Selective permeability

Many fluorine containing polymers are being evaluated for possible use as permeable selective membranes. The fluorine containing polymers have been shown to increase permeation rates without decreasing in the selec-... 

Table 2.8. Permeability, Selectivity and Separation Characteristics of Various Polymeric and Molecular Sieve Carbon (MSC) Membranes (Koresh and Soffer 1983) ...

The permeability selectivity ratio for P < /Pci estimated to be 5 for the channel of 5 pS. The selectivity again favors cation and the ratio resembles the value obtained by supramolecular oligoether channels. Open and closed transitions were relatively slow and the time distributions were 300 ms and 400 ms, respectively. Therefore, the ion pair, trans-2l, can be concluded to show typical characteristics of the single-ion channel in all respects. 

Table 13.2 Permeability, selectivity and permeating driving force of some transport mechanisms. 

Wijers, M.C., Jin, M., Wessling, M. and Strathmann, H. (1998) Supported liquid membranes modification with sulfonated poly(ether ether ketone) - permeability selectivity and stability Journal of Membrane Science, 147, 117. 

Freeman, B. D. (1999). Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes, Macromolecules 32(2),... 

Luminal thiol oxidation is facilitated by ascorbate (vitamin C) (45) or FAD (12, 13), so the physiologic role of their transport has been proposed. ER membrane is permeable selectively to dehydroascorbate, the oxidized form of ascorbate (10, 11). Luminal reduction of dehydroascorbate to ascorbate is associated with thiol oxidation and leads to ascorbate entrapment (46). 

The first generation of gas separators has achieved an impressive penetration into markets traditionally dominated by cryogenic, chemical and physical sorption processes. Competition from these processes is strong. Membranes with higher permeabilities, selectivities and resistance to penetrant attack are required to meet challenges from these traditional processes and to permit expansion into additional application areas. 

An investigator in this area typically has precise information on composition of casting solutions and other physicochemical factors affecting membrane formation. Functional measurements of transport in terms of convective permeability, selectivity or diffusive permeability are usually also available. However, without proper techniques for quantitative description of membrane pore structures, and their shape and size distributions, membrane development efforts remain largely empirical. 

A follow-up study demonstrated that Qi phases further enhance the performance of LLC-BR composite membranes in both water transport and harmful chemical vapor rejection [170]. A cross-linkable, gemini phospho-nium amphiphile (Fig. 23) was blended with BR and cross-linked to form films exhibiting a Qi-phase nanostructure. Materials with a Qi-phase showed 300 times greater water vapor permeability and 500 times greater permeability selectivity for water/CEES than pure cross-linked BR. Furthermore, these Qi-phase composite films were far superior to their Hu and L analogues in both water vapor permeability and water/CEES transport selectivity. Further studies were planned to process thinner films as well as test their rejection properties against other types of chemical agents. 

Membrane processes are modular in their nature, so expansion and/or replacement can be spread out over time, based on market needs and capital availability. The needed performance capabilities required of membranes vary with the nature of the application and generally involve the need for high permeability, selectivity, low cost, slow degradation, wide temperature operating range, and adequate mechanical strength. Membrane processes are successful only when the associated engineering... 

Solvent casting of polymer blends and controlled evaporation can also lead to SD. This technology has been used for industrial production of semi-permeable, selective membranes. The product characterized by co-continuity of phases also showed excellent mechanical performance. The type of solvent, concentration, temperature and method of casting are used to control the blend morphology and its final performance [Inoue et al., 1985, 1987 Nauman et al., 1986]. 

The Mn(ii)-based SOD mimics have numerous potential advantages over the SOD enzymes as potential therapeutic agents, including membrane permeability, selective reactivity for superoxide, immunogenicity effects, stability, and cost. Table 1 compares characteristics of the Mn(ii)-based SOD mimics with R = H versus those of bovine erythrocyte Cu,Zn SOD. 

Other methods of combining reaction with separations, such as extraction, crystallization, and adsorption, are being explored, but none have been used on a large scale. Using reactors with membranes that selectively remove a reaction product is a very promising development, but improvements in membrane permeability, selectivity, and high-temperature stability are needed for practical processes. 

Compound Sorption coefFicient S,- Diffusion coefFicient D, [m s ] Sorption selectivity i/ water Diffusion selectivity I / water Permeability selectivity t//twater... 

In the second method, ultrafiltration, centrifugal force is applied for a few minutes to a sample of the compound in plasma, pushing the unbound fraction through a permeability selective membrane. LC/MS/MS analysis of the ultrafiltrate and retentate is then done. This has the advantage of being faster than dialysis. Methods like 96-well plate have been developed here too. As in HTS, edge effects have been noted with these, and one study comparing such a method with results from individually run samples for a dozen research compounds showed an value of 0.60. ... 

This section intends to provide a review on the advanced materials used in the recent development of TFC-NF membranes and the effects of the advanced materials on the improvement of composite membrane properties with respect to permeability/selectivity, chlorine tolerance, solvent stability, fouling resistance, etc. The advanced materials that are used in composite membrane fabrication to improve either the top active layer properties or substrate properties can be generally categorized into (a) active monomer, (b) surfactant/ additive, (c) nanoflller, and (d) polymeric substrate. 

B.D. Freeman, Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes, Macromolecules, 32 (1999) 375 -380. 

Obviously, membrane reactor performance is strongly dependent on selective membrane behavior in terms of permeability, selectivity, and stability. Chapter 2 reports several data about hydrogen permeation performance of different metal-based and particularly of Pd-based membranes. 

For successful commercialization of Pd-based membranes, the membrane must have sufficient permeability, selectivity, robustness and durability in relevant environments, preferably in the presence of common contaminants such as H2S. Moreover, the production cost of membranes and modules must be low enough for integration in the process resulting in beneficial cost-effectiveness compared to alternative technologies and processes. 

The carbon-based membranes show superior permeability-selectivity combination to polymeric ones and are categorized in three classes carbon membranes, carbon molecular sieve membranes and carbon nanotubes. ... 

---