Membrane material selection criteria

Koros WJ and Heliums MW. Gas separation membrane material selection criteria Differences for weakly and strongly interacting feed components. Fluid Phase Equilibria 1989 53 339-354. 

A simple yet valuable criterion for candidate membrane material selection is the characteristic membrane thickness [41 ]. In the theory, the transport equations for diffusion in the solid, and for the surface exchange are linearized. It should therefore strictly be used when small Po -gradients are imposed across the membrane. In the next section, methods for measuring are briefly discussed. 

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

Tubular porous metal supports with 1 and 2 inch outside diameters (OD) were selected because they can be easily assembled with normal welding techniques, are not fragile like ceramic materials, and do not require special fittings to achieve leak-tight seals. CRI-Criterion currently produces membranes of ODs up to 2 inches and lengths up to 24 inches. Current focus is to extend the viable individual unit length to approximately 1 meter. 

It has been known for some time that infection with various types of viruses impairs the permeability barrier function of the host cell cytoplasmic membrane, allowing ordinarily impermeable large molecules to enter the cell from the surrounding medium and allowing particulate intracellular material to leak out. A good example of this is penetration of supravital dyes, such as trypan blue, to enter cells, usually late after viral infection as a criterion of cell death. In addition, certain viral infections induce earlier and more subtle changes by altering membrane transport of small ions into or out of the infected cell. A few selected examples of altered membrane permeability of the cytoplasmic membrane are cited here as an example of readily measured cytopathic effects of well-known virulent viruses. 

The application of HSPPO as a material for gas separation membranes is limited because of thermal instability of SPPO in protonated form. Substitution of the proton in sulfonic groups by metal cations helps to overcome this problem. The question arises is there any particular metal cation or a group of metal cations that would be preferred for such substitution The most obvious criterion in the selection of metal cation is its valence. While the reaction of HSPPO with Na Li and other monovalent cations is expected to result in a direct substitution of proton by metal cation, the situation complicates for multivalent cations, because such cations can attach to more than one sulfonic group, which may result in cross linking of the polymer. Consequently, depending on the valence and the size of cation, the physical structure and hence the gas transport properties of MSPPO of similar DS can vary significantly. 

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