C2 yield. One must recognize that the development of such a membrane reactor structure at the higher temperatures needed is a formidable task. 3.2. Multiple Membranes in a Laminate. Here, we consider a number of cases: 3.2.1. Product separation and enrichment 3.2.2. Separation of multiple products 3.2.3. Supported liquid-phase catalytic membrane reactor-separator for homogeneous catalysis

3.2.1. Product Separation and Enrichment. Figure 19a illustrates a membrane laminate consisting of a two-layer sandwich of a permselective membrane and a catalytic membrane.101 On the permselective side, a feed solution containing reactant R (e.g., an ester) and inerts flows. Reactant R partitions into the permselective membrane (e.g., an organic nonpolar liquid membrane, the feed solution being aqueous) and then partitions into the aqueous environment of the catalytic membrane where the reaction R f P leads to the product P. The product P, in this case an acid, is rejected

3734


by the nonpolar liquid membrane and therefore accumulates in the sweep aqueous liquid flowing on the other side. If the sweep liquid flow rate is much lower than that of the feed, it is possible to increase the concentration of P in the sweep stream to a very high value compared to the feed concentration of the reactant R. Thus, the membrane laminate allows simultaneous separation of the product from the feed stream and its considerable enrichment in the product stream. Any inert in the feed, if it partitions through the liquid membrane, will be present in the product stream at a concentration lower than that in the feed. Matson and Quinn101 have experimentally demonstrated this concept by resolving a racemic aqueous mixture of a D,Lester where the L-ester was converted to the L-acid enzymatically in the catalytic membrane containing the enzyme chymotrypsin. The product stream was highly enriched in the L-acid. 3.2.2. Separation of Multiple Products. The concept may be illustrated by considering the schematic of Figure 19b. Reactant A in the feed solution is highly permeable though the permselective membrane exposed to the feed solution. On the other side of this membrane, reactant A enters a catalytic membrane in an aqueous environment and undergoes the following reaction:

AfB+C

(24)

It so happens that product C is also very permeable through the permselective membrane but product B is rejected by this membrane. On one hand, if now the sweep solution on the other side of the catalytic membrane has a flow rate much lower than that of the feed, product B concentration will be very high in this sweep stream. On the other hand, the concentration of C will be essentially the same in both solutions; however, most of C will appear in the feed stream since its flow rate is much higher. Thus, products B and C are effectively separated; further, product B concentration is also much higher than that of A. Matson and Quinn101 have demonstrated the utility of this concept by carrying out the hydrolysis of Nacetyl-L-tyrosine ethyl ester into the corresponding acid (product B) and ethanol (product C); the catalytic membrane employs chymotrypsin covalently bound to a Millipore filter to catalyze this hydrolysis. The permselective membrane was an immobilized liquid membrane of decanol through which alcohol, product C, as well as the reactant ester are freely permeable. The bipolar ion-exchange membrane described in section 2.1.1.6 may also be considered as a membrane laminate where a cation-exchange membrane and an anion-exchange membrane have been joined together, even though there is some space between them where water comes in from both sides. Further, under the applied potential difference between the two electrodes (Figure 7), water dissociates in this space and two products of the reaction, H+ and OH-, are effectively separated by the two ion-exchange membranes. 3.2.3. Supported Liquid-Phase Catalytic Membrane Reactor-Separator for Homogeneous Catalysis. If instead of a covalently bound enzyme catalyst in the membrane, a homogeneous catalyst is necessary, say, for the hydroformylation of ethylene to form propionaldehyde by the reaction

H2 + CO + CH2dCH2 f CH3CH2CHO

(25)

Kim and Datta97 have suggested using a porous support disk to load the catalyst by evaporation of a volatile solvent in the catalyst solution impregnated into the pores. During the reaction, an essentially nonvolatile solvent, dioctylphthalate, is used. To protect this solution from being forced out, a second nonselective membrane is used (Figure 19c) whereas the first membrane must be freely permeable to all the reactants but not the product. With a low flow rate of the sweep stream in contact with the second membrane, the product concentration can be raised to a high level compared to the feed concentration level (as in Figure 19a,b). Unfortunately the membrane employed as the first membrane was a ultrafiltration-type membrane of 50 Å pore size; therefore, it had only a Knudsen diffusion-type selectivity (1.44) between ethylene and propionaldehyde. If a reaction

AfBTC

(26)

is desired to be carried out where C is the desired product, then the schematic of Figure 19c can be employed, except that membrane 2 must also reject the intermediate product B and be freely permeable to product C (but not to reactant A). This has been termed as the “trapped intermediates” membrane bioreactor since the intermediate B is trapped between the membrane 1 and 2 and its increasing concentration forces the reaction in the direction of C.101 Contrast this scheme with those of reactions 4a and 4b where the intermediate B is the desired product:14 the membrane plays quite a different role for species B in these two cases. 4. Concluding Remarks A membrane in a reactor is capable of performing a wide variety of useful functions. These capabilities can result in the performance levels of the membrane reactor and the process to be considerably higher than those of a conventional reactor under similar operating conditions. Membrane functions that are particularly useful and unique in this regard are separation of product(s) from the reaction mixture, separation of a reactant from a mixed feed for introduction to the reactor, controlled addition of a reactant or two reactants, and a solid-electrolyte membrane acting as support for the electrodes, allowing the conduction of ions and having reactions on its surfaces. When compared with conventional reactors, there are additional unique functions, e.g., nondispersive phase contacting, segregation of a catalyst (and cofactor) in a reactor, catalyst immobilization, etc., which impart considerable operational and economic advantages to devices in reaction processes employing membrane(s). Simultaneous achievement of a number of such functions by a membrane in a reactor has made membrane reactors quite attractive. Even when the membrane is located outside the reactor and connected to it in a recycle mode, such coupled reaction-separation processes provide considerable improvements in the overall reaction performance. Many investigations on membrane reactors have been conducted in biochemical processing, petrochemical applications, and environmental pollution control. A few processes employing polymeric membranes have been commercialized. Utilizing more than one membrane in a reactor imparts additional functional capabilities. For effective large-scale utilization of the diverse functional


capabilities of a membrane in a reactor, considerable research and development on membrane lifetime, available space time, module design, membrane fouling, membrane poisoning, and membrane cost are essential. This is especially true for inorganic membranes and higher temperature applications where the hurdles for industrial use are considerable. Acknowledgment This article is dedicated to George Keller for his many contributions. Along with his colleague M. Bhasin at Union Carbide, George Keller carried out a pioneering study on oxidative coupling of methane to C2 hydrocarbons. Many current and large efforts are focused on achieving improved oxidative coupling of methane via membrane reactors. The senior author wishes to acknowledge invaluable discussions with Ka Ng, Paul Bryan, D. Bhattacharyya, T. Tsotsis, Steve Matson, W. S. Winston Ho, Arvind Varma, and Richard Noble on various aspects of a membrane in a reactor. P.V.S. would like to thank Asim K. Guha, Sudipto Majumdar, and Zhi-Fa Yang for sharing their insights and thoughts on membranes and their applications. Literature Cited (1) Hsieh, H. P. Inorganic Membranes for Separation and Reaction; Elsevier Science: Amsterdam, 1996. (2) Falconer, J. L.; Noble, R. D.; Sperry, D. P. Catalytic Membrane Reactors. In Membrane Separations Technology. Principles and Applications; Noble, R. D., Stern, S. A., Eds.; Elsevier Science: Amsterdam, 1995. (3) Govind, R., Itoh, N., Eds. Membrane Reactor Technology; AIChE Symposium Series 268; AIChE: New York, 1989; Vol. 85. (4) Lo`pez, J. L.; Matson, S. L.; Stanley, T. J.; Quinn, J. A. Liquid-Liquid Extractive Membrane Reactors. In Extractive Bioconversions; Mattiasson, B., Holst, O., Eds.; Bioprocess Technology; Marcel Dekker: New York, 1991; Vol. 11. (5) Matson, S. L.; Quinn, J. A. Membrane Reactors. In Membrane Handbook; Ho, W. S. W., Sirkar, K. K., Eds.; Chapman and Hall: New York, 1992. (6) Cheryan, M.; Mehaia, M. A. Membrane Bioreactors. In Membrane Separations in Biotechnology; McGregor, W. C., Ed.; Bioprocess Technology; Marcel Dekker: New York, 1986; Vol. 1. (7) Tsotsis, T. T.; Champagnie, A. M.; Minet, R. G.; Liu, P. K. T. Catalytic Membrane Reactors. In Computer-Aided Design of Catalysts; Becker, E. R., Pereira, E. R., Eds.; Chemical Industries; Marcel Dekker: New York, 1993; Vol. 51. (8) Armor, J. N. Applications of Catalytic Inorganic Membrane Reactors to Refinery Products. J. Membr. Sci. 1998, 147, 217. (9) Saracco, G.; Versteeg, G. F.; van Swaaij, W. P. M. Current Hurdles to the Success of High Temperature Membrane Reactors. J. Membr. Sci. 1994, 95, 105. (10) Ho, W. S. W., Sirkar, K. K., Eds. Membrane Handbook; Chapman and Hall: New York, 1992. (11) Guha, A. K.; Shanbhag, P. V.; Sirkar, K. K.; Vaccari, D. A.; Trivedi, D. H. Multiphase Ozonolysis of Organics in Wastewater by a Novel Membrane Reactor. AIChE J. 1995, 41, 1998. (12) Whu, J.; Baltzis, B. C.; Sirkar, K. K. Modeling of Nanofiltration-Aided Organic Synthesis. J. Membr. Sci. 1999, in press. (13) Raich, B. A.; Foley, H. C. Ethanol Dehydrogenation with a Palladium Membrane Reactor: An Alternative to Wacker Chemistry. Ind. Eng. Chem. Res. 1998, 37, 3888. (14) Agarwalla, S.; Lund, C. R. F. Use of a Membrane Reactor to Improve Selectivity to Intermediate Products in Consecutive Catalytic Reactions. J. Membr. Sci. 1992, 70, 129. (15) Frank, G. T.; Sirkar, K. K. Alcohol Production by Yeast Fermentation and Membrane Extraction. Biotechnol. Bioeng. Symp. Ser. 1985, 15, 621. (16) Kang, W.; Shukla, R.; Sirkar, K. K. Ethanol Production in a Microporous Hollow-Fiber Based Extractive Fermentor with Immobilized Yeast. Biotechnol. Bioeng. 1990, 34, 826.

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Received for review January 29, 1999 Revised manuscript received July 14, 1999 Accepted July 19, 1999 IE990069J

Membrane in a Reactor: A Functional Perspective - Industrial

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