Transport and separation properties of a silicalite-1 membraneði. operating conditions

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Chemical Engineering Science 54 (1999) 245—258

Transport and separation properties of a silicalite-1 membrane—I. Operating conditions
L.J.P. van den Broeke*, W.J.W. Bakker, F. Kapteijn, J.A. Moulijn
Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 12 November 1997; received in revised form 23 July 1998; accepted 14August 1998

Abstract Results are reported for the one-component permeation of a number of gases through a silicalite-1 membrane. The effect of several operating conditions, like the temperature, the feed pressure, the sweep gas flow rate, and the orientation of the membrane, on the flux are discussed. The optimal experimental conditions are identified for both steady state and transient permeation.Results for the flux as function of the pressure, 10—900 kPa, and as function of the temperature, 200—700 K, are modelled taking two diffusion mechanisms into account. Permeation results are reported for two different silicalite-1 membranes. A small difference in the permeance is observed, indicating a high reproducibility of the zeolite membrane synthesis. The contribution of the equilibriumisotherm to the flux and the permeance is demonstrated. A comparison is made between results obtain with helium and argon as the sweep gas. Finally, results are reported for the permeation of pure gases through a silicalite-1 membrane modified by silanation. 1998 Elsevier Science Ltd. All rights reserved. Keywords: Zeolite membrane; Silicalite-1; Diffusion mechanisms; Adsorption; Modelling

1.Introduction Inorganic membranes, especially membranes with micropores, are interesting for they can combine separation and catalysis over a large temperature range. However, to employ zeolite membranes on a large scale a proper understanding of the synthesis procedure, the mass transport properties, and the separation behaviour is required. Most of the zeolite membranes reported in the literature are of theMFI type, which include silicalite-1 and ZSM-5. An overview of the various zeolite membranes synthesised has been given by Bein (1996). Recently, also permeation experiments with a mordenite membrane (Nishiyama et al., 1995), with ferrierite membranes (Nishyama et al., 1997; Lewis et al., 1997), and

*Corresponding author. Present address: Process Development Group, Department of ChemicalEngineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands. Tel.: 0031 40 247 3675; fax: 0031 40 244 6104; e-mail: l.j.p.van.den. broeke@tue.nl. Present address: Akzo Nobel Central Research, Department RTA, P.O. Box 9300, 6800 SB Arhnem, Netherlands.

with zeolite Y membranes (Kita, 1997; Kusakabe, 1997) have been reported. Mastukata and co-workers (1993, 1994)studied the synthesis of zeolite membranes from aluminosilicate gels under vapour atmosphere. Depending on the synthesis time a ferrierite or a ZSM-5 membrane was obtained. If the synthesis was stopped after three days a ferrierite membrane was obtained while after nine days a ZSM-5 membrane was obtained. It was found that for a successful synthesis, i.e. a gas-tight zeolite layer, the pH was themost important factor. Yan et al. (1995) studied the synthesis of ZSM-5 layers on -Al O porous disks using   a hydrothermal procedure. The effect of a number of parameters, including the TEOS content, the water content, the effect of NaOH, and the effect of aluminium, on the synthesis and the permeation was studied. Concerning the description of the transport and the separation properties of zeolitemembranes a proper understanding of the diffusion as well as of the equilibrium adsorption is needed. The mass transport in microporous media can be understand in terms of diffusion of molecules in an adsorbed phase. The diffusion is described in terms of molecules jumping between minima in the potential energy field of the pore (Barrer and Jost, 1949; Riekert,

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