Selectivity and Permeability

Vapor Sorption and Diffusion in Mixed Matrices Based on Teflon s AF 2400 139 

Mixed matrix membranes based on fluorinated high free volume matrices loaded with fumed silica nanoparticles show attractive features, especially if compared to unloaded polymeric membranes. The addition of filler changes the FFV of the glassy polymeric matrix, thus positively affecting both solubility and diffnsivity. A simple procedure has been proposed to calculate the relevant properties of the MMM, which is based on limited experimental data represented by the solubility isotherm of a test penetrant and few values of diffusion coefficients. The nnexpected behaviour shown by the MMM obtained by using nano-filler loadings can be properly described by the method developed. The 

We believe that the procedure can be properly appUed also for the case of MMM obtained by loading glassy matrices with other nano-fillers and can be a useful tool to develop better mixed matrix membranes. 

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FIG. 22-76 Constant -cost lines as a function of permeability and selectivity for CO2/CH4, Cellulose-acetate membrane mscf is one thousand standard cubic feet, CouHesy VP R. Grace.)... 

High-temperature or Potential for lower cost and Low permeability and selectivity for Laboratory scale... 

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. 

A very effective way to improve the pertraction performances in permeability and selectivity is to incorporate extractants into the hydrophobic phase, which react with a given solute reversibly and selectively. 

Table 6.2 shows some gas permeabilities and permselectivities for several gases through a membrane, activated at 9S0°C. From these data it is clear, that by simple thermochemical treatment, permeability and selectivity can be influenced. The product of permeability and selectivity is among the highest ever reported. 

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

Large area membranes that demonstrate high permeability and selectivity Reproducibility of synthesis ... 

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... 

Mixed matrix membranes have been prepared from ABS and activated carbons. The membranes are intended for gas separation. A random agglomeration of the carbon particles was observed. A close interfacial contact between the polymeric and filler phases was observed. This morphology between inorganic and organic phases is believed to arise from the partial compatibility of the styrene/butadi-ene chains of the ABS copolymer and the activated carbon structure. A good permeability and selectivity for mixtures of carbon dioxide and methane has been reported (91,92). 

Considerable research and development effort is being placed on a chlorine-resistant membrane that will maintain permeability and selectivity over considerable time periods (years). 

This so-called "active" layer has characteristics similar to those of cellulose acetate films but with a thickness of the order of 0.1 micrometer (jjm) or less, whereas the total membrane thickness may range from approximately 75 to 125 ym (see Figure 1). The major portion of the membrane is an open-pore sponge-like support structure through which the gases flow without restriction. The permeability and selectivity characteristics of these asymmetric membranes are functions of casting solution composition, film casting conditions and post-treatment, and are relatively independent of total membrane thickness. 

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. 

Table 8.4 Permeabilities and selectivities of polymers of interest in air separation...

Selective barrier structure. Transport through porous membranes is possible by viscous flow or diffusion, and the selectivity is based on size exclusion (sieving mechanism). This means that permeability and selectivity are mainly influenced by membrane pore size and the (effective) size of the components ofthe feed Molecules... 

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. 

Figure 8.3 Oxygen/nitrogen selectivity as a function of oxygen permeability. The upper-bound line represents the point above which no better membranes are known [12]. This line shows the trade-off relationship between membrane permeability and selectivity.

Some of the variables that are important for the subsequent discussion are recalled here. The membrane properties are related to the mass transport of the different chemical species through the membrane itself or its separating layer (for an asymmetric or multilayer membrane). Permeability and selectivity were defined for the mass transport by permeation both depend on the membrane nature and morphology that impose the specific transport mechanism driving the permeation of which it is characteristic. Table 13.2 reports the permeability coefficient, selectivity and permeating driving force of some permeation mechanisms. 

A controllable biodegradability, desirable mechanical properties, suitable gas permeability and selectivity would extend the potential application areas of aliphatic polyesters not only in agriculture or in the greenhouse or packaging industry but also as a substitute for human skin. There is a need for such focused studies in the future. 

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]. 

To conclude this section, it is necessary to state that Pd and Pd-based membranes are currently the membranes with the highest hydrogen permeability and selectivity. However, the cost, availability, their mechanical and thermal stabilities, poisoning, and carbon deposition problems have made the large-scale industrial application of these dense metal membranes difficult, even when prepared in a composite configuration [26,29,33-37],... 

The five remaining compounds were specifically acting on the mitotic spindle, and did not alter its components in interphase cells. One of them in particular prevented the formation of the spindle in most mitotic cells, replacing it with a monoastral microtubule formation surrounded by chromosomes the compound (9.6, Fig. 9.3) was thus named monastrol. The authors compared monastrol effects with several published effects (21-23) related to inhibition of Eg5, a member of the BimC kinesin family, and showed monastrol to be the a selective Eg5 inhibitor (Eg5-driven microtubule motility inhibition =14 pM). This compound is both the first permeable and selective inhibitor of a specific kinesin, and may have many possible applications as a tool or as the starting point for a chemical optimization program. 

Fig. 8. Dependences of oxygen permeability and selectivity on the alkyl group of polyf 1 -(alkyldimethylsilyl)-1 -propy ne] (membrane thickness 30 pm, 25 °C)...

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 ... 

Table 2. Rubbery polymer permeabilities and selectivities for various gases ... 

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. 

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