Permeable materials selection

Our screening and testing of multicomponent capsules/beads is incomplete. However, it offers a novel approach for the material selection for immobilization devices, which permits the simultaneous control of permeability, mechanical stability, and compatibility. The alternative multicomponent systems presented herein offer new possibilities for biomaterials, particularly those employed in bio artificial organs. 

Despite concentrated efforts to innovate polymer type and tailor polymer structure to improve separation properties, current polymeric membrane materials commonly suffer from the inherent drawback of tradeoff effect between permeability and selectivity, which means that membranes more permeable are generally less selective and vice versa. 

Shinji, Remmen and Chiang (1970). Authors state that further improvements can be achieved by selecting materials with higher permeability and selectivity. 

The membrane performance for separations is characterized by the flux of a feed component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a feed mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity or separation factor. Selectivity (0 can be defined as the ratio of the permeabilities of the feed components across the membrane (i.e., a/b = Ta/Tb, where A and B are the two components). The permeability and selectivity of a membrane are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent... 

Later Henis and Tripodi [73] showed that membrane defects in anisotropic Loeb-Sourirajan membranes could be overcome in a similar way by coating the membrane with a thin layer of a relatively permeable material such as silicone rubber. A sufficiently thin coating does not change the properties of the underlying selective layer but does plug defects, through which simple convective gas flow can occur. Henis and Tripodi s membrane is illustrated in Figure 3.29. The silicone rubber layer is many times more permeable than the selective layer and... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

Another type of gas separation membrane is the multilayer composite structure shown in Figure 8.9. In this membrane, a finely microporous support membrane is overcoated with a thin layer of the selective polymer, which is a different material from the support. Additional layers of very permeable materials such as silicone rubber may also be applied to protect the selective layer and to seal any defects. In general it has been difficult to make composite membranes with... 

The hydrophilicity-hydrophobicity balance of the membrane polymer is another important parameter that is mainly influenced by the functional groups of the polymer. Hydrophilic polymers have high affinity to water, and therefore they are suited as a material for nonporous membranes that should have a high permeability and selectivity for water (e.g., in RO or hydrophilic PV). In addition, hydrophilic membranes have been proven to be les s prone to fouling in aqueous systems than hydrophobic materials. 

Fluorinated polyimides have achieved great importance as barrier materials during the last few years. Many experimental polyimides prepared from fluorine-containing monomers, mainly novel diamines, show an advantageous balance of permeability and selectivity for technical gases and vapours, which makes them very attractive for the fabrication of permselective membranes [119]. This is an application field showing very rapid expansion, where there exists a strong demand for new polymeric materials, and where soluble aromatic polyimides are considered as a real alternative [136-146]. 

Permeable materials for the production of membranes have been employed for the treatment of a diversity of fluids [16,59-64], A membrane is a perm-selective barrier between two phases capable of being permeated owing to a driving force, such as pressure, concentration, or electric field gradient... 

Polyelectrolyte complexes are very promising materials for preparing semi-permeable membranes of definite permeability and selectivity The methods of preparation and the properties of membranes made of polyelectrolyte complexes based on strong polyelectrolytes, e.g. poly(sodium sterene sulfonate) and poly(vinylbenzyl-trimethyl ammonium chloride) were described These membranes may be applied for reverse osmosis in the desalting of sea-water, for dialysis and ultrafiltration in purifications and concentration of water solutions containing coUoids or micro-and macroparticles ... 

A considerable array of membrane materials exist, notably for various gaseous separations, some more effective than others (Hoffman et al., 1988). That is, some are more permeable and more selective than others. It will also depend on the system to be separated. In other words, materials are not yet available for the full array of gaseous mixtures encountered. As to other mixtures, a partial hsting is shown in Table 19.3 for reverse osmosis, and in Table 19.4 for ultafiltration, with performance data in Tables 19.5 and 19.6. Table 19.7 pertains to gas permeation, giving permeabilities and selectivities or relative permeabilities. Much more information is furnished in Appendix 1 of Hoffman (2003) as well as in other references. 

The evaluation of the commercial potential of ceramic porous membranes requires improved characterization of the membrane microstructure and a better understanding of the relationship between the microstructural characteristics of the membranes and the mechanisms of separation. To this end, a combination of characterization techniques should be used to obtain the best possible assessment of the pore structure and provide an input for the development of reliable models predicting the optimum conditions for maximum permeability and selectivity. The most established methods of obtaining structural information are based on the interaction of the porous material with fluids, in the static mode (vapor sorption, mercury penetration) or the dynamic mode (fluid flow measurements through the porous membrane). 

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