Inorganic composite membrane comprising molecular sieve crystals

Details Inorganic composite membrane comprising molecular sieve crystals Inorganic composite membrane comprising molecular sieve crystals

1993-08-04

1995-07-04

Dawson, Robert A.

Liquid purification or separation

Filter

Supported, shaped or superimposed formed mediums

21050022, 21050025, 210506, 264 455, 264139, 502 4, 502 64, 502 71, 96 4, 96 11, B01D 2900, B01D 3900

Patent

active

054297435

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an inorganic composite membrane containing molecular sieve crystals and to methods for producing such a membrane. For separation on a molecular level, such as gas separation, vapor permeation and pervaporation, mainly membranes on the basis of organic polymers have been proposed so far for use on an industrial scale. A wide variety of macromolecular (almost exclusively organic) materials have been found to be suitable for use as a membrane material. Reasonable separation factors can be achieved, and the throughput of such membranes is sufficiently large.
However, these polymer membranes have the disadvantage of a relatively short service life. Owing to the sensitivity of the materials to solvents (swelling) and the low stability at high temperatures, the range of application is limited. Moreover, regeneration by oxidative removal of impurities is not possible.
Also known are so-called ceramic membranes composed substantially of inorganic materials, which, compared with polymer membranes, have the advantage that they are resistant to high temperatures, so that regeneration is possible, and moreover are relatively inert. Such membranes are usually produced starting from multi-layered systems, in which a relatively thick macroporous layer serves as a support for a microporous top layer which is much thinner relative to the supporting layer and exhibits the separation properties. The production of such membranes, in which the so-called sol-gel or dip-coating techniques can be used successfully for providing the separating layer, is described inter alia in the following publications: A. Larbot, A. Julbe, C. Guizard, L Cot, J. Membr Sci., 93, (1989), 289-303; A. Larbot, J. P. Fabre, C. Guizard, L. Cot, J.Am.Ceram.Soc., 72, (1989), 257-261; W. A. Zeltner, M. A. Anderson, "Chemical Control over Ceramic Membrane Processing: Promises, Problems and Prospects", in: Proc. 1st Int.Conf.Inorg.Membr., (eds. J. Charpin, L. Cot), Montpellier, France, July 3-6, 1989, 213-223; A. Leenaars, Preparation, Structure and Separation Characteristics of Ceramic Alumina Membranes, PhD thesis, University of Twente, Netherlands, (1984); H. M. van Veen, R. A. Terpstra, J. P. B. M. Tol, H. J. Veringa, "Three-Layer Ceramic Alumina Membrane for High Temperature Gas Separation Applications", in: Proc. 1st Int.Conf.Inorg.Membr., (eds. J. Charpin, L. Cot), Montpellier, France, July 3-6, 1989, 329-335.
A disadvantage of such ceramic membranes is that the separation efficiency is low. In most ceramic membranes developed so far, separation takes place on the basis of Knudsen diffusion. In that case, the rate of transport is inversely proportional to the square root of the molecular weight. The selectivity of the separation process is sufficient only if molecules having widely divergent molecular weights are to be separated from each other.
Improved insights have led to separation processes on the basis of ceramic membranes exhibiting material transport mechanisms other than Knudsen diffusion, such as surface diffusion or capillary condensation: R. J. R. Uhlhorn, "Ceramic Membranes for Gas Separation; Synthesis and Transport Properties", PhD thesis, University of Twente, Netherlands, (1990). In the case of surface diffusion, use is made of differences in chemical and/or physical properties of the molecules to be separated. The surface of the separating (or active) part of the membrane is modified in such a manner that one type of molecule is transported much more rapidly than the other as a result of a difference in surface diffusion. However, the insight into the mechanism of surface diffusion is still poor, so that it is difficult to make appropriate use of differences in chemical and/or physical properties.
In capillary condensation and multilayer diffusion, use is made of the formation of a liquid phase in the separating part of the membrane. Here, too, it may be advantageous to modify the surface of the membrane. Although the separation efficiency can be high, the implementation of the separation process is strongly

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