membrane in a reactor- a functional perspective

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 EnvironmentalScience, 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 separationprocesses, the novel characteristics of membrane structures, and the geometrical advantagesoffered by the membrane modules have been employed to enhance and assist reaction schemesto attain higher performance levels compared to conventional approaches. In these, membranesperform a wide variety of functions, often more than one function in a given context. Anunderstanding of these various membrane functions will be quite useful in future developmentand commercialization of membrane reactors. This overview develops a functional perspectivefor membranes in a variety of reaction processes. Various functions of the membranes in a reactorcan be categorized according to the essential role of the membranes. They can be employed tointroduce/separate/purify reactant(s) and products, to provide the surface for reactions, to providea 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 havedifferent physical/chemical structures and geometries. The functions of the membrane in areaction 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 betterappreciation of their role in the improvement of overall process performance.

1. Introduction

Membrane reactors have been investigated since the1970s. The early investigations employed primarilypolymeric membranes and enzymatic reactions. Laterinvestigations show an abundance of petrochemicallyrelevant systems and inorganic membranes. Whole cellfermentation-based chemical and biochemical produc-tions as well as degradation of pollutants biologicallyor otherwise have also been studied in membranereactors. Polymer membrane-based reactors have beenblessed with some commercial success. Much of thisresearch has been discussed in a number of reviews.1-9

In these investigations a variety of membrane separa-tion processes as well as membranes have been used.More importantly, the membrane inside the reactor hasserved a variety of functions. In some studies, themembrane has a single well-defined function. In others,the membrane allows two or more functions to becarried out. The variety of functions achievable via amembrane in a reactor is very broad. An understandingof the breadth of the roles capable of being performedby a membrane is likely to be quite useful in the futuredevelopment of membrane reactors. This overviewproposes to develop a functional perspective of a mem-brane(s) in a reactor. This perspective is developed byemploying a variety of contexts including differentmembrane separation processes, different membranes,chemical/electrochemical reactions, enzymatic processes,fermentations, catalyst immobilization/segregation, cata-lytic membranes, integration of functions, etc.

A brief enumeration of different membrane separationprocesses and different classes of membranes investi-

gated in the literature is useful in the Introductionbefore we present the functional perspective. Of themany types of membrane separation processes andmembrane-based equilibrium separation processes avail-able for separation,10 membrane reactors have beenstudied 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 moleculardiffusion), gas membrane, membrane solvent extraction,and membrane gas absorption/stripping.

An extraordinary variety of membranes have alsobeen used. Membranes are employed in gross physicalforms as flat films, hollow fibers, tubules, and tubes,while their physical structures can be as follows: mi-croporous symmetric and asymmetric membranes, non-porous membranes, and composite membranes. Mem-branes can be of the polymeric variety or be inorganicin nature, which would include zeolitic, ceramic, andmetallic membranes. Membranes can also conductelectrical charges and can be chosen from one of follow-ing categories: ion-exchange membranes, bipolar mem-branes, mixed conducting membranes, proton-conduct-ing membranes, etc. In many cases, the membraneshave catalysts incorporated in their porous structure oron the surfaces. The membranes in such cases aretermed as catalytic membranes. Of course, the mem-brane can be catalytic by itself without the addition ofany catalyst materials from external sources. The termcatalytic membrane reactor sometimes includes theabove cases as well as a catalytic reactor enclosed by amembrane, which is noncatalytic.2

In the next section, we will first present a compactlist of membrane functions in a reactor. Often, thegeneric membrane function identified will affect thereaction processes in different ways. Such effects on the

* 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.

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

10.1021/ie990069j CCC: $18.00 1999 American Chemical SocietyPublished on Web 09/18/1999

reaction will be identified. Each generic membranefunction will then be illustrated in a separate subsectionusing examples from the literature. This illustrativeexposition will employ different membrane separationprocesses, reaction-phase systems, and other distin-guishing features to elaborate briefly on particularmembrane reactors. The goal is to develop a perspectiveon the range and the utility of each membrane functionin a membrane reactor rather than a review of allinvestigations. Frequently, a membrane (or two mem-branes) incorporated in a reactor serves more than onedesired function, only one of which may involve amembrane separation process where membrane fluxand selectivity are important. The development of sucha multifunctional perspective of membranes in a reactoris an additional objective of this paper.

The reaction processes of interest in this paper mayinvolve the production of a particular chemical or abiochemical product. Alternately, it may involve thedestruction of some organic species in a phase for thepurpose of controlling environmental pollution. Reactionprocesses are sometimes employed to purify a particularfluid stream without destroying the undesirable speciesinto simple compounds such as CO2, H2O, etc. (e.g.,enzymatic resolution processes). Although such pro-cesses are within the scope of this paper, our main focuswill be on those reactive processes where a particularcompound or two are produced by the reactions. Wespecifically exclude those processes where reactions areused to enhance separation of a mixture.

2. Membrane Functions in a Reactor

Figure 1 schematically identifies many of the majorgeneric functions performed by a membrane in a reactor.One should not conclude from the figure that a given

membrane in a given reactor is capable of all functionsidentified in the figure. However, a given membraneunder appropriate circumstances can perform more thanone generic function. The introduction of another mem-brane into the reactor can increase the number ofgeneric membrane functions in the reactor or achievethe same generic membrane function vis-a`-vis someother species. Figure 1 also indicates other activitiesconcurrently taking place in the so-called nonreactor (orpermeate) side of the membrane as well as in the reactorside of the membrane. A list of the generic membranefunctions performed by a membrane or two in a reactoris provided next:

2.1. Separation of products from the reaction mixture2.2. Separation of a reactant from a mixed stream forintroduction into the reactor2.3. Controlled addition of one reactant or two reac-tants2.4. Nondispersive phase contacting (with reaction atthe phase interface or in the bulk phases)2.5. Segregation of a catalyst (and cofactor) in areactor2.6. Immobilization of a catalyst in (or on) a mem-brane2.7. Membrane is the catalyst2.8. Membrane is the reactor2.9. Solid-electrolyte membrane supports the elec-trodes, conducts ions, and achieves the reactions onits surfaces2.10. Transfer of heat2.11. Immobilizing the liquid reaction medium

Membranes in a reactor existing as membrane lami-nates or physically separated membranes with a fluidphase between have also been studied. They can provideparticular combinations of the above functions some-times with added and novel benefits;5,11 these novelbenefits include product separation and simultaneousconcentration, separation of multiple products, reactionintensification, and physically containing the reactionmedium in multiphase reaction systems.

Before proceeding further, it is necessary to point outthat there are many studies where the membrane isphysically located in a device external to the reactorproper (structure). The reaction medium is then circu-lated over the membrane and back to the reactor in arecycle mode (Figure 2). This configuration is frequentlyemployed in reaction processes based on enzymes andwhole cells; it is also being proposed for organic syn-theses. The reactor vessel in such case is sometimesoperated as a batch reactor or more frequently as acontinuous stirred tank reactor (CSTR). In many cir-cumstances, the system behavior here can be considered

Figure 1. Schematic of possible functions of a membrane in areactor.

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

3716 Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999

to be de facto equivalent to that with a membrane insidea reactor. Therefore, such recycle membrane reactorswill also be included in the following treatment: pri-mary emphasis will however be on systems where amembrane (or two) is physically located in the reactor.

One must recognize the major advantages of thesedifferent arrangements:

(1) The mixing conditions and the flow velocities (andtherefore the extent of consequent concentration polar-ization in membrane devices involving liquid-phasesystems) can be maintained at different levels in thereactor and the membrane separator if recycle mem-brane reactors are employed; conditions can be opti-mized for each. The reactor may require long residencetimes whereas the membrane device may need a shortresidence time.

(2) Building a reactor with a membrane in it or usinga membrane device as the reactor can sometimes be verydemanding on the membrane, especially for highertemperature systems. The recycle membrane reactorallows the reactor and the membrane unit to operateat two different temperatures by using heat exchangersin between.

(3) Recycle membrane reactors allow use of existingequipment, namely, a separate reactor and a separatemembrane device.

(4) For fast reactions, the membrane in a reactor islikely to be a more desirable configuration.

2.1. Separation of Products from the ReactionMixture. Separation of products from the reactionmixture is one of the most common functions of amembrane in a reactor. The separation may be purifica-tion, enrichment, or concentration. Consider the follow-ing elementary reversible reaction (see Figure 1):

where D is a product needed to be removed via themembrane to the permeate side. The separation processemployed may produce a permeate side stream wherethe mole fraction of D is much higher than that in thereactor side. In the case of species D being H2 and apalladium membrane, pure H2 is obtained in the perme-ate side. The H2 partial pressure on the permeate sidewill be lower than the partial pressure of H2 on the feedside. If species C also permeates to some extent throughthe membrane, the permeate stream is enriched in Dvis-a`-vis the product species C: permeation of D leadsto partial purification of the product C in the reactoroutlet stream.

Removal of D via the separation function of themembrane has the following effects on reaction (1) andthe reactor performance:

(a) The equilibrium condition indicated in the revers-ible reaction (1) is shifted to the right, i.e., leading tohigher equilibrium conversion of A and B to C and D.

(b) If there is an undesirable side reaction as shownbelow,

taking place in the reactor (see Figure 1), the separationof product D from the reaction mixture reduces the lossof reactant B to the side reaction, increasing theselectivity of conversion to product C (or D) (modeledby Whu et al.12 for nanofiltration-aided liquid-phaseorganic synthesis). An experimental study by Raich andFoley13 of ethanol dehydrogenation in a palladium

membrane reactor whereby the product H2 is withdrawnthrough the palladium membrane to shift the reaction

to the right showed that the deleterious effect of the sidereaction

can be drastically reduced, provided the reaction (3a)is shifted to the right via the Cu/SiO2-aq catalyst andH2 removal by the membrane.

(c) In consecutive catalytic reactions,

where B is the desired intermediate product, if the rateconstant for reaction 4b is significantly larger than therate constant for reaction 4a, it is difficult to achieve ahigh selectivity to B using a conventional packed bed,plug flow reactor. By using an inert sweep gas on theoutside of a permeable tube having the catalysts andthe reaction taking place inside the tube, the intermedi-ate product B may be selectively removed from thereaction zone, leading to increased selectivity.14 Removalof the intermediate products (methanol/formaldehyde)via a membrane in the partial oxidation of methane isan example; this strategy will prevent further oxidationof these products to CO and CO2.

(d) In fermentation processes, one of the products maybe inhibitory to the fermentation process. Removal ofthe product from the fermentation broth via a mem-brane can substantially reduce product inhibition andincrease volumetric productivity of the fermentor.15,16Further, one can use higher concentrations of thesubstrate in the feed (e.g., glucose for ethanol fermenta-tion) since the product is being removed as it is beingformed.17

The separation of a reaction product(s) (C or D orboth) can be implemented using a variety of membraneprocesses. The nature of the membrane process isobviously influenced by the phase of the reactionmedium exposed to the membrane and the desiredphase of the permeated product stream. Examples ofsuch processes will be provided under two categories,namely, (1) liquid reaction medium/liquid feed phaseand (2) gaseous reactions/gaseous feed phase.

2.1.1. Separation from a Liquid Reaction Mix-ture. We will briefly mention and/or illustrate the useof the following membrane separation processes forremoving products from the liquid reaction medium:reverse osmosis, nanofiltration, ultrafiltration, pervapo-ration, gas membranes, electrodialysis, and liquid mem-branes.

2.1.1.1. Reverse Osmosis. Vasudevan et al.18 havedescribed a membrane sandwich reactor in which theSaccharomyces cerevisae (ATCC 4126) cells were ef-fectively placed between an ultrafiltration (UF) mem-brane and a reverse osmosis (RO) membrane; thereactor was fed with a solution of glucose at a highpressure from the UF membrane side and the productsolution was forced out through the RO membrane. Theproduct solution concentration progressively increasedin ethanol; the glucose in the feed solution was ef-fectively rejected by the RO membrane. The reactorstructure shown in Figure 3 has a microfiltration

C2H5OH T CH3CHO + H2 (3a)

C2H5OH + CH3CHO T CH3COOC2H5 + H2 (3b)

A f B (4a)

B f C (4b)

A + B T C + D (1)

B + D T E (2)

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3717

membrane and a coarse filter paper on two sides of theyeast cells between the UF and the RO membrane toimmobilize and provide physical protection in a high-pressure (up to 400 psig) environment. The membranesin this reactor separate the product ethanol from thereactant glucose and effectively immobilize the biocata-lyst (whole cell).

2.1.1.2. Nanofiltration. Whu et al.12 have modeledthe performance of a semibatch/batch reactor coupledto an external nanofiltration (NF) unit for the synthesisof the desired product C (a hydroxyester of MW 400)from reactants A (a diketone of MW 400) and B (analkoxide of MW 40-100) present in an organicsynthesis solvent, methanol. The membrane removesthe low-molecular-weight product, D (MW 40-100)and the solvent which significantly improves the selec-tivity if the reaction system consists of the reactions (1)and A + D T E. Figure 4 illustrates the role of themembrane in improving the selectivity of the reactionfor the product C. This figure shows that a much higher

selectivity is achieved when the semibatch reactor isexternally coupled with a nanofiltration unit to removethe solvent and the product D (in the manner of Figure2). Operation as a continuous flow stirred tank reactoras in Figure 2, coupled with a NF unit, could alsoprovide a way to increase the concentration of thereactants in the reactor from a dilute feed if thereactants are rejected by the NF membrane. In such acase, separation of products by the NF unit mayfacilitate conversion of the reactants.

2.1.1.3. Ultrafiltration. Cheryan and Mehaia6 haveprovided a comprehensive review of enzyme-based andwhole-cell-based membrane bioreactors where ultrafil-tration is often the predominant mode of membraneseparation. Many systems have been described. Someexamples are hydrolysis of proteins leading to modifiedproteins with smaller molecular weights appearing inthe permeate; hydrolysis of carbohydrates, e.g., starch,cellulose to produce lower molecular weight sugars;hydrolysis of sugars, e.g., lactose. Perhaps the earliestexperimental study was carried out in a stirred tankreactor coupled to an ultrafiltration membrane cell forthe hydrolytic breakdown of cellulose to the membranepermeable product glucose using cellulase enzymes.19The utility of a thin channel membrane reactor linedwith UF membranes for the enzymatic reduction ofstarch to glucose is shown in Figure 5. The figureillustrates the lengthwise variation of the performanceindicator fA of the plug flow membrane reactor definedby

with that for a solid tube reactor; the membrane reactorhas a much higher conversion for the cases analyzedwhere the enzyme is completely rejected.20

2.1.1.4. Pervaporation. The pervaporation (PV)process is used to remove volatile reaction products fromthe reaction mixture; generally, a vacuum on the

Figure 3. Membrane sandwich: (1) UF membrane, (2) coarsefilter paper (>10 m), (3) cell mass, (4) fine filter (0.2 m,microporous), and (5) RO membrane (reprinted from Vasudevanet al.18 with permission).

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 Whuet al.12 Copyright 1999, with permission from Elsevier Science).

Figure 5. Reactor performance as a function of enzyme concen-tration and distance along the membrane for enzymatic reductionof starch to glucose; uj0 ) 1 cm/s (reprinted with permission fromClosset et al.20 Copyright 1973, John Wiley & Sons, Inc.).

fA )(inlet flow of reactant ( flow of reactantacross the tube wall - flow of reactant

at a given axial distance)inlet flow of reactant

(5)


permeate side is employed to create the needed partialpressure difference. The most common reaction systemstudied for the application of pervaporation is anesterification reaction between an alcohol and an acidin the presence of a highly acidic catalyst (e.g., concen-trated sulfuric acid):

This reversible reaction in industrial processing isdriven to high conversion by adding a large excess ofalcohol. Adding a poly(vinyl alcohol) (PVA)-based water-selective PV membrane to the esterification reactorallows one to shift the equilibrium in reaction 6 to theright (thus reducing the need for excess alcohol beyondthat needed for solubilization of the acid). Figure 6illustrates how the conversion to the ester is increasedin a batch reactor with time as the ratio of themembrane surface area (S) to the reaction volume (V)is increased for the esterification reaction betweenpropionic acid and 1-propanol studied by David et al.21Note that in this case the equilibrium value of theconversion in the absence of any product removal is 0.7;the membrane allows a much higher conversion to beattained much more rapidly. If the temperature of theesterification reaction is high, it will be necessary toemploy vapor permeation membranes to remove H2Ovapor. Another application of pervaporation studied22involved selective removal of alcohol from a fermenta-tion broth in a recycle configuration using siliconecapillary membranes. Silicone (PDMS) is a highlybiocompatible material. Further, some of the otherbyproducts of fermentation (which can inhibit thefermentation if their concentrations build up in a recyclesystem) like acetaldehyde, butanol, etc. are also easilyremoved. Removal of ethanol decreased product inhibi-tion and increased fermentor productivity.

2.1.1.5. Gas Membranes. A hydrophobic microporousor porous membrane having gas-filled pores and twononwetting aqueous solutions on two sides will allowspontaneous transfer of volatile species from one solu-tion to the other solution through the pore as long asthere is a partial pressure difference.23 Such a gas

membrane-mediated selective transfer of volatile spe-cies has been used to remove volatile product speciesfrom a reactor solution to that on the receiving side.Twardowski and McGilvey24 have used a porous poly-(tetrafluoroethylene) (PTFE) membrane to transferproduct ClO2, from the reactant stream in a reactor tothe aqueous solution on the other side of the membranewhere it is ultimately used to bleach wood pulp, etc.Removal of the ClO2 from the reactor solution isnecessary to reduce the high partial pressure of ClO2inevitably occurring in the gas space of commercial ClO2generators which leads to localized decomposition ofClO2.

2.1.1.6. Electrodialysis. In electrodialysis processesusing bipolar membranes, a solution of a salt, e.g., NaCl,is converted to a pure solution of NaOH and anotherpure solution of HCl. This acid and base production iscarried out first by the production of H+ and OH- ionsfrom water and their collection into separate aqueoussolutions into which are fed the corresponding ions, viz.,Cl- and Na+, respectively, via the electrodialysis pro-cess. The splitting of water into H+ and OH- ions inseparate aqueous solutions is carried out in a bipolarion-exchange membrane-based reactor shown in Figure7.25 The thin space between a cation-exchange mem-brane and an anion-exchange membrane laminatedtogether and placed between a cathode and an anode isfilled with water. Any ions, e.g., Na+ and Cl-, in thiswater are quickly removed through the correspondingion-exchange membrane. This leads to deionized waterin the space between the two laminated ion-exchangemembranes. The resistance of the aqueous solutionbecomes very high, which leads to H+ and OH- ionsparticipating in the transport of electrical chargesthrough the membranes. In the water dissociationequilibrium process,

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

Figure 6. Influence of the variation of the membrane area to thesolution volume ratio on the esterification rate of propionic acidwith 1-propanol. T ) 50 C; 1 wt % catalyst; Noac/Noalc ) 1 (withpermission21).

no. 1 2 3 4S/V (cm-1) 1 2 4 8

R1COOHacid

+ R2OHalcohol

{\}Cat-H+ R1COOR2

ester+ H2O v (6)

Figure 7. Schematic diagram showing the configuration and thebasic function of a bipolar membrane for water splitting (reprintedfrom Strathmann.25 Copyright 1992, Kluwer Academic Publish-ers).

H2O T H+ + OH- (7)


brane, respectively, more water is dissociated. Thus,separation of products, H+ and OH-, through themembranes is essential to continue the water-splittingreaction.

2.1.1.7. Liquid Membranes. Enantioselective hy-drolysis of (R,S)-phenylalanine isopropyl esters by theenzyme subtilisin Carlsberg in solution in a reactor wasshown by Ricks et al.26 to selectively convert the (S)-ester to (S)-Phe-COO- while leaving the (R)-esteressentially unchanged. Part of the enzymatic solutionwas circulated on the shell side of a hollow fiber modulein which a liquid membrane of N,N-diethyldodecana-mide in dodecane is immobilized as a supported liquidmembrane in the pores of hydrophobic Celgard polypro-pylene hollow fibers. The (R)-ester is permeable throughthis liquid membrane into an acidic strip solution whereit gets protonated and cannot partition back into thefeed phase. The (S)-Phe-COO- is essentially imperme-able through the liquid membrane and is recovered fromthe feed solution. A part of the feed solution is recircu-lated through the reactor and the membrane separatorin the recycle mode. If the (R)-ester accumulates in thereaction media, it can inhibit the rate of conversion ofthe (S)-ester, which can also permeate through themembrane. The retention time of any ester fed to thereaction vessel is adjusted such that hydrolysis isessentially completed (12 min) before the solution isintroduced in the membrane module in the recirculationmode (Figure 8). This is an example of an unconvertedreactant being removed from the reaction mass as if itwere an inhibitory product. Further, a reaction (in thiscase protonation) is carried out in the permeate phase(Figure 1),

to maintain a large driving force for the permeation ofD.

2.1.2. Separation from a Gaseous Reaction Me-dium. The membrane processes employed include thoseusing liquid membranes, Knudsen diffusion, gas per-meation, molecular sieving, etc. We will briefly identifysome typical examples to illustrate product removal bymembranes in each case.

2.1.2.1. Liquid Membrane. One of the earlieststudies by Ollis et al.27 involved acetaldehyde synthesisin a multiphase catalytic liquid membrane-based reac-tor. The feed gaseous mixture of O2 and C2H4 as theinner gas phase surrounded by an aqueous catalyticliquid membrane layer was prepared by dispersing thegas as bubbles into an aqueous liquid membranereservoir containing the palladium-based catalyst PdCl2and the oxidizing agent CuCl2. The individual gasbubbles surrounded by the aqueous liquid membranelayer then spontaneously rose through a solvent layerkept above the aqueous catalyst reservoir. Any reactionproduct was extracted into the outer solvent layer. Theoverall reaction and the continuous extraction of theproduct acetaldehyde into a solvent phase (e.g., ethylacetate, n-butanol, etc.) can be indicated by

The product acetaldehyde is recovered from the solventin a separate flash drum, while the solvent phase isrecycled back to the reactor to form the outer continuoussolvent phase in the double-emulsion liquid membraneemployed in the reactor. The liquid membrane in thiscase allows not only product separation but also thesegregation of the soluble catalyst in the aqueousmembrane phase (function 2.6) in addition to being theactual site where the reaction is taking place (function2.8). The product separation here does not lead to a pureproduct for which an additional step (flash) is necessary.

2.1.2.2. Knudsen Diffusion. Since the mid 1980s, alarge number of studies have been carried out usingreactants in the gas phase and an inorganic membranethrough which one or more of the gaseous products(usually H2) is withdrawn. The membranes used oftenwere microporous/mesoporous, e.g., -alumina, Vycorglass, etc., with or without a catalyst deposited in thepores. Tubular noncatalytic membranes were also usedpacked with catalyst particles.

Sun and Khang28 have compared the performancesof a catalytic membrane reactor, an inert membranereactor, and an ordinary reactor without any membrane

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

D + F f G (8)

O2 (w) + C2H4 (w) f CH3CHO (w) fCH3CHO (s) (9)


for the dehydrogentation of cyclohexane:

They employed a porous Vycor glass membrane tubewith and without a Pt catalyst in the pores (whichexhibited Knudsen flow for H2 and N2 but had surfacediffusion and other effects for C6H6 and C6H12). For thenoncatalytic membrane, catalyst particles were packedin the tube. Their simulation results shown in Figure 9indicate that at high space-time operations the cata-lytic membrane reactor is superior to the inert mem-brane reactor which performs better than a conventionalequilibrium-limited reactor without any selective sepa-ration of products. Propane dehydrogenation studies ina packed-bed membrane reactor by Ziaka et al.29employed a 40 -Al2O3 membrane in a porous multi-layered alumina tube containing Pt/-Al2O3 catalystparticles; the membrane reactor at a residence time of10 s provided 1.8 times higher propylene yield than thecorresponding equilibrium conversion.

Numerical studies30 of catalytic dehydrogenation ofethylbenzene using microporous ceramic membranespossessing Knudsen diffusion behavior indicate only ag5% increase in styrene yield over the thermodynamiclimit by a hybrid system, i.e., a fixed-bed reactorfollowed by a membrane reactor. Numerical analysis byMohan and Govind31 of a cocurrent packed bed mem-brane reactor suggest among others, the use of amembrane with high permselectivity to achieve largevalues of conversion; the loss of reactants to the perme-ate side through a membrane with not-too-high aselectivity is a serious concern. Hsieh1 has provided adetailed review of the many investigations based onporous/mesoporous membranes.

2.1.2.3. Gas Permeation/Molecular Sieving. Aconsiderable number of membrane reactor studies withgas-phase reactions have employed denser membraneswith selectivities much higher than Knudsen diffusionmembranes; these membranes include metallic mem-branes of Pd, molecular sieve membranes, and densesilica membranes among others.1 Catalytic dehydroge-nation of ethanol to acetaldehyde in a palladium mem-brane reactor with H2 removal through the membraneincreased 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%.13In the catalytic dehydrogenation of propane using silicamembranes (having H2/N2 selectivity of 10-19, muchhigher than Knudsen diffusion-based selectivity), apropylene yield of 39.6% was obtained at 823 K com-pared to a yield of 29.6% in a conventional packed-bedreactor operated at the same flow rate.32 Catalyticisobutane dehydrogenation in a dense silica membranereactor yielded somewhat higher isobutene yield andselectivity than a conventional reactor.33

Catalytic decomposition of NH3 in a gas feed to N2and H2 in a composite Pd-ceramic membrane reactorachieved an NH3 conversion of over 94% at 873 K and1618 kPa compared to 53% in a conventional reactor.34A water-gas shift reaction carried out at 673 K in apalladium membrane reactor enclosing an iron-chro-mium oxide catalyst achieved higher CO conversionthan the equilibrium level due to selective removal ofH2.35 A metal membrane-based catalytic membranereactor for facilitating the water-gas shift reaction ata temperature of 500 C was run successfully andcontinuously for 6 months for the economical productionof H2 in a 0.4 ft2 plate-and-frame module.36 Recovery ofH2 isotopes (tritium, deuterium) is being studied in Pd/Ag-based membrane reactors from impurities presentin fusion reactor exhaust streams.37 Dixon et al.38 havemodeled the performance of membrane reactors havingmixed conducting O2 permeable ceramic membranes forthe very high temperature reactions:

Removal of O2 through the membrane resulted indramatic improvements in conversion over the non-membrane tubular reactors.

Armor8 has provided a critical review of the needs inmany dehydrogenation membrane reactor applications.The problem areas are: defects in metallic membranesat higher temperatures, phase changes in metallicmembranes causing catastrophic failure, leakage be-tween the membrane and device, low surface area perunit volume of commonly used membranes, severemass-transfer limitations, very low feed flow ratesresulting in high residence times, carbon deposition onmembrane pores and surfaces, no method (currentlyavailable) for repairing defective membranes in situ, andthe low turnover number of commercially availabledehydrogenation catalysts. The approach adopted byRezac et al.39 is of great interest in this respect. Theypropose existing plug flow reactors and heat-exchangeequipment to be used in series with an optimized higher-temperature stable polyimide-ceramic composite H2removal membrane module. Thus, C4H10 dehydrogena-tion is carried out at 480 C, the product mixture iscooled to 180 C, and H2 selectively permeated throughthe polyimide-ceramic composite membrane (H2/C4H10selectivity > 75). Unreacted C4H10 is passed on to thesecond-stage reactor after heat exchange and so on. Thisresulted in a substantial increase in conversion and ahigh selectivity for the production of n-C4H8; membranepoisoning is also substantially avoided.

2.2. Separation of a Reactant from a MixedStream for Introduction into the Reactor. Figure1 identified a particular function of the membrane aspurify reactant A from species F before addition to the

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

C6H12 {\}Pt

C6H6 + 3H2 (10)

CO2 T CO +1/2O2 (11a)

NO2 T NO +1/2O2 (11b)


reactor on the left-hand side. The effect of this separa-tion on the reaction system is generally quite differentfrom that of a reaction product from the reactionmixture. The purification may lead to pure A beingintroduced into the reactor; a direct effect of this isprevention of dilution of the reaction mixture. It can alsolead to rejection of a class of compounds by the mem-brane while reactant species (one or a class) may beintroduced by the membrane preferentially into thereactor from the feed stream; the species rejected caninhibit the reaction. An additional possibility involvessimultaneous operation of two different reactions on twosides of the membrane wherein the products of onereaction feeds the other and vice versa; the latter couldbe in a coupled mode as well (to be explained later).

2.2.1. Pure O2 from Air. Mixed conducting denseceramic membranes (see sections 2.3 and 2.9 as well)allow O2 transport from air to a lower O2 partialpressure side without allowing N2 to be transportedthrough the membrane. When such a membrane is usedwith air on one side and CH4 on the other side, partialoxidation of CH4 to syngas can be carried out at a hightemperature (800 C) without contaminating the reac-tion gas mixture with N2. Further, the need for an O2plant is eliminated,40 improving the economics consider-ably. When a similar dense ceramic membrane in asolid-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 membrane41as an anion O2- into the reaction zone; in this case airis fed on the outside of the tubular reactor to produceoxygen throughout the reactor length (function 2.3). Hadair been used directly, the auxiliary byproduct of H2 +CO, a high-quality fuel stream, would have been dilutedwith N2.

2.2.2. Purify Organic Pollutants from Wastewa-ter for Biodegradation. Point-source industrial waste-waters containing a variety of priority pollutants wereoften considered recalcitrant from a biodegradationperspective. These industrial wastewaters are fre-quently released from organic synthesis and containhigh salts, extreme pHs, and residual catalysts, all ofwhich are either singly or jointly very harmful to thegrowth of microbial cultures used for biodegradation.Brookes and Livingston42 have employed a siliconecapillary membrane-based device to extract organicpriority pollutants from these demanding wastewaters.The biological reaction medium is circulated between abioreactor and one side of this membrane device. Onthe other side of the silicone capillary membranes flowsthe wastewater. Priority organic pollutants, e.g., aniline,4-chloroaniline, 3,4-dichloroaniline, etc., from the waste-

water (having high pH 9-11) are partitioned throughthe silicone membrane into the biological reactionmedium. Reactors operated over 3000 h show very highreductions of the pollutants without any form of pre-treatment, pH adjustment, or dilution of the wastewa-ter. The membrane essentially isolates the bioreactorenvironment from the vagaries of the industrial waste-water properties as the pollutants get extracted out intothe bioreaction medium for degradation by the ap-propriate microorganisms. Successful pilot plant studieshave been conducted using this technique.

2.2.3. Purify Organic Compound from a Synthe-sis Medium. To prepare a concentrated aqueous solu-tion of diltiazem malate by reacting diltiazem (sparinglysoluble in water) with malic acid, a liquid membranewas utilized to recover diltiazem from an aqueousreaction mixture containing diltiazem, NaCl, NaHCO3,etc. A contained liquid membrane of decanol wasutilized by Basu and Sirkar43 to extract diltiazem intothe reaction zone where it reacted with L-malic acid inwater to form diltiazem malate (in solution). When avery high concentration of L-malic acid is maintained,the aqueous concentration of diltiazem malate achievedcould be 3 orders of magnitude higher than the very lowconcentration of diltiazem in the feed solution. Themembrane not only facilitated production of a highlyconcentrated and purified solution of diltiazem malatebut also avoided two steps of solvent extraction and backextraction which was very problematic due to the ordersof magnitude difference in the two aqueous phase flowrates, namely, the very high flow rate of a very dilutefeed solution and the very concentrated reactor effluenthaving a low flow rate.

2.2.4. Coupling of Two Chemical Reactions. Inenzymatic methods for the production of pure enanti-omers from achiral precursors using, say, dehydroge-nase enzymes, specialized and costly reagents, e.g.,nicotinamide cofactors (NAD+ or NADH), are required.A process for efficient and continuous regeneration ofthese nicotinamide cofactors by a chemical reactionisolated from the main reaction by a membrane parti-tion is illustrated in Figure 10.44 The membrane em-ployed is a gas membrane (see 2.1.1.5) utilizing amicroporous hydrophobic polypropylene membrane whosepores (0.03 m) are gas-filled. In the main reactioncompartment on the left side, D-lactic acid is producedin an aqueous solution by catalyzing the reduction ofpyruvic acid using D-lactic acid dehydrogenase. TheNAD+ produced by this reaction reacts with ethanol toregenerate NADH required for the main reaction. In anadjacent compartment, sodium borohydride reductionof acetaldehyde leads to ethanol which, being volatile,

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


enters the main reaction chamber whereas acetaldehydeproduced in the regeneration of NADH in the mainreaction chamber enters the adjacent chamber (throughthe membrane). Thus, the membrane allows separationof the volatile species from the rest of the nonvolatilereaction mixtures in both compartments and feeds eachvolatile species at a controlled rate into the adjacentreaction chamber (function 2.3).

A weaker coupling between two reactions on two sidesof a membrane (see Figure 1 for A + B T C + D and D+ F f G where the membrane controls the addition ofreactant D to the permeate side reaction) was explorednumerically by Itoh and Govind.45 On the basis of earlierproposals by Gryaznov and Itoh, they modeled thecatalytic dehydrogenation of 1-butene to butadiene inthe main packed bed reactor. The permeate side wasbeing continuously fed with O2 in air which oxidizes thepermeated H2 to produce water and liberate heat by anexothermic reaction. This reaction is surface-catalyzedby the palladium membrane used to remove hydrogenfrom the main reactor. The heat so generated is con-ducted through the palladium membrane to the mainendothermic dehydrogenation reaction. The membranehas many functions: separate product of main reaction,H2 (function 2.1); provide purified reactant (H2) tosecond reaction (function 2.2); add reactant H2 to secondreaction in a controlled way (function 2.3); supply heatto main endothermic reaction (function 2.10) and therebycontrol both reactions; and catalyze the second reaction(function 2.7).

2.3. Controlled Addition of a Reactant or TwoReactants. Control of the reaction pathway is a majorconcern in reaction engineering. Partial oxidation reac-tions of hydrocarbons are especially relevant here. Inparticular cases, possibilities of thermal runaway andcatalyst poisoning do exist. In biodegradation processesfor toxic organics, microorganism growth may be af-fected by inhibition from the toxic organics unless theirconcentrations are controlled. In an aerobic wastewatertreatment process, high O2 utilization with minimumwaste to the atmosphere requires controlled but efficientintroduction of O2 to the system. In processes usingreactants having limited half-lives, e.g., ozonation ofwastewater or for water purification, efficient andlocalized introduction of O3 at a controlled rate can leadto higher O3 utilization. Using a membrane to introducea reactant or two in a controlled fashion in the reactorcan facilitate achievement of the desired reaction condi-tions. A number of examples provided below will il-lustrate the role of a membrane in such processes.

2.3.1. Gas-Phase Reactions. Gas-phase reactionswhere reactants are introduced into the reaction zoneby membranes at a controlled rate(s) (see Figure 1,purify reactant A from species F before addition) caninvolve three types of membranes: (1) inert porousmembranes which provide no selectivity; (2) microporousmembranes with some selectivity; (3) nonporous/densemembranes having much higher selectivity. The firsttwo types of membranes may employ catalysts in thepores; the nonporous membrane can be inherentlycatalytic. All three types of membranes have beenstudied with O2 as one of the reactants introduced in acontrolled manner for partial oxidation or oxidativedehydrogenation reactions.

Tonkovich et al.46,47 employed 50 -Al2O3 (effectivelayer) or 200 R-Al2O3 membrane tubes packed with amagnesium oxide catalyst doped with samarium oxide

to study the oxidative dehydrogenation of C2H6 to C2H4at 600 C. Air was introduced via permeation through-out the length of the reactor from the outside of the tube.Controlling the ratio of C2H6 to O2 was found to becrucial to selectivity (with respect to CO2, CO, etc.) andconversion. At low-to-moderate C2H6 to O2 feed ratios(98%, CO selectivitywas 90%, and H2 produced was twice that of CO.

Zaspalis et al.49,50 have experimentally studied thedehydrogenation of methanol in a microporous -Al2O3membrane with methanol fed from one side and O2 fromthe other (Figure 1, reactants A and B introduced intothe pore from opposite sides). This arrangement avoideddirect contact between the two streams (i.e., an alcoholand an oxidant), thereby minimizing undesirable gas-phase reactions. Further, the inner surface areas of themembranes were used. Zaspalis et al.50 used silverparticles deposited on the -Al2O3 membrane poresurfaces; CH3OH was fed from one side and O2 fromthe other so that activation of the Ag catalyst occurredsimultaneously with the methanol conversion to form-aldehyde. This opposed flow mode of feeding two reac-tants must be carried out in a controlled manner so asnot to decrease the selectivity for the desired product.

2.3.2. Gas-Liquid Reactions. Catalytic reactionsbetween gas and liquid phases pumped concurrentlydown a bed of catalyst particles in conventional reactorsencounter mass-transport limitations due to intrapar-ticle mass-transfer resistance, liquid film resistance,liquid maldistribution, channeling, etc., apart from hotspots and undesirable side reactions. To overcome theseproblems, Cini and Harold51 have studied hydrogenationof R-methylstyrene (diluted in mesitylene) to cumenein a porous (6 m) tubular -Al2O3 membrane impreg-nated with a Pd catalyst on the pore surface area. H2was supplied on one side of the porous membrane andthe liquid reactant flowed on the other side of the


membrane (Figure 1). The results demonstrated anefficient supply of the volatile reactant, H2, which wasalso the limiting reactant when compared with thecatalyst pellets conventionally used in a trickle-bedreactor. There were no operational difficulties encoun-tered in the membrane-based operation; further, therate increased by up to a factor of 20. From a study ofhydrogenation of nitrobenzene to produce aniline in atubular -Al2O3 membrane (50 pore size) having a Ptcatalyst deposited on the pore surfaces, Torres et al.52have also concluded that catalytic membrane reactorsare efficient for three-phase reactions as a result of theeasy access of the gas to the catalytically active phase.

Biodegradation processes to destroy organic andinorganic contaminants in air or water employ organ-isms in a biofilm attached to a support. The efficientsupply of nutrients, e.g., O2 and pollutants, to thebiofilm is demanding without incurring excessive powerconsumption and with minimum loss of O2 to theatmosphere. Hydrophobic porous hollow fibers mem-branes are especially useful. For example, Brindle etal.53 have immobilized a biofilm on the fiber outerdiameter. The shell side is fed with, say, the wastewatercontaining NH4+ which is oxidized by the nitrifyingbiofilm into nitrite and then nitrate; the tube side is fedwith pure O2. Exceptionally high O2 utilization efficien-cies were achieved via efficient interfacial oxygentransfer to the biocatalyst in the biofilm, even at lowO2 supply rates. Parvatiyar et al.54 have demonstratedefficient biodegradation of toluene in a hollow fibermembrane-based biofilter: air containing toluene wasfed on the lumen side of porous polysulfone hollow fiberson the outer surface of which the biofilm was im-mobilized. Nutrients were supplied via an aqueousstream on the shell side. High conversion of toluene wasachieved because of efficient contact of the biomasscatalyst with O2 and toluene through the membranepores at controlled rates of supply of the reactants inthe gas phase.

Noncatalytic gas-liquid reactions are employed usingO3 to destroy pollutants and bacteria in water purifica-tion and wastewater treatment processes. Since O3 hasa very low solubility in water, kla (volumetric mass-transfer coefficient) controls the mass-transfer rate ofO3 and thereby the reaction rate. Further, O3 has a verylimited half-life in a moist gaseous phase. Membrane-based nondispersive ozonation studied by Shanbhag etal.55,56 provides a much higher value of kla (g5 times)compared to conventional dispersive bubble-based ozo-nation, eliminating the possibility of gas-phase degrada-tion of the unstable O3 in bubbles and efficientlybringing O3 in contact with the aqueous pollutants alongthe length of reactor.

2.3.3. Liquid-Phase Reactions. Lee et al.57 carriedout simultaneous biodegradation of the pollutants tolu-ene and p-xylene in a completely mixed and convention-ally aerated bioreactor using the microorganismPseudomonas putida. Under aerobic conditions, themicroorganisms utilize toluene and xylene as carbonsources. The pollutant species (toluene and p-xylene)were introduced into the reactor in a controlled mannerthrough silicone capillary membranes. The removalefficiency of these species increased at the beginningwith an increase in the transfer rate of the pollutantmixture (increased by, for example, the impeller speedand not by the aeration rate or the circulation rate ofthe solvents in the capillary); however, beyond a certain

rate, the removal efficiency decreased since the limitingsubstrate shifted from carbon to O2. At this time, thesolvent loss in the exit gas also increased. For givenimpeller speeds, the membrane can be designed tocontrol the rate of introduction of organic pollutants tothe biomass-containing medium.

2.4. Nondispersive Phase Contacting. In manyreactions, aqueous and organic phases are frequentlyused together. One phase is dispersed as drops in theother phase followed by coalescence after the process isover. This can be problematic if there are tendenciesfor emulsification. Microporous/porous membranes canbe particularly useful here since the two immisciblephases can be kept on two sides of the membrane withtheir phase interfaces immobilized at the membranepore mouths. Solvent extraction is conventionally usedto isolate and concentrate dilute organic productsobtained from whole cell-based fermentation processes.If the fermentation suffers from product inhibition, thenextraction of the product(s) during fermentation in-creases the fermentor productivity. However, solventdispersion can lead to a phase-level toxicity58 problemfor the whole cells. Nondispersive phase contactingusing microporous/porous membranes can resolve thisproblem.

In nondispersive phase contacting employing mi-croporous/porous hydrophobic membranes, the organicphase wets the membrane pores; the aqueous phase ismaintained outside the pores at a pressure equal to orhigher than that of the organic phase. As long as thisexcess pressure does not exceed a breakthrough pres-sure, the aqueous-organic interface remains immobi-lized on the aqueous side of the membrane with eachphase flowing on a particular side of the membrane.59,60For hydrophilic microporous/porous membranes, theaqueous phase is inside the pores; the organic phase iskept outside the pores at a pressure higher than thatof the aqueous phase (Figure 11).

This technique has been employed in four types ofreaction systems: fermentor-extractor; enzymatic fatsplitting; phase transfer catalysis; extractive membranebioreactor for enzymatic resolution of isomers. Theadvantages of these techniques are: no dispersion andtherefore no need for coalescence; no need for densitydifference between the two phases; known interfacialarea; modular systems leading to easy scale-up; mass-transfer rates independent of interfacial tension; noflooding and no loading, allowing widely different phaseflow rate ratios to be used. Further, the membrane mayprovide a very large interfacial area per unit reactorvolume.

Nondispersive phase contacting advantages are alsopresent in gas-liquid systems already discussed under

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


the category of gas-liquid reactions (section 2.3.2) forthe controlled addition of one or two reactants. Toachieve nondispersive operation, one has to maintainthe proper pressure in each phase. For example, forhydrophobic microporous membranes, the aqueous phaseis usually outside the pores (which are gas-filled) andmaintained at a pressure23 higher than that of the gas.For hydrophilic membranes, the aqueous or organicphase is usually inside the pores and the outside gasphase is at a higher pressure.

2.4.1. Fermentor-Extractor. In fermentation pro-cesses for producing ethanol, acetone-butanol-ethanol(ABE), etc., microporous hydrophobic hollow fiber mem-branes have been introduced into tubular reactors inwhich whole cells are immobilized on appropriate sup-ports on the shell side; through the bore of the hydro-phobic hollow fibers, O2 is supplied in ethanol fermen-tation and N2 is supplied in ABE fermentation.15,61,62The gases supplied help the cells grow, maintain theneeded anaerobic condition, and remove gases such asCO2 and H2 produced by fermentation. When a sub-stantial concentration of the desired product has beenachieved in the shell-side broth, an organic solvent,passed through the fiber lumen at a pressure lower thanthat in the broth, extracts the products (e.g., ethanol,ABE, etc.) nondispersively (Figure 12). This reducesproduct inhibition and can lead to considerably in-creased volumetric fermentor productivity.16 Other in-hibitory side products, e.g., acetic acid, etc., are simul-taneously extracted out. Although the fermentation-extraction process has not been commercialized yet, themembrane-based solvent extraction technique is alreadybeing commercially employed in at least two largeinstallations (one in Europe and the other in Japan).

2.4.2. Enzymatic Fat Splitting. In enzymatic split-ting of olive oil using hydrophobic microporous mem-branes and the enzyme lipase immobilized at theaqueous-organic interface63 (Figure 13), olive oil flowson one side of the membrane and wets the pores; anaqueous buffer solution flows on the other side at ahigher pressure which immobilizes the aqueous-organicinterface. The enzyme Candida cylindracea lipase,spontaneously adsorbed at the aqueous-organic inter-face of the microporous membrane, catalyzes the fol-lowing hydrolysis reaction:

Glycerine is removed in the aqueous phase; fatty acids

are removed in the oil phase. The membrane allowsaqueous-organic phase immobilization, enzyme im-mobilization, and localized product separation into theappropriate phase. Molinari et al.64 have shown thatthis reactor was better than a conventional emulsion-based reactor: the specific enzymatic activity washigher, the specific rate was more constant with time,and the two products were separated after the reaction.

2.4.3. Phase Transfer Catalysis. Stanley andQuinn65 have studied the reaction of bromooctane in thesolvent chlorobenzene with aqueous iodide to form thedisplacement products, iodooctane in the solvent chlo-robenzene and aqueous bromide. The phase transfercatalyst (PTC), tetrabutylammonium (TBA) ion, wasintroduced as the bromide salt in the organic feed. Theaqueous feed was passed on one side of the microporoushydrophobic flat membrane of poly(tetrafluoroethylene)(PTFE); the organic phase wetting the membrane poreswas passed on the opposite side at a pressure lower thanthat of the aqueous phase. Conventional emulsification/coalescence problems were avoided in this PTC-facili-tated reaction. Further, since the membrane area(therefore the aqueous-organic interfacial area) isknown, operation of the reactor can be carried out withgreater flexibility.

2.4.4. Extractive Membrane Bioreactor for En-zymatic Resolution. In a multiphase/extractive en-zyme membrane reactor66 used for the industrial pro-duction of diltiazem chiral intermediate, an asymmetricwater-filled hydrophilic hollow fiber membrane of acopolymer of acrylonitrile having a 30000 MWCO (mo-lecular weight cutoff) skin on the fiber internal diameteris employed. The enzyme is immobilized in the poroussubstructure from the shell side via ultrafiltration(function 2.6). The organic phase in which the enzymehas limited solubility flows on the shell side at a higherpressure, immobilizing the aqueous-organic interfaceat the pore mouth, thus containing the enzyme in the

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

triglycerides + 3H2O {\}enzyme

glycerine + 3 fatty acids (12)

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


water-filled substructure bounded by the membraneskin impermeable to the enzymes. If the enzyme activityis reduced substantially, the enzyme can be flushed outeasily and fresh enzyme loaded in the absence of theorganic phase. The organic flow is then restarted.Stereoselective enzymatic hydrolysis of the undesiredisomer from a racemic mixture of glycidic esters and itsremoval in an aqueous buffer is carried out continu-ously. Here, the membrane provides reversible im-mobilization of the hydrolytic enzymes in the pores ofthe hydrophilic asymmetric hollow fiber, immobilizationof the aqueous-organic interface on the fiber outsidediameter, reaction product extraction, and a very largeaqueous-organic interfacial area.

2.5. Segregation of the Catalyst (and Cofactor)in a Reactor. A membrane incorporated in a catalyticreaction system can perform, among others, a numberof functions related to the catalyst. If the catalyst ismobile in the reaction fluid, the membrane can preventits escape from the system. If the catalyst is to beimmobilized with easy access to the reactants andconvenient exit for the products, a porous/microporousmembrane structure may have the catalyst immobilizedon/within its structure (function 2.6). Alternately, themembrane material itself may act as the catalyst(function 2.7). We focus here on cases where the catalystis mobile in the reaction fluid. Examples of suchcatalysts are enzymes (and cofactors where applicable),whole cells, and homogeneous catalysts (in organicsynthesis). The segregation of particulate heterogeneouscatalysts by filters is not under consideration.

The production of organic compounds by synthesis inorganic solvents or an aqueous-organic biphasic reac-tion medium is very common. Many use homogeneouscatalysts whose molecular weights are considerable(e.g., in the range of, say, 300-800). Very few mem-branes are capable of retaining such species whileallowing the organic solvent to pass at appreciable rates.Nanofiltration membranes, just becoming available,have the necessary solvent resistance and rejectionbehavior in a few cases. Whu et al.12 have identifiedsome of these capabilities including retaining the ho-mogeneous catalysts while passing the organic solvents.More extensive use of such nanofiltration membraneswill allow their use in organic synthesis for, amongothers, retaining the homogeneous catalysts.

The use of membranes to segregate enzymes used ascatalysts for biosynthesis or biocatalysis is practiced ina wide scale. Primarily, ultrafiltration membranes hav-ing molecular weight cutoffs in the range of 5000-100000 are used to retain the enzymes (molecularweight range 10000-100000 Da) in the CSTR reactoras smaller products are removed with water throughthe membrane (Figure 2). Originally suggested byMichaels67 and implemented as early as 1970,19 thistechnique is used commercially for the production ofamino acids. Jandel et al.68 have illustrated continuousproduction of L-alanine from fumaric acid in a two-stagemembrane reactor using the enzyme aspartase in thefirst stage,

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

Some enzymatic synthesis reactions are carried outin hollow fiber ultrafiltration membrane devices in whatis called the perfusion reactor mode. The enzymes orthe whole cells are packed on the shell side with theshell side ports being closed off; substrate-containingfeed is pumped through the tube side. The substratediffuses through the membrane pores to the shell sideand reacts with the enzymes/whole cells, and thesmaller molecular weight products diffuse back to thetube side and are carried away. The enzymes or wholecells may also be kept in the tube side with both endsclosed off: the substrate-containing solution will thenflow into and out of the shell side. This latter mode isnot commonly used. A review of membrane bioreactorswherein the membranes segregate enzymes/whole cellsis available in Cheryan and Mehaia.6

Some enzyme-based reactions, however, require low-molecular-weight coenzymes or cofactors in addition tothe main enzyme to carry out the overall enzymaticreaction. Typical examples of such coenzymes are nico-tinamide adenine dinucleotide (NAD+), the reducedform of NAD+, viz., NADH, NADPH (the reduced formof NAD phosphate), etc. Figure 10 shows one suchreaction where the D-lactate dehydrogenase enzymeneeds 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 fromNAD+, one has to continuously supply NADH fromexternal sources. NADH is costly ($1000/mol). A numberof 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 aregenerating enzyme, in this case an alcohol dehydro-genase, to regenerate the NADH from NAD+. However,this requires a cosubstrate (e.g., ethanol) and yields acoproduct (e.g., acetaldehyde). To design a continuousprocess to retain the enzymes as well as the cofactorsin the system by a membrane as the substrates comeinto the reaction chamber and the product and coprod-uct leave the reaction chamber, one needs a very specificmembrane. Figure 10 provides a special example forvolatile cosubstrate and coproduct. We consider here aseparate situation, but one more commonly encountered;viz., enzymes have molecular weights > 40000 Da andthe substrates are 5-12 carbon sugars; note that thecofactor molecular weights are around 700 Da. Thus, ifwe use too tight of a membrane, the substrate introduc-tion into the reaction zone will encounter considerableresistance, although NAD+ and NADH will be retainedby the membrane.

To solve the problem, Nidetzky et al.69,70 have selecteda charged nanofiltration (NF) membrane having -vecharges on the surface (as shown in Figure 14) thatpreferably retains the cofactors almost completely with-out binding the enzymes or cofactors. However, the NFmembrane had a size exclusion (1 kD) slightly higherthan those molecules having the molecular mass of thecofactor; this ensured higher substrate fluxes. Thecharge of the NF membrane (-ve) in this case allows forcofactor retention since both NAD and NADH carrynegative net charge at pH values greater than 3. Thisfumaric acid + NH3 {\}

aspartaseL-aspartic acid (13a)

L-aspartic acid98L-aspartate--decarboxylase

L-alanine + CO2 (13b)


process has been demonstrated for the conversion offructose to mannitol by mannitol dehydrogenase69 (orxylose to xylitol by xylose reductase70) as the cosubstrateglucose was converted by the regenerating enzyme,glucose dehydrogenase, to glucono--lactone (ultimatelyconverted to gluconic acid in a separate device). Pilotscale results show a TTN (total turnover number) ofcoenzyme to be in the range of 75000-100000.

This approach to cofactor retention or segregation bythe membrane appears to be considerably simpler thanthe earlier strategy of binding the cofactors to a mac-romolecule, e.g., poly(ethylene glycol), dextran, etc.,which allowed a regular UF membrane to retain thecofactor within the reactor.71 (A similar strategy72 hasbeen pursued for the enantioselective addition of diethylzinc to benzaldehyde using a homogeneously solublecatalyst retained by a solvent-stable polyaramide UFmembrane within the reaction vessel; the R,R-diphenyl-1-prolinol used as the chiral ligand was coupled to acopolymer made from 2-hydroxyethyl methacrylate andoctadecyl methacrylate (96000 MW) and the solventused was hexane.) However, the charged NF membrane-based retention of NAD+ is at this time somewhat lower,around 0.65-0.85. Optimization of the NF membranevis-a`-vis the cofactor retention is still needed unless theproduct happens to have a very high value so that alower TTN (1000-5000) may be tolerated.

2.6. Immobilization of a Catalyst in (or on) aMembrane. Four basic types of catalysts are relevant:(a) enzymes and (b) whole cells for biocatalysis; (c)oxides and (d) metals for nonbiological synthesis. Bio-catalysts will be considered first since their immobiliza-tion 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 fibermatrix; (2) enzyme immobilized on the membranesurface by gel polarization; (3) enzyme adsorbed on themembrane surface; (4) enzyme immobilized in themembrane pore by covalent bonding; (5) enzyme im-mobilized in the membrane during membrane formationby the phase inversion process of membrane making.

Of these, technique (1) is used commercially. Anenzyme solution is ultrafiltered from the spongy side(porous substructure) of an asymmetric ultrafiltrationmembrane to the skin side; this introduces the enzymesin the spongy matrix. Feed solution is also supplied from

the same side; product solution goes out through themembrane skin (see Jones et al.73 for a brief introduc-tion to the literature of hollow fiber enzymatic reactorsemploying this method of immobilization). An exampleof this type of immobilization in the context of aqueous-organic enzymatic processing has been illustrated insection 2.4.4. Technique (2) whereby the enzymes areimmobilized on the pressurized ultrafiltration mem-brane surface where an adherent gel layer of concen-trated enzymes are formed was suggested by Drioli andScardi.74

Lipase enzymes (e.g., C. cylindracea) are spontane-ously adsorbed (technique (3)) on a hydrophobic mi-croporous polypropylene membrane surface and areutilized in aqueous-organic enzymatic processing (sec-tion 2.4.2). Enzymes can be covalently bound to hydro-philic membranes by the cyanogen bromide procedure.This technique has been known for a long time and hasbeen used, for example, for binding chymotrypsin to aMillipore filter membrane by Matson and Quinn75 intheir membrane reactor studies. Site-directed mutagen-esis has been introduced into enzymes so that the activesite of the enzyme is away from the surfaces of mem-branes such as hydrophobic poly(ether)sulfone; im-mobilized mutant enzymes on such membranes havemuch higher activity than randomly immobilized en-zymes for catalytic conversions.76 Finally, it is possibleto incorporate particular enzymes into the organiccasting solution for polymeric membranes and thenprepare a membrane by the phase inversion processwherein the final membrane has the enzyme dispersedthroughout the membrane structure.

Chopped microporous hydrophobic or hydrophilic hol-low fibers have been used to grow whole cells in the fiberlumen and the fiber outside surface.77 Such choppedhollow fibers with immobilized cells were later utilizedin a tubular fermentor to carry out yeast-based ethanolfermentation for an extended period. The chopped fibersof appropriate lengths provide adequate nutrients aswell as O2 to the immobilized cell mass; such a biore-actor could also have continuous lengths of hydrophobicmicroporous hollow fibers for gas supply and removalas well as for in situ product extraction by dispersion-free solvent extraction.16

Nonbiological synthesis of most products involvechemical and thermal conditions too harsh for almostall of the current polymeric membranes available;therefore, the membranes investigated are primarilyinorganic in nature, either ceramic or metallic. Thecatalysts are predominantly oxides and/or metals. Ofthe numerous oxides and metals used as catalysts,membrane reactor studies, where the membrane hadimmobilized catalysts on/in it, have primarily used thoseemployed for dehydrogenation reactions, viz., platinum,palladium, etc., on metal oxides such as alumina andsilica.1 Examples are a Pt-impregnated Vycor glasstube28 for cyclohexane dehydrogenation, Ag-modified 40 -Al2O3 membrane obtained via wet impregnation ofAgNO3, and then calcination under a reduced atmo-sphere for dehydrogenation of methanol.50 For the Clausreaction,

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

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

2H2S + SO2 T3/8S8 + 2H2O (14)


studies, palladium was deposited on microporous -Al2O3deposited on 6 m R-Al2O3 by soaking the tube in anaqueous solution of ammonium tetrachloropalladium,drying it, and calcining it.51 For nitrobenzene hydroge-nation, the platinum catalyst was deposited in a 50 -Al2O3 membrane tube by ion exchange with H2PtCl6.52Although the effect of catalyst distribution in a mem-brane-enclosed catalytic reactor has been studied,79 nostudies have been made with a variation of catalystdistribution in the catalytic membrane itself.

Polymeric membrane-based studies involving thedistribution of nanosized clusters or micron-sized par-ticles in porous or nonporous polymeric membranes hasbeen carried out at GKSS (Geesthacht, Germany) byFritsch;80 10% Pd on charcoal powder particles (40 m)were used as catalysts and dispersed in the castingsolution for the preparation of porous polyetherimide(PEI) membranes. Transfer hydrogenation of N-CBZ-L-phenylalanine to L-phenylalanine was successfullycarried out at low temperatures. Hydrogenation ofpropene to propane was carried out by using a nonpo-rous membrane containing nanoclusters of Pd in a densepoly(amide-imide) membrane. The mechanism of sucha membrane function has been described in section 2.8.

Bellobono et al.81 have described the performance ofmicrofiltration membranes formed with 30% TiO2 aswell as some 6% of particular photocatalysts. Themonomer, prepolymer blend with the semiconductorTiO2 and the photcatalysts were photografted onto aperforated polyester support; the pore size of photosyn-thesized membranes were 2.5-4 m. The membraneswere placed coaxially with a UV lamp in a stainless steelcasing having a mirror-like surface for reflection. Thesolutions processed permeated through the membrane;before entering the reactor, the wastewater solutionswere saturated with O2 or O3.

2.7. Membrane Is the Catalyst. Most catalyticmembrane reactors for higher temperature operationsemploy ceramic membranes in the pores/micropores ofwhich catalysts were deposited. The base membranes,e.g., silica and alumina, are generally not catalysts forthe reactions studied. There are, however, a number ofmembranes which are inherently catalytic for particularreactions; no catalyst needs to be deposited on or in themembrane. Particular examples are cation-exchangemembranes, Nafion membranes, palladium membranes,and zeolite membranes.

Consider a cation-exchange membrane and an esteri-fication reaction (15) in which a carboxylic acid, R1-COOH (e.g., oleic acid), reacts with an alcohol, R2OH(e.g., methanol), under the influence of an acidic catalystproviding a proton:

In the schematic shown in Figure 15, CH3OH and H2-SO4 are on one side of the membrane and the reactants,viz., CH3OH and oleic acid, are on the other side of themembrane.82 In this case protons are the counterionsin the cation-exchange membrane introduced from theleft-hand side of the membrane; naturally, a layer ofprotons appears on the other membrane surface exposedto the reaction mixture of the alcohol and the acid. Theseprotons catalyze the esterification reaction. No separatecatalyst, e.g., H2SO4, p-toluenesulfonic acid, etc., isrequired, eliminating the need for catalyst separationafter reaction if homogeneous catalysts are employed

(anions, e.g., HSO-4, do not get transported through thecation-exchange membrane). The cation-exchange mem-brane also separates a reaction product, viz., water, fromthe reaction mixture. Water is transported from thereaction mixture to the catalyzing mixture, thus achiev-ing equilibrium shift to the right in reaction 15 (section2.1). If solid ion-exchange beads were used as thecatalyst, such a function would not have been possiblein a continuous process.

The acid form of a Nafion membrane is also capableof carrying out both functions.83 The acid form of themembrane prepared by boiling tubular Nafion mem-branes in concentrated HNO3 and then in water wasfound to catalyze the esterification of methanol orn-butanol to methyl acetate or butyl acetate, respec-tively, with acetic acid. These membranes were alsofound to be effective in separating water from n-butanol(water/alcohol selectivity 8.0; water/acetic acid selec-tivity 9.0). The Cs+ form of the Nafion membrane wasfound to have a much higher selectivity for both water/alcohol (71) and water/acetic acid (149). Simulta-neous catalysis and separation by a catalytically activemembrane can potentially increase the membrane fluxfor the products produced and removed since the reac-tions occur within the membrane compared to a passivemembrane over which the solution is passed after thereaction is completed.

Palladium is known to be a catalyst for hydrogenationand dehydrogenation reactions. A palladium membraneis also infinitely selective for H2. Thus, for dehydroge-nation reactions, a palladium membrane simultaneouslyacts as a catalyst and allows the product H2 to beremoved through the membrane and obtained in thepure form (see section 2.1 for references). Zeolites arewell-known as catalysts. Thin zeolite membranes arebeing developed for the selective transmission of speciespreferentially adsorbed or smaller than the pore size.A number of reactions have been studied.2

2.8. Membrane Is the Reactor. In a membranereactor, catalysts are used frequently. The membranemay physically segregate the catalyst in the reactor(function 2.5) or have the catalyst immobilized in theporous/microporous structure or on the membranesurface (function 2.6). The membrane having the cata-lyst immobilized in/on it functions almost in the sameway as a catalyst particle in a reactor does exceptseparation of the product(s) (function 2.1) takes place,in addition, through the membrane to the permeateside. All such configurations involve the bulk flow of thereaction mixture along the reactor length while diffusionof the reactants/products takes place generally in aperpendicular direction to/from the porous/microporouscatalyst.

R1COOH + R2OH {\}Cat-H+

R1COOR2 + H2O (15)

Figure 15. Concentration profiles around a cation-exchangemembrane. Example at the beginning of run no. 11. Catalyzingsolution, 0.9 g of H2SO4 in 180 mL of methanol; reacting mixture,25 g of oleic acid and 75 g of methanol (reprinted from Chemsed-dine and Audinos.82 Copyright 1996, with permission from ElsevierScience).


When the bulk flow of a reaction mixture takes placethrough the membrane from one membrane surface tothe other, the membrane is the reactor. Generally, themembrane in such a case will be porous/microporous toreduce the pressure drop for practical flow rates. Thelength of the pores/transport corridors is the reactorlength; the reactions may take place on the surfaces ofsuch macropores or there may be radial diffusion ofreactants into the micropores and products out of themicropores into the main porous corridors where con-vective motion occurs. The convective motion of thereaction mixture through the membrane created by anapplied pressure drop across the membrane thicknessmay involve Knudsen diffusion, Poiseuille flow, or atransitional regime for gaseous reaction mixtures.

Such a reactor can have an exceptionally high valueof reactor L/d for even thin membranes since d valuescan be very small (from 0.2 m to 4 nm). As a result,extremely high conversions are possible, as shown byrecent theoretical studies.84 Such a reactor can beidentified as a pore flow through reactor (PFTR) sinceeach macropore that traverses the membrane thicknessis a reactor tube. Further, the mass-transfer resistanceencountered by the reactants to reach the catalytic sitesare significantly reduced because of the bulk flowthrough the membrane pores.85 Pina et al.85 haveexperimentally demonstrated extremely efficient re-moval of toluene and methyl ethyl ketone from air bylow-temperature oxidation (100-320 C) using a Pt/-Al2O3 catalytic membrane operating under the Knudsendiffusion regime, supporting the earlier work by Pinaet al.86

The membrane in this case does not possess appar-ently the separation capability characteristic of a mem-brane in a tubular reactor enclosed by a membrane. Themembrane can still separate, for example, particles froma gas mixture as the gas mixture enters the membraneand is convected through the pores. One can have stacksof disks of such membrane reactors with spaces betweenthe disks being used for product separation or reactantaddition or heat exchange.

Fritsch80 deposited nanoclusters of catalysts such asPd, Pt/Ag, and Pd/Co in the size range of 1-3 nm inpolymeric membranes such as poly(amide-imide) andpoly(dimethylsiloxane) (PDMS). Reactions were carriedout in the flow through mode except that the mem-branes were essentially nonporous. In this case, themembrane treats every gas in the feed mixture accord-ing to the normal gas permeation selectivity displayedin the polymeric membrane. For example, PDMS has aselectivity of 8.4 from pure gas measurements for C3H6(propene) over H2. Therefore, a feed gas ratio of at least8.4 for H2/C3H6 is required for complete conversion ofC3H6 to C3H8. In fact, Fritsch80 has illustrated completeconversion of C3H6 to C3H8 using Pd cluster-containingpore-free membranes of PDMS. The metallic catalyticnanoclusters were prepared during the polymeric mem-brane formation itself.

Such thin membranes acting as the reactor are quiteuseful for fast exothermic reactions or for reactionswhere one of the intermediates should be the mainproduct. They possess very high catalyst surface areaper unit membrane area. However, the throughput perunit membrane area would be low. For example, Fritsch80has observed permeance values in the range of 1-3.2 10-6 cm3/cm2 s cmHg for PDMS membranes. Thereactor, therefore, has to have a large area and will have

the shape of a thin disk of large diameter. A standardindustrial reactor will have the shape of a long narrowtube.

In an alternate strategy, Wu et al.87 had used asomewhat similar membrane as a reactor via interphasecontacting. They employed PDMS membranes modifiedappropriately and containing titanium silicalite zeoliteas a catalyst. Oxyfunctionalization of n-hexane to amixture of hexanol and hexanone was carried out bybringing in n-hexane and 30 wt % of an aqueous H2O2solution: the silicone membrane acted as the reactionmedium and the reactor.

2.9. Solid-Electrolyte Membrane Supports theElectrode, Conducts Ions, and Achieves the Reac-tions on the Surface. Solid electrolytes are solid-statematerials possessing ionic conductivity. The two ionsof the greatest relevance are H+ and O2-, although otherions, Cl-, F-, Ag+, etc., have been found to be conductedas well. Solid polymer electrolytes such as perfluori-nated ionomer membranes (e.g., Nafion) allow transportof H+ ions in the presence of water and are often calledproton-exchange membranes. Solid solutions of oxidesof di- or trivalent cations (e.g., Y2O3) in oxides oftetravalent metals such as ZrO2 can conduct O2- overa wide temperature range. Nonporous disks of such asolid electrolyte can act as membranes for such ionicspecies and are quite useful for fuel cells and as O2-conductors.

Consider a solid-polymer-electrolyte fuel cell: porousgraphite gas-diffusion electrodes hot pressed onto bothsides of a thin polymer membrane (e.g., Nafion) aboveits glass transition temperature. This cell is fed withwet air on one side and wet H2 on the other side (derivedfrom the reforming of CH3OH in an adjacent reformerand therefore contains CO2 also). The membrane andthe electrode assembly is schematically shown in Figure16.88 The gas-diffusion electrode is made from porousgraphite impregnated with a Pt catalyst. H2 gas diffusesthrough the porous electrode and is oxidized on Ptcatalyst sites at the anode in a three-phase region(Figure 16) containing a polymer electrolyte, gaseousreactants, and a carbon matrix89

Protons transferred through the membrane react withO2 at similar catalyst sites at the cathode to form water

Figure 16. Membrane and electrode assembly. The five regionsof the model are shown (not to scale) (adopted from refs 88 and89).

H2 T 2H+ + 2e- (16)

O2 + 4H+ + 4e- T 2H2O (17)


Thus, the membrane supports the electrodes on twosides, transports H+, and achieves reactions on itssurfaces. The electrons in the external circuit obtainedfrom the chemical energy of the oxidation of H2 providethe required current.

A solid ceramic proton conductor tube from a stron-tium-ceria-ytterbia (SCY) perovskite of the formSrCe0.95Yb0.05O3 has been employed with two porouspolycrystalline palladium films deposited on the twosides of the ceramic tube to carry out NH3 synthesis atatmospheric pressure.90 The two electrodes were con-nected to an external galavanostat-potentiostat bywhich the appropriate current was applied. At theanode, gaseous H2 is converted to H+:

The protons are transported through the solid electro-lyte to the cathode where the half-cell reaction

takes place to complete the overall reaction

The reaction rate in this case was strictly controlled bythe rate of H+ supply since H2 was the limiting reactant,N2 being present in abundance. The membrane func-tions in this case again are quite similar to those in thefuel cell example given earlier. At 570 C and atmo-spheric pressure, greater than 78% of the electrochemi-cally supplied H2 was converted into NH3. This rateexceeds that in a conventional catalytic reactor underequivalent conditions of compositions, pressure, andtemperature by 3 orders of magnitude.

Using a yttria-stabilized zirconia-based (YSZ-based)conducting solid electrolyte which transports O2-, McK-enna et al.41 have studied the synthesis of HCN in asolid-electrolyte-cell reactor. At the cathode, an O2-containing gas (containing N2) passes; adsorbed O2 getsconverted to O2- via

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

producing HCN. In such a scheme, N2 in the air on thecathode side is rejected by the membrane.

For partial oxidation reactions using, for example,CH4, it is not necessary to apply a voltage. Instead, oneemploys mixed conducting materials having both ionicand electronic conductivities. A gradient of O2 partialpressure across the ceramic membrane induces a gradi-ent of O2- in the same direction and electrons in theopposite direction. The mixed conducting membrane hasa catalyst on the O2 side to facilitate the formation ofO2-. The catalytic properties on the other surface of themembrane allows CH4 to react in the manner of

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

of the ceramic membranes. See Figure 17 from EltronResearch for CH4 conversion to synthesis gas.91 Themembrane in this case separates O2 from air throughthe membrane, while supporting the catalyst for re-forming CH4 and distributing O2 in a controlled fashionthroughout the reactor. The study jointly sponsored byDOE and Argonne National Laboratories40 used arhodium-based partial oxidation catalyst inside themixed-conducting ceramic tube with O2 from the airbeing present on the outside of the tube. Such arrange-ments also reduce the possibilities of an explosivemixture.

2.10. Transfer of Heat. The most recent studies ofmembrane reactors have been in the context of thepetrochemical industry.8 They take place at highertemperatures (>200 C) and there likely is a need forconsiderable heat transfer because the reaction may beexothermic or endothermic. Dehydrogenation reactionsstudied frequently are endothermic. The membrane, ifinert, is in a catalytic reactor, packed bed, or fluidizedbed.92 Thus, the membrane may have to participate inheat transfer. Itoh and Govind45 have studied an en-dothermic dehydrogenation reaction on one side of apalladium membrane coupled with an exothermic hy-drogen oxidation on the other side. Heat was transferredfrom the oxidation reaction side to the dehydrogenationside through the membrane. They have concluded thatheat transfer across the membrane leading to anadiabatic reactor resulted in a higher conversion thanwhat was possible under isothermal conditions.

In actual practice, there will be one particular reactiongoing on and heat is going to be supplied from a firedheater, molten salt baths, or thermal fluid jackets.Therefore, the membrane is most likely going to bedecoupled from the heat transfer process. A commonconfiguration of some interest in a packed bed mem-brane reactor consists of multiple membrane tubesinside tubular catalyst beds, placed in turn, in anotherenclosure for heat exchange.7,92 Thermal expansionproperties of the membrane tube, sealing at the header,and protection from abrasional damage from catalystparticles are of much greater importance.8

2.11. Immobilizing the Reaction Medium. Manyreactions are carried out in an organic solvent. Theseinclude two-phase reactions, e.g., those encountered inphase transfer catalysis, gas-liquid reactions, etc. Aporous/microporous membrane can immobilize an ap-propriate reaction medium in the pores. The two dif-ferent phases containing reactants can be brought to

3H2 f 6H+ + 6e- (18)

N2 + 6H+ + 6e- f 2NH3 (19)

N2 + 3H2 T 2NH3 (20)

3/2O2 + 6e- f 3O2- (21)

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

- (22)

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

- (23)

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


the two sides of the membrane. As long as the two feedphases are immiscible with the reaction medium, reac-tants can partition into the reaction medium and reactand then the products can partition back into theflowing phases on opposite sides of the membrane.Unfortunately, such a configuration, usually termed asthe supported liquid membrane (SLM), has limitedstability93 because of a variety of reasons including afinite solubility of the reaction medium in the twodifferent reactant-containing phases. Therefore, as dis-cussed in the next section, Guha and Sirkar,94 Chen etal.,95 and Guha et al.11,96 have employed two differenthollow fiber membranes for bringing two reactant-containing streams into the membrane reactor while thereactants partitioned into the liquid reaction mediumcontained (confined) in the shell side between the twosets of hollow fibers.

Kim and Datta97 have used a porous support disk toimmobilize a homogeneous catalyst in a high-boilingorganic solvent. However, they did not completely wetthe pores so that the gas space was left in the pore. Thereaction was hydroformylation of ethylene to give pro-pionaldehyde. With an appropriate support membrane,the capabilities of the reaction medium within themembrane could be substantially enhanced. The onlyissue is the lifetime of the reaction medium.

3. Functions of Multiple Membranes in aReactor

Although more than two different types of membranescan be accommodated in a reactor, this section willconsider primarily the functions of only two membranes(differe

membrane in a reactor- a functional perspective

Documents

membrane technologies

membrane modules

various membrane functions

wide variety of functions

different membranes

primarilypolymeric membranes

variety of contexts

inorganic membranes