Gas permselectivity

Several studies have appeared in the literature (5-9) describing the reactions of various compounds with PPO in order to change the properties of this polymer but none of them have valued the chemical structure in terms of gas permselectivity behavior. 

A membrane can be described as a semipermeable barrier between two phases which prevents intimate contact. This barrier must be permselective which means that it restricts the movement of molecules in it in a very specific way. The barrier can be solid, liquid or gas. Permselectivity can be obtained by many mechanisms ... 

Fig. 2. Schematic structure of a Clark-type enzyme electrode A, electrode body B, working electrode (Pt) C, reference electrode (Ag/AgCl) D, inner gas-permselective membrane (polypropylene) E, enzyme layer F, outer membrane.

The thermal rearrangement of a-hydroxyl-PI membranes improves the gas permselectivity properties in comparison to a neat PI. By intro-dueing segments within the polymer that do not undergo thermal rearrangement, the gas separation properties of the thermally rearranged membrane can be modified. PI copolymers based on 4,4 -hexafluoroisopropylidene diphthalic anhydride and diamines 3,3 -dihydroxy-4,4 -diamino-biphenyI with 2,3,5,6-tetramethyI-l,4-phenyIenediamine or 9,9 -bis(4-aminophenyl)fluorene, thermally rearranged into poly(benzoxazole-co-imide), were tested [107]. 

Iwata M, Adachi T, Tomidokoro M, Ohta M, Kobayashi T (2003) Hybrid sol-gel membranes of polyacrylonitrile-tetraethoxysilane composites for gas permselectivity. J Appl Polym Sci 88(7) 1752-1759... 

Suda and Haraya [10] were successful in preparation of flat, asynunetric carbon molecular sieve membranes, which exhibited the highest gas permselectivities among those fabricated in the past research by pyrolysis of a Kapton type PI derived from pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA). They used the permeation measurements and X-ray powder diffraction to relate the relationship between the gas permselectivity and microstucture of the CMSM. They proposed that the decrease of the interplanar spacing, amorphous portion and pores upon heating might be the origin of the molecular sieving effect . 

Kita H, Yoshino M, Tanaka K, Okamato K (1997) Gas permselectivity of carbonized poly-pyrrolone membrane. J Chem Soc Chem Commun 1051-1052... 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Gas Separation. During the 1980s, gas separation using membranes became a commercially important process the size of this appHcation is stiH increasing rapidly. In gas separation, one of the components of the feed permeates a permselective membrane at a much higher rate than the others. The driving force is the pressure difference between the pressurized feed gas and the lower pressure permeate. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

The permeability of dense membranes is low because of the absence of pores, but the permeance of Component i in Equation 10.20 can be high if SM is very small, even though the permeability is low. Thickness of the permselective layer is typically in the range 0.1 to 10 tm for gas separations. The porous support is much thicker than this and typically more than 100 tm. When large differences in PM exist among species, both high permeance and high selectivity can be achieved in asymmetric membranes. 

Koresh, J.E. and A. Sofer, Molecular sieve carbon permselective membrane, Part I. Presentation of a new device for gas mixture separation, Sep. Sci. Technol., 18, 723, 1983. 

Today it is claimed that the surface fluorination of polymers using F2 gas mixtures enhances a wide range of properties, e.g., low permeability to nonpolar liquids4 improved permselectivity,5-6 excellent wettability and adhesion,7 low friction coefficient (especially for elastomers),8 and chemical inertness.9 Obviously, these properties depend on the chemical composition ofthe fluorinated layer, which in turn is determined by the chemical structure ofthe base polymer, the composition of the F2 gas mixture, and the fluorination parameters. 

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. 

When the membrane performs only a separation function and has no catalytic activity, two membrane properties arc of importance, the permeability and the selectivity which is given by the separation factor. In combination with a given reaction, two process parameters are of importance, the ratio of the permeation rate to the reaction rate for the faster permeating component (c.g. a reaction product such as hydrogen in a dehydrogenation reaction) and the separation factors (permselectivities) of all the other components (in particular those of the reactants) relative to the faster permeating gas. These permselectivities can be expressed as the ratios of the permeation rates of... 

Sloot, H. 1991. A non-permselective membrane reactor for catalytic gas phase reactions. Thesis, University of Twcntc, Enschede. 

Gas Permeability Fluorine substitution often results in greater permselectivity than imfluorinated analogues... 

Membrane pervaporation (permselective evaporation of liquid molecules) is the term used to describe the extraction of volatile organics from an aqueous matrix to a gas phase through a semipermeable membrane. 

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