Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Patent
1994-07-02
1994-08-30
Spitzer, Robert
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
95 51, 96 13, B01D 5322
Patent
active
053424328
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a composite membrane constructed from three layers, which is highly suitable for the separation of gas mixtures and which contains a permselective layer of regularly arranged amphiphilic molecules.
In industry, the task is often faced of completely separating gas mixtures or at least concentrating one component of the gas mixture. This task is carried out to an increasing extent with the aid of semipermeable membranes. Depending on solubility and diffusion coefficients, these membranes allow gases to pass through at a variable rate.
A composite membrane for gas separation having a three-layer structure is already known from DE-OS 3,415,624. It contains a supporting membrane layer made of porous polymer (A), a nonporous, gas-permeable intermediate layer (B) made of polyorganosiloxane and a thin layer of a special polymer (C) which has a favorable O.sub.2 /N.sub.2 permeability coefficient ratio (selectivity). This layer can be obtained by application of a thin film, which can be produced by spreading on a water surface.
For the industrial utility of such gas separation membranes, both their permeability and their selectivity are of particular importance.
The permeability of a membrane for a certain gas depends both on the thickness of the active layer (=permselective layer) and on the permeability coefficient for this gas. The gas permeability of the gas-permeable intermediate layer (B) is in general substantially greater than that of the permselective layer.
The separation selectivity of a membrane is primarily determined by the material of the layer (C). However, experience tells us that materials of high selectivity have a low permeation coefficient.
Therefore all attempts to find polymer materials for C which have both a high selectivity and a high permeability coefficient have hitherto been unsuccessful. The choice is therefore only between highly permeable and slightly selective or selective and slightly permeable membranes. The latter are preferably employed today for industrial applications.
In order to be able to obtain acceptable permeation rates, efforts have been made to make the thickness of the active layer (=permselective layer) as thin as possible. In this case, however, the occurrence of defects, so-called pinholes, sets limits on the efforts towards ever smaller layer thicknesses.
Thus, today it is possible to produce permselective layers having layer thicknesses of about 0.05 to 0.5 .mu.m (=50-500 nm). The use of membranes having such layer thicknesses became possible after it had been learnt virtually to block the pinholes always occurring in such thin (permselective) layers with silicone rubber (Henis, J. M. S.; Tripodi, M. K.; Sep. Sci. Technol. 1980, 15, 1059).
The silicone layer can thus be applied to the membrane as an outer layer. It can also be enclosed, however, by the supporting membrane layer and the permselective layer (cf. DE-OS 3,415,624).
Nevertheless, especially with respect to their permeability, these known membranes are still in need of improvement. The aim therefore existed of providing a composite membrane suitable for gas separation which has a considerably improved permeability combined with good selectivity.
A composite membrane has now been found, wherein the permselective layer is composed of regularly arranged amphiphilic molecules which contain one or two alkyl groups each having 7-25 carbon atoms per polar hydrophilic group. The layer (A) of the composite membrane is a porous micro- or ultra-filter. The gas-permeable intermediate layer (B) should be composed of an amorphous polymer having a high gas permeability. Suitable polymers are, for example polymethylpentene, polysiloxane-polycarbonate block copolymers, polytrimethylsilylpropyne, EPDM rubber or chlorinated polyethylene.
The amphiphilic molecules of the permselective layer should be water-insoluble and should form stable monolayers on the water/air interface. Such monolayers are known, for example, from DE-OS 3,724,688. Amphiphilic molecules in each case contain a hydrophi
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Haubs Michael
Prass Werner
Hoechst Aktiengesellschaft
Spitzer Robert
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