A high temperature catalytic membrane reactor for ethane dehydrogenation

A high temperature catalytic membrane reactor for ethane dehydrogenation

Chemical Engineering Science, Printed in Great Britain. Vol. 45, No. A HIGH 8, pp. 24232429, 1990 TEMPERATURE FOR Q CATALYTIC ETHANE MEMB...

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Chemical

Engineering

Science,

Printed in Great Britain.

Vol.

45, No.

A HIGH

8, pp. 24232429,

1990

TEMPERATURE FOR

Q

CATALYTIC

ETHANE

MEMBRANE

0009-2509/90 S3.00 + 0.00 1990 Pcrgamon Press plc

REACTOR

DEHYDROGENATION.

A. M. CHAMPAGNIE,

T. T. TSOTSIS,

R. G. MINET

and I. A. WEBSTER’

Department ‘Union

of Chemical

Oil Company

Engineering,

of California,

University

Corporate

of Southern

Division,

California,

1201 West

5”

Los Angeles,

California,

Street, Los Angeles,

90089-1211,

California,

90051,

U.S.A. U.S.A.

Al3STRACT A high temperature catalytic membrane reactor, containing a Pt impregnated alumin a ceramic membrane tube in a shell-andtube configuration, was used to study dehydrogenation reactions. Experiments in this membrane reactor in the temperature range of 450-600°C, with the ethane dehydrogenation reaction to produce ethylene, show reactor conversions up to 6 times higher than equilibrium conversions. This shift in equilibrium is due to the selective permeation of one of the reaction products, i.e. hydrogen, according to Knudsen difision. In the experiments we have utilized a tram-membrane pressure difference and an inert sweep gas on the low pressure side of the membrane. KEYWORDS Membrane

reactor; sol-gel alumina membrane;

dehydrogenation

of ethane.

INTRODUCTION

.

Ceramic membranes were first produced in the early seventies (Yoldas, 1975), but until recently had not found any extensive industrial applications. In recent years ceramic membranes have become commercially available with excellent pore Such membranes are finding broad applications in the food, size uniformity and good thermal and mechanical properties. pharmaceutical and electronic industries, for waste water treatment and in bioreactor applications (Hakuta, 1988). High temperature ceramic membranes are also today finding use in catalytic and reaction engineering applications (Tsotsis et al., 1989a). Catalytic membrane reactors have the inherent capability of combining reaction and separation in a single operation. This, in itself, is a significant advantage over having two completely separate unit operations of reaction and separation. Membrane reactors, however, offer other advantages over traditional reactors (packed, fluidized and trickle beds). The membrane provides for selective removal of one or more products and/or stable intermediates in parallel with the reaction. This drives the reaction (to use equilibrium limited reactions as an example) continuously towards the product side and results in higher than equilibrium conversions, thus limiting the need for high temperatures and pressures, reducing recycle and downstream separation requirements. The membrane, furthermore, allows one to adjust and influence the surface concentration of reactant and intermediate species and provides the flexibility for the unique control of the reactor selectivity and product distribution. In 1979 Kameyruna et d. utiThe earliest applications of membrane reactor technology utilized porous glass membranes. lised porous Vycor Glass tubes to study the decomposition of RsS to produce HZ and S. Conversions as high as twice the equilibrium limit were observed. Glass membranes have since then been utilieed in other catalytic studies for the catalytic dehydrogenation of cyclohexane to bcnsene over supported Pt catalysts. Most of the current literature (prima& from the Soviet Union and Jaoan) on catalvtic membrane reactor anrrlications involves the use of Pd. Pd allovs with Ru. Ni and various metals Scam goups VI to VIII, andPd coated Zirconia membranes. The Soviet literature, in particular, is impressive and includes (by 1986) 58 Soviet inventors certificates and 86 European and herican patents. They cover a number of hydrogenation and dehydrogenation reactions such as CHd steam reforming (Nasarkina and Kiiichenko, 1979), dehydrogenation of butene to butadlene (Gryaznov ez al., 1970), acetylene hydrogenation (Gul’yanova et al., 1978), dehydrogenation of I-, 2-cyclohexanediol (Michenko et al., 1977), dehydrocyclization of n-hexane (Lebedeva et al., 1981), hydrogenation of cyclopentadiene to cyclopentene (Zhernoskek et al., 1979), and the production of many other specialty chemicals. Zhao et al. (1989) recently described experiments utilizing an oxidative palladium membrane reactor for dehydrogenation reactions. The application of Pd membranes is based on the fact that Pd is highly permeable to Hs (a fact known since Thomas Graham first observed the phenomenon in 1866) but virtually impermeable to other gases and, of course, liquids. The development of Pd catalytic membranes has had a significant impact on proving and populariaing the concept of catalytic membrane reactors. The inherently low trans-membrane fluxes, however, combined with the high cost of these membranes and the phenomena of metal sintering, imbrittlement and fatigue have so far hindered the widespread industrial application of these membranes. Some of the earlier membrane reactor efforts also involved the use of materials such as nonporous Ag and CaO-stabilized sirconks exhibiting enhanced oxygen anionic conductivity. A variety of reaction processes, primarily partial oxidation reactions, have been tested, some with considerable success (Hasbun, 1988).

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A. M. CHAMPAGNIE et al.

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The use of ceramic membranes in catalytic membrane reactor applications has been reported only recently. The ceramic membranes utilired were alumina membranes prepared either by anodic or Sol-Gel techniques. A Sol-Gel alumina membrane was utilized by Bitter (19%8), in an ceramic tubular reactor for propane dehydrogenation. The use of Sol-Gel ceramic membranes for &bane dehydrogenation has been reported in a patent filed by Minet et ol. (1989). Open literature reports of the use of ceramic membranes in catalytic membrane reactor applications are more recent. Tsotsis et al. (1989b) have reported the use of flat anodic alumin a membranes during cyclohexane dehydrogenation in a membrane reactor. Anodic alumina membranes are ideally suited for fundamental investigations of transport and reaction because their porous structure consists of straight nonintersecting cylindrical pores. They are commercially available, however, only in flat disk confgnrations, and their utility for industrial scale applications remains to be proven. At the recent ACS symposium on new catalytic materials and techniques, Zaspalis et ol. (1989) reported on the use of Sol-Gel alumina and titauia membranes in catalytic membrane reactor applications for dehydrogenation of methanol and Moser et al. (1989) reported the use of Sol-Gel tubular alumiua membranes during ethylhenzene dehydrogenation to styrene. Conversions close to equilibrium were reported in a region of experimental conditions. The ethybenzene to styrene reaction is also currently under study by a number of other industrial and academic groups. At the recent AIChE annual meeting in San Francisco, Liu et al. (1989) outlined their results of their studies. of the ethylbenzene to styrene reaction using alumin a catalytic membrane reactors. In this paper, we present results of our studies of the catalytic dehydrogenation of ethane to ethylene in a tubular catalytic membrane reactor utilizing a Sol-Gel ceramic- alumin a membrane. Ethane dehydrogenation is, of course, a very important industrial reaction used to produce ethylene which is becoming a valuable chemical commodity. The predominant industrial process for producing ethylene is homogenous thermal cracking of ethane at high temperatures, which produces considerable amounts of by-products, such as methane, acetylene and higher hydrocarbons. The typical selectivity to ethylene, in an ethane steam cracker, is 78 to 82 mole percent with recycle. Heterogeneous catalytic processes have also been developed using supported platinum on alumina catalysts, resulting in higher selectivities to ethylene of up to 98 percent (Vera et al., 1986). However the very high temperatures necessary to obtain adequate yields result in cataiyst deactivation, due to metal sintering, and coke formation. EXPER.IMENTAL

APPARATUS

The experimental apparatus used in the studies reported in this paper is shown in Fig. 1. It basically consists of the reactant handling and delivery systems, the catalytic membrane reactor and its control hardware, and the product collection and measurement systems.

Fig. 1. Experimental

apparatus

for ethane dehydrogenation.

The schematic of the catalytic membrane reactor is shown in Fig. 2. It consists of a stainless steel casing inside of which is placed the ceramic membrane tube. The ceramic tube is sealed inside the stainless-steel reactor body using graphite-string (Fiber Materials Inc., Maine). The string is typically wrapped six to seven times around the alumina tube ends, which are then sealed to the reactor body by Swagelok compression fittings. A high temperature cylindrical band heater (Watlow Inc.) provided the heating necessary to carry out the endothermic reaction. Four Omega ‘cement-on ’ thermocouples, each 2.5 in. The ceramic tube temperature was controlled within l°C apart monitored the outside alumin a ceramic tube temperature. of the set point by an Omega CN-2010 programma ble controller using the third thermocouple from the tubeside feed inlet. The reactor was well insulated so that during all experimental runs reported here, all thermocouples indicated the same temperature, i.e. the reactor was operating under isothermal conditions.

High temperature

D3

F’k.

2. Detair

catalytic membrane reactor for ethane dehydrogenation

of ceramic membrane

2425

reactor.

The reactant gases utilized were Hz (ultra-high pure grade, Matheson), Ct& (99 percent purity, MG Industries), and argon (ultra-high pure grade, Mathes on). Before being sent to the reactar, the reactants were dried using calcium snlphate crystals and purified using charcoal and reolite. The reactants were fed through the ‘tube-side’ inlet (see Fig. 2). The individual flowrates, the mixture composition as well as the overall flow rate were controlled by Condyne flow controllers (Vici Condyne Inc., C&f.). The flowrates were measured by Omega gas flowmeters. An inert gas, acting as a sweep gas, was allowed into the reactor ‘she&de’, which was at a distance of 5cm from the ‘shellside’ outlet. The reactor was operated in an intermediate ‘cross-flow’/Lcounter-flow’ mode, which allowed for good mixing of the gases in the ‘shellside’ and resulted in a fiat temperature profile along the length of the ceramic tube. The inert gas flow rate was also controlled by a Condyne Aow controller and measured by an Omega flow-meter. The ‘tubeside pressure was controlled by a needle valve on the ‘tubeside’ outlet and measured by a pressure transducer. The ‘shellside’ outlet was maintained at atmospheric pressures. The composition of the ‘tubeside’ and ‘shellside’ inlet and outlet streams were analysed using a UT1 1OOC mass spectrometer with an attached atmospheric sampling unit. The overall flowrates of the outlet streams were also measured using glass bubble flowmeters. The ceramic alumina membrane tube (MembraloxTM) was supplied to us by ALCOA. It consists of a multilayered composite porous alumina tube of inner diameter 7and outer diameter lOmm, and 250mm in length. The 1” layer insjde the tube is 5pm thick and has a unimodal pore structure with diameter of 40A. Successive layers are thicker, with progressively larger pores, supported on a thick support layer approximately 1.5rnm thick with a pore size in the range lO-15pm. Ninety five percent of the resistance to flow and over 90 percent of the surface area lies in the first two top layers. The alumin a membrane tube was wet impregnated with a chloroplatinic acid solution (Alfa products, Morton Thiokol) up to 5 wt. percent. It was dried overnight and subsequently the tube was placed in the reactor and a 60 percent argon, 40 percent oxygen gas mixture was passed over the catalyst overnight at 13OOC. Subsequently, hydrogen was passed through the reactor at 35O’C for 12 hours. We found that the impregnation procedure did not noticeably lower the membrane permeabiity. Permeability studies of the various gases through the ceramic membrane were run in situ after the ceramic tube was sealed inside the stainless a permeability 1 atm V (m3/s)

a bubble

a function ‘shellside’

(psi) is the room pressure, Z’, (K) is the room temperature, pressure, T,,, (K) is the membrane temperature,

(1) (2) EXPERIMENTAL Permeability

RESULTS

Studies.

Figure 3 shows the pressure dependence of volume Aow rate for various gases at 400°C through a 40B platinum impregnated membrane. The straight lines are indicative of the fact that ethaue, ethylene, hydrogen and argon permeate through the membrane with no significant bulk flow, for pressure differences between the ‘tube’ and ‘shell’ sides of up to 15-20 psi. Above 20 psi of trams-membrane pressure difference, however, bulk fioow starts becoming significant.

A. M. CHAMPAGNIE et al.

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Fig. 3. Pressure dependence

D3

of gas volume flow rates.

For temperatures above 4OO“C (which is the region of interest in this study), all four gases (i.e. hydrogen. ethane, ethylene and argon) follow a Knudsen diffusion mechanism. This means that the ratio of difisivities for any pair of these four gases is inversely proportional to the square root of the ratio of their molecular weights. At lower temperatures, however, surface diffusion seems to play a sign&ant role. At room temperature, for example, for ethane, ethylene and argon surfaw diffusion contributes more than 25 percent to the overall difLsional flux. Membrane

Reactor Studies

For the experiments reported here the membrane reactor was operated in a mixed ‘counter’/‘cross-flow’ mode, i.e. with the reactant gases and the inert sweep gas flowing in the same direction. A series of experimental runs were also performed without any sweep gas at ail, i.e. with the ‘shellside’ inlet kept closed, and the outlet open and maintained at atmospheric pressures. The experimental observati&s are plotted in Fig. 4, which shows the membrane reactor conversion in terms of ethane for a temperature range of 450 to 600°C. (The trans-membrane pressure drop for all these experiments was 10 psi). The conversions obtained in the membrane reactor, were higher than the corresponding equilibrium conversions calculated at the ‘tubeside’ or ‘shellside’ total pressures, with ratios of membrane to corresponding equilibrium conversions being as high as 6 at the lower temperatures. The reaction selectivity to ethylene was greater than 96 percent, with negligible amounts of methane and acetylene produced as by-products.

Fig. 4. Ethane conversion

to ethylene vs. temperature.

<.

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High temperature

catalytic membrane

reactor for ethane dehydrogenation

2427

The effect of increasing the flow rate of the inert sweep gas on the membrane reactor conversions was investigated in a series ofexperimentr in which the temperature of the reactor was maintained constant at 550°C and the trans-membraae pressure difterences was approximately 15 psi. The results are plotted in Fig. 5, and show the conversions initially increasing with increasing sweep rate, and &ally leveling OR to a value approxima tely 6 times bigher than the equilibrium conversion. This is due to the fact that the increase in the inert gas sweep rate creates an increased hydrogen flux across the membrane, whkb in turn causes an enhanced shift of the reaction towards the product side. The increased sweep gas flow rates also enhance the diffusional flux of ethane across the membrane. Since the diffusion of ethane is, however, one fourth of that of hydrogen, the beneficial effects of enhanced hydrogen flux outweigh the detrimental effects of the accompaning increased ethane 5x1.x, at least for the range of experimental conditions of Fig. 5.

Fig.

5. Experimental

conversions

of ethane to ethylene at 55OOC.

The effect of the feed-side residence time on conversion was also investigated in another series of experiments in which the membrane reactor temperature was maintained at 500°C (Fig. 6). The sweep gas flow rate to the feed gas 50x17 rate ratio was maintained at values close to 1.0 and the tram-membrane pressure drop was kept constant at 15 psi. The results show that at residence times above 10 seconds, the conversion to ethylene is constant at about 11.5 percent. Below 10 seconds the conversion decreases continuously to a constant value of of 6.5 percent at residence times close to 1 sec. This is still significantly higher thk the corresponding equilibrium conversion.

3 ?I

F%JlJILl5RlUM

2

1

6 NBESDE

Fig. 6. Effect of reactor residence

I

CONVeRSION

IO

RESlDENC!5,,M%

12 SEC

time on conversion.

II

16

I

IS

A.

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CHAMPAGNIE er al.

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A detailed theoretical model of the membrzuxe reactor has been developed, which accounts for the diEusion and reaction phcnoin the catalytic membrane and for the transport mechanism of the ‘tube’ and ‘shell’ sides. This model utiIizes no a&stable pammeters and performs reasonably weIl in terms of fitting the available experimental data. CONCLUSIONS We have described here a laboratory scale catalytic membrane reactor, which utilizes platinum impregnated Sol-Gel type alumina membranes. This reactor has been shown to signi&antly enhance product yields during the ethane dehydrogenation The reactor conversions obtained reaction by selectively allowing product hydrogen to permeate through the membrane. Conversions in the membrane reactor increase with increasing inert sweep gas flow rates through the membrane ‘she&de’. are up to 6 times higher than those obtained at equilibrium, without removal of product hydrogen. The experiments reported here are intended only to show that the catalytic membrane reactor concept works, using a simple but nevertheless very important industrial catalytic reaction. The catalyst loading and preparation techniques and reactor configuration and operating conditions reported here are, however, far from being optimum. Research towards this goal is continuing in our laboratory. ACKNOWLEDGEMENT We express our appreciation to Dr. Paul Liu of ALCOA Separations Technology Division for many helpful technical discussions and for providing the MembraloxTM membrane elements. We also wish to thank Ms. Connie Anasis, for assisting with the expernnental measurements. REFERENCES Bitter, .I. G. A. (1988). Process and apparatus for the dehydration of organic compounds. Brit. Patent GB 2,201,159, 24 August. Gryaznov, V. M., V. S. Smirnov, L. h. Ivanova and A. P. Mishchenko (1970). Coupling of reactions resulting from hydrogen transfer through the catalyst. D&l. Alsad. Nauk. SSSR, m(l), 144-147. Gul’yanova, S. G., V. M. Gryaznov, A. Kh. Morales and A. M. Fiippov (1985). T+. d-go Sov.-frants. Seminara po Kataku, Met. 2. Katditich. Akfimwst ionov Perekhodn. Met. v KompleksaM i Olsisn. 1. Katalitich. Aktiunosi Chist. i Nanensen. A4atritsakh Tbilis< 126-130. From Re$ Zh., Khim. 1979, Abstr. No. 15B1273. Hakuta, T. (1988). Advancement in membrane separation technology. Kagaku Kogak- z(2), 112-114. Hazbun, E. A. (1988). Ceramic catalytic membrane for hydrocarbon conversion. U.S. Patent US 4,791,079, 13 December. Itoh, N., Y. Shindo, K. Haraya, and T. Halcuts (L988). A membrane reactor using microporous glass for shifting equilibrium of cyclohexane dehydrogenation. J. C&m. Eng. Jpn., a(4). 399-404 Itoh, N. (1988). Dehydrogenation reaction apparatus. Jpn. Patent JP 63,X4,624, 27 June. Itoh, N., Y. Shindo, K. Haraya, and T. Hakuta (1989). A novel method of dehydrogenation of cyclohexane with a Pd. membrane. Sehyu Gakkatihi, a(l), 47-50. Kameyama, T., M. Dokiya, K. Fukuda and Y. Kotera (1979). Differential permeation of hydrogen through a microporous Vycor-type glass membrane in the separation of hydrogen sulfide. Sep. Sci. Technol., l4(10), 953-957. Kameyama, T., M. Dokiya, M. Fujishige, H. Yokokawa and K. Fukada (1981). Possibility for effective production of hydrogen from hydrogen sulfide by means of a porous Vycor glass membrane. Ind. Eng. Chem. Fundam., a(l), 97-99. Lebedeva, V. I. and V. M. Gryaznov (1981). Effect of hydrogen removal through a membrane catalyst on the dehydrocyclzation of n-hexane. Izv. Akad. Nat& SSSR Ser. Khim., (3), 611-613. Liu, P. K. T., J. C. Wu, E. S. Martin, R. R. Bhave (1989). Use of microporous ceramic membranes as catalytic reactors for dehydrogenation processes. AIChE Annual Meeting 1989, San Francisco, 5-10 November. Paper No. 178d. Lugo, H. J., and J. H. Lunsford (1985). J. Catal., 91, 155.166. P recess for production of ethylene from ethane. US Patent file Minet, R. G., T. T. Tsotsis and A. M. Champagnie (1989). no. 427,517. Mischenko, A. P., M. E. Sarylova, V. M. Gryaznov, V. S. Smirnov, N. R. Roshan, V. P. Polyakova and E. M. Savitskii (1977). Hydrogen permeability and catalytic activity of membranes made of palladium-copper alloys in relation to the dehydrogenation of 1,2-cyclohexanediol. Isv. Akad. Nauk. SSSR, Ser. Khrm., (7), 1620-1622. Moser, W. R., Y. Liu, Ma Y. Hua and A. G. Dixon (1989). Inorganic membranes for shifting equilibria in the catalytic dehydrogenation of ethylbenzene to styrene. Paper presented at the ACS symposium on new catalytic materials and techniques Miami, Fla., lo-15 September. Nazarkina, E. B. and N. A. Kirichenko (1979). Improvement in the steam catalytic conversion of methane by hydroger liberation via palladium membranes. Klzim. Teknol. Topl. Masel., 2, 5-7. Sun, Y. M., and S. J. Khang (1988). Catalytic membrane for simultaneous chemical reaction and separation applied to 2

D3

High

temperature

dehydrogenation reaction. Ind. T. T., N. Nourbarkhsh,

Tsotsis,

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membrane

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Eng. Chem. Res., 27, 1136-1142. A. M. Champagnie and I. A. Webster

for cthane

(1989).

dehydrogenation

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membranes. AIChE Symposium Sties, f&, 286, 78-85. Tsotsis, T. T., N. Nourbarkhsh and I. A. Webster (1989). Ceramic membranes for studies of reaction and diffusion. Prep. Am. Chem. Sot., Div. Pet. Claem., a(3), 502-508. Vera, B. V., P. R. Pujado, and R. F. Anderson (1986). Oleflex: 62 - C5 dehydrogenation updated. Energy Pmg., c(3), 171-176. Yazawa, T. (1989). Separative glass. Osaka Kogyo Gqutsu Shikensho Hokokt~, 377, 1-233. Yoldas, B. E. (19’i5). Cemm. Bull., M, 289-294. Zaspalis, V. T.. W. van Praag, IL Eeizer, .I. G. van Omen, J. FL H. Ross and A. J. Burggraaf (1989). Inorganic porous membranes as catalytic active materials : concept and experiments. Paper presented at the ACS symposium on new catalytic materials and techniques, Miami, Fla., lo-15 September. Zhao, FL., N. Itoh and R. Govind (1969). Exp erimental investigation of a novel oxidative membrane reactor for dehydrogenation reactions. Paper presented at the ACS symposium on new catalytic materials and techniques, Miami, Fla., lo-15 September. Zhernosek, V. M., N. Mikhalenka, Iokhannes Seyum, E. V. Khrapova and V. M. Gryaznov (1979). Kinetic model of a hydrogen permeable catalyst. H-in& K&al., a(4), 921.924.