Membranes in reverse osmosis

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

Polymer Plasticizer. Nylon, cellulose, and cellulose esters can be plasticized using sulfolane to improve flexibiUty and to increase elongation of the polymer (130,131). More importantly, sulfolane is a preferred plasticizer for the synthesis of cellulose hoUow fibers, which are used as permeabiUty membranes in reverse osmosis (qv) cells (131—133) (see Hollow-FIBERMEMBRANEs). In the preparation of the hoUow fibers, a molten mixture of sulfolane and cellulose triacetate is extmded through a die to form the hoUow fiber. The sulfolane is subsequently extracted from the fiber with water to give a permeable, plasticizer-free, hoUow fiber. 

Reverse Osmosis Membrane Cleaning. Citric acid solutions are used to remove iron, calcium, and other cations that foul ceUulose acetate and other membranes in reverse osmosis and electro dialysis systems. Citric acid solutions can solubilize and remove these cations without damaging the membranes (94—96). 

Table 2. Effect of the Isophthaloyl Trimesoyl Chloride Ratio on the Performance of NS-300 Membranes in Reverse Osmosis Tests...

Rozelle, L.T. Kopp, C.V.,Jr. Cadotte, J.E. Kobian, K.E. "Nonpolysaccharide Membranes for Reverse Osmosis NS-lOO Membranes," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council Canada, Ottawa, 1977, p.249. 

Figure 3.20 Schematic of the interfacial polymerization process. The microporous film is first impregnated with an aqueous amine solution. The film is then treated with a multivalent crosslinking agent dissolved in a water-immiscible organic fluid, such as hexane or Freon-113. An extremely thin polymer film forms at the interface of the two solutions [47]. Reprinted from L.T. Rozelle, J.E. Cadotte, K.E. Cobian, and C.V. Knopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada (1977) by permission from NRC Research Press...

L.T. Rozelle, J.E. Cadotte, K.E. Cobian and C.V. Kopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada, pp. 249-262 (1977). 

W.M. King, D.L. Hoemschemeyer and C.W. Saltonstall, Jr, Cellulose Acetate Blend Membranes, in Reverse Osmosis Membrane Research, H.K. Lonsdale and H.E. Podall (eds), Plenum Press, New York, pp. 131-162 (1972). 

Y. Nakagawa, K. Edogawa, M. Kurihara and T. Tonomura, Solute Separation and Transport Characteristics Through Polyether Composite (PEC)-1000 Reverse-Osmosis Membranes, in Reverse Osmosis and Ultrafiltration, S. Sourirajan and T. Matsuura (eds), ACS Symposium Series Number 281, American Chemical Society, Washington, DC, pp. 187-200 (1985). 

Marcinkowsky et al. (16) were the first to use dynamic secondary membranes in reverse osmosis for rejection of salts. Giiell et al. (17) later investigated protein transmission and permeate fluxes in microfiltration of protein mixtures using yeast to form a predeposited secondary membrane, and they observed higher flux and protein transmission in the presence of the secondary layer. Kuberkar and Davis (18) also observed higher flux and transmission of BSA in the presence of a cake layer of yeast,... 

Applegate, L. E., and C. R. Antonson, "The Phenomenological Characterization of DP-Membranes," in Reverse Osmosis and Membrane Research, H. K. Lonsdale and H. E. Podall eds.. Plenum Press, New York, New York, 1972. 

Ridgeway, Harry F., "Microbial Adhesion and Biofouling of Reverse Osmosis Membranes," in Reverse Osmosis Technology, Parekh Bipin S., Marcel Dekker, Inc., New York, New York, 1988. 

Ridgway, H.F., Microbial adhesion and biofouling of reverse osmosis membranes. In Reverse Osmosis Technology, Applications for High Purity Water Production, Pakekh, B.S. and Dekker, M., Eds., Marcel Dekker, New York, 1988, p. 429. 

Tabk 8. Performance of cellulose acetate and polybenzimidazole membranes in reverse osmosis test ... 

What is the function of a semipermeable membrane in reverse-osmosis desalination (26.2)... 

Model, F. S., Lee, L. A. Polybenzimidazole Reverse Osmosis Membranes, in Reverse Osmosis Membrane Research, Lonsdale, H. K., and Podall, H. E., Eds., Plenum Publishing Co., New York 1972... 

The insoluble cellulose derivatives utilized for permeation control of various species (e.g. oxygen and water vapor transport in coated pharmaceuticals, contact lenses, packaging, or water and solute transport through semi-permeable membranes in reverse osmosis, as well as drug release from reservoir systems) differ considerably in their permeability characteristics according to the type and extent of substitution, as well as their molar mass. However, very few comparative data are available from the literature on the polymers actually used in biological applications. Recently, new results have been published. Thus, Sprockel et al. [142] determined the water vapor transmission through various CA, CAT, CAB and CAPr films at different relative humidities (Table 22). 

H. Yasuda, C.E. Lamaze and A. Schindler, Salt rejection by polymer membranes in reverse osmosis. II. Ionic polymers, J. Polym. Sci., A-2, 1971, 9, 1579. 

M. Urairi, T. Tsuru, S-I. Nakao and S. Kimura, Bipolar reverse osmosis membrane for separating mono- and divalent ions, J. Membr. Sci., 1992, 70, 153-162 T. Tsuru, S.-I. Nakao and S. Kimura, Ion separation by bipolar membranes in reverse osmosis, J. Membr. Sci., 1995,108, 269-278. 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

Table A1.17 Performance of Cellulose Acetate Membranes in Reverse Osmosis...

Direction of soivent and soiute transport through membrane in reverse osmosis... 

Figure 3.4.6. Solvent and solute transport through a membrane in reverse osmosis with well-mixed feed and permeate solutions.

So far, the integrated flux expressions for the transport of solvent and solute through a membrane in reverse osmosis assumed well-mixed feed and permeate solutions. Therefore any transport resistances in the two liquid phases on two sides of the membrane were eliminated. In practical reverse osmosis, there is, however, significant transport resistance for salt on the feed side. This subject will be briefly treated now. 

A major difference between the solute transfer from feed solution to permeate solution through a membrane in reverse osmosis (RO) and the interphase solute transfer shown in Figures 3.4.1 and 3.4.2 is the following even though there is some solute transfer through the RO membrane, the solute concentration builds up at the feed-membrane interface. It is possible to have such a low value of kii and such a high value of Cj that AP = Ajt, and the reverse osmosis process stops. Such a situation is unlikely to be encountered in conventional nonmembrane interphase transfer processes. 

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