Membranes sorption

The selectivity (amcm) of pervaporation membranes critically affects the overall separation obtained and depends on the membrane material. Therefore, membrane materials are tailored for particular separation problems. As with other solution-diffusion membranes, the permeability of a component is the product of the membrane sorption coefficient and the diffusion coefficient (mobility). The membrane selectivity term amem in Equation (9.11) can be written as... 

Bulk water Aqueous js Membrane Sorption phase... 

Figure 2. General view of Module Membrane-Sorption Concentrating Facility (MMSF) ...

V.V. Shatalov, V.G. Solovev, A.Yu. Shcheglov, R.A. Penzin, D.I. Smirnov, V.M. GeUs, V.V. MUyutin, E.A. Kozlitin, A.A. Svittsov, Tests of membrane-sorption decontamination of the reservoir cascade of the Techa River, Atomic Energy 105, 2008, 357-366. 

Koltuniewicz, A.B., Witek, A. Bezak, K. (2004) Efficiency of membrane-sorption integrated processes. Journal of Membrane Science, 239, 129-141. 

Transport properties of PVA/PEl/PEG composite membranes sorption and permeation characterizations. Cent. Eur. J. 

Lagorsse S, Magalhaes FD, Mendes A(2004) Carbon molecular sieve membranes Sorption, kinetic and structural characterization. J Membr Sci 241 (2) 275-287 Lagorsse S, Leite A, Magalhaes FD, Bischofberger N, Rathenow J, Mendes A (2005) Novel carbon molecular honeycomb membrane module Configuration and membrane characterization. Carbon 43 (4) 809-819... 

Sorption, a thermodynamic factor, occurs when a gas molecule in the bulk state adsorbs physically onto a membrane surface with respeet to the concentration difference, while diffusion, a kinetie factor, is related to how fast a gas molecule can pass through the membrane. Sorption and diffusion are activated energy proeesses, whieh are affeeted substantially by temperature, thus, the relationships for D, S and Pe are expressed as a funetion of temperature ... 

Lagorsse, S., MagaUiaes, F. D., and Mendez, A. (2004). Carbon molecular sieve membranes sorption, kinetic and structural characterization. J. Membr. Sci. 241, 275-287. 

Polymeric catalytic membranes, when compared with inert membranes, offer the possibility of tuning the sorption of reactants and products in the close vicinity of the catalytic active sites, by a careful selection of the polymeric environment. As was mentioned above, in porous catalytic membranes the choice of the polymer is of less importance, since permeation does not take place through the polymer matrix. However, in the case of dense membranes, sorption and transport properties are crucial for the catalytic performance and are strongly affected by the polymeric matrix. 

Permeability P, can be expressed as the product of two terms. One, the diffusion coefficient, reflects the mobility of the individual molecules in the membrane material the other, the Henry s law sorption coefficient, reflects the number of molecules dissolved in the membrane material. Thus equation 9 can also be written as equation 10. 

Fig. 38. Permeability as a function of molar volume for a mbbery and glassy polymer, illustrating the different balance between sorption and diffusion in these polymer types. The mbbery membrane is highly permeable the permeability increases rapidly with increasing permeant size because sorption dominates. The glassy membrane is much less permeable the permeability decreases with increasing permeant size because diffusion dominates (84).

Reverse osmosis models can be divided into three types irreversible thermodynamics models, such as Kedem-Katchalsky and Spiegler-Kedem models nonporous or homogeneous membrane models, such as the solution—diffusion (SD), solution—diffusion—imperfection, and extended solution—diffusion models and pore models, such as the finely porous, preferential sorption—capillary flow, and surface force—pore flow models. Charged RO membrane theories can be used to describe nanofiltration membranes, which are often negatively charged. Models such as Dorman exclusion and the... 

Plasticization and Other Time Effects Most data from the literature, including those presented above are taken from experiments where one gas at a time is tested, with Ot calculated as a ratio of the two permeabihties. If either gas permeates because of a high-sorption coefficient rather than a high diffusivity, there may be an increase in the permeabihty of all gases in contact with the membrane. Thus, the Ot actually found in a real separation may be much lower than that calculated by the simple ratio of permeabilities. The data in the hterature do not rehably include the plasticization effect. If present, it results in the sometimes slow relaxation of polymer structure giving a rise in permeabihty and a dramatic dechne in selectivity. 

Several selective interactions by MIP membrane systems have been reported. For example, an L-phenylalanine imprinted membrane prepared by in-situ crosslinking polymerization showed different fluxes for various amino acids [44]. Yoshikawa et al. [51] have prepared molecular imprinted membranes from a membrane material which bears a tetrapeptide residue (DIDE resin (7)), using the dry phase inversion procedure. It was found that a membrane which contains an oligopeptide residue from an L-amino acid and is imprinted with an L-amino acid derivative, recognizes the L-isomer in preference to the corresponding D-isomer, and vice versa. Exceptional difference in sorption selectivity between theophylline and caffeine was observed for poly(acrylonitrile-co-acrylic acid) blend membranes prepared by the wet phase inversion technique [53]. 

Petropoulos, J. H. Membranes with Non-Homogeneous Sorption Properties. Vol. 64, pp. 85-134. Pino, P. Optically Active Addition Polymers. Vol, 4, pp. 393-456. 

Another major drawback of polysaccharides is their hydrophilic nature leading to low degrees of adhesion between fiber and matrix [11]. Moisture absorption takes place by three types of mechanisms namely diffusion, capillarity, and transport via micro cracks [2]. Among the three, diffusion is considered to be the major mechanism. Water absorption largely depends on the water-soluble or hygroscopic components embedded in the matrix, which acts as a semipermeable membrane. While, fiber/matrix adhesion and fiber architecture also affect the moisture absorption. The results of the water sorption experiment showed an interesting trend. The extent of water uptake was not very significant and also did not increase linearly with amount of filler (Table-2). 

For gas and vapor systems, by combining the laws of sorption and diffusion in the sequence (l)-(3), general permeation equations are obtained. For sheet membrane samples of polymers above Tg, if the definition is made that permeation coefficient Q = Ds,... 

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. 

HB Hopfenberg, A Apicella, DE Saleeby. Factors affecting water sorption in and solute release from glassy ethylene-vinyl alcohol copolymers. J Membrane Sci 8 273-281, 1981. 

WA Hunke, LE Matheson. Mass properties of Co(polycthcr) polyurethane membranes. Part 2. Permeability and sorption characteristics. Int J Pharm 1313-1318, 1981. 

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