Inorganic Membrane Reactors

ELSEVIER Journal of.Membrane Science 92 ( 1994) 1-28

Review

Inorganic membrane reactors

J. Zaman, A. Chakma* Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Canada TZN IN4

(Received August 27, 1993; accepted in revised form February 15, 1994)

Abstract

The availability of inorganic membranes which can withstand high temperatures has resulted in a wide ranging opportunity for the application of membranes in chemical reactors. Increased conversions, better selectivity, milder operating conditions and decreased separation load are some of the attractive features which are going to promote membranes as chemical reactors in many established and novel reaction systems. Established and emerging technologies in ceramics, semiconductors and metal plating such as slip casting, electroless plating, sputtering, and chemical and electrochemical vapor deposition techniques are being successfully adapted in the laboratory scale to produce membranes with high permeabilities and improved separation factors. It should soon be possible to categorize membrane technologies for different types of reactions. Further advancements in modifying the surface of the membranes to tailor them to specific catalytic and separation requirements will greatly enhance the versa- tility of the membranes as chemical reactors and separators in the future. This article reviews various applications of membrane reactors with particular emphasis on their application in high-temperature gas-phase reactions.

Keywords: Ceramic membranes; Inorganic membranes; Membrane reactors; Membrane preparation and structure; Gas separation

1. Introduction

The development of inorganic membranes, particularly the ceramic membranes having consistent quality and narrow pore size distribution, in recent years paved the way for the application of membranes in high-temperature reactors. As opposed to polymeric membranes, the inorganic membranes are characterized by high resistance to temperature and corrosive environments, and good mechanical stability. The use of membranes in chemical reactors is motivated principally by the equilibrium shift caused by selective or preferential permeation of reaction products,

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leading to a higher conversion in a single pass. The equilibrium shift also allows attaining a given conversion at less severe conditions of temperature and pressure. As reaction and permeation proceed simultaneously, the separation of product can be accomplished in the reactor unit itself, or at least the downstream separation load is reduced. Selective or preferential permeation may prevent further reaction of a product and this may improve the yield of a desired component in a multiple reaction system. On the other hand, the ability to introduce a reactant in a controlled manner through a permeating membrane may allow the regulation of the reaction leading to better yield and selectivity and improved control. In addition, the membrane may allow hot separation of products and eliminate

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2 J. Zaman, A. Chakma /Journal of Membrane Science 92 (1994) l-28

the need for quenching a reaction to prevent back reaction. The membrane itself may act as a catalyst or catalyst may be impregnated on the membrane. The membrane reactor may be bifunctional and two complementary reactions may take place on either side of the membrane, the product of reaction on one side acting as a reactant on the other side, while the endothermicity of one reaction is compensated by the exotherm- icity of the other.

There are basically two types of inorganic membranes which can be used for membrane reactors: dense (nonporous) and porous membranes. Palladium and its alloys with ruthenium, nickel or other metals from groups VI to VIII, silver and zirconia are examples of dense membranes. Palladium-based membranes are permeable only to hydrogen while silver and zirconia are permeable only to oxygen. These membranes have high selectivity, but low permeability. Zir- conia membranes are solid oxide electrolytes and their permeability depends on ionic conductivity. Examples of commercial porous inorganic membranes are ceramic membranes, such as alumina, silica and titania, glass and porous metals, such as stainless steel and silver. These membranes are characterized by high permeability, but low selectivity. Recently, attempts are being made to combine the selectivity of the dense membranes and the permeability of porous membranes by supporting dense membranes on porous supports using various techniques. In addition, new materials are being developed and preparation techniques devised to produce thin- ner membranes and/or smaller pore-sized membranes. Methods are also being developed to modify the pore structure, producing smaller pores and increasing the stability of the membrane.

A number of excellent reviews came out recently on inorganic membranes [ l-7 1. The purpose of this review is to concentrate on the application of membrane reactors for high- temperature gas-phase reactions. The discussion will center around the most promising inorganic membranes developed so far: the palladium- based membranes, ceramic and glass membranes, solid oxide membranes and dense membranes supported on porous substrates.

2. Membrane reactor configuration

The membrane reactors are usually operated in parallel or cross-flow mode, with the reactants on one side and the permeate on the other. The permeate is driven by vacuum or a sweep gas. Quite often, the membrane is inert and the reactor may be packed with catalyst or the catalyst may be fluidized. The former is called the inert membrane packed bed reactor (IMPBR) while the latter is called inert membrane fluidized bed reactor (IMFBR). When the membrane itself acts as a catalyst or catalyst is impregnated in the membrane and the reactants pass through the empty tube having the catalytic membrane wall, it is called a catalytic membrane reactor (CMR ) . When a CMR is also packed with catalysts, it is called a packed bed or fluidized bed catalytic membrane reactor (PBCMR or FBCMR). In most laboratory work, tubular reactors have been used with either cocurrent or countercurrent operation. In some cases, membrane discs have been used with flow across the disc. In industrial reactors, shell and tube configuration with an as- sembly of single tubes or multichannel mono- liths may be incorporated into a large shell. Hol- low fibers offer a much greater packing density, but suffer from poor mechanical strength. A number of other shapes have been suggested for palladium-based membranes [ 5 1. The ends of the membranes have been sealed by glazing or by very line powders. The membrane tube has been sealed by carbon or graphite strings pressed against Swagelok-type compression fittings.

A schematic layout for a membrane reactor system is presented in Fig. 1. The arrangement shown is for the decomposition of hydrogen sulfide to hydrogen and sulfur in the IMPBR mode. The ceramic tube is a multilayered membrane with the top layer having pore size 5 nm, inside diameter 7 mm, length 250 mm and outside diameter 10 mm. It is enclosed in a quartz tube shell sealed by a high-temperature adhesive. The reactor is placed in a furnace. The reactants and sweep gases are allowed at specified flow rates by mass flow meters and the pressures are controlled by the needle valves at the exit of the reactor and sweep sides. The membrane tube is

J. Zaman, A. Chakma /Journal ofMembrane Science 92 (1994) 1-28 3

t I SWEEP a PERYEATUIN GAS SWEEP G&S

Fig. 1. A schematic layout of a membrane reactor system.

loaded with catalysts and thermocouples are placed to measure the temperature at different points in the reactor.

The permeation studies are done in situ by closing the sweep gas inlet and measuring the exit gas flow rates by a soap bubble flow meter. The kinetics of the reaction can also be studied in situ by closing the inlet and exit valves for the sweep gas. It is possible to operate the system as a CMR by replacing the membrane with a catalyst-impregnated membrane or a membrane which itself acts as a catalyst. The PBCMR can be prepared by packing the CMR with catalysts.

3. Palladium-based membranes

The palladium-based membranes are highly selective to hydrogen and have been used with hydrogenation [8-lo], dehydrogenation [ ll- 141, dehydrogenation-oxidation [ 15-l 71 and steam reforming [ 18,191 reactions. Palladium alloys are often preferred, because pure palladium tends to become brittle after repeated cycles of hydrogen absorption and desorption. Exten- sive studies have been made over the years on the permeability as well as mechanical properties and durability of these membranes [ 20-241. Much of the early studies of the applications of palladium-based membranes have been carried out in the former Soviet Union [ 25-301. Several small scale commercial applications of these membranes have been reported [ 26,3 l-341. Shu

et al. [ 5 ] provide an extensive coverage of the palladium-based membranes, their properties, preparation techniques and applications. The discussion below on these membranes will be brief and will focus primarily on recent applications.

Schmitz et al. [ 181 used a palladium-silver alloy membrane in the lower part of a reformer tube at N 700’ C and 1 O-35 bar pressure. A 25% increase of conversion over the equilibrium value was obtained. They also investigated the permeation of H2 in membranes made of palladium, palladium-silver, stoichiometric titanium-nickel alloy coated with nickel, membranes of vanadium with a coating of copper and palladium, and a double-layer membrane of Pd-Ag alloy (atomic ratio 75/25 ) coated with vanadium. The titanium-nickel membranes had poor permeation rates and the coated vanadium membranes performed better than pure palladium, while the Pd-Ag membranes had the best permeation rate. Itoh et al. [ 14 ] recently conducted a series of experiments to study the mechanism of hydrogen permeation through the palladium membrane, its long-term stability and the validity of the reactor models postulated earlier. The results showed that the hydrogen permeation obeyed Siebert’s law and that the amount of dehydrogenation reaction occurring on the membrane surface was small. The membrane reactor operated under stable conditions for more than 120 h. Countercurrent mode of operation resulted in better performance and this confirms the findings of their previous simulation studies [ 13 1. A palladium alloy membrane (67% Pd, 20% Au, 13% Ag) used by them gave a permeation rate constant 1.7 times higher than that obtained with a pure palladium membrane. The alloying generally improves permeability and/or durability of the membrane. Itoh and coworkers [ 15- 17 ] demonstrated both theoreti- cally and experimentally the feasibility of carry- ing out the dehydrogenation of 1-butene to butadiene on the reaction side and the oxidation of permeated hydrogen by the sweep gas oxygen to water on the permeation side of a palladium membrane reactor. The palladium membrane in this case acted as catalyst for both the reactions.

4 J. Zaman, A. Chakma /Journal of Membrane Science 92 (1994) 1-28

The oxidation reaction increases the hydrogen flux across the membrane and supplies the thermal energy for the endothermic dehydrogenation reaction.

Edlund and Pledger [ 35 ] used a composite metal membrane to carry out the decomposition of hydrogen sulfide at 700°C and a H2S partial pressure of 793 kPa abs. Vanadium was used as the base metal which, unlike palladium does not suffer from hydrogen embrittlement, has good permeability for hydrogen and is less expensive. Platinum was used on the feed side to protect the vanadium from attack by hydrogen sulfide. It also acts as a catalyst for the decomposition reaction. On the permeate side, the vanadium was coated with palladium to protect it from oxidation during start up and shut down. An intermetallic barrier of Si02 on each side of the membrane was found to provide good stability for constant hydrogen flux. The detailed procedure of preparing the composite membrane has been discussed by them. The platinum coating on the feed side offered the greatest resistance to hydrogen permeation. They found that when used on the feed side, palladium quickly gets corroded by hydrogen sulfide while platinum remained unaffected. Nearly complete conversion of H2S was achieved in the membrane reactor, with a feed concentration of 1.5 to 100% H$.

The greatest difficulty in developing metal- based membrane reactors is the difficulty in obtaining a thin membrane with sufficient mechanical strength and satisfactory permeability. These two objectives act opposite to each other and commercial metal membranes have usually permeabilities lower than what would be required for the profitable operation of a membrane reactor/separator system.

4. Ceramic and glass membranes

The advent of porous ceramic and glass membranes spurred applications of inorganic membranes as separators and reactors. The ceramic membranes are characterized by high permeation rates, but low selectivity, because separation is primarily governed by Knudsen diffusion. In

order to provide mechanical stability, the sepa- rating membrane is usually supported on a coarse support membrane. Recently, multilayered composite alumina membranes are being extensively used in chemical reactor and separation applications. A typical composite tubular membrane will consist of an innermost layer, typically of 5 pm thickness and having an average pore size of 4 nm. Successive layers are thicker, with progres- sively larger pores (200 nm and 800 nm), supported on a thick support layer, N 1.5 mm thick, with a pore size in the range of 1 O-l 5 pm. The glass membranes are usually prepared by a com- bination of heat treatment and chemical leach- ing, while ceramic membranes are made by slip casting. The details of the preparation methods are closely guarded; several reviews and patent information provide useful guidelines [ 361. The sol-gel technique emerged as the most versatile method of obtaining a thin, uniform and pin- hole-free top layer and the intermediate layers on the ceramic support; this technique has been extensively covered in the recent literature [ 36- 47 1. The quality of the membrane is very much dependent on the preparation procedures and a close adherence to a rigid protocol is necessary to obtain membranes of consistent quality. The membrane developed has to undergo a series of characterization tests using techniques based on adsorption, X-ray diffraction, SEM, TEM, etc. These are well covered in the literature on preparation techniques [ 39-441.

4.1. Applications of ceramic and glass membranes

The primary applications of ceramic and glass membranes, similar to the palladium-based membranes, have been the hydrogenation-dehydrogenation reactions. The most favorite reactions in recent times have been the industrially important reactions of the decomposition of hydrogen sulfide [48-531 and the dehydrogenation of lower paraffins [ 54-671, cyclohexane [ 68-741 and ethylbenzene [ 75-791. Increasing attention is being focused on the benefits of selectivity in multiple reactions, the ability to lower the reaction temperatures and the possibility to

J. Zaman, A. Chakma /Journal of Membrane Science 92 (1994) l-28 5

attain practically 100% conversions in environmentally important reactions [ 80-921. A number of applications of these membranes are listed in Table 1.

Interests in the application of porous glass membranes have been shown principally by Jap- anese workers [ 48-521. Compared to the ceramic membranes, glass membranes suffer from fragility and unsuitability beyond 800°C. Alu- mina occupies the dominant position in research and development in ceramic membranes. Silica and titania membranes have also been investigated by some authors, their developments following the same pattern as alumina [ 40,411. There have been a few other ceramic materials referred to in the literature, but not much information is available on them. These materials in- clude mullite, cordierite, mica, silicon carbide, silicon nitride, tin oxide, etc. [ 4 1. Among other inorganic membranes, porous stainless steel has been considered a promising material particularly to act as a substrate for a more selective film of membrane.

4.2. Applications to decomposition reactions.

Kameyama and coworkers [48,49] used Vy- car glass and alumina membrane reactors at 600- 800°C for the decomposition of hydrogen sul- tide. The reactor tube was 600 mm in length, 15 mm in outside diameter and 3 mm in thickness. It had a porosity of 0.79, a mean pore diameter 4.5 nm and a surface area 19 1 m*/g. The membrane tube was loaded with MoS, catalyst and was surrounded by a stainless steel shell protected by a nonporous alumina tube placed close to it. The ends of the porous glass tubing was sealed at both ends by carbon packing. The feed gas was introduced into the membrane side at high pressure (404 kPa) with the permeate side kept at 10 1 kPa. The glass membrane was tested for durability and no degradation was observed in hydrogen and hydrogen sulfide atmosphere between 873 and 1073 K for 2 16 h of operation. However, a remarkable shrinkage occurred beyond 1073 K, thus setting this as the limiting value of temperature for this membrane. The catalytic activity of the porous glass particles on

the decomposition was tested and this showed no more conversion than in an empty tube. The Vy- car glass membrane, therefore, did not have any catalytic activity on the reaction. Permeation of sulfur was found to be low and it was concen- trated mainly on the retentate side. Significant equilibrium shift caused a conversion of double the equilibrium value. They also worked on a porous alumina tube reactor having higher temperature resistance and 30 times greater permeability than the Vycor glass tube having smaller pores [ 49 1. However, the separation factor was poorer. A film coating composed of alumina sol on the tube improved the separation factor apprecia- bly. An early Japanese patent [ 501 for this reaction suggested a ceramic membrane ( Si02, A1203 or Si3N4) with 3-10 nm pore diameter and with the sulfide of MO, W or Ru as catalyst at temperatures higher than 600°C and pressures 19.6-98 Pa for the H2S decomposition reaction. A hollow fiber construction of ceramic or glass membranes was described in another patent [ 5 11. A European patent [ 53 ] described membranes of various materials such as A1203, Si02-A1203, ZrO,, zeolite, porous glass or C supported on A1203, Si02-A1203, mullite, cordierite, Zr02 or C. The catalyst for the decomposition was MoS, or precious metals such as Pt and Pd [ 53 1.

4.3. Applications to dehydrogenation reactions.

This is the most widely studied class of reactions in membrane reactors of all kinds. Most of the reactions have been taken up because of their industrial importance, there being already established conventional reactor systems operated commercially. The impetus for the study of these reactions has been the possibility of enhanced conversions, improved selectivity, decreased downstream separation load and milder operating conditions. The milder operating conditions are perceived to be beneficial not only as energy- saving technology, they might be a remedy for coking and catalyst deactivation problem inherent in many commercial processes. The recent applications are discussed in this review.

6

Table I

J. Zaman, .-t. Chakrna /Journal qf.Vernbrane Science 92 (1994) 1-28

Application of porous inorganic membrane reactors

Reaction Membrane Reactor configuration Operating conditions Main conclusions Ref.

Decomposition of hydrogen sulfide

Dehydrogenation of ethane Dehydrogenation of propane Dehydrogenation of n-butane

Dehydrogenation of cyclohexane



Dehydrogenation of ethylbenzene

Dehydrogenation Composite of ethylbenzene alumina

Dehydrogenation of methanol

Glass

Dehydrogenation Composite of methanol alumina

Reduction of nitrogen oxide with ammonia

Steam reforming Composite of methane alumina

Glass, alumina, composite alumina Composite alumina Composite alumina Composite alumina

Glass

Glass

Composite alumina

Composite alumina

Composite alumina and composite titania

IMPBR 600-800°C

CMR. PBCMR

IMPBR

IMPBR

CMR

IMPBR, PBCMR

IMPBR

IMPBR

IMPBR

IMPBR

CMR, IMPBR

CMR

IMPBR

450-600’ C, Pt catalyst 480-625”C, Pt/y- alumina catalyst 400-5OO’C, Pt/SiO, catalyst

197-297”C, Pt catalyst

187’ C, Pd catalyst

197°C Pt/alumina catalyst.

600-64O’C, iron oxide catalyst

555-602’C, iron oxide catalyst

300-4OO”C, Ag catalyst

300-500”c, catalysts: y-alumina and y-alumina modified with silver 300-350°C v,os catalyst

445-590°C. Ni/ alumina catalyst

Double the equilibrium yield could be obtained

Conversion up to six times the equilibrium obtained Conversion I .8 times the equilibrium obtained Simultaneous catalyst regeneration by addition of oxygen on permeate side Equilibrium shift depends on space time and reaction stoichiometry. Back reaction affects the performance of the membrane reactor, which in turn depends on the nature of impregnation of the catalyst particles on the membrane Hybrid reactor performed better than membrane reactor alone Membrane reactor gave 15% more conversion compared to a PBR. Hybrid reactor gave 4% higher yield compared to two PBRs in series Inadequate permselectivity and deterioration of membrane permeability were identified to be the main problems Lower temperature operation is possible in membrane reactor Membrane reactor perform better than PBR. Novel configurations are presented

Reactants can be supplied to the membrane from opposing sides and the slip of the reactant across the membranes can be arrested by creating a pressure difference across the membrane High conversion at lower temperature will enhance the use of the reaction in nonconventional applications such as fuel cells

48,49

54-51

58,59

82

71.72

74

73

76-78

79,80

90

81-83

84

62-64

J. Zaman, A. Chakma /Journal of Membrane Science 92 (I 994) l-28 I

Dehydrogenation of lower paraffins In a series of papers, Tsotsis and coworkers

[54-651 presented the results of their experimental and simulation work for the dehydrogenation of ethane and propane, and of steam reforming of methane. They used a commercially available multilayered porous alumina tube as reactor. The reactor had a shell and tube construction with a stainless steel shell sealed at the ends of the membrane tube by graphite string and compression fittings. The membrane reactor was impregnated with up to 5 wt% platinum (based on the weight of the microporous layers). The reactor was operated as a CMR in the temperature range of 450-600°C with transmembrane pressure difference and inert sweep gas as the adjustable variables. Reactor conversions up to six times the equilibrium conversions were obtained for ethane dehydrogenation. The dehydrogenation of propane [ 58,591 was carried out in the IMPBR mode with the membrane tube filled with 14 mesh commercial 5 wt% Pt/y-alumina (Johnson Matthey). Mg was added to the catalyst by a wet impregnation technique to improve upon the sintering and coking character- istics of the catalyst. The catalyst was dried in situ in the reactor at 130’ C overnight and then calcined in two stages: first at 400°C overnight and then at 550°C under flowing hydrogen for 24 h. Hydrogen was added to the propane and argon feed mixtures to prevent catalyst deactivation due to coking. A single feed composition consisting of 80 mol% propane and 20 mol% hydrogen was used in the temperature range 480- 625 o C. Both the conversion and selectivity were found to increase compared to a conventional reactor. At a residence time of 10 s, the yield to propylene was 1.8 times higher than the corresponding equilibrium conversion. The conversion increases with sweep gas flow rate, but this is associated with a decrease in selectivity. The steam reforming of methane [ 6 1-65 ] was carried out in an IMPBR with 8 g of a commercial catalyst (15% NiO on calcium aluminate) ground to 1 mm size. The experiments were conducted at 445-590°C with argon as the sweep gas. The outlet pressure at the reactor was kept at 1 kPa while the permeation side was maintained

at atmospheric pressure. Conversions beyond equilibrium values were obtained in all cases.

Dehydrogenation of cyclohexane Okubu et al. [ 73 ] used a thin y-alumina layer

supported on the inner surface of an a-alumina hollow fiber, with the catalyst loaded around the hollow fiber. Significant equilibrium shift for cyclohexane dehydrogenation was obtained at much shorter space time compared to a Vycor glass reactor. The reaction was studied in a PBCMR with 0.5 wt% Pd/A1203 catalyst gran- ules packed into the bore of a Vycor glass tube [ 741. The tube was impregnated with palladium catalyst in two different ways. It was found that palladium was preferentially deposited on the inner and outer surfaces of the membrane if the impregnation was done by the adsorption of palladium nitrate solution, while uniform deposition throughout the membrane was obtained by reaction of palladium ally1 chloride dimer with hydrogen peroxide treated Vycor glass. The catalytic membrane obtained by the latter method performed worse than the IMPBR. A CMR was used for the same reaction by Sun and Khang and a mathematical model developed for the system predicted the experimental results well [ 7 1,721.

Dehydrogenation of ethylbenzene The dehydrogenation of ethylbenzene to sty-

rene [ 75-801 was carried out in a multilayered alumina tube similar to the one used by Tsotsis et al. [ 56-571. The reaction was carried out at 600-640°C and the conversion increased 15% compared to a conventional reactor. The reactor was operated in the IMPBR mode with an iron oxide catalyst prepared in the laboratory. SEM micrography of the used and fresh membranes showed no visual change in the layer thickness and the overall structure of the membrane. A mathematical model was developed to simulate the reactor. Because of the loss of reactant through permeation, a hybrid system was explored with a normal packed bed reactor followed by a membrane reactor. The hybrid reactor performed well with an increase ( > 5%) in styrene yield over the thermodynamic limit. The proposed membrane reactor showed a different

8 J. Zaman, A. Chakma /Journal of Membrane Science 92 (1994) 1-28

behavior in the generation of key byproducts, i.e., benzene and toluene. The side reaction for toluene was inhibited as a result of the selective removal of hydrogen, while the generation of benzene continued at a reduced rate. An experimental hybrid system was constructed and results in conformity with the simulation were obtained. The initial permeability of the membrane was found to be severely reduced because of carbon deposition. A steady value of permeability was obtained after 40 h of operation. Le- chuga et al. [ 801 also suggested the use of a hybrid system to overcome the detrimental effect of permeation at high reactant concentration in the dehydrogenation of ethylbenzene. To avoid the problem of carbon deposition, they suggested that either a noncoking membrane material or a noncoking operating regime has to be found. A hybrid reactor system was also used by Okubu et al. [ 731 for cyclohexane dehydrogenation as well as by Schmitz et al. [ 181 for steam reforming.

4.4. Novel applications.

Zaspalis et al. [ 8 l-841 used a cross-flow CMR, utilizing the membrane as catalyst as well as par- titioning wall to separate the reactants, allowing controlled addition of one reactant. This could be achieved by regulating the resistances of the two sides of the membrane or by regulating the pressure on each side, the increase in pressure on one side opposing the flow from the opposite side. For the dehydrogenation of methanol [ 8 l-83 1, the sol-gel layer of y-alumina acted as catalyst and methanol was introduced on this side while oxygen, when used, was introduced on the support side. Before the introduction of sol coating, a-alumina was tested to be noncatalytic so far as the dehydrogenation reaction was concerned. With the catalyst, a total conversion of methanol of 75% was obtained at 500°C. At low temperatures, the main product was dimethyl ether. At 450°C the selectivity to methanol was found to be 16% while above 450°C hydrogen and carbon monoxide were the principal products. Sim- ilar results were obtained in conventional reactors using the membrane top layer material as

catalyst. The incorporation of an oxidant can be exploited to run a second independent process on the catalytic surface such as for regenerating the catalyst by burning the carbon that forms on the surface and deactivates the catalyst [ 821. The catalytic activity of the membrane catalyst was found to be better ( 10 times higher) than the activity of the same catalyst when packed. This was considered to be due to the fact that every gas molecule has to pass through the catalytic membrane while in a packed bed, the gas molecules can follow a far less effective path. Introduction of the reactant through the support side was found to give lower conversion. This was due to an external mass transfer resistance created by the support which lowers the concentration pro- file of methanol, decreasing its conversion. Sil- ver-modified y-alumina membranes were found to exhibit higher selectivities than nonmodified membranes. The reduction of nitric oxide with ammonia to nitrogen and water was also carried out in a similar system [ 841. It was found that the supply of reactants from opposite sides and a pressure difference between the two sides allow flexibility in the level of conversion achieved. The dehydrogenation of butane in a similar system increased by a factor of 1.5 and selectivity to butene increased by a factor of 1.6. The regeneration of catalyst by burning off carbon through a supply of oxygen from the separation side was partially successful [ 8 3 1.

The ability of the membrane to act as a barrier between reactants was also utilized by Sloot et al. [ 85,861 for the Claus reaction. Two reactants were introduced on the opposing sides of a CMR and the reaction took place within the thin film of the catalytic membrane. If the reaction rate is fast enough compared to the diffusion of the reactants, the permselectivity is not important. A small reaction zone inside the membrane occurs and the slip of the reactants to the opposing sides of the membrane is prevented, while the product diffuses to either side. The same concept was used by Harold et al. [ 87-891 for a multiphase reaction system. The volatile reactant passes through one side of a catalytically impregnated membrane while the nonvolatile reactant passes along the other side. Reaction occurs in

J. Zaman, A. Chakma /Journal of Membrane Science 92 (I 994) I-28 9

the catalytic film. This configuration is very use- have found widespread applications in fuel cells, ful for systems where there is a large external oxygen pumps, oxygen sensors and chemical re- mass transfer resistance on the gas side as in a actions of various kinds [93-l 18 1. New mate- conventional gas-liquid system in a catalytic rials with improved conductivities are being de- packed bed. The hydrogenation of a-methylstyr- veloped which will significantly enhance the ene on Pd/A1203 was selected as a test reaction application of these membranes as chemical re- and a rate enhancement factor of 20 was ob- actors [ 93- 1001. These membranes may act as tained for the system. The membrane in these re- inert semi-permeable membrane reactor (ISMR ) actors provides the interface for the reaction and where only the permselectivity of the membrane the permselectivity is not an important factor. due to ionic conductivity is utilized. They may The membrane used in this mode is referred to also be employed as electrochemical reactors, the in the literature as catalytic nonpermselective membrane acting as electrolyte and the elec- membrane reactor (CNMR). The slip of one of trodes as catalysts. The two most significant areas the reactants in a CNMR can be controlled by of current research interests with these mem- regulating the feed rate and the pressure. This branes are in the utilization of natural gas to pro- feature is very significant for environmentally duce useful chemicals and in the generation of important reactions such as NO,, SOz and H$ energy carriers in high-temperature reactions. removal systems. These are summarized in Table 2.

The successful application of the sol-gel technique for the production of high quality films of nanosized pores spurred interest in evaluating the viability of commercialization of membrane reactor applications. It appears that the hybrid reactor system with a conventional reactor followed by a membrane reactor holds out great promise for future applications. Practical application of such a hybrid reactor may occur as a retrofit of some existing plants. Many of the reactions for which the ceramic membrane reactors have been proposed are now being carried out commercially in conventional reactor systems. The processes have already been opti- mized and are being maintained competitive by constant input of research and development. The full scale commercial application of composite ceramic membrane reactors alone may have to wait for the development of techniques for obtaining even smaller sized pore structure, allowing greater selectivity or some degree of molecular sieving.

5. I. Utilization of natural gas.

The oxidative coupling of methane to produce Cz hydrocarbons is a reaction that has generated intense interests in recent years [ 10 l-l 041. It has been found that the lattice oxygen of lead oxide and manganese oxide converts methane to C2 hydrocarbons with high selectivity. The use of oxygen selective zirconia membrane has been explored to provide the lattice oxygen which pre- vents large dilution of the reaction mixture by nitrogen when air is used as the oxidant. Com- mercially available nonporous tubes of CSZ ( 11 mol% Ca), MSZ ( 15 mol% Mg) and YSZ (8 mol% Y) were used as support for catalysts. A catalyst layer was formed on the supporting oxygen ion conducting membranes by impregnating them with an aqueous solution of lead nitrate or a methanol slurry of metal oxide with alkali compounds and calcining at 800°C. High selectivity was obtained and this was due to the conversion of methane exclusively to Cz hydrocarbons by lattice oxygen. The rate of the reaction was dependent on the electron conductivity of the membranes. However, with YSZ having much higher conductivity than CSZ or MSZ, the rate was not enhanced to the same degree. It was con- cluded that the overall reaction was limited by the surface reaction rate and not by the supply of

5. Solid oxide membrane

Solid oxide membranes are electrolytes that exhibit ionic activity, among which yttria-stabilized zirconia (YSZ), calcia-stabilized zirconia (CSZ) and magnesia-stabilized zirconia (MSZ)

10 J. Zaman, A. Chakma /Journal ofMembrane Science 92 (1994) l-28

Table 2 Application of solid oxide membranes

Reaction Membrane Reactor configuration Operating conditions Significant conclusions Ref.

Decomposition of water CSZ ISMR

Decomposition of carbon dioxide

CSZ ISMR

Decomposition of carbon dioxide

YSZ ISMR

Oxidative coupling of csz, YSZ, CMR methane MSZ,

perovskites

Dehydro-dimerization Bi,Oj-LazO, CMR of propylene

Oxidation of methane YSZ Electrochemical

Steam reforming of methane

csz, YSZ Electrochemical

Steam reforming of methane

Decomposition of methane

Nd-doped Electrochemical BaCeO,

SeCe0.95Ybo.0503

Electrochemical

1200-1800°C no catalyst

168 1 “C, no catalyst

131 I-1509”C,no catalyst

750°C PbO modified by alkaline metal or earth compounds, perovskites 600°C

700-900°C iron catalyst

600- 1 OOO”C, catalyst: Ag, Ni, Pt, Pd and InaO,-SnOz 9oo-looo”c, Pt catalyst

900°C Pt catalyst

HZ production limited by the ionic conductivity of the membrane At feed CO,/CO=O.Ol conversion was 2 1.5Oh CO2 compared with 1.2% equilibrium value Back permeation has a significant effect on reactor performance Oxygen permeation is adequate without the application of external electrical potential

The membrane acts as oxygen ion conductor as well as catalyst Selectivity to CO and H2 up to 86% obtained. Bypasses endothermic steam reforming to produce synthesis gas Pure hydrogen can be obtained in a one-step process CH4 conversion exceeded 30% even at open circuit condition Methane decomposed into carbon and hydrogen

113,114

115

116

101-103

118

107,112

110

96

109

the oxide ion. It was, therefore, considered un- necessary to accelerate the transportation of the oxide ion by an outer circuit or electrical power, as was done in some earlier work. The reaction was also carried out using nonporous tubes of solid ion conductors ( CaCo,,,Fe,,zO, and SrCe,,ssYb,,osOj) which themselves had catalytic activity. The selectivity for C2 hydrocarbons for these membranes were not high, but this could be enhanced by modifying the surfaces with lead oxide and alkali compounds. The solid oxides can also be utilized in the partial oxidation [ 105- 108 1, decomposition [ 107- 109 ] and reforming of methane [ 107,110-l 121.

Otsuka et al. [ 1 lo] produced pure hydrogen by high-temperature electrolysis of methane and ethylene. In their experiment, the anode and cathode were placed on the inner and outer walls of a CSZ or YSZ tube surrounded by a quartz tube reactor. The hydrocarbon was introduced into the membrane tube and water vapor mixed with helium was fed into the annular space. The reaction temperature was 873-1073 K. A number of catalysts such as Ag, Pt, Ni and Inz03- SnO, were tested as electrode material. The system produced high purity hydrogen, because CO, CO2 or unreacted methane could not come in contact with hydrogen at the cathode.

J. Zaman, A. Chakma /Journal ofMembrane Science 92 (1994) 1-28 11

5.2. Production of energy carriers 6. Supported membranes

The direct thermal decomposition of water [ 113,114] or carbon dioxide [ 115,116] in a high-temperature solar furnace has been considered as an option in the utilization of solar energy for future energy needs. In addition to the problems associated with high temperature and low conversions (e.g., 4% dissociation of water at 2300 K at normal pressure), the prevention of recombination reactions at lower temperatures is also a severe problem. The separation of the reaction product in both cases can be carried out by the use of a membrane semipermeable to oxygen but impervious to all other gases. Nigara and Cales [ 115 ] used a reaction cell made of a closed- end tube of CSZ, 2 mm thick and 10 mm inner diameter, which acts as an oxygen semipermeable membrane. The cell is placed in a high-temperature furnace, the carbon dioxide fed partially dissociates and the oxygen formed diffuses through the zirconia membrane causing the separation (thus preventing recombination), and at the same time effecting a greater amount of dissociation because of equilibrium shift. They obtained up to 21.5% dissociation at 1954 K in place of 1.2% that could be obtained in the absence of the membrane. The effectiveness of the process was found to be limited by the electronic conductivity of the stabilized zirconia. A solid solution of Zr02-Ce02-YzO, having a higher oxygen semipermeability would be more promising.

The microporous inorganic membranes do not meet the dual requirements of high selectivity and high permeability. On the other hand, the dense membranes meet the criteria of high selectivity, but satisfying the criteria of high permeability is limited by the thickness of the membrane, which again is constrained by the mechanical strength and durability of the membrane. To meet the dual challenge of selectivity and permeability, there has been a recent thrust to support thin layers of highly selective membrane material on a porous support with high permeability [ 119-l 501. Among the various methods to accomplish this, different thin film preparation techniques such as electroless plating, sputtering, chemical vapor deposition, electrochemical vapor deposition and high-temperature spraying have been employed. Some of the applications are summarized in Table 3.

6. I. Electroless plating.

Interests in the application of solid oxide membranes for the oxidative coupling of low molecular weight paraffins as well as the partial oxidation reactions are increasing. Develop- ments of materials with improved conductivities and the ability to produce thin membranes with high permeabilities will determine the possibili- ties for commercial applications. Improvements in this type of materials are being actively sought in the field of solid oxide fuel cells (SOFC) which themselves constitute a special group of membrane reactors.

The technique basically involves dipping the target surface in a bath which contains a complex salt of the support material and a reducing agent. The deposition takes place by the autoca- talyzed reduction of the metallic salt complex on the target surface. In order to have a uniform coating, the target must be cleaned, sensitized and activated. In the case of palladium deposition, amine complexes such as [ Pd( NH3)4]Clz may be used in the presence of a reducing agent like hydrazine or sodium hypophosphate. The method is simple and can be applied to surfaces of any geometry. Palladium deposition on porous glass and ceramic surfaces by this technique has been particularly promoted by the Japanese workers in recent years [ 12 I- 1281. Uemiya et al. [ 122,123] supported a porous glass tube (300 nm pore size) with a thin membrane of palladium using a chemical bath of following composition:

Component P’dWH~LtlClz 2 NaEDTA

Concentration 5.4 g dm-3 67.2 g dmM3

12 J. Zaman. A. C’hakma /Journal of Membrane Science 92 (I 994) 1-28

NH40H H2NNH2

350.0 g dmp3 4.6 mm3 dmp3

butane, aromatization of propane and steam reforming of methane. The permeation rate of hydrogen was measured at 673-773 K and it was found that there was a ten-fold increase in the rate compared to a metallic palladium membrane. The increase was attributed to the low thickness of the supported palladium membrane ( 13 pm ) as opposed to the usually available metallic membrane ( 150 pm). The supported membrane thus eliminates the difficulties in fab- rication techniques and poor mechanical properties of thin metallic membranes. These membranes, however, still had the problem of hydrogen embrittlement when exposed to hydrogen below 573 K. The hydrogen embrittlement temperature was brought down to 473 K by adding copper or silver with palladium [ 126,141]. The combined film was deposited by successive electroless plating in a palladium-containing bath and a copper- or silver-containing bath. The

Prior to dipping in the bath, palladium nuclei were supported on the outer surface of the clean support tube by sensitization and activation treatments with stannous chloride and palladium chloride solutions, respectively. This was repeated ten times to ensure the autocatalytic deposition of palladium in the bath. The glass tube was immersed for 17 h in one case and the thickness of the palladium was calculated to be 13 pm. The plating solution was renewed every hour to keep the plating rate constant. The rate of plating was dependent on the activation of the surface, the concentrations of the metal salts and the reducing agent, and the temperature and pH of the bath. Membranes prepared following this procedure were used for hydrogen separation, water gas shift reaction, dehydrogenation of iso-

Table 3 Summary of application of supported membranes

Membrane Ref. Substrate Technique

Glass Electroless plating

Application

Hydrogen separation, water gas shift reaction, steam reforming Hydrogen separation

Pd 121-124

130

125-129

Pd Silver Electroless plating

Electroless plating Pd, Pd-Ag Alumina Hydrogen separation, dehydrogenation of isobutane, aromatization of propane Hydrogen separation Stainless steel

Glass

Electroless plating

CVD

CVD

CVD

Pd-Ag

SiOZ, TiOz, ALO3, &03 Silica Glass

YSZ Alumina

131

133-136

138

139

141

141

143

Hydrogen separation

Dehydrogenation of isobutane

He permeability measurement

Pd

Ni, Pd

Anodic Sputtering and alumina permeation Alumina Ion plating

Sputtering

Hydrogen separation

Hydrogen purification

Hydrogen separation, hydrogenation of carbon monoxide, hydrogenation of pentadiene Hydrogen separation

Oxygen permeation

Pd alloys with Mn, Co, Sn and Pb

Polymer, metal, oxide

Pd-Ag

YSZ

Alumina Spray pyrolysis

Alumina EVD

147

150

J. Zaman, A. Chakma /Journal ofMembrane Science 92 (I 994) 1-28 13

plated tube was treated for 12 h in a stream of argon at 773 K. The hydrogen permeability for the membrane was measured and found to be the same as for pure Pd, indicating that the pores of the porous glass (PG) tube offered little resistance to hydrogen permeation. Both Pd-Cu/PG and Pd-Ag/PG composite membranes gave high permeability for hydrogen at 473 K compared to commercial Pd membranes, but lower than supported Pd membranes. The lowering of the permeability was attributed to the nonhomo- geneity of the films. This was remedied by supporting Pd-Ag on ceramics. The membrane supported on ceramics was treated at 1073 K, which yielded a miscible Pd-Ag alloy. Hydrogen embrittlement temperature was reduced to 473 K with the film having 23 wt% silver. The permeability of this Pd-Ag membrane was also higher than the Pd membrane alone. The thickness of the supported Pd-Ag membrane was 5-8 pm compared to the commercial Pd-Ag membrane having a thickness of 150 pm.

Shu et al. [ 13 1 ] investigated the co-deposition behavior of palladium and silver on porous stainless steel in an electroless plating bath. It was found that simultaneous deposition was passi- vated by the preferential deposition of silver. An improved deposition procedure with effective Pd activation achieved co-deposition in separate phases. The heterogeneity of the composite membrane can be considerably eliminated by annealing in a hydrogen atmosphere. Govind and Atnoor [ 1301 deposited palladium on a porous silver support, The membrane showed good mechanical strength and did not suffer any degradation of mechanical properties due to thermal and pressure cycling around 400” C. The permeability of the composite membrane was found to be comparable to the theoretical permeability for pure palladium. The hydrogen flux obtained for the composite membrane was found to be equiv- alent to that obtained for a 5 pm palladium membrane.

means of a chemical reaction. Gavalas and coworkers [ 133-l 38 ] deposited SiOz films within the walls of porous Vycor tubes by SiH4 oxidation in an opposing reactant geometry. In this method, SiH, was passed inside the Vycor tube (pore size 4 nm) while O2 was passed outside the tube. The two reactants diffused opposite to each other and reacted within a narrow front inside the tube wall to form a thin SiOz film. Once the pores were plugged, the reactants could not reach each other and the reaction stopped. At 450’ C and SiH4 and 0, partial pressures of 10.1 and 33.33 kPa, respectively, the reaction was complete within 15 min. The thickness of the SiOz film was estimated to be N 0.1 mm. Tsapat- sis et al. [ 1351 used a CVD technique to deposit films of SiOz, TiOz, A1,03 and B203 by reacting the respective chloride precursors with water at l OO-800°C. All the membranes showed good thermal stability. The H2:N2 permeation ratios for SiOz membranes were 1000-5000, the titania and alumina membranes having somewhat lower values. The SiOz-coated membranes were used for the dehydrogenation of isobutane to isobutene. The reaction is accompanied by extensive side reactions and catalyst deactivation by coking. In a membrane reactor, the coking condition is aggravated by the loss of hydrogen from the reaction side. In spite of that, the IMPBR performed better than the PBR. The IMPBR performed even better with an increase in residence time.

The method has also been applied to deposit thin films of ion conducting solid oxides on different porous support materials [ 1391. Films of yttria-doped zirconia were deposited on porous supports by reacting ZrCl, (and YCl,) with water at 800- 1000’ C. The effects of substrate pore di- mension and structure, bulk-phase reactant concentration, reactant diffusivity in pores and deposition temperatures were investigated and the experimental results were explained qualita- tively by theoretical modeling.

6.2. Chemical vapor deposition (CVD) 6.3. Sputtering

The method involves deposition of a desired component in the vapor state on a substrate by

This technique involves bombarding a target with energetic particles which cause surface at-

14 J. Zaman, A. Chakma /Journal of Membrane Science 92 (I 994) l-28

oms to be ejected and then deposited on a substrate close to the target. The method has recently been used for the deposition of a thin film of a semipermeable membrane on porous substrates [ 140- 1441. Gryaznov et al. [ 143 ] deposited thin films of binary and ternary alloys of palladium with manganese, cobalt, ruthenium, tin and lead on asymmetric polymeric membranes, porous stainless steel sheets and oxide supports by the sputtering technique. The sputtering was performed at argon pressures from 0.1 to 1.0 Pa. The target disc had a diameter of 130 mm. The rate of alloy deposition was - 1 pm per minute. The membrane was maintained at the same temperature to provide a continuous film. All the membranes proved permeable to hydrogen only. The stability of the value of permeability of a composite membrane can often be improved by introducing a thin film of a third material such as cobalt, nickel or molybdenum before sputtering the alloy. An asymmetric membrane coated with a film of a palladium alloy performed well for the vapor phase hydrogenation of 1,3-pentadiene. Meunier and Man- aud [ 142 ] deposited a nickel film on a composite a-alumina tube by reactive ion plating. First, a 0.3 pm titanium hydride film was deposited by evaporating titanium using an electron beam through a gas mixture of 50% Ar and 50% H2 under 0.4 Pa pressure. The substrate was polarized at 600 V with a radio frequency power supply. A further heat treatment at 400°C under vacuum provided a good binding between the ceramic and the metallic films. A 2 or 3 pm nickel film was then deposited by the same technique. A film with a very small porosity was obtained; the gas- tightness for hydrogen was then achieved by electrolytic plating. The resulting 10 to 15 pm thick nickel film was tested and permeability measurements were made between 98 and 227°C.

4.4. Spray pyrolysis

This method involves spraying a solution of metal salts into a heated gas stream where it is pyrolyzed. The method has been successfully applied for the production of fine metals or metal

oxide particles [ 145-1471. Li et al. [ 1471 obtained a Pd-Ag alloy membrane on the outer surface of a porous alumina hollow fiber by spray pyrolysis of a Pd ( N03) 2 and AgNO, solution in a Hz-O2 flame. The mass fraction of silver in the membrane at a support-surface temperature of 1240- 1340 K was as low as 0.04. An additional spray pyrolysis with a silver nitrate solution produced a 24 wt% alloy with palladium. The thickness of the alloy membrane was 1.5-2 pm and the separation factor of hydrogen to nitrogen was - 24 at 773 K. This was quite low compared to the competing methods and the technique needs further development.

6.5. Electrochemical vapor deposition (EVD)

The wide scope for the application of the solid oxide electrolyte in catalytic membrane reactors led to interests in producing thin films on porous support. However, this is a more difficult task, because of the crystalline nature of the oxides. The electrochemical vapor deposition (EVD ) technique has proved to be an effective process in depositing a gastight film of electrolytes on a porous support [ 148-1501. This method is es- sentially a variation of the CVD technique. In the EVD process, the porous substrate separates a mixture of chloride vapors ( ZC13, YC13, etc. ) and an oxygen source (water vapor or oxygen). Ini- tially, the reactants from both sides of the substrate interdiffuse into the pores and form the solid oxides, as in the CVD process. When the pores are closed, oxygen ions are conducted across the solid oxide and the oxide film grows on the chloride side. Membranes with films of solid oxides deposited by the EVD technique have been used in various catalytic chemical reactions [ 1071.

The supported membranes hold the promise for reaping all the benefits that were envisaged during the early stages of the development of metal membrane reactors. The methods for producing gastight, mechanically stable thin coat- ings of semipermeable materials on porous substrates require standardization and long-term studies are needed to establish a given method. It is quite possible that the choice of a particular

J. Zaman, A. Chakma /Journal ofMembrane Science 92 (I 994) 1-28 15

method may depend on the substrate and the support as well as on the nature of the application. No clear choice of a particular technique can yet be made based on the information available in the open literature.

7. Surface modification

The surfaces of porous ceramic membranes may be modified to obtain improved separation or catalytic activity [ 15 l-l 691. In general, the benefits of surface modification may be one or more of the following: (a) metal or oxide deposited on the surface may act as a catalyst, (b) it may promote the permeability of a specific component, (c) it may control the pore size of the membrane, and (d) it may increase membrane stability. The common procedures for the modification of the surface of a membrane have been impregnation, dip coating, surface reaction or CVD. Some of the work on surface modification is presented in Table 4.

Impregnation with a metal salt is the most common method for obtaining a catalytic membrane reactor. Tsotsis et al. [ 56-581 used a mi- cropipette to put the impregnating solution drop by drop on the surface of the membrane. Harold

Table 4 Application of surface modification techniques

et al. [ 87-891 impregnated palladium catalyst on an y-alumina supported on a-alumina by soak- ing the ceramic tube into an aqueous solution of ammonium tetrachloropalladium (II). Uzio et al. [ 1511 deposited platinum catalyst within the porous structure of y-alumina by an ion-exchange method using an aqueous solution of hexachloroplatinic acid. The membrane was first dipped in water and then in the acidic solution (0.1 g Pt/l) and contact time was between 1 and 4 h. After this step, the material was twice washed in dilute nitric acid solution (pH 4) for 20 min and then dried for 4 h by passing dry nitrogen through the inner part of the tube. The sample was then calcined under nitrogen at 723 K (temperature 1 K min- ’ ) for 2 h. Pt was then reduced to the metallic state under flowing hydrogen (80 cm3 min- ’ ) at 673 K for 2 h with a temperature increase of 1 K min- ‘. The permeability and other properties of the membrane were found to be unaffected by the Pt deposition. Raman et al. [ 1581 examined two ap- proaches to disperse rhodium metal particles onto a composite silica membrane. In one approach, the surface was treated with a sililating agent and then with an organo-metallic com- pound, followed by reduction in hydrogen. The second approach was the impregnation of a

Membrane Technique Deposition Application Ref.

Composite alumina

Composite alumina

Composite alumina

Composite alumina

Composite alumina

Composite alumina

Glass

.SiO,/composite alumina

Impregnation

Ion exchange

Sol-gel

Sol-gel

Sol-gel

Reverse micelles

Silylation

Silylation/dispersion

Catalyst

Catalyst

(Pt) Catalyst (Ni) Silica

Ru

Zirconia

Silanol group Gas separation

Rh Gas separation

Dehydration and other reactions

Hydrogenation of toluene

Dehydrogenation of methanol

Gas separation ( C02-CH4, hydrocarbons, inerts) Gas separation, steam reforming reaction Gas separation

57,71,14, 81-84

151

159.160

161

163,164

165

152-154

158

16 J. Zutnan. .3. Chaktna /Journal qf‘Metnhrane Science 92 (1994) 1-28

metal-organic precursor solution into the ceramic support followed by calcination and reduction. The latter method led to pore penetra- tion and eventually to defect or crack formation in the SiOz membrane.

Okubu and Inoue [ 152-l 541 obtained pore size control and catalytic activity by adding nickel nitrate solution to the boehmite sol. The BET surface area was found to be unaffected by the addition of nickel. However, the pore radius decreased with Ni/Al ratio up to 1.0, but remained constant at higher ratios. The larger ag- glomerates formed initially by the nickel addition were broken down during the gelling or drying stage and were not observed after firing. The pore size distribution was found to be mon- odisperse in all cases. The catalytic activity of the doped membrane was dependent on the nickel content, rising as the nickel content increases beyond 10%. Impregnated nickel catalysts were found to be active even at lower nickel content, but without the benefit of pore size reduction. Kusakabe et al. [ 165 ] modified the porous alumina membrane by incorporating ultraline zirconia particles prepared by the reverse micelles method. The permselectivity of the membrane for hydrogen to nitrogen increased from 3.4 to 4.5 at the cost of a drastic reduction of permeability (5% of the original value ) .

Chai et al. [ 163,164] developed a procedure to prepare Ru-dispersed alumina membrane with a procedure of dip coating a porous tube in an alkoxide-derived boehmite sol. The dip coating was repeated 40 times, until a membrane with a thickness of N 15 ,um and having a Ru-content of 1.33 wt% on the inside wall was obtained. There was no change in the microstructure of the resulting membrane, but the separation factor (a) for Hz/N2 was found to range from 4.5 to 6.0. This was significantly higher than that expected from Knudsen diffusion ( cy = 3.7 ) . This increase was attributed to an additional transport mechanism of surface diffusion resulting from the chemisorption of hydrogen on the Ru-deposits. The performance of this membrane in the steam reforming reaction in the temperature range 300- 500°C was found to be better than a similar membrane without the metal dispersion.

The thermal stability of unsupported alumina membrane top layers was studied by determin- ing the pore structure (mainly pore size) change of alumina gels prepared by sol-gel methods, after sintering at temperatures ranging from 450 to 1200°C [ 1661. The average pore size of pure alumina membranes and PVA-added membranes increased sharply beyond sintering temperatures higher than 1000°C. The addition of 3% lanthanam, either by mixing lanthanam nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by heat treatment, can considerably stabilize the pore structure of the alumina membrane top layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200” C for 30 h, about one-sixth the pore diameter for pure alumina membranes. The substantial increase in pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200°C was accompanied by the phase transformation from y- to (Y- alumina. The addition of lanthanum can raise this transformation temperature by N 200’ C.

Uhlhorn et al. [ 1701 deposited a film of polymeric silica sol on y-alumina porous membrane and obtained pore sizes below 1 nm and large permselectivity. In particular, the separation factors of COJCH, and hydrocarbon/inert gas mixtures increased with good permeabilities. Mulder et al. [ 1711 obtained a microporous, tightly bonded silica film (4 pm thick) on a metal substrate by dipping in a solution of silicones in a volatile organic liquid, followed by heating. A catalytically active membrane on the top of a porous metal substrate was prepared by a one-step process using a mixture of silicones and a metal precursor in the solvent.

The modification of the surface of the membrane is often associated with a significant reduction of the permeability. This is partially compensated by increased selectivity. There has been a significant growth of interest in the use of catalytic membrane reactors and metal-modi- lied ceramic membranes in recent times. Addi- tion of the modification agent during the sol preparation is the most widely used approach.

.I. Zaman, A. Chakma /Journal ofMembrane Science 92 (1994) l-28 17

8. Gas separation in inorganic membranes

The performance of membranes as chemical reactors is generally dependent on their gas separation capability. The mechanisms involved in the separation of gases are widely different for dense and porous membranes and are discussed separately.

8.1. Dense membranes

The effectiveness of gas separation by dense membranes of palladium and other metals is dependent on two opposing factors: permeability and selectivity. The dense membranes have high selectivities, but low permeabilities. The gases are dissolved in dense films depending on the solubility, transport occurs because of a concentration gradient and dissolution takes place on the other side of the membrane. The permeability is low because of the very low diffusion coefficients for gases in solids. Extensive literature is available on diffusion in solids, particularly for palladium [ 51.

The gas separation in solid electrolytes is dependent on the ionic activity of the membrane material. The development of electrolytes with improved conductivities is currently a matter of intense research activity [ 93-1001. The application of a thin film of a dense membrane, metal or solid oxide, on a porous ceramic support can drastically decrease the thickness of the membrane. The permeability is inversely propor- tional to the thickness and hence the reduction in the thickness improves the permeability of the dense membranes. This enhances the prospects for the application of these membranes in reactor/separator systems.

8.2. Porous membranes

Five different mechanisms may be involved in the transport of gases across a porous membrane: Knudsen diffusion, surface diffusion, capillary condensation, laminar flow and molecular sieving [ 172- 187 1. The contribution of the different mechanisms are dependent on the properties of the membranes and the gases as well

as on the operating conditions of temperature and pressure. In a commercial ceramic membrane with pore sizes greater than 4 nm, Knudsen diffusion is likely to be the dominant mechanism of gas transport at low pressures and elevated temperatures. Capillary condensation and surface diffusion are unlikely to exist at elevated temperatures in membranes with pore sizes in the range of 2 nm. Molecular sieving does not take place, because the pore sizes are much larger than the gas molecules. The contribution of viscous flow, resulting from a pressure difference across the pores, will be quite small and even if it is present, it does not contribute to the separation process. This leaves Knudsen diffusion as the only transport mechanism contributing to the separation of various components in a gaseous mixture at elevated temperatures in a porous membrane.

Gas permeation by Knudsen diffusion varies inversely with the square root of the molecular weight. The ideal separation factor for binary gas mixtures therefore equals the inverse of the square root of the ratio of the molecular masses. The actual separation factor, however, is found to be smaller, this being attributed to back diffusion, nonseparative diffusion, concentration polarization on the feed or permeate side and/or the occurrence of viscous flow (in large pores). Predictive models for permeation and separation have been suggested with various simplify- ing assumptions [ 170- 178 1.

The transport of gases through porous membranes by Knudsen diffusion alone imposes a severe constraint on the selectivity in a reaction/ separation system. To overcome this difficulty, various efforts to promote other modes of transport have been made by surface modification. By plugging the pores of an alumina membrane by hydroxide and reducing them to less than 2 nm, Asaeda and Du [ 155 ] improved the separation factor for water-alcohol mixtures to above 60. Uhlhorn et al. [ 16 1 ] obtained a separation factor of 27 for NJ&H6 mixture at 0°C. In both cases, the increase was due to the additional effect of capillary condensation, which requires a condensable component and hence restrictive in application.

18 J. Zaman, A. Chakma /Journal qfhlemhranc Suewe 92 (1994) 1-28

Surface diffusion has been studied as another means of enhancing separation factors [ 180- 1 82 ]. It has been observed that the contribution of surface diffusion can be significant at low temperatures if the pore diameters are less than 2 nm. This contribution decreases as the temperature is raised.

Various degrees of pore plugging in alumina membranes by silica led to considerable improvement in selectivity [ 186-l 87 1. Gavalas et al. [ 133- 1381 prepared such membranes by the CVD technique and obtained a separation factor of 2000 for N2/H2 mixtures. The permeation of hydrogen occurred by an activated diffusion mechanism, but the rate was low.

For high-temperature membrane reactors, molecular sieving offers the most probable route for the enhancement of separation factors. Effec- tive modification of the porous membrane to re- duce the pore size remains an active area of research. Further, new materials having inherent size exclusion properties, such as zeolites [ 188 1, molecular sieve carbons (MSC ) [ 189- 193 ] and polyphosphazenes [ 194,195 1, are being developed. Koresh and Soffer [ 190-1931 carried out extensive work on the preparation and characterization of MSC and obtained a separation factor of 30-50 for a CH4/H2 mixture. Polyphos- phazenes are polymers with an inorganic backbone and can be made dense or porous and can also be supported on an inorganic membrane [ 194,195].

9. Modeling and process evaluation

There has been a significant amount of modeling work done with membrane reactors. The purpose of the modeling has been either to simulate the experimental reactors or to explore the operation of a prospective system in order to gain an understanding and to develop guidelines for the design of such a system [ 196-2 12 1. In general, the models for the membrane reactors have been kept quite simple with the assumption of plug flow and the absence of heat and mass transport limitations. Based on the membrane

configuration, three classes of models are discussed in this section.

9.1. IMPBR models

Itoh and coworkers [ 12- 17,196-l 98 ] developed simple one-dimensional models with assumptions of steady state, constant pressure, plug flow and isothermal operation of both palladium and porous membrane reactors. These models were quite successful in simulating and predicting the dehydrogenation of cyclohexane, dispro- portionation of propylene and the decomposition of hydrogen iodide. Similar models were used by other investigators for predicting the dehydrogenation of ethylbenzene [ 771, partial oxidation of methanol to formaldehyde [ 921, decomposition of hydrogen sulfide [ 2041 and partial oxidation of methane [206,207]. These models were extended for nonisothermal operation and applied to bifunctional reactors with dehydrogenation on one side and oxidation on the other [ 15-l 7 1. Mohan and Govind [ 200- 2021 used models similar to those developed by Itoh and coworkers in order to obtain the limiting strategies in the design and operation of the porous membrane reactors. Criteria for the most desirable configurations of the reactor for cocurrent, countercurrent, mixed or recycle opera- tions were established by simulation. Tsotsis and his group used similar models for IMPBR operation of their composite ceramic membrane reactor for various dehydrogenation reactions

[551. Prokopiev et al. [ 191 modeled the steam re-

forming of methane in an IMPBR and investigated the influence of temperature (800-l 000 K), pressure (0.1-2.0 MPa) and thickness of a palladium membrane (0.02- 1 .O mm) on the ex- tent of methane conversion, hydrogen production rate and H2:C0 ratio in the syn gas produced. The composition of the syn gas produced could be manipulated by a suitable choice of variables. It was found that the use of a membrane had a greater impact at conditions where the rate of reaction was slower. Adris et al. [ 2031 numerically simulated the steam reforming of methane in fluidized bed reactors using two novel

J. Zaman. A. Chakma /Journal qfMernhrane Science 92 (1994) 1-28 19

configurations. Simulation was carried out at industrial operating conditions. Both the conligu- rations resulted in a drastic reduction of reactor size and heat transfer area compared to a conventional industrial reactor with equal performance.

Collins et al. [ 2 111 carried out modeling studies of an IMPBR for the decomposition of low concentrations of ammonia which may be present in coal gasification gases in a power plant. The NH3 formed in the gasification process must be removed in order to prevent the formation of NO, during combustion in the gas turbine. It was found that a selectivity of HZ over Nz greater than 50 was needed for high NH3 conversion. The choice between cocurrent and countercurrent flow depended on the selectivity of the membrane, sweep gas flow rate and pressure difference between the sweep side and retentate side. They evaluated the interphase and intraphase mass transfer resistances on the performance of the membrane reactor. The presence of such resistances effectively reduces the reaction rate with respect to the permeation rate and generally lim- its the maximum conversion achievable in a system. The issue assumes a greater importance in situations where the objective is to control the emissions in the atmosphere. Gokhale and coworkers [ 2 10 ] incorporated mechanical energy balances in the IMPBR model for the dehydrogenation of butane and found that the pressure variation along the length of the reactor was small and its effect on reactor performance negligible. For maximum conversion, they proposed a hybrid reactor with a space time of 0.5 s. A sufti- ciently long space time was required in order to profitably utilize the conversion of the back-permeated reactants.

9.2. CMR model

In the CMR mode of operation of the reactor, the reaction takes place on the catalytic wall and hence there exists a concentration gradient in the radial direction through the catalytic wall. For flow through the tube and the shell, temperature and concentration changes occur in the axial direction only. Sun and Khang [ 7 1,72 ] were the

first to propose a CMR model for their experimental reactor for the dehydrogenation of cyclohexane. They successfully simulated their experimental results and attempted to obtain some generalized conclusions about the performance of CMR with respect to conventional reactors for reactions of different stoichiometries. Champag- nie et al. [ 571 were able to represent their data on ethane dehydrogenation quite well with a similar model.

9.3. PBCMR model

For PBCMR operation, the concentration and temperature in the tube side vary both radially and axially. Tsotsis and coworkers [ 651 obtained a general PBCMR model, two dimensional on the tube side and one dimensional on the shell side (axial) and in the membrane layer (radial). With the thickness of the membrane layer as the only adjustable parameter, the model predicted well the experimental results for the dehydrogenation of ethane and steam reforming of methane. Becker et al. [ 2121 found signili- cant mass transfer resistance across the support, though it has been considered negligible by most investigators. While the models developed for the CMR have been generally successful in predicting the experimental results, those for the PBCMR have not been always successful. For example, in the case of a PBCMR for the dehydrogenation of ethylbenzene to styrene, only 2% increase in conversion was predicted while the experimental value was 10% [ 2 121.

Recently, Tsotsis et al. [ 2 13 ] summarized the modeling work in the literature and attempted to arrive at a general model for the membrane reactor. The generalized model could be reduced to different forms corresponding to the membrane configuration used. However, the general model is also based on plug flow behavior and assumes the absence of heat and mass transfer effects on reactor performance.

There obviously exists a great opportunity for advancing sophisticated models for membrane reactors, as has been done for conventional packed bed reactors. However, except in one in- stance for a PBMCR [ 2 12 1, the simple models

20 J. Zaman, A. Chakma /Journal of Membrane Science 92 (I 994) 1-28

have quite adequately described the performance of quite a number of reaction systems. What is missing currently is the conceptual design of membrane reactor-based processes and their economic evaluation. Some work in this direction has been done for the separation of gases as in the coal-based power station [ 2 141, but noth- ing so far has been reported for membrane reactors.

10. Current developments and direction of research

An examination of the applications of membranes as chemical reactors shows that they have been used in the following modes of operation: 1. IMPBR (Inert Membrane Packed Bed

Reactor) 2. CMR (Catalytic Membrane Reactor) 3. PBCMR (Packed Bed Catalytic Membrane

Reactor) 4. CNMR (Catalytic Nonpermselective Mem-

brane Reactor) 5. ISMR (Inert Semipermeable Membrane

Reactor ) In the majority of the cases of dehydrogena-

tion/decomposition systems, the motivation for the use of membrane reactors has been the increased conversion and/or selectivity. The reactor has been operated in IMPBR, CMR or PBCMR mode. ISMR operation with solid oxide membranes is used for reactions such as the decomposition of water or carbon dioxide while CMR or PBCMR mode would be suitable for the partial oxidation of methane. CNMR operation seems particularly attractive in environmentally important reactions, because this has the potential to eliminate completely the slip of the unde- sirable reactant. CNMR also provides a novel contacting method for two-phase reactions.

cation of the companies involved in their development and the gradual relaxation in licensing their production, these membranes looked for market elsewhere and eventually penetrated the cross filtration market. There is now a considerable level of application of these membranes in biotechnology, food and wastewater treatment [6,36]. With the demand for these membranes vanishing in the nuclear industry because of the changes in technology for isotope separation, there is a distinct possibility that the membrane manufacturers will push for entry into the market in petroleum and chemical processing industries. However, further improvements are needed for gas-phase applications in terms of narrowing the pore sizes down to 1 nm or so. Various techniques have been attempted of which the partial plugging of the pores by siliceous materials by the sol-gel or CVD technique appears to have gained some greater attention and success. Modilica- tion of the surface by catalytic functionalities to impart catalytic activity or induce surface diffusion has also been attempted by various techniques. It appears that PBCMR operation of these membranes stand out as the ultimate objective. The primary hurdle on the way is the nonavailability of any commercial alumina membrane with pore size liner than 5 nm. In the absence of a suitable membrane, the researchers are forced to undertake the dual task of membrane development as well as reactor development requiring expertise in surface chemistry, materials science, chemistry of catalysis and reaction engineering. However, even with a basic membrane available, researchers will not be in an enviable position because of the continual need to improve the material and add catalytic functionalities. The situation is made all the more complicated because of the absence of any stan- dard method or procedure and the highly empir- ical nature of all the relevant techniques.

There seems little doubt that the basic mate- The dense membranes directly are not attrac- rial of construction of the membrane reactors in tive as chemical reactors. However, available the foreseeable future will be the multilayered techniques of obtaining a thin film of dense composite porous alumina membranes. These membrane of palladium or a solid oxide on a po- materials were developed in the nuclear industry rous alumina support allows an increase in the in order to separate the isotopes of UF6 in the gas permeability while retaining the selectivity of the phase [215]. Because of the need for diversili- dense membrane. This makes the research in this

J. Zaman, A. Chakma /Journal of Membrane Science 92 (I 994) 1-28 21

area interesting and worthwhile. In the case of providing a thin film of Pd or Pd-based alloy, porous stainless steel may also act as the support. This has not been investigated much in the literature, but certainly demands careful evaluation.

On a longer term, opportunities for developing new materials and improving existing materials are many and varied. The need for better materials in SOFC continues to provide the impetus for developments in solid oxides which will enhance their suitability as chemical reactors. The zeolitic and molecular sieve membranes can be further developed so that an optimal combi- nation of permeability and selectivity is achieved. The high permeability of the inorganic membranes can be combined with the good selectivity of the organic membranes [ 216-2191. The concept of liquid-immobilized membranes (LIM) can be adapted in the field of inorganic membranes with molten salts incorporated in their porous matrix [ 220,22 11.

Research on membrane reactors requires in- genuity because of the presence of a large number of variables in membrane preparation and modification as well as in reactor configuration and operation. It will also remain multidiscipli- nary for a long time to come and the researchers have to have access to a wide variety of analyti- cal techniques.

11. Conclusions

Chemical reactions occur over a wide range of temperature and pressure under a variety of corrosive environments. Inorganic membranes have been applied to quite a large number of reactions. There seems to be little doubt that the membrane reactors will be suitable for some or all of these reactions. However, the work reported so far in the literature in many cases seems exploratory and tentative in nature and many of them fail to bring in new ideas or break new grounds. It is the opinion of the authors that while developments in membrane materials take place, various issues of reactor design and operation should be addressed based on the available

membranes. Some work has been done compu- tationally, but more should be done both com- putationally and experimentally. The issues of cocurrent and countercurrent flow, feed location and recycle, nature of sweep gas and its implica- tions in terms of downstream separation and effects on heat load require resolution. Based on these findings, an optimal design and an optimal choice of operating conditions should be sought. An economic analysis of a conceptual membrane reactor-based plant may then be made. This will point to the inadequacies in membrane properties and suggest areas for further improvement of the membrane materials.

The membrane technology is really based on long-standing technologies in materials development. The application of these technologies is proving generally successful in the laboratory scale. Concern is being expressed regarding tech- nological problems in module development for commercial reactors. Again, the problems are being tackled successfully in the laboratory en- vironment. With the long experience of chemical industries in designing chemical reactors in the highly demanding conditions of temperature, pressure and corrosion combined with the experience in the nuclear industry with membrane separators, the industry may not be far off in the technology for developing modules for membrane reactors. The exotic configuration of hollow fibers and other more complex shapes to increase the surface to volume ratio may not be on the board, but the practical design and imple- mentation of tubular modules in a shell and tube configuration may not be too far for commercial realisation.

What is definitely missing at this point in research is long-term data of membrane reactor operation. We now have no information about the long-term stability and mechanical integrity of the membrane, the effect of fouling and deactivation and the durability of the module and the sealing. The primary need at this point, besides the research on membrane developments and novel applications, is to obtain longer term data in the laboratory units and move to piloting potential systems.

22 J. Zarnan, A. Chakma /Journal ofMembrane Science 92 (1994) l-28

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