[membrane science and technology] fundamentals of inorganic membrane science and technology volume 4...

Fundamentals of Inorganic Membrane Science and Technology Edited by A.I. Burggraaf and L. Cot

�9 1996, Elsevier Science B.V. All rights reserved

Chapter 11

Current developments and future research in catalytic membrane reactors

Jose Sanchez ~ and Theodore T. Tsotsis 2

1Laboratoire des Materiaux et Proc6d@s Membranaires, UMR 9987 CNRS ENSCM UMII, 2, Place E. Bataillon, cc 024, 34095 Montpellier Cedex 5, France 2Department of Chemical Engineering, University of Southern California, Los Angeles, CA 90089-1211, USA

11.1 INTRODUCTION

Membrane reactors combine two distinctly different functions, i.e., reaction and separation into a single operation. For the high temperature catalytic membrane reactors, which are the main topic of this chapter, the reaction function is most often carried out by a conventional bed (packed, fluidized or moving) of catalyst particles; the membrane (metal or ceramic especially suited for the high temperature operation) is placed inside the reactor and carries out the separation function. The membrane reactor concept has evolved from the simpler design concept shown in Fig. 11.1, where the reaction and separation functions are carried out by two different processing units, the second d_ unit being a membrane separator. For bioengineering applications the process of Fig. 11.1 is finding widespread applications [1-3]. For the high temperature catalytic applications discussed here the process economics are only recently beginning to look attractive with the development of permselective, high temperature resistant membranes. It will become obvious to the reader that membrane reactors, where reaction and separation are carried out simultaneously in a single physical unit, provide significantly better design options than the concept of Fig. 11.1. Combining reaction and separation in the same unit often creates a synergy.

530 11 - - CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

Feed Reactants v'-- l Reactor

t I ! Membrane ] Separator ]

Reactants Recycle

Products

Fig. 11.1. A conventional reactor and a membrane separator with reactant recycle.

The presence of the membrane enhances the "per-pass" conversion and, in turn, enhanced "per-pass" conversions diminish the downstream separation requirements. Throughout this chapter, the reader should look for the need for this synergy. It is where a difficult separation problem exists, coupled to a "per-pass" conversion or selectivity or equilibrium limitation problem that the application of membrane reactors makes the best sense.

The earlier applications of membrane reactors involved for the most part various equilibrium limited dehydrogenation reactions, where the role that the membrane played was well defined and simple to describe; remove hydrogen efficiently and by so doing increase the yield of the reaction, by shifting the conversion to the right of the equilibrium. The earlier reactor designers envisioned two main reasons why the membrane reactor concept would improve upon the economics of the more conventional processes. These are the capital and operational savings realized by eliminating the processing steps required for separating the hydrocarbons from the hydrogen rich stream, and the energy savings realized from the lower operating temperatures required due to the reactor yield enhancement. Three decades or so later the promise still remains mostly unrealized for the high temperature catalytic dehydrogenation applications the early designers envisioned. Though small-scale commercial applications exist we are not aware of any large-scale commercial units. The reasons are many and varied and we will touch upon them throughout this chapter.

In the meantime membrane reactors are finding increasing utilization in bioengineering applications both for the production of fine chemicals via the use of both enzyme and whole-cell bioreactors [2] and for large-scale environmental clean-up type applications [4]. Membrane bioreactors remain an exciting area with many important new processes coming on line or in the pilot-plant stage. The topic, however, goes beyond the scope of this chapter; furthermore membrane bioreactors use for the most part polymeric or organic membranes, which are outside the theme topic of this book. Those interested for further reading on the topic (and everyone working in the area of catalytic/high temperature membrane reactors should be since many of the basic concepts are similar and some of the smart ideas developed could have widespread applications) can get started with a number of fairly recent review papers [1-3].

Some of the exciting things that are recently happening in the area of high temperature catalytic membrane reactors (which we will discuss in more detail

11 ~ CURRENT DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 5 3 1

later in this chapter) no longer involve equilibrium limited dehydrogenation reactions. The membrane instead aims to separate intermediates and products from the reacting zone so that they do not deactivate the catalyst or undergo further undesirable reaction. In some applications the membrane is not required to be permselective since its only role is to provide a controlled reactive interface between reactants flowing on opposite sides of the membrane. [5,6].

Whether such devices (which is this chapter are referred to by the acronym CNMR, namely catalytic nonpermselective membrane reactors) should be called membrane reactors in the first place will stay a matter of debate among the purists; nevertheless these 'membrane reactors' are attracting a growing share of attention with a number of hydrogenation, oxidation and partial oxidation reactions studied.

Membrane reactors as a concept date back to the 1950s. In the limited space of a book chapter, one cannot, of course, even begin to do justice to the topic and the many interesting studies that have been published. Fortunately the field has been blessed with a number of well written review papers focusing on both general and special aspects of the problem [7-16]. For those who are beginning in this area in additionto these papers we would also recommend the proceedings volumes of the International Conferences on Inorganic Membranes and a Volume 25 of Catalysis Today on the topic, which gives a good snapshot of what is currently happening in the area. It is a challenging feat to compose a chapter on a topic as old but also as vibrant as catalytic membrane reactors are. The risk is real that one is obsolete by the time the ink dries. The approach in the writing of this chapter is motivated by the belief that a technical book is first and foremost a reference guiae; we have, therefore, tried to be inclusive by attempting to cover as wide an area of topics as the space allows. For the benefit of breadth, we have sometimes sacrificed depth. For those desiring further in-depth study the long (but by necessity very selective) list of technical citations at the end of the chapter should provide welcome additional insight. The choice of illustrations in the discussion of various topics, furthermore, is more a reflection of personal taste rather than of a judgment of technical quality. More often than not the choice was among many equally illustrative and important examples.

Before proceeding further it would be appropriate for our readers to famil- iarize themselves with the few additional acronyms that will be used in this chapter and which are listed in Table 11.1. They are used to describe some of the most common membrane reactor configurations that have been studied in the technical literature. By far the most commonly referred to reactor is the PBMR, in which the reaction function is provided by a packed bed of catalysts in contact with the membrane. The membrane is not itself catalytic at least not intentionally so. Some of the commonly utilized inorganic and metal membranes, on the other hand, are intrinsically catalytically active. The PBMR classification, therefore, should be assigned with caution. When the packed bed


T A B L E 11.1

Types of membrane reactors

Acronym Description

CMR CNMR PBMR PBCMR FBMR FBCMR

Catalytic membrane reactor Catalytic nonpermselective membrane reactor Packed-bed membrane reactor Packed-bed catalytic membrane reactor Fluidized-bed membrane reactor Fluidized-bed catalytic membrane reactor

is replaced by a fluidized bed the FBMR configuration results. The concept is really interesting but FBMRs have yet to gain widespread acceptance. In the CMR configuration the membrane provides both the separation and the reaction function. The concept, however, has found wider acceptance in the bioreactor area than with catalytic reactors. Finally, the PBCMR (and FBCMR) uses both a catalytic bed and a permselective membrane. This configuration would appear to be ideal for situations where a bifunctional catalytic function is desirable; we are not aware, however, of many examples of such PBCMR use.

Other reactor configurations and concepts have also been discussed in the technical literature. Most commonly cited are hybrid concepts, where the membrane reactor is used as an add-on stage to an already existing conventional reactor. This particular configuration has a number of attractive features, especially for applications involving conventional type porous membranes, which are characterized by moderate (Knudsen-type) permselective properties. Staged membrane reactors have received mention and so have reactors with multiple feed-ports and recycle. To facilitate the transport across the membrane in laboratory studies one often applies a sweep gas or a vacuum in the permeate side or a pressure gradient across the membrane. It is unlikely that the first two approaches, effective as they may be in laboratory applications, will find widespread commercial application.

11.2 DENSE METAL MEMBRANE REACTORS

The earlier membrane reactors involved the use of Pd and Pd alloy membranes. Pd together with a handful of other metals is permeable to hydrogen but virtually impermeable to other gases and, of course, liquids. The diffusion process through Pd, furthermore, is an activated process and at high temperatures such membranes show very reasonable permeances. The pioneering work on Pd membrane reactors was done by Gryaznov and coworkers in the former Soviet Union and some industrial groups in the U.S. and Europe. Gryaznov and

11 - - CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 533

coworkers have studied a great many systems involving both hydrogenation and dehydrogenation reactions. Their work has been very ably reviewed by Gryaznov et al. [17], Gryaznov [18] and more recently Shu et al. [19]. Their focus on recent years has been on the production of high added value, specialty chemicals. Some examples of these applications involve: the synthesis of vita- min K from quinone and acetic anhydride [17] and the cis/trans-2-butene-l,4 diol hydrogenation to cis/trans-butanediol [20].

(a)

(b)

I t

SECTION " A A "

Flat double spiral tube membrane reactor.

Double spiral coiled plate membrane reactor.

Corrugated Spacer Plate

Pd- Based Vlembrone

Dehydrogenation

Side Hydrogenation Side

Schematic of a cross-flow catalytic membrane reactor

Fig. 11.2. Various types of Pd membrane reactors. Reproduced from Shu et al. [19] with permission.

5 3 4 11 m CURRENT DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

Gryaznov and coworkers have paid particular attention to the mechanical design aspects of the metal membranes and have come up with very elaborate designs as can be seen in Fig. 11.2. Small scale Pd-alloy membrane reactors became available in the eighties for CH4/CH3OH reforming for H2 production for stationary fuel-cell type applications in remote locations [14,21]. Large-scale industrial applications have yet to materialize, however, because of a number of concerns. These include:

11.2.1 Cost and Availability

Self-supporting, mechanically resistant Pd or Pd-alloy dense membranes must be typically a few hundred ~t thick. Pd being an expensive metal the cost of such membranes is, of course, non-trivial. What is often not mentioned, furthermore, in the discussion about dense metal membranes is the effect that the increased demand of the metal will have on worldwide markets and prices, if metal membrane reactors were to be adopted for large scale chemical processes like for example methane reforming. In recent years various groups have instead focused their attention on the development of composite metal membranes whereby a thin (a few ~t) film of the metal is deposited on (or in the pores of) an underlying porous support. This kind of membrane aims to have the mechanical resistance of the inorganic porous matrix and the high selectivity of a dense membrane with better permeabilities. Porous glass, ceramic and metal supports have been utilized [19], the metal film deposited by a variety of conventional techniques like vacuum sputtering [14], pyrolysis [23] and electroless plating [24-25]. Metal composite membranes are not free of problems themselves. In addition to the phase transition problem to be discussed below other problems include loss of mechanical stability, due to the significant differences in thermal expansion coefficients between the metal film and the support and metal atom/ion counter diffusion (this latter phenomenon is also of concern for some of the advanced microporous membranes, see discussion in the appropriate section). Such problems are being extensively investigated by Edlund and coworkers [26] at Bend research and by other groups [22].

11.2.2 Mechanical and Thermal Stability

Pure Pd membranes become brittle upon thermal cycling in a H 2 atmosphere due to a phase transition between the different Pd hydride phases (c~ and ~) with distinctly different crystal lattice parameters (see Fig. 11.3). Alloying with various other metals (for example Ag, Ru, Rh) tends to lower the phase transition temperature and undersome circumstances improves the hydrogen permeability. The phase transition problem is not technically insurmountable, if good care is exercised during start-up (when the membrane is first exposed to hydrogen)

11 -- CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 535

o I:L

o4 2:

0.

lOJ- /Fzeo'c r 288, "C

0.1 , \ I

o.o11 I i . . . . . i ,m

0.001 20"C

. . . . . . . . . . \ /

0 0.1 0.2 ~ 0.:5 0.4 0.5 0.6 0 .7 n

Fig. 11.3. Equilibrium relationship between the hydrogen pressure and the H / P d ratio (n) in the metal. Shown are the domains of existence of the R and ~ palladium hydride phases. Reproduced

from Shu et al. [19] with permission.

and shutdown operations (when the membrane is purged completely by hydrogen). The concern is, however, still there. Mechanical and thermal stability problems real or perceived have plagued the future of dense metal membranes in terms of large scale chemical process applications. Membrane cost and reactor safety issues notwithstanding, the shutdown costs of a large chemical plant brought upon by the need for membrane replacement can be astronomical.

11.2.3 Poisoning and Carbon Deposition Problems

Poisoning by S and C1 containing gas-phase impurities and deactivation due to carbon deposition are matters of concern with Pd membranes. Ali et al. [27], for example, recently studied the effect of gas phase impurities on Pd-Ag composite membranes during their use in a membrane reactor for methylcyclo- hexane (MCH) dehydrogenation to toluene (TOL). The presence of small amounts of dimethyl disulfide (DMDS) in the MCH feedstream were shown to significantly reduce the membrane permeability. For example, after exposure to the MCH-DMDS feed for about I h the I--I 2 permeation rate decreased to 12% of its value with the S free feedstream. Similar behaviour was observed with a C1 containing gas phase impurity (CC14). The results of Ali et al. are consistent with reports from the Bend Research labs [26] which show that poisoning by

536 11 -- CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

H2S can have significant effects on dense Pd membranes resulting in some instances in complete membrane failure in short periods of time. Short of replacing Pd with other metals, it is unclear at this point how one overcomes the problem of Pd sensitivity to sulphur and chloride impurities, which are unfortunately present in many industrial hydrocarbon streams.

100

80

60 @

~ 40 @ r..)

20

f

J I t

" , , , ,

" ' ",, ,,,, .,, .,,

" - ' .,, . . , , . . .

" ' - . . . . .

equilibrium

I I , I I

2 4 6 8

Reaction pressure / arm

10

r..) @

Q r,.9

100

8 0 -

6 0 -

4 0 -

2 0 -

0 v 600

O , , , , , . " " , . ,, .,. " S ' ' ' ' ' S

am_

I . . I , , I I

650 700 750 800

Temperature / K

Fig. 11.4. Top figure shows the effect of pressure on the reaction side of the membrane on methane conversion in the Pd membrane reactor; bottom figure shows the effect of temperature. The solid line and the symbols (o) are for the Pd membrane reactor. The dotted line is the calculated equilibrium conversion and the symbols (r'l) are for a membrane reactor using a porous Vycor glass

membrane. Reproduced from Uemiya et al. [29] with permission.

11 - - CURRENT DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 5 3 7

Less understood is the effect of carbon deposition. Ali et al. [27], for example, showed that after an initial pretreatment, membrane exposure to MCH + H 2 or TOL + H 2 mixtures did not significantly influence its permeability. Surface carbon on the other hand, which often results from exposure to hydrocarbons, has a tendency to dissolve in the bulk of the metal. In a study on the topic [28] this resulted eventually in membrane mechanical failure.

Their problems notwithstanding Pd membrane reactors have been shown very effective through the years in enhancing the yield of both hydrogenation and dehydrogenation and dehydrocyclodimerization reactions. Figure 11.4 is from the study of CH4 steam reforming by Uemiya et al. [29] using a Pd membrane coated on a Vycor glass tube by an electroless plating technique. The significant enhancement in methane conversion over the calculated equilibrium is evident. Interestingly the effect of reactor pressure in the membrane reactor is quite the opposite of what would be expected for the same reaction being run in a conventional reactor. According to Kikuchi and coworkers this is indicative that what limits the membrane reactor is the hydrogen permeation through the membrane, which is a strongly activated process, thus the significant effect of temperature. This is not always, however, true and in some of the studies reported where the membrane reactor failed to produce significant enhancements in yield [30] slow kinetics have been blamed for the failure. An interesting application of palladium membranes to C H 4 reforming was proposed by Adris et al. [31]. They combined a fluidized catalytic bed with palladium membranes for the C H 4 steam reforming. C H 4 o r CH3OH steam reforming remains the reaction of choice with no less than six different research group reporting studies using Pd membrane reactors over the last year alone [32-37].

The only other metal that has received some serious attention for membrane reactor applications is Ag [14,38] which is permeable to oxygen. Ag has similar thermal/mechanical stability problem as Pd and in addition its oxygen permeability is orders of magnitude lower.

11.3 POROUS INORGANIC MEMBRANE REACTORS

The earlier applications of membrane reactors using porous inorganic membranes involved the use of glass membranes, i.e., Vycor glass. Reactions studied included HI and H2S decomposition [39-41]. The latter reaction is still receiving attention for porous glass membrane reactor applications [41]; H2S catalytic decomposition is a very important but also truly challenging experimental reaction system. Porous Vycor glass is a mesoporous system (average pore diameter ~40 ~. Self-supporting, mechanically resistant Vycor glass membranes must have a thickness of ~1 ram. The permeances of such membranes are, therefore, 2-3 orders of magnitude less than the conventional mesoporous asymmetric sol-gel A1203 membranes.

5 3 8 11 ~ CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

A few years ago PPG produced an exciting new microporous glass membrane, which exhibits molecular sieving properties [42]. The membranes became available in a hollow fibre configuration and have since proven an interesting topic of fundamental research by a number of U.S. groups [43]. These glass membranes are plagued by mechanical stability problems, however, and to the best of our knowledge PPG is not pursuing their further commercialization. Hollow fibre inorganic membranes have from time to time been an- nounced and soon after forgotten; it appears, at least to those not intimately involved with the technology, that it will be a challenging technical task to produce a hollow ceramic fibre that will be able to withstand the kind of environments one expects to encounter in high temperature catalytic reaction applications. Mesoporous Vycor glass membranes have been used as a porous substrate for the deposition of various metal or other inorganic films in order to create asymmetric type highly permselective membranes. We have already discussed the membranes made by Kikuchi and coworkers [24,25] who deposit Pd films on Vycor glass membranes by electroless plating. As shown in Fig. 11.4 for the C H 4 steam reforming reaction carried out in a PBMR using the unmodi- fied Vycor glass membrane alone conversions are at or slightly above the calculated equilibrium; on the other hand the composite Pd/Vycor glass membrane attains conversion significantly higher than equilibrium.

Using CVD techniques various groups are also depositing both dense and m i c r o p o r o u s SiO 2 films within the porous structure of Vycor glass membranes [44,45]. Dense SiO 2 like Pd is permeable to hydrogen but not to other gases, though in SiO 2 it appears that transport involves molecular rather than atomic hydrogen. Ioannides and Gavalas [46] used such a membrane to study isobutane dehydrogenation in a PBMR but problems with the catalyst itself seemed to overshadow the beneficial membrane effects. Dense SiO 2 has very low permeability and membranes with reasonable permeances must be very thin (~ a few hundred ~). Such membranes have been recently produced by Gavalas and Jiang [47] using a technique which involves the deposition of an organic template within the membrane pores which is then burned away after the deposition of the SiO 2 film by CVD.

A number of concerns still remain with the dense SiO 2 membranes. Some of these are not unique to SiO 2 but concern all dense or molecular sieve type membranes. Due to their small permeabilities such membranes are sensitive to microcrack development. A single crack of an area of few ~t 2 allowing a convective flow can potentially neutralize the permselective properties of a membrane area many orders of magnitude its size. Poisoning and coking is always a concern when membrane transport is preceded by dissolution/adsorption type processes. One unique concern for dense SiO 2 [48] membranes is the fact that these membranes undergo a densification process, especially in the presence of steam. It is unclear what is the true long term effect of this densification process;

11 ~ CURRENT DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 539

the data that have been reported so far are for a limited set of conditions and relatively short periods of time.

Until very recently all membrane reactor applications using porous inorganic membranes (other than glass) involved alumina membranes. With the exception of the use of alumina membranes for H2S decomposition in the late seventies and early eighties [40,49-50] all other applications are more recent. In the middle to late eighties Alcan Laboratories in England developed an efficient technique for producing both symmetric and asymmetric type anodic alumina membranes. These membranes are ideally suited for fundamental studies of transport and reaction because they have straight nonintersecting pores. Alcan workers reported on the use of such membranes in a catalytic membrane reactor for C2H 6 dehydrogenation and C2H 4 hydrogenation and hydrogenolysis [51]. Tsotsis and coworkers [52] prepared their own anodic membranes by anodiza- tion of aluminum in various electrolytes. They used the resulting membranes for studying cyclohexane dehydrogenation in a PBMR [53]. Though anodic aluminas are ideal for fundamental investigations of membrane reactor systems they do not appear promising for large-scale industrial applications. The films that are available today are appropriate for small-scale laboratory filtration type applications; they do not appear to have the mechanical strength required for large-scale industrial reactor applications.

More appropriate for reactor applications are asymmetric sol-gel type alumina membranes. These typically consist of a mesoporous thin 7-A1203 film placed on the top of one or more macroporous o~-A1203 support layers. The 7-A1203 film is deposited by a sol-gel process [54-56]. The commercially available alumina membranes consist of several macroporous support layers each layer with a successively smaller pore diameter. This technique seems to help in avoiding crack formation and to eliminate significant infiltration of the deposited layer into the underlying support layer. Unless special precautions are taken, however, some infiltration is unavoidable [57], and in modelling calculations this must be taken into account [13]. The Process and Media Technology Group has recently reported [58] a series of studies in which they measured the thickness of the permselective layer along the membrane length of commercial A1203 membranes. They report significant variations for the same membrane and among different membranes. This is an alarming finding pointing out the difficulty in standardizing the preparation of such membranes and the need for additional fundamental studies and insight. In focusing on the challenges involved in the preparation of the thin permselective sol-gel films what one often ignores is the fact that a major component in determining the final cost of such membranes is the quality and cost of the underlying macroporous support layers. Several groups are recently doing work in this area [59-60] and a critical need exists for major developments here.

Mesoporous alumina membranes have been studied extensively mostly for

60 "@ @

55

5O

45

40

"N 35

o 30

o 25

0 ~

I~, 20

15

[0 600


650 700 750 800

Temperature (K) Fig. 11.5. Formaldehyde yields in various reactors under similar experimental conditions. V, conventional plug-flow reactor; e, a PBMR using a Pd/A1203 dense composite membrane; A, a PBMR using a mesoporous A1203 membrane; O, a CMR ushlg a mesoporous A1203 membrane catalytically

impregnated by a sol-gel technique. Reproduced from Deng and Wu [61] with permission.

equilibrium limited dehydrogenation of various lower molecular hydrocarbons and alcohols [7-16]. Figure 11.5, for example, is from a very recent study by Deng and Wu [61]. These authors studied the CH3OH dehydrogenation and compared the performance of a variety of membrane reactors including a PBMR using a Pd/A120 3 composite dense membrane, a PBMR using a mesoporous alumina membrane and a CMR in which the mesoporous alumina membrane was itself rendered catalytic during sol-gel processing by the incor- poration of catalytic components. Though the PBMR using the Pd/alumina composite membrane showed the best performance it proved to be sensitive to temperature cycling. The CMR improved the formaldehyde yield by on the average 10%, which in the lower temperature region corresponds to almost doubling the attained yield in the conventional reactor.

Chai and coworkers [62] have recently used mesoporous alumina membranes to study the CH 4 steam reforming reaction. The mesoporous alumina membranes were impregnated by a variety of catalytic metals. As can be seen in Fig. 11.6 under the same experimental conditions including the same space velocity and catalyst bed weight and the same sweep gas flow rate the PBCMRs, in which the membrane was impregnated by Ru and Rh, showed significant better performances than the PBCMRs using Pd or Pt impregnated membranes and also the PBMR using the original unimpregnated mesoporous alumina


100 . . . . . . , -- , . . . . . . . , . . . . . . .

fpj.,. o

80

C .o_ 60

r

C

8 4o

20

0 200 3 0 0 400 5 0 0 6 0 0

Temperature I "(3

Fig. 11.6. Effect of temperature on reactor conversion with all other experimental conditions the same. The catalyst hi the bed is Ru/A1203. e, Rh-A1203 membrane; A, Ru-A1203 membrane; O, Pd-A1203

membrane; V, Pt-A1203 membrane; A, A1203 membrane; r-l, conventional PFR reactor. Reproduced

from Chai et al. [62] with permission.

membrane. The study of C H 4 steam reforming in a membrane reactor using a porous alumina membrane was reported some years back by Vasileiadis et al. [63]. This group is studying the application of membrane reactors to the CH4 steam reforming reaction in the context of the Chemically Recuperated Gas Turbine (CRGT) concept, which we will briefly describe later in this chapter.

Porous membranes, in contrast to their dense counterparts, allow other species to penetrate through in addition to the preferentially permeable component. Typically this, all other things being equal, will result in some loss of attainable yield (see for example Fig. 11.5). Reactant permeation through the membrane, furthermore, negatively impacts on the economics of the membrane reactor processes because of the need for additional downstream separation requirements, which membrane reactors promise to eliminate in the first place. The latter problem is a serious one and can only be improved upon by the use of more permselective membranes. Several such membranes are currently under development by various groups. The earlier efforts involved the use of conventional sol-gel techniques [14] for the deposition of microporous A 1 2 0 3

(as well as zirconia and titania [64,65]) films within the pores of various mesoporous and macroporous alumina supports. Some of the resulting films proved to be prone to densification and sintering. Techniques to avoid this involve adding various additives, which are thought to inhibit the sintering process [66]. A number of groups are experimenting with techniques which

542 11 ~ CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

involve the use of various organic templates [67]. In the final preparation stage the organic additives are burned away to create the porous structure. Recently, microporous silica (and silica-titania) membranes have also been developed by de Lange et al. [131,132] following Uhlhom [140]. These are reported to combine high hydrogen fluxes with large separation factors.

We have mentioned in passing zirconia and titania porous membranes. These membranes are finding applications in ultra and nanofiltration but little use, if any, in membrane reactor applications. A type of microporous membranes that have attracted considerable attention are zeolite-type membranes. These membranes are made by hydrothermal techniques traditionally prac- tised in the area of zeolite synthesis. The earlier efforts involved the preparation of unsupported membranes [68]. Most recent efforts, and they are many, involve the deposition of zeolite films on various macroporous and mesoporous supports including porous metals [69-70] and various aluminas [71-77]. The preparation of silicalite, ZSM-5 and various A type zeolites has been reported. Zeolite membrane preparation represents a very active new thrust for the field of inorganic membranes one that has attracted significant attention. A number of reports exist on the measurement of the transport properties of such membranes and recently even some membrane reactor applications [78] have been reported. Our fundamental understanding of these materials still remains in- complete and much more remains to be learned. Zeolites being catalytically active systems care must be exercised in their catalytic reaction applications. Zeolites like ZSM-5 are prone to coking due to their tendency to catalytically crack various hydrocarbons. Being polycrystalline materials one must strike a fine balance between intra- and intercrystalline transport. Due to their nature these materials are sensitive to poisoning, plugging of the narrow pores due to coking (particularly true for the zeolites with the finer pores) and metal a tom/ion counterdiffusion from the underlying mesoporous/macroporous support layers. Without doubt zeolite membranes represent a very new exciting research area.

We will return to the discussion of micro and mesoporous membranes but before we do so we will discuss a number of recent studies which utilize macroporous alumina membranes, where the membrane itself acts as a means of bringing together reactants flowing on its opposite sides and to create a well controlled reactive interface. Earlier efforts involved the use of macroporous membranes as efficient contactors for various gas/liquid phase reactions. These studies were motivated by the desire to improve upon the mass transfer characteristics of multiphase reactors. Mass transfer in these reactors is one of the most important parameters controlling efficiency. The volatile reactant limits the reaction extent due to its poor mass transfer through the liquid film covering the catalyst particles. Strategies that have been used to overcome this limitation include varying the catalyst size and shape, the operating parameters


and the reactor configuration. Typically one utilizes, slurry, trickle bed, fluidized bed and cross flow reactors. The application of a membrane reactor to multi-phase catalytic reactions aims to improve upon the mass transfer characteristics of conventional reactors. Here the role of the membrane, is to increase the gas-liquid-solid interface through the high surface of the porous membrane acting as catalyst support.

Akyurtlu et al. [79] and Harold and coworkers [80-81] used this concept first for hydrogenation reactions. Cini and Harold studied [81] ~-methylstyrene hydrogenation to cumene. The membrane consisted of a ~-A1203 layer deposited on an ~-A1203 tube and impregnated with Pd. Hydrogen passed through the active layer side whereas the liquid phase passed along the support side. The ceramic tube was completely filled with liquid and the reaction occurred within the catalytically active layer. The membrane reactor allowed for good temperature control and a well defined reactants/catalyst interface. Cini and Harold report significant rate enhancements when compared with a single pellet reactor. Torres et al. [82] used a similar type reactor for the nitrobenzene hydrogenation reaction. Their results suggest that depending on the conditions, the conversion can be either kinetically or mass transfer (hydrogen or nitrobenzene) controlled. Under diffusional limitations the best reactor configuration was the one where the volatile reactant was supplied in the reactor inner side.

Van Swaaij and coworkers [5,6] proposed the use of nonpermselective macroporous membranes for gas phase reactions, which by their nature require strict stoichiometric conditions. Such reactions include selective catalytic reduction of NO by NH3, and the Clauss reaction. The principle for the CNMR operation is shown in Fig. 11.7. By creating a sharp reaction front within the membrane one avoids the slip by either reactant (NH3 or NO, SO 2 or H2S) on either side of the membrane. Small perturbations in the feed could be accom- modated in principle by a shift in the reaction front within the membrane. The success of the CNMR concept depends, as one would expect, on the sharpness of the reaction front created within the membrane. For a non-instantaneous reaction the front created is rather diffuse (see Fig. 11.7) and there is, as a result, a reactant slip.

A number of research groups [83-86] have used a rather related concept for carrying out various selectivity limited reactions in a membrane reactor. The concept is illustrated schematically in the top part of Fig. 11.8 which is from a study by Harold and coworkers [84]. It applies to partial oxidation reactions where the desired reaction product can further react with oxygen to produce an undesirable total oxidation product. In some instances it makes better sense (in terms of maximizing the reactor yield) to feed the two reactants separately on either side of the membrane rather that to co-feed them on either side. This is shown in the bottom part of Fig. 11.8 which shows the yield to the desired product as a function of the Thiele modulus as the degree of feed segregation


THE CNMR

3 2H2S + S O 2 ~ --~ S 8 -!- 2H20

stagnant stal t gas / / ~ ' ~ . , . , ~ ~ gasfiim

- ~ : : ~ - - - - -

Ei2S ~ 2

0'.0 �9 0'.2 " 0'.4 0'.6 0'.8 1.0 Iocadon (~..,)

stagnant gasfilm H20

stagnant gasfilm

t U .UU t ~

0.0 0.2 0.4 0.6 0.8 1.0 location (x/L)

Fig. 11.7. Calculated mole fraction profiles for the CNMR. The upper figure is under conditions for which the reaction is instantaneous; in the lower figure the reaction is non-instantaneous and the

front created is rather diffuse. Reproduced from Sloot et al. [5] with permission.

changes (higher values of CA indicate a higher degree of segregation with CA 0 corresponding to the completely segregated feed and CA ~ ~ to the total co-feed). For large values of the Thiele modulus the segregated feed clearly shows a better performance. The PBMR equivalent of the concept of Fig. 11.8, where a catalytically inactive membrane in contact with a catalyst bed is utilized has been applied to direct CH4 activation for the production of C2+ hydrocarbons [85,86].

11 ~ CURI~NT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 545

A + v B B ~ VR1R

A + VR2 B ~ VpP

Feed" A, diluimt ,~CTI:VE' ' Feed: B, diluent

Support side Active layer side

bulk streaml A n A t ~ / / / I / ~ / J ~ i B~pR bullk stream

Emue EITIuent

Reaction Structure: Example:

A + B ..... > R A: oxygen B: ethylene

A + R ..... > P R: acetaldehyde P: carbon oxides, water

. 8 . . . . . . . . . . .

-x_ 0.2 ;>.,

- 0.4 " " , 1

,?.J # . . ,

>" 0.0

" ~ , r "

~ -0.4

-0.8 . I i i0 I00

Thiele ~lodulus (Rcn. 2 Basis), ~,_

Fig. 11.8. The use of membrane reactors in partial oxidation reactions. Upper figure represents a schematic of the concept. Lower figure gives the calculated yield as a ftmction of the Thiele modulus.

Reproduced from Harold et al. [84] with permission.

Some of the major challenges facing the area of catalytic membrane reactors are of a more conventional nature. We have briefly discussed the need for s tandardizat ion in the membrane preparat ion techniques and for better quality control; the preparat ion of inexpensive good quality macroporous supports still remains a challenge as are reliable chemically inert and cost effective seals (an excellent discussion on the latter topic can be found in the review paper by Saracco and Specchia [14]). For highly exothermic (endothermic) reactions

546 11 m CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

providing the required heating (cooling) poses some interesting technical ques- tions, especially for multi-tubular porous membranes. These are some, but not necessarily all, the technical hurdles that loom in the horizon we face as we move away from the laboratory and bench-scale studies towards process development units (PDU) and commercial-scale installations.

Before we proceed further with our discussion focusing on dense, solid oxide type membranes we would like to mention a class of porous membranes, which though not strictly inorganic in nature, they themselves show good potential for high temperature membrane reactor applications. These are carbon membranes [87,88]. Carbon membranes are, of course, not a recent development. Koresh and Softer [89-92], for example, prepared carbon membranes with molecular sieve type properties more than ten years ago. Mesoporous and macroporous carbon membranes are available commercially by GFT mostly for various liquid and gas phase separations. The first known application of carbon membranes in catalytic membrane reactors was the one reported recently at the First International Workshop on Catalytic Membranes [93] for Fischer-Tropsch synthesis. There the carbon membranes (in the form of hollow fibres) were used as a means to provide a high surface area catalytic support for the reaction. Carbon membranes are very exciting materials, since by a relatively simple modification of the preparation technique one can prepare materials, which span the full range from molecular sieves to macroporous membranes. Carbon membranes are not free of limitations however. They cannot be utilized, for example, in oxidative atmospheres; caution should be, furthermore, exercised even under reductive atmospheres in the presence of various metal impurities. Nevertheless carbon membranes, in our opinion, show great potential for use in catalytic membrane reactors especially for the CNMR type applications we have already discussed.

11.4 SOLID OXIDE MEMBRANES

Another class of dense inorganic membranes that have been used in membrane reactor applications are solid oxide type membranes. These materials (solid oxide electrolytes) are also finding widespread application in the area of fuel cells and as electrochemical oxygen pumps and sensors. Due to their importance they have received significant attention and their catalytic and electrochemical applications have been widely reviewed [94-98]. Solid materials are known which conduct a variety of cationic/anionic species [14,98]. For the purposes of the application of such materials in catalytic membrane reactor applications, however, only 0 2- and H + conducting materials are of direct relevance.

The earlier solid oxide electrolytes were solid solutions of divalent or triva- lent metal oxides ( Y 2 0 3 , YB20 3 o n C a O ) in oxides of tetravalent metals having a fluorite type structure A408 like ZrO2, ThO 2 or CeO 2 [97]. The introduction of

11 ~ CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 547

aliovalent cations in the lattice creates oxygen vacancies. Yitria and Calcia stabilized ZrO 2 (YSZ or CSZ) in particular have found most of the applications. Pyrochlore type electrolytes with an A2B20 7 structure and Bi20 3 based systems have received lesser attention. Solid oxide membrane reactors have been operated in three different modes. Their primary use so far has been as solid oxide fuel cells (SOFC), where the chemical energy of the reaction is converted into electrical energy (with the co-production in some instances of useful chemicals [99,100]). The reverse of this case is when the solid oxide membrane reactors are operated as electrochemical oxygen pumps, whereas electrical energy is expended to enhance the oxygen transport towards (or away from) the anodic membrane surface. With the development of perovskite type membranes, which exhibit good ionic and electronic conductivities, reports are appearing with a greater fre- quency where solid oxide membrane reactors are operated in the conventional mode without the need for imposing an external electric voltage.

ABO3 type perovskites either with a 3:3 (like LaA103) or 2:4 (like SrTiO3) structure can accommodate various metals in the A, B position. Their conductivity can vary from totally electronic (metallic-like) like in LaNiO3 to ionic as in doped LaA103 [97]. Materials like BaCe0.9Gd0.102.95 and BaTh0.9Gd0.102.95 have good protonic conductivity at low temperatures (~10 -~ s /cm -1 at 600~ Acceptor doped perovskites have received the greatest attention. Cation substitution in the A or B position significantly increases anion disorder. Perovskites with a structure like Lao.2Sro.sCOl_yFeyO3_x or Lao.6Sro.4Coo.sBo.203-x (B=Fe, Ni, Cu) have been shown by Yamazoe and coworkers [101-104] to have high ionic conductivities (~1 s / c m -1 at 800~ but also very highelectronic conductivities (,-'100 s/cm-1). For such high ionic conductivities gas-phase exchange seems to often be the controlling mechanism [97].

Hydrogen and CH 4 (and most recently CH3OH) SOFCs have been the subject of intensive investigations since the early sixties and a complete review of the area certainly goes beyond the scope of this chapter. Hydrogen SOFCs have received the lion's share of attention. CH4 (and more recently CHBOH) SOFCs have shown promise but concerns still remain with carbon deposition and low catalytic activity.

Solid oxide membrane reactors operating as electrochemical oxygen pumps have been the subject of several laboratory investigations. Reactions studied include CH4 reforming and partial oxidation to synthesis gas; CH4 direct activation for the production of C2 and higher hydrocarbons and partial oxidation to CH3OH and formaldehyde, various decomposition reactions like those of CO2 and H20, NO and SO2. The latter two decomposition reactions have received attention by a number of research groups in the USA due to their importance in environmental applications. The earlier work was done by Ma- son and coworkers [105,106] and Giir and Huggins [107] who used various stabilized zirconias to study the decomposition of NO. YSZ electrolytes were


utilized by Ceramatech for NO and SO2 decomposition [108]. Most recently Helipump Corporation and Research Cotrell [109] have reported on the use of yitria stabilized ceria solid electrolytes for the SO2 and NOx reduction from a simulated flue gas from coal combustion containing up to 2% 02. Good rates of NOx and SO2 reduction were reported in the presence of a strodium ruthenate perovskite electrocatalyst. Fly-ash which is present in coal combustion flue gas seemed to have little effect on the solid electrolyte performance.

An assortment of other reactions like various hydrogenation reactions, the production of HCN from CH 4 and NH3, etc. have also been studied [98]. Problems resulting from the estimated cost of the required electric energy and difficulties in scaling up the laboratory reactors [110] have hindered further commercialization. For a number of chemical reactions it has been shown recently that by cofeeding oxygen on the anodic side the rate of the electrochemical reaction far exceeds the rate of electrochemical oxygen pumping [111]. This effect called NEMCA (non-faradaic electrochemical modification of catalytic activity) shows promise in reducing the associated electrical energy costs.

A significant recent effort in this area is a collaborative study by Amoco and the Argonne National Laboratory utilizing solid oxide type membranes [112- 113]. The newly developed membranes show improved mechanical and thermal characteristics and are reported to remain stable for over 21 days at 900~ under CH4 partial oxidation conditions. The membrane used was tubular in shape. A CH4/Ar mixture was allowed to flow in the tubeside which was packed with a Rh based catalyst. Air was the source of oxygen on the outside

o~" 9 6 ~- >

"6 92

"~ 8 4 * H 2 SelJ2 o = �9 CO Sel. 7 o t - -

80 �9 CH4 Conv. o ~k 02 Perm. 6 "~ 76 r

= E _0.o 72 -5 '- e

e 68 ~ _ :> I:::: = E 4 o o 64 0 ~ >, x

60 3 0 1 3 5 7 9 11 13 15 17 19 21

Time (days) Fig. 11.9. Methane conversion, CO mid H2 selectivities and 02 permeation in a solid oxide m e m b r a n e

reactor. Reproduced from Balachandran et al. [113] with permission.


of the membrane. Figure 11.9 shows the CH4 conversion and CO/H2 selectivities during a 21 day run. They all remain in the 90+% range throughout the whole run.

The use of solid oxide membranes in partial oxidation reactions aims to avoid the complete oxidation of the desired partial oxidation products. When compared to similar efforts using microporous membranes solid oxide membrane reactors, are at a disadvantage (except for reactions that take place at high temperatures) because oxygen transport through the oxide lattice is generally low when compared with the permeability of porous materials. The synthesis of non-porous ceramics with good oxygen permeability and selectivity at lower temperatures is not a simple task. The application of membrane reactors to partial oxidation is often complicated by the fact that the desired product is often more reactive with oxygen than the reactant itself as was observed by Julbe et al. [114,115] for the methane oxidative coupling using lanthanum oxychloride membranes.

11.5 THEORETICAL CONSIDERATIONS

The modelling and simulation of catalytic membrane reactors has attracted the interest of many investigators over the last ten years. Most studies have focused on particular membrane reactor systems aiming to simulate their performance in terms of attainable yield and selectivities. The considerable body of modelling work in this area has been reviewed by Tsotsis et al. [13]. The authors of this paper presenteda discussion of the pre-1993 modelling literature in terms of a general mathematical model of a PBCMR, which is shown schematically in Fig. 11.10, with catalytic beds present both in the inner and outer membrane regions. The model takes into account mass and energy balances in the tubeside

OUTER ~ " TUBE

IS

TUBESm~ CATALYST BED

z

~MBRANE

SHELLSIDE CATALYST BED

Fig. 11.10. Schematic of m e m b r a n e reactor for PBCMR model . Reproduced f rom Tsotsis et al. [13]

wi th permission.

5 5 0 11 ~ CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

and shellside and in the membrane itself and accounts for the existence of pressure drops in the shellside and tubeside. The membrane is considered to consist of a single permselective layer either dense or mesoporous following a Knudsen type diffusion mechanism, an assumption utilized by most pre-1993 modelling investigations.

There are a number of modelling efforts, however, which cannot be discussed in the context of the generalized model of Tsotsis et al. [13]. Van Swaaij and coworkers, for example, have modeled the behaviour of CNMR reactors using the Dusty Gas Model description of transport [5,6]. They have shown that when bulk diffusion and convective flows must be taken into account the Dusty Gas Model provides a more accurate description of transport through the membrane. The earlier studies of the group modelling the application of CNMR to reactions requiring strict stoichiometric ratios have been reviewed in detail by Tsotsis et al. [13]. More recent efforts by the same group deal with the application of the CNMR Dusty Gas Model to the combustion of CO and hydrocarbons.

Membrane reactors utilizing multilayered membranes have been modelled in recent studies by Becker et al. [116] and Tayakout et al. [117,118]. In contrast to prior efforts these models account for mass transport both through the mesoporous permselective layer and the underlying macroporous support layer(s). Both are isothermal models. Becker et al. [116], however, in their analysis utilized the experimentally measured temperature gradients along the reactor length in the calculation of reaction constants and transport coefficients. Both models assume dilute reactant mixtures and, therefore, neglect complica- tions resulting from changes in the number of moles due to the reaction. The reaction studied was ethylbenzene dehydrogenation in the model of Becker et al. [116] and cyclohexane dehydrogenation in the model of Tayakout et al. [117,118]. A schematic of the reactor analyzed by both groups is shown in Fig. 11.11 (in the model of Becker et al. [116] there is no catalyst bed in region 4).

At steady-state Becker et al. [116] write the following set of equations. On the tubeside (region 1)

3C~ 1 3 ! 3Ct] - rl U T --~- = D t r -~r r ---~-r ) p b O~ i (11.1)

O<_r<_r i or O < Z < L

where C~ is the concentration of species i in region I (ethylbenzene, styrene and hydrogen in the Becker et al. model; cyclohexane, benzene and hydrogen in the Tayakout et al. model). D~ is an effective bed radial diffusivity, Pb the reactor bed density (g/cm3), c~i the stoichiometric coefficient and r 1 the reaction rate (gmol/g s) in region 1. UT the superficial fluid velocity is considered constant in the model of Becker et al. [116] (an assumption which is relaxed in the model of Tayakout et al. [117,118], see discussion to follow).


4 shell side OO._.O % 0 0 0 0 0 0 l -I'-~ . . . . . . . . . O O U O --- ,., 0 0 0

I

membrane 2 ] ~ - -/'hii

~, 1 feedside 0 O 0 F%00C)~o0 J i iiltt- i- 0 O 0 " ~ u 0 " - ' 0 0 ...........

i i iil i ii ! iiill o o o o o o o o o oooO~

Z O ............................................................................. k

O catalyst packed bed ~ active layer Fig. 11.11. Schemat ic of the m u l t i l a y e r e d m e m b r a n e reactor .

In the reactive membrane (region 2)

1 3 ( 3C21= r2 D2 r ~ r ~ j (x i pm (11.2)

r i <__ r <_ r m

with Pm being the membrane density. Becker et al. [116] utilize a Fickian description of transport both in the mesoporous and macroporous (see below) membrane regions with D~ (D 3) being an overall effective transport coefficient. In the macroporous support (region 3)

D 3 1 3 I 3C31= r ~ r r--~-r J 0 (11.3)

r m <_ r < rs

On the shellside of the reactor (region 4)

3C 4 1 3 [ OC4 / Us - - ~ - D 4 r -~r r --~-r ) (11.4)

rs <- r < r e and 0 < Z _< L

Equations (11.1)-(11.4) are complemented by a set of corresponding initial and boundary conditions.


At Z - 0 (reactor inlet)

c =cb

c4-c4 o

(11.5)

(11.6)

At r - 0 (symmetry condition)

- 0 3r

(11.7)

At r - r e (wall no-flux condition)

4

3r (11.8)

In addition to Eqs. (11.5)-(11.8), one must account for the continuity of fluxes and concentrations at the interfaces between the various regions. Becker et al. [116] first converted the system of Eqs. (11.1)-(11.8) into an equivalent system of dimensionless equations by defining appropriate dimensionless variables like C~/Clo, r/ri, z /L , etc. By discretizing the first and second order derivatives the dimensionless equations were reduced to a mixed system of first order ordinary differential and algebraic equations which were solved by the DASSL numerical package [116]. The model was used to fit experimental data with the ethylbenzene dehydrogenation reaction. The agreement between experimental data and theory was good (see Fig. 11.12). As previously mentioned for fitting the data of Fig. 11.12, Becker et al. [116] incorporated in their model the experimentally measured temperature gradients along the reactor length.

100

8 0

0 *~ 80 i -

> C 0

0

Qs 0.75 cm=/s

t

�9 Experimental convers ion - - S imula ted convers ion

0.2 o., o's o's 1?o 1.'2 ~., 1;, Or (cm31s)

Fig. 11.12. Convers ion vs. volumetr ic tubeside f low rate for a PBMR. Qs is the shellside f low rate.

Rep roduced f rom Becker et al. [116] wi th permiss ion.

11 - - CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 5 5 3

The model of Tayakout et al. [117,118] in addition accounts for the possibility of axial dispersion effects in the tubeside and shellside. The inclusion of axial dispersion effects in regions (1) and (4) necessitates a different set of initial conditions at Z = 0 and a companion set of conditions at Z = L. The effect of pressure drop through the catalytic bed could be included in this type of model using Ergun's equation.

A nonisothermal membrane reactor for the partial oxidation of C H 4 to

synthesis gas utilizing a multilayered membrane was recently presented by Tsai et al. [119]. The membrane is considered to consist of three layers. A macroporous support layer, a dense perovskite film permeable only to oxygen on the top of which a porous catalytic layer is placed. In modelling such a reactor Tsai et al. [119] distinguish five regions. These are the tubseside and shellside of the membrane and the three membrane layers. For each region Tsai et al. write the appropriate mass and energy balance equations. For the tubeside and shellside regions the mass balances are similar to Eqs. (11.1) and (11.4) above. For the macroporous support region the equation is the same with Eq. (11.3) and for the catalytic layer the same with Eq. (11.2). The dense perovskite layer allows only oxygen to transport through and Tsai et al. [119] opt for a Fickian description of transport with an activated diffusion coefficient. In cylindrical coordinates they write the following equation from the flux of 02 No2r (km~ s) through the dense perovskite layer.

A exp I~T] T - In (P~II~ / (11.9)

N O 2 " r/~b) r ln )

In the above equation r is the radial position within the perovskite layer, r0 and rb the outer and inner radii and Po, ro and Po2,rb, the corresponding oxygen partial pressures at these positions. The preexponential factor A and the activation energy E were fitted to the experimental data of Teraoka et al. [101].

The energy balance equations in every region account for energy changes due to the flow and diffusional transport of the various species and the ener- getic effects associated with the reaction. Tsai et al. [119] describe C H 4 partial oxidation to syngas as the direct outcome of the total oxidation of C H 4 coupled with CO2 and steam reforming.

The model of Tsai et al. is of direct relevance to the experimental study of Balachandran and coworkers [112,113] that we have previously discussed. The same group [120] have also recently presented a modelling study of the application of solid oxide membrane reactors in the area of environmentally benign processes. With the emergence of solid oxide membranes and their use in membrane reactor applications, a number of models have appeared recently to


model such reactors. Wang and Lin [121] and Lu et al. [122] presented models of membrane reactors utilizing solid oxide membranes for C H 4 activation; so did Nozaki and Fujimoto [123] to model their data of selective oxidative methane coupling in a membrane reactor utilizing a PbO oxide membrane impregnated on a porous silica-alumina tube.

In addit ion to the studies of Becker et al. [116], Tayakout et al. [117,118] and Tsai et al. [119], a number of other groups have presented models which take into account the possibility of external mass transport limitations [124-126]. Such effects could become of concern in industrial membrane reactors utilizing larger size membranes.

Several other recent modell ing membrane reactor studies are also wor th discussing, Varma and coworkers [127] have analyzed the effect that nonuniform catalyst distribution on the membrane itself (for CMR and PBCMR applications) and in the catalyst bed (for PBMR applications) has on membrane reactor performance. Conventional membrane reactor models were utilized by a number of groups to model their experimental data. Shu and co-workers [33]

0 r , .

o ~D

A

V In

r

-I-

0 , r tern

0 r " 0 0 r

e . . . , u) t _

(1) > o

0 L)

C) q r - -

o

membrane not important membrane important

1 10 100 P e r m e a t i o n / R e a c t i o n rate (H)

1000

Fig. 11.13. Cyclohexane conversion vs. the (permeation/reaction rate) ratio. Curves 1 and 2 for mesoporous membranes with Knudsen separation factors. Curves 3 and 4 for microporous membranes with a separation factor of 100. Curves 5 and 6 for membranes permeable only to hydrogen. Odd (even) numbered curves correspond to an hlert sweep gas rate of I (10) times the cyclohexane flow. The temperature is 477 K, Pfeed = 100 kPa. From Harold et al. [130] with permission.

11 - - CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 5 5 5

modeled the data in their studies of CH 4 steam reforming in a catalytic membrane reactor using a Pd-Ag membrane deposited on porous stainless steel. Similar models were used by Gobina and Hughes [128] to model alkane dehydrogenation in a catalytic membrane reactor using a Pd membrane. Gokhale et al. [129] presented a model for isobutane dehydrogenation in a membrane reactor using a porous membrane.

Some recent models have also appeared discussing the operation of three phase catalytic membrane reactors by Torres et al. [82]. The models which represent extension of prior models by Akyurtlu et al. [79] and Cini and Harold [80] are numerically analyzed and appear to simulate well the experimental results of the nitrobenzene hydrogenation reaction in a three phase catalytic membrane reactor.

Some of the important findings of the modelling studies so far are nicely summarized in Fig. 11.13 which is from a paper by Harold et al. [130]. This figure is the result of a modelling investigation of a PBMR for the cyclohexane dehydrogenation reaction but the main findings, are generally applicable. Curves (1) to (4) correspond to meso/microporous [131,132] membranes and curves 5 and 6 to microporous membranes which are only permeable to hydrogen. Note that for porous membranes the conversion passes through a maxi- mum the result of the detrimental influence of reactant losses; this is not true, however, for the perfect (i.e., only permeable to hydrogen) membranes. Such membranes (i.e., dense metal) on the other hand have problems of their own which we have already discussed. As one would expect the higher the permselectivity of the membrane to hydrogen the better the performance of membrane reactors especially in the region of higher (permeation/reaction rate) ratios.

11.6 EMERGING APPLICATIONS

Though most of the applications of membrane reactors that we have discussed so far involve the production of useful chemicals some of the most interesting applications currently under consideration relate to energy production, transfer and utilization. High temperature membrane reactors are, for example, being looked upon in the context of Chemical Energy Transmisssion Systems (CETS).

Such systems utilize reversible catalytic reactions associated with large energy effects, typically hydrogenation/dehydrogenation type reactions. During the endothermic reaction (dehydrogenation) the heat of the reaction is provided by an energy source (like solar, geothermal, etc). During the exothermic reaction the heat (which in the endothermic part of the cycle is stored in the form of chemical energy) is released back to the user. To increase the energy efficiency of the process the hydrogenation reactor pressure must be increased with


Rdh

T

t

I

1 i Ii I /i I 1 9 Rh

~ F c ~3 HE2

, F 6

~ j I - I

/ CHE1 M4

, r j '

HE5 7

, " t C3 i J

12

Fig. 11.14. Process flow sheet of cyclohexane/benzene heat pump using hydrogen permeable membranes: Reh and R4rdehydrogenation and hydrogenation reactors; C, compressors; T, turbine; HE, heat exchangers; CHE, counter-current heat exchangers; P, liquid pump; M, hydrogen membranes.

Reproduced from Cacciola et al. [133] with permission.

respect to the dehydrogenation reactor pressure. High temperature hydrogen permselective membranes offer many advantages here. Figure 11.14 is from the study of Cacciola et al. [133] and represents a conceptual design of a CETS system involving a high temperature membrane reactor utilizing the cyclohexane dehydrogenation reaction.

The process utilizes four hydrogen permselective membrane modules and a condensation/evaporation unit in addition to the two reactors, compressors and associated other hardware. The dehydrogenation reactor containing the high temperature membrane increases the yield of the dehydrogenation reaction while simultaneously allowing the removal of product hydrogen. To further increase the coefficient of performance (COP) Cacciola et al. [133] propose the use of three additional membrane modules. The mixture of benzene, unreacted cyclohexane and the remainder of hydrogen exiting the dehydrogenation reactor Rdh are cooled down in CHE1 and condensed in HE4

11 - - C U R I ~ N T DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 557

releasing energy; hydrogen is separated in HE4 by means of the membrane module M4 and mixed at point 14 with the hydrogen coming form Rdh. This hydrogen is then compressed to the level of its partial pressure in the hydrogenation reactor Rh and cooled down in the heat exchanger HE1 releasing additional energy; it is then mixed into the excess hydrogen circulating in the loop 16-9-10-19, which includes the dehydrogenation reactor Rh. Note that Cacciola et al. [133] envision Rh to be a conventional reactor with the membrane module M3 acting as a simple mixing device of the hydrogen with the cyclohexane and benzene vapour from the evaporator HE5 and heat exchanger CHE2; in fact M3 and Rh could be combined into a single unit as is true with Rdh and M2. The mixture exiting Rh is passed through the membrane module to partially remove the hydrogen, the retentate (consisting primarily of cyclohexane and some benzene and hydrogen) is cooled in CHE2 expanded in turbine T and heated to the dehydrogenation temperature Tdh to complete the cycle.

The role of the membrane reactor is obvious. The membrane module M1 is important because it decreases the total amount of hydrogen flowing through Rdh; this in turn decreases the ratio of reactor pressures Ph/Pah, conversions (Xdh, Xh) and temperatures (Tdh, Th) remaining the same. M3 is also important from an energy efficiency point of view, since it allows one to keep the pressure at 17 at the level of the hydrogen partial pressure at Rh (rather than the total pressure Ph) thus reducing the load on compressor C2.

It should be obvious from the above discussion that though the ability of the high temperature membrane reactor to increase the reactor yield is important equally important is the membrane's ability to efficiently separate hydrogen at high temperatures and to sustain a transmembrane pressure drop.

A somewhat related concept is the Chemically Recuperated Gas Turbine (CRGT), see Fig. 11.15 [63,134]. Here C H 4 is being reformed in a catalytic membrane reactor. The purpose of the reforming, however, is not to completely convert C H 4 to H 2. Rather the goal is to convert C H 4 to a degree sufficient to create a fuel mixture with better flaming characteristics which would then allow higher steam to fuel ratios lower burner temperature and reduced NOx production. The success of this concept is strongly dependent on its ability to utilize waste heat, thus the need for efficient membrane reactor reformers.

Several of the membrane reactor efforts under development are motivated by environmental regulations and factors. We have already briefly described the CRGT concept and the solid oxide membrane reactors for SO2 and NOx removal. With the growing awareness and public fear for the risk of global warming and climate change have come calls for the reduction of greenhouse gases, particularly CO2 from stationary power sources. This has led to the initiation of various research efforts for the development of high temperature membranes for H 2 - C O 2 separation in the context of integrated coal gasification combined cycle (IGCC) and molten carbonate fuel cells (IGMCFC) technology.


B ) _

Air

_ [ ,,

' to' ~ u2o c

C H 4. Energy of Combustion (BTU/ft3)

650~

C H 4 + H 2 0 ---> C O + 3 H 2

1000 300 999

Fig. 11.15. The C R G T concept .

Figure 11.16a, from a recent study by Jansen et al. [135], is a conceptual flow sheet of an IGCC power plant with CO2 removal; Fig. 11.16b is the corresponding flowsheet for an IGMCFC system. The plant of Fig. 11.16a consists of four basic subsystems; the gasifier, a high temperature cleaning system (HTG), a subsystem for CO2 removal (which includes a CO shift reactor, a high temperature hydrogen permselective membrane module and a CO2 compression/liquefaction unit) and the gas turbine together with the Heat Recovery Steam Generation System (HRSG). The study by Jansen et al. [135] considers a conventional Shell en- trained bed gasifier operating at 30 bar, 1500~ and utilizing 95% pure oxygen. Though, to the best of our knowledge, not currently under development, the potential for adopting solid oxide membrane technology to gasification processes should be investigated. The conventional shift reactor of Fig. 11.16a can be replaced by a membrane shift reactor, as Jansen et al. themselves suggest.

The basic difference between the systems in Figs. 11.16a and 11.16b is that the hydrogen burning turbine is replaced by a molten carbonate fuel cell system (MCFC). In the process of Fig. 11.16b the shift reactor and the membrane unit are placed after the MCFC unit. Jansen et al. [135] estimate energy efficiencies from the ICGG and IGMCFC systems with CO2 removal (assuming a 95% hydrogen recovery in the membrane modules) of 37.5 and 47.5% as opposed to 46.4 and 53.1% without CO2 removal. The energy efficiency losses (and the incremental additional per kWh costs) are small enough to warrant further study for the development of more efficient membranes.

11 - - CURRENT DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 559

(a)

I

I ! ! I

I I

, _~ I

I ! | . . . . .

!

(b)

~ _ ~ o . _ ~ ~ ~ ~ . , f~ i _ 7 " " I ~ , , , . 2 } ~ . . . . . . . . . - ' I - i . J . ' . . . . I i . . . . .

, i I ~ l , / '1 -' I ~1 r162 A C _ ,, -I ~ t I ! 1 ~ , , "-~ i' ' ~ ~ - j ~ ~~i",

~-~!-- '~ ' ~ , ~:. ~

Fig. 11.16. Upper figure (a) is a simplified flowsheet of an IGCC power plant ,with CO2 removal; lower figure (b) the same for an IGMCFC plant. Reproduced from Jansen et al. [135] with permission.

An interesting application of catalytic membrane reactors [14,136] relates to the product ion of tritium which together with deuter ium will be the fuel for the fusion reactors of the future. Tritium is produced by means of a nuclear reaction between neutrons and lithium atoms in a breeder reactor. The trit ium thus produced must be further purified to reach the puri ty levels that are required in the fusion reactor. For the extraction and purification process Basile and

560 11 - - CURRENT DEVELOPMENTS A N D FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS

coworkers [136] propose the use of two membrane reactors. In the first reactor using a hydrogen permselective membrane, tritium is removed from a He stream with the aid of an oxidation reaction. The water thus produced is subsequently decomposed in a membrane shift reactor to release the tritium.

The use of membrane reactors in homogeneous catalysis is attracting attention and will most likely be a very active research area in the future. Here the porous membrane can act as a solid matrix containing an active catalyst or as a separation barrier for reactants and products for the reactions taking place in the liquid phase. Several such applications have begun to appear using organic membranes [137-139]. It is possible that inorganic membranes will provide some advantages in this area over the organic ones, currently utilized.

Multi-phase catalytic reactions have attracted some attention but the area has not in our opinion been fully exploited. Previous studies have demonstrated that the yields obtained with the catalytic membrane reactors are often better than the yields obtained with more conventional reactors. Future research in this area must involve reactions with more immediate industrial applications. Examples of such reactions could be the hydrogenation reactions studied by Gryaznov and co-workers with dense metallic membranes which we discussed earlier. New materials like zeolite membrane could offer some advantages here with their enhanced regio- or chemioselectivity.

11.7 C O N C L U D I N G REMARKS

As we hope has become apparent to the reader of this chapter catalytic membrane reactors have been fully demonstrated at the laboratory scale, the most important systems studied being dehydrogenation and oxidation reaction. Though some small industrial installations already exist the concept of coupling catalytic reaction and separation by membranes has yet to find widespread industrial application. The reasons for this and ways to overcome the existing barriers to commercialization are discussed in a recent paper by Saracco et al. [15]. It seems to us that the most important factor hindering further progress are the membranes themselves. Their synthesis and production meth- ods must be improved. Membranes must not only have good permselectivity and flux, they must also be thermally stable, long-term reliable and affordable. Many of the activities in this area give reason for hope. A number of research groups are working on the direct synthesis of microporous solids, until now unavailable on a commercial scale (silica or alumina); others are focusing on the modification of the existing commercial membranes by the addition of very thin metallic or other films producing membranes with good mechanical strength and permeability combined with high selectivity provided by the thin film. Both approaches are very attractive and additional research in this area is certainly worthwhile. One must not forget, however, that by far the overriding

11 m CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 561

factor determining the commercial viability of any new membrane will be its cost and long term reliability under industrial operating conditions and temperature cycling. The synthesis of new thermally stable microporous materials with molecular sieving characteristics like zeolites shows a good potential for gas phase reactions.

The availability of good, reliable membranes will not, of course, eliminate the need for optimal reactor design and process analysis, necessary to determine the type of membrane to be used and the optimal operating conditions. As was discussed previously, some reactions do not need permselective membranes. Other process parameters like the reactor configuration or the amount of sweep gas utilized can affect dramatically the observed performance.

The use of inorganic membranes in catalytic reactors is providing opporttmity for good fundamental applied research in catalysis and chemical reaction engineering. The coupling of reaction and separation in one unit is an attractive concept but also a complex technological problem. Though for the moment widespread industrial application of the concept is not yet a reality the potential is certainly there. The reactor system more easily amenable to high temperatures industrial application is the PBMR, because in this reactor configuration the membrane does not require extensive regeneration and activation steps. Even for the PBMR applications, however the development of crack-free, uniform, microporous and high temperature resistant membranes is a key step for future developments.

Three-phase catalytic membrane reactor systems, in our opinion, show significant promise, for near term application to hydrogenation reactions for fine chemicals synthesis. These reactions generally require mild operating conditions which will place less stringent requirements on the available and future commercial membranes.

Acknowledgements

Professor Tsotsis wishes to acknowledge the support of the National Science Foundation. The assistance of Karen Woo and Ravi Kumar in typing the manuscript are also gratefully acknowledged.

REFERENCES

1. G. Belfort, Membranes and bioreactors: A technical challenge in biotechnology. Biotech- nol. Bioeng., 33 (1989) 1047.

2. H.N. Chang and S. Furusaki, Membrane bioreactors: present and prospects. Adv. Biochem. Eng. Bioeng., 44 (1991) 27.

3. E. Drioli, G. Iorio and G. Katapano, in: M.C. Porter (Ed.), Handbook of Industrial Mem- brane Technology, Noyes, 1990, p. 401.

4. P.R. Brookes and A.G. Livingston, Biological detoxification of 3-chloronitro benzene manufacture wastewater in an extractive membrane bioreactor. Water Res., 28 (1994) 1347.


5. H.J. Sloot, G.F. Versteeg and W.P.M. van Swaaij, A nonpermselective membrane reactor for chemical processes normally requiring stoichiometric feed rates of reactants. Chem. Eng. Sci., 45 (1990) 2415.

6. H.J. Sloot, C.A. Smolders, W.P.M. van Swaaij and G.F. Versteeg, High temperature membrane reactor for catalytic gas-solid reactions. AIChE J., 38 (1992) 887.

7. J.N. Armor, Catalysis with permselective inorganic membranes. Appl. Catal., 49 (1989) 1. 8. J.N. Armor, Challenges in membrane catalysis. Chemtech, 22 (1992) 557. 9. S. Ilias and R. Govind, Development of high temperature membranes for membrane

reactor: an overview. AIChE Syrup. Ser., 85, 268 (1989) 18. 10. N. Itoh, Dehydrogenation by membrane reactors. Sekiyu Gakkaishi, 33 (1990) 136. 11. H.P. Hsieh, Inorganic membrane reactors. Catal. Rev. Sci. Eng., 33 (1991) 1. 12. V.T. Zaspalis and A.J. Burggraaf, Inorganic membrane reactors to enhance the produc-

tivity of processes, in: R.R. Bhave (Ed.), Inorganic Membranes: Synthesis, Characteristics and Applications. Reinhold, New York, 1991, Ch. 7.

13. T.T. Tsotsis, R.G. Minet, A.M. Champagnie and P.K.T. Liu, Catalytic membrane reactors. in: E.R. Becker and C.J. Pereira (Eds.), Computer Aided Design of Catalysts. Dekker, New York, 1993, pp. 471-551.

14. G. Saracco and V. Specchia, Catalytic inorganic membrane reactors: Present experience and future opportunities. Catal. Rev. Sci. Eng., 36 (1994) 305.

15. G. Saracco, G.F. Versteeg and W.P.M. van Swaaij, Current hurdles to the success of high temperature membrane reactors. J. Membr. Sci., 105 (1994) 95.

16. J. Zaman and A. Chakma, Inorganic membrane reactors. J. Membr. Sci., 92 (1994) 1. 17. V.M. Gryaznov and A.N. Karavanov, Hydrogenation and dehydrogenation of organic

compounds on membrane catalysts (review). Khim.-Farm. Zh., 13(7) (1979) 74. 18. V.M. Gryaznov, Hydrogen permeable palladium membrane catalysts. Plat. Met. Rev.,

30 (1986) 68. 19. J. Shu, B.P.A. Grandjean, A. van Neste and S. Kaliaguine, Catalytic palladium-based

membrane reactors: A Review. Can. ]. Chem. Eng., 69 (1991) 1036. 20. V.M. Gryaznov, A.N. Karavanov, T.M. Belosljudova, A.M. Ermolaev, A.P. Maganjunk

and I.K. Sarycheva, U.S. Patent 4,388,479 (1983). 21. J.E. Philport, Hydrogen diffusion technology. Commercial applications of palladium

membrane. Platinum Met. Rev., 29 (1985) 12. 22. V.M. Gryaznov, O.S. Serebryannikova, M.Yu. Serov, M.M. Ermilova, A.N. Karavanov,

A.P. Mishchenko and N.P. Orekhova, Preparation and catalysis over palladium composite membranes. Appl. Catal., 96 (1993) 15.

23. Z.Y. Li, H. Maeda, K. Kusakabe, S. Morooka, H. Anzai and S. Akiyama, Preparation of palladium-silver alloy membranes for hydrogen separation by the spray pyrolysis method. J. Membr. Sci., 78 (1993) 247.

24. S. Uemiya, Y. Kude, K. Sugino, N. Sato, T. Matsuda and E. Kikuchi, A palladium/porous-glass composite membrane for hydrogen separation. Chem. Lett. 1687 (1988).

25. S. Uemiya, T. Matsuda and E. Kikuchi, Hydrogen permeable palladium-silver alloy membrane supported on porous ceramics. J. Membr. Sci., 56 (1991) 325.

26. D.J. Edlund and W.A. Pledger, A study of long-term stability of composite metal membranes. Paper presented at the 3rd International Conference on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

27. J.K. Ali, E.J. Newson and D.W.T. Rippin, Deactivation and regeneration of Pd-Ag membranes for dehydrogenation reactions. J. Membr. Sci., 89 (1994) 171.

28. A.F.Y. A1-Shammary, I.T. Caga, J.M. Winterbottom, A.Y. Tata and I.R. Harris, Palla-


dium-based diffusion membranes as catalysts in ethylene hydrogenation. J. Chem. Tech. Biotechnol., 52 (1991) 571.

29. S. Uemiya, N. Sato, H. Ando, T. Matsuda and E. Kikuchi, Steam reforming of methane in a hydrogen permeable membrane reactor. Appl. Catal., 67 (1991) 223.

30. B.A. Raich and H.C. Foley, Super equilibrium conversion in palladium membrane reactors. Kinetic sensitivity and time dependence. Appl. Cat.A: General, 129 (1995) 167.

31. A.M. Adris, J.R. Grace, C.J. Lim and S.S.E.H. Elnashaie, Fluidized bed reaction system for steam/hydrocarbon gas reforming to produce hydrogen. U.S. Patent Appl. 07965011, Canad. Patent Appl. 2081170 (1992).

32. S. Laegsgaard and P.E. Jorgensen, H. Nielsen and P. Lehrmann, Stream reforming of methane in a hydrogen-permeable membrane reactor. Paper presented at the 1st Interna- tional Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

33. J. Shu, B.P.A. Grandjean and S. Kaliaguine, Asymmetric Pd-Ag/SS catalytic membranes for CH4 steam reforming. Appl. Catal. A, 119 (1994) 305.

34. E. Kikuchi, Palladium/ceramic membranes for selective hydrogen permeation and their application to membrane reactor. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

35. H.M. Adris, C.J. Lim and J.R. Grace, The fluidized bed membrane reactor system: A pilot-scale experimental study. Paper presented at ISCRE 13, September 1994, Baltimore, MD, USA.

36. Y. Ohta, M. Gondaira, K. Kobayashi, Y. Fujimoto and K. Kuroda, Study of the performance of a steam reformer of town gas equipped with Pd membranes. Paper presented at the 1994 AIChE Annual Meeting, November 1994, San Francisco, CA, USA.

37. K. Jarosch, J.D. Hazlett and H.I. de Lasa, A novel catalytic reactor for steam reforming of CH4. Paper presented at the 1994 AIChE Annual Meeting, November 1994, San Francisco, CA, USA.

38. D. Eng and M. Stoukides, Catalytic and electrocatalytic methane oxidation with solid oxide membranes. Catal. Rev. Sci. Eng., 33 (1991) 375.

39. T. Kameyama, M. Dokiya, M. Fujishige, H. Yokokawa and K. Fukuda, Possibility for effctive production of hydrogen from hydrogen sulfide by means of a porous vycor glass membrane. Ind. Eng. Chem. Fundam., 20 (1981) 97.

40. T. Kameyama, M. Dokiya, M. Fujishige, H. Yokokawa and K. Fukuda, Production of hydrogen from hydrogen sulfide by means of selective diffusion membrane. J. Hydrogen Energy, 8 (1983) 5.

41. Y.H. Ma, W.R. Moser, S. Pien and A.B. Shelekhin, Experimental study of H2S decomposition in membrane reactor. Paper presented at the 3rd International Conference on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

42. J.J. Hammel, W.P. Marshall, W.J. Robertson and H.W. Barch, Porous-siliceous containing gas enriching hollow fibers and process of manufacture and use. Eur. Pat. Appl. 248,391 (1987).

43. M. Hassan, P.M. Thoen, J.D. Way and A.C. Dillon, Separation of gas mixtures using hollow film silica membranes. Paper presented at the 3rd International Conference on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

44. G.R. Gavalas, C.E. Megiris and S.W. Nam, Deposition of H2-permselective SiO2 Films. Chem. Eng. Sci., 44 (1989) 1829.

45. C.E. Megiris and J.H.E. Glezer, Preparation of silicon dioxide films by low-pressure chemical vapor deposition on dense and porous alumina substrates. Chem. Eng. Sci., 3925 (1992) 47.


46. T. Ioannides and G.R. Gavalas, Catalytic isobutane dehydrogenation in a dense silica membrane reactor. J. Membr. Sci., 77 (1993) 207.

47. G.R. Gavalas and S. Jiang, Use of carbon barriers in chemical vapor deposition of silica membranes. Paper presented at the 1994 AIChE Annual Meeting, November 1994, San Francisco, CA, USA.

48. M. Tsapatsis and G.R. Gavalas, Structure and aging characteristics of H2 permselective SiO2-Vycor membranes. ]. Membr. Sci., 87 (1994) 281.

49. T. Kameyama, K. Fukuda, M. Dokiya and Y. Kotera, Separation of hydrogen from hydrogen sulfide and hydrogen mixtures. Japan Patent 78,99,078 (1978).

50. M. Dokiya, J. Kameyama and K. Fukuda, Separation of hydrogen from hydrogen sulfide. Japan Patent 78,130,291 (1978).

51. A.D. Davidson and M. Salim, Initial studies of a novel ceramic membrane reactor. Br. Ceram. Proc., 119 (1988) 43.

52. N. Nourbakhsh, T.T. Tsotsis and I.A. Webster, Model planar alumina catalyst preparation and aging under hydrotreating conditions. Appl. Catal., 50 (1989) 65.

53. N. Nourbakhsh, Anodic Alumina Films: Preparation, Characterization and Investiga- tion of Reaction and Transport Properties. Ph.D. Thesis, USC, May 1990.

54. A.F.M. Leenaars, K. Keizer and A.J. Burggraaf, The preparation and characterization of alumina membranes with ultrafine pores. J. Mater. Sci., 19 (1984) 1077.

55. R.J.R. Uhlhorn, M.H.B.J. Huis In't Veld, K. Keizer and A.J. Burggraaf, Synthesis of nonsupported and supported ~'-alumina membranes without defects. J. Mater. Sci., 27 (1992) 527.

56. A. Julbe, C. Guizard, A. Larbot, L. Cot and A.G. Fendler, The sol-gel approach to prepare candidate microporous inorganic membranes for membranes reactors. J. Membr. Sci., 77 (1993) 137.

57. J. Randon, A. Julbe, P. David, K. Jaafari and S. Elmaleh, Computer simulation of inorganic membrane morphology, 2. Effect of infiltration at the membrane support interface. ]. Colloid Interface Sci., 161 (1993) 384.

58. C.L. Lin, D.L. Flowers and P.K.T. Liu, Characterization of ceramic membranes. II. Modified commercial membranes with pore size under 40 ~. J. Membr. Sci., 92 (1994) 45.

59. J. Luyten, J. Cooymans, R. Persons, R. Leysen and J. Sleurs, A New Ceramic Support Material for Gas Separative Membranes. Paper presented at the 3rd International Confer- ence on Inorganic Membranes, July, 1994, Worcester, MA, USA.

60. P.K.T. Liu, Private communication, 1995. 61. J. Deng and J. Wu, Formaldehyde production by catalytic dehydrogenation of methanol

in inorganic membrane reactors. Appl. Catal. A, 109 (1994) 63. 62. M. Chai, M. Machida, K. Eguchi and H. Arai, Promotion of hydrogen permeation on

metal dispersed alumina membranes and its application to a membrane reactor for methane steam reforming. Appl. Catal. A, 110 (1994) 239.

63. R.G. Minet, S.P. Vasileiadis and T.T. Tsotsis, Experimental studies of a ceramic membrane reactor for the steam/methane reaction at moderate temperatures 400-700~ in: G.A. Huff and D.A. Scarpiello (Eds.), Proceedings of the Symposium on Natural Gas Upgrading, 37 (1992) 245.

64. V.T. Zaspalis, W. van Praag, K. Keizer and J.R.H. Ross, Synthesis and characterization of primary alumina, titania and binary membranes. J. Mater. Sci., 27 (1992) 1023.

65. K.N.P. Kumar, K. Keizer and A.J. Burggraaf, Stabilization of the porous texture of nano structured titania by avoiding a phase transformation. J. Mater. Sci. Lett., 59 (1994) 13.

66. Y.S. Lin, C.H. Chang and R. Gopalan, Improvement of thermal stability of porous

11 -- CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 565

nanostructured ceramic membranes. I\ &EC Res., 33 (1994) 860. 67. C.J. Brinker, R. Sehgal, N.K. Raman, S. Prakash, S.L. Hietala and S. Wallace, Sol-gel

strategies for amorphous inorganic membranes exhibiting sieving characteristics. Paper presented at the 3rd International Congress on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

68. W.O. Haag and J.G. Tsikoyiannis, Membrane composed of pure molecular-sieve. U.S. Patent 5,019,263 (1991).

69. W.J.W. Bakker, G. Zheng, F. Kapteijn, M. Makkee, J.A. Moulijn, E.R. Geuss and H. van Bekkum, in: M.P.C. Weijnen and A.H.H. Drinkenburg (Eds.), Single and Multicompo- nent Transport through Supported MFI Zeolite Membranes. Kluwer, Dordrecht, 1993, p. 425.

70. F. Kapteijn, W.J.W. Bakker, G. Zheng, J.A. Moulijn and H. van Bekkum, Permeation and separation behavior of a silicalite MFI membrane. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

71. K. Kusakabe, A. Murata and S. Morooka,, Permeation of inorganic gases thoughts zeolite membranes at high temperatures. Paper presented at the 3rd International Confer- ence on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

72. M.D. Jia, B. Chen, R.D. Noble and J.L. Falconer, Ceramic-zeolite composite for the separation of vapor/gas mixtures. J. Membr. Sci., 90 (1994) 1.

73. S. Xiang and Y.H. Ma, Formation and characterization of zeolite membranes from sols. Paper presented at the 3rd International Conference on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

74. J. Wang, Y. Wang, S. Fan and X. Shi, Permeation and gas permeabilities of zeolite membranes. Microporous Mater., 4 (1995) 213.

75. P. Meriaudeau, A. Thangaraj and C. Naccache, Preparation and characterization of silicalite molecular sieve membrane over supported porous sintered glass. Paper presented at the 1st International Workshop of Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

76. D. Uzio, J. Peureux, A.G. Fendler, J.-A. Dalmon and J. Ramsay, Formation and pore structure of zeolite membranes, in: J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger (Eds.), Characterization of Porous Solids III. Studies in Surface Science and Catalysis, Vol. 87. Elsevier, Amsterdam and New York, 1994, p. 411.

77. Z.A.E.P. Vroon, K. Keizer, M.J. Gilde, H. Verweij and A.J. Burggraaf, Transport properties of alkanes through ceramic thin zeolite MFI membranes. J. Membr. Sci.,113 (1996) 293.

78. D. Cazanave, A.G. Fendler, J. Sanchez, R. Loutaty and J.-A. Dalmon, Control of transport properties with a microporous membrane reactor to enhance yields in dehydrogenation reactions. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

79. J.F. Akyurtlu, A. Akyurtlu and C.E. Hamrin, A study on the performance of the catalytic porous-wall three-phase reactor. Chem. Eng. Comm., 66 (1988) 169.

80. M.P. Harold, P. Cini and B. Patenaude, The catalytically impregnated tube: an alternative multiphase reactor. AIChE Symp. Ser., 268 (1989) 26.

81. P. Cini and M.P. Harold, Experimental study of the tubular multiphase catalyst. AIChE l., 37, 7 (1991) 997-1008.

82. M. Torres, J. Sanchez, J.-A. Dalmon, B. Bernauer and J. Lieto, Modeling and simulation of a three-phase membrane reactor for nitrobenzene hydrogenation. Ind. Eng. Chem. Res., 33, 10 (1994) 2421.


83. A. Pantazidis, J. A. Dalmon and Mirodatos, Oxidative dehydrogenation of propane in catalytic membrane reactor. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

84. M.P. Harold, V.T. Zaspalis, K. Keizer and A.J. BurggraaL Intermediate product yield enhancement with a catalytic inorganic membrane, I. Analytical model for the case of isothermal and differential operation. Chem. Eng. Sci., 48 (1993) 2705.

85. D. Lafarga, J. Santamaria and M. Menendez, Methane oxidative coupling using porous ceramic membrane reactors. I. Reactor development. Chem. Eng. Sci., 49 (1994) 2005.

86. J. Coronas, M. Menendez and J. Santamaria, Methane oxidative coupling using porous ceramic membrane reactors. II. Reaction studies. Chem. Eng. Sci., 12 (1994) 2015.

87. Y.D. Chen and R.T. Yang, Preparation of carbon molecular sieve membrane and diffusion of binary mixtures in the membrane. Ind. Eng. Chem. Res., 33 (1994) 646.

88. M.B. Rao and S. Sircar, Nanoporous carbon membranes for separation of gas mixtures by selective surface flow. J. Membr. Sci., 85 (1993) 253.

89. J.E. Koresh and A. Softer, Study of molecular sieve carbons. Part I. Pore structure, gradual pore opening and mechanism of molecular sieving. J. Chem. Soc. Faraday Trans. /, 76 (1980) 2457.

90. J.E. Koresh and A. Softer, Molecular sieve carbon permselective membrane. Sep. Sci. Technol., 18 (1983) 723.

91. J.E. Koresh and A. Softer, Mechanism of permeation through molecular sieve carbon. ]. Chem. Soc. Faraday Trans. I, 82 (1986) 2057.

92. J.E. Koresh and A. Softer, The carbon molecular-sieve membranes, general properties and the permeability of CH4/H2 mixture. Sep. Sci. Technol., 22 (1987) 972.

93. V.M. Linkov, R.D. Sanderson, E.P. Jacobs, A.L. Lapidus and A.J. Krylova, Hollow fibre carbon membranes in Fischer Tropsch synthesis. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

94. T.H. Etsell and S.N. Flengas, The Electrical properties of solid oxide electrolytes. Chem. Rev., 70 (1970) 339.

95. A. Clearfield, Role of ion exchange in solid state chemistry. Chem. Rev., 88 (1988) 125. 96. P.J. Gellings, H.J.A. Koopmans and A.J. Burgraaf, Electrocatalytic phenomena in gas

phase reactions in solid electrolyte electrochemical cells. Appl. Catal., 39 (1988) 1. 97. B.C.H. Steele, Oxygen ion conductors and their technological applications. Mater. Sci.

Eng., B13 (1992) 79. 98. D. Eng and M. Stoukides, Catalytic and electrocatalytic methane oxidation with solid

oxide membranes. Catal. Rev. Sci. Eng., 33 (1991) 375. 99. M. Stoukides, Application of solid electrolytes in heterogeneous catalysis. Ind. Eng.

Chem. Res., 27 (1988) 1745. 100. S.H. Langer, Electrogenerative systems. Potential uses include clean-up of flue gases

from coal fired stationary power plants. Plat. Met. Rev., 36 (1992) 202. 101. Y. Teraoka, H. M. Zhang, S. Furukawa and N. Yamazoe, Oxygen permeation through

perovskite type oxides. Chem. Lett. (1985) 1743. 102. Y. Teraoka, H. M. Zhang, K. Okamffoto and N. Yamazoe, Mixed ionic electronic

conductivity of Lal-xSrxCOl-yFeyO3-~ perovskite-type oxides. Mater. Res. Bull., 33 (1988) 51. 103. Y. Teraoka, T. Nobunaga and N. Yamazoe, Effect of cation substitution on the oxygen

semipermeability of perovskite type oxides. Chem. Left. (1988) 503. 104. Y. Teraoka, N. Nobunaga, K. Okamoto, N. Miura and N. Yamazoe, Influence of

constituent metal cations in substituted LaCoO3 on mixed conductivity and oxygen permeability. Solid State Ion., 48 (1991) 207.


105. S. Pancharatnam, R.A. Huggins and D.M. Mason, Catalytic decomposition of nitric oxide on zirconia by electrolytic removal of oxygen. J. Electrochem. Soc., 122 (1975) 869.

106. D.M. Mason, Method and apparatus for catalytic dissociation of NO. US Patent 4,253,925, March 3, 1975.

107. T.M. Gur and R.A. Huggins, Decomposition of nitric oxide on zirconia in a solid state electrochemical cell. J. Electrochem. Soc., 126 (1979) 1067.

108. A.V. Joshi, An alternative method for the removal of oxides of nitrogen and sulfur from combustion processes. Final Report, US DOE contract DE-AC22-85PC81003, 1986.

109. J.W. Cook, L.P. Cornell, M. Keyrani, M. Neyman and D. Helfritch, Simultaneous particulate NOx, SOx removal from flue-gas by all solid-state eletrochemical technology. Final Report, US DOE Contract, DE-AC22-87PC79856.

110. D.J. Clark, R.W. Losey and J.W. Suitor, Separation of oxygen by using zirconia solid electrolyte membranes. Gas Sep. Purif., 6 (1992) 201.

111. C.G. Vayenas, S. Bebelis, I. V. Yentekakis, P. Tsiakaras and H. Karasali, Non-faradaic electrochemical modification of catalytic activity. Platinum Metals Rev., 34 (1990) 122.

112. U. Balachandran, J.T. Dusek, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich and A.C. Bose, Dense ceramic membranes for partial oxy- genation of methane. Paper presented at the 3rd International Congress on Inorganic Mem- branes, July 10-14, 1994, Worcester, MA, USA.

113. U. Balachandran, J.T. Dusek, S.M. Sweeney, R.B. Poeffel, R.L. Mievile, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich and A.C. Bose, Methane to syngas via ceramic membranes. Am. Ceramic. Soc. Bull., 74 (1995) 71.

114. A. Chanaud, A. Julbe, C. Larbot, L. Guizard. H. Cot, A. Borges, A.G. Fendler and C. Mirodatos, Catalytic membrane reactor for the oxidative coupling of methane, Part 1: Preparation and characterization of LaOC1 membranes. Paper presented at the 1st Inter- national Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

115. H. Borges, A.G. Fendler, C. Mirodatos, P. Chanaud and A. Julbe, Catalytic membrane reactor for the oxidative coupling of methane, Part 2: Catalytic properties of LaOC1 membranes. Paper presented at the the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

116. Y.L. Becker, A.G. Dixon, W.R. Moser and Y.H. Ma, Modelling of ethylbenzene dehydrogenation in a catalytic membrane reactor. J. Membr. Sci., 77 (1993) 197.

117. M. Tayakout, B. Bernauer, Y. Toure and J. Sanchez, Modeling and simulation of a catalytic membrane reactor. J. Simul. Pract. Theory, 2 (1995) 205.

118. M. Tayakout, J. Sanchez, D. Uzio and J.A. Dalmon, Catalytic membranes for cyclohexane dehydrogenation reaction. Paper presented at the 1st International Workshop on Cata- lytic Membranes, September 1994, Lyon-Villeurbanne, France.

119. C.Y. Tsai, Y.H. Ma, W.R. Moser and A.G. Dixon, Simulation of nonisothermal catalytic membrane reactor of CH4 partial oxidation to syngas. Paper presented at the 3rd Interna- tional Congress on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

120. A.G. Dixon, W.R. Moser and Y.H. Ma, Waste reduction and recovery using 02 permeable membrane reactors. Ind. Eng. Chem. Res., 33 (1994) 3015.

121. W. Wang and Y. S. Lin, A theoretical analysis of oxidative coupling of CH4 in a tubular dense membrane reactor. Paper presented at the 3rd International Congress on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

122. Y. Lu, A. Ramachandra, Y.H. Ma, W.R. Moser and A.G. Dixon, Reactor modeling of the oxidative coupling of methane in membrane reactors. Paper presented at the 3rd Interna- tional Congress on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.


123. T. Nozaki and K. Fujimoto, Oxide ion transport for selective oxidative coupling of methane with new membrane reactor. AIChE J., 40 (1994) 870.

124. N. Itoh, W. C. Xu and K. Haraya, Radial mixing diffusion of hydrogen in a packed-bed type of Pd membrane reactor. Ind. Eng.Chem. Res., 33 (1994) 197.

125. V.A. Papavassiliou, J.A. McHenry, E.W. Corcoran, H.W. Deckman and J.H. Meldon, High flux asymmetric catalytic membrane reactors. Optimization of operating conditions. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon- Villeurbanne, France.

126. A.B. Bindjouli, Z. Dehouche, B. Bernauer and J. Lieto, Numerical simulation of catalytic inert membrane reactor. Computers Chem. Eng., 18 (Suppl.) (1994) 5337-5341.

127. A. Varma, K.L. Yeung, J. Szegner and G. Cao, Nonuniform catalyst distribution for catalytic membrane reactors. Paper presented at the 3rd International Congress on Inorganic Membranes, July 10-14, 1994, Worcester, MA, USA.

128. E. Gobina and R. Hughes, Equilibrium-shift in alkane dehydrogenation using a high temperature catalytic membrane reactor. Paper presented at the 1st International Workshop on Catalytic Membranes, September 1994, Lyon-Villeurbanne, France.

129. Y.V. Gokhale, R.D. Noble and J.L. Falconer, Analysis of a membrane-enclosed catalytic reactor for butane dehydrogenation. J. Membr. Sci., 77 (1993) 197.

130. M.P. Harold, C. Lee, A.J. Burggraaf, K. Keizer, V.T. Zaspalis, and R.S.A. de Lange, Catalysis with inorganic membranes. MRS Bull., XIX (4) (1994) 34.

131. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Formation and characterization of supported microporous ceramic membranes prepared by sol-gel modification techniques. J. Membr. Sci., 99 (1995) 57.

132. R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Permeation and separation studies sol-gel modified ceramic membranes. Microporous Mat., 4 (1995) 169.

133. G. Cacciola, Y.I. Aristov, G. Restuccia and V.N. Parmon, Influence of hydrogen-permeable membranes upon the efficiency of the high temperature chemical heat pumps based on cyclohexane dehydrogenation, benzene hydrogenation reactions. Int. J. Hydro- gen Energy, 18 (1993) 673.

134. R.G. Minet and T.T. Tsotsis, Catalytic membrane steam/hydrocarbon reformer. U.S. Patent 4,981,676, January 1,1991.

135. D. Jansen, A.B.J. Oudhuis and H.M. van Veen, CO2 reduction potential of future coal gasification based power generation technologies. Energy Conserv. Manage., 33 (1992) 365.

136. A. Basile, V. Violante and E. Drioli, The membrane integrated system in the fusion reactor fuel cycle. Paper presented at the 1st International Workshop on Catalytic Mem- branes, September 1994, Lyon-Villeurbanne, France.

137. J. Feldman and M. Orchin, Membrane-supported rhodium hydroformylation catalysts. J. Mol. Catal., 63 (1990) 213.

138. J. Kim and R. Datta, Supported liquid-phase catalytic membrane reactor-separator for homogeneous catalysis. AIChE J., 37 11 (1991) 165.

139. S. Chen, H. Fan and Y.-K. Kao, A membrane reactor with two dispersion-free interfaces for homogeneous catalytic reactions. Chem. Eng. J., 49 (1992) 35.

140. R.J.R. Uhlhorn, K. Keizer, A.J. Burggraaf, Gas transport and separation with ceramic membranes, Part II. Synthesis and properties of microporous membranes. J. Membr. Sci., 66 (1992) 271.

[membrane science and technology] fundamentals of inorganic membrane science and technology volume 4...

Documents