Zeolite Membrane Separations

The present review of zeolite membrane technology covers synthesis and characterization methods as well as the theoretical aspects of transport and separation mechanisms. Special attention is focused on the performance of zeolite membranes in a variety of applications including liquid-liquid, gas/vapor and reactive 

Zeolites in Industrial Separation and Catalysis. Edited by Santi Kulprathipanja Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-32505-4 

Zeolite Maximum pore size (A) Thermal stability (°C) Typical conditions for in situ synthesis methods  

5-TIPB o/m-xylene Ethylbenzene Double branched alkanes p-xylene SF6 

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I 70 Zeolite Membrane Separations In situ growth method... 

Bernardo, P., Algieri, C., Barbieri, G. and Drioli, E. (2008) Hydrogen purification from carbon monoxide by means of selective oxidation using zeolite membranes. Separation and Purification Technology, 62, 631—637. 

Heng S, Yeung K-L, Julbe A, Ayral A, Schrotter J-C. Preparation of composite zeolite membrane separator/con-tactor for ozone water treatment. Micropor Mesopor Mater 2008 115 137-146. 

Wee, S.-L., Tye, c.-T. and Bhatia, S. (2008) Membrane separation process— pervaporation through zeolite membrane. Separation and Purification Technology, 63, 500-516. 

Recently, high-quality SOD membranes for water separation have been developed by Khajavi etal. [21, 52]. These zeolite membranes should allow an absolute separation of water from almost any mixture since only very small molecules such as water, hydrogen, helium, and ammonia can theoretically enter through the six-membered window apertures. Water/alcohol separation factors 10 000 have been reported with reasonable water fluxes up to 2.25 kg nr h at 473 K in pervaporation experiments. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

At low temperatures, adsorptive separation becomes important for zeolite membranes as sorption of one species can effectively hinder permeation of other species. 

Hong, M., R.D. Noble, and J.L. Falconer, Highly selective H2 Separation Zeolite Membranes for Coal Gasification Membrane Reactor Applications, Annual Technical Progress Report, U.S. DOE Contract DE-FG26-02NT41536, December 2005. 

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