Reverse osmosis porosities

Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electro dialysis (ED), and electro dialysis reversal (EDR). These processes (with the exception of UF) remove most ions RO and UF systems also provide efficient removal of nonionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to remove contaminants, such as oil and grease, and suspended soHds. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

The semi-permeable membrane is the heart of the reverse osmosis separation process. Semi-permeable membranes for reverse osmosis are broadly divided into two types. The earhest practical membrane was of the asymmetric type [3-6]. It consisted of an osmotically active surface layer with very small pores (less than 1 nm) with a thickness of 30-100 nm. This layer was physically supported on a porous substructure, whose porosity increased with distance from the surface layer. In such a membrane, the... 

All symbols are defined at the end of the paper. Equation 10 defines the pure water permeability constant A for the membrane which is a measure of its overall porosity eq 12 defines the solute transport parameter D /K6 for the membrane, which is also a measure of the average pore size on the membrane surface on a relative scale. The Important feature of the above set of equations is that neither any one equation in the set of equations 10 to 13, nor any part of this set of equations is adequate representation of reverse osmosis transport the latter is governed simultaneously by the entire set of eq 10 to 13. Further, under steady state operating conditions, a single set of experimental data on (PWP), (PR), and f enables one to calculate the quantities A, Xy 2> point... 

The object of the foregoing discussion is two-fold eq 19 to 24, together with Figure 12, show how one can obtain the values of Daji/k6 of solutes for a very large number of membrane-solution systems from Dam/k6 data for a single reference solute such as sodium chloride they also show how the physicochemical parameters characterizing solutes, membrane-materials and membrane-porosities are integrated into the transport equations in the overall development of the science of reverse osmosis. 

Three different membrane processes, ultrafiltration, reverse osmosis, and electrodialysis are receiving increased interest in pollution-control applications as end-of-pipe treatment and for inplant recovery systems. There is no sharp distinction between ultrafiltration and reverse osmosis. In the former, the separation is based primarily on the size of the solute molecule which, depending upon the particular membrane porosity, can range from about 2 to 10,000 millimicrons. In the reverse-osmosis process, the size of the solute molecule is not the sole basis for the degree of removal, since other characteristics of the... 

Kastelan-Kunst, L., et al. (1997). ET30 membranes of characterized porosities in the reverse osmosis organics removal from aqueous solutions. Water Res. 31, 11, 2878-2884. 

The aspect of hole filling by plasma deposition can be demonstrated by the transport characteristics of LCVD-prepared membranes. First, the porosity as porous media calculated from the gas permeability dependence on the applied pressure can be correlated to the salt rejection of the composite membrane as shown in Figure 34.13. The effective porosity s/, where s is the porosity and q is the tortuosity factor, is measured in dry state and may not directly correlate to the porosity of the membranes in wet state. The effective porosity of LCVD-prepared membranes was measured before the reverse osmosis experiment. The decrease of porosity (as porous media) is clearly reflected in the increase in salt rejection in reverse osmosis. 

The six flat cast membranes were shrunk at different temperatures (from 68 to 85°C) prior to loading the membranes into the reverse osmosis test cells. This treatment adjusts the average surface pore size of each membrane so that a range of porosities could be studied. A prepressurization at a pressure of 11 720 kPa for 2 hours was used to stabilize the membranes for subsequent use at pressures of 6900 kPa or lower. (All pressures listed are gauge pressure.)... 

The frictional force is expressed by a function of the ratio of a distance associated with sterlc repulsion at the Interface, to the pore radius. The frictional function Increases steeply with increase in the latter ratio. The method of calculating reverse osmosis separation data by using the surface potential function and the frictional function so generated, in conjunction with the transport equation is illustrated by examples involving cellulose acetate membranes of different porosities and AO nonionized organic solutes in single solute aqueous solution systems. 

Reverse Osmosis Separation of Various Organic Solutes Using Membranes of Different Porosities. Since Rj for all membranes, g and Q for all solutes Involved in this work are now available (Table I and II), it is possible to calculate the solute separation, f, for all membranes other than those used for the determination of B and D and for all solutes other than the reference solute. 

The physicochemical criteria approach to reverse osmosis separations Involving the surface excess free energy of solvation for ionized and nonlonized solutes has been demonstrated by this work to include nonaqueous solutions. The parameters and correlations presented in this work permit the prediction of reverse osmosis separations and permeation rates for different alkali metal halides for cellulose acetate OEastman E-398) membranes of different surface porosities from only a single set of experimental data for a sodium chloride-methanol reference feed solution system. 

Reverse osmosis separations of 12 alkali meteil halides in methanol solutions have been studied using cellulose acetate membranes of different surface porosities. Data for surface excess free energy parameters for the ions and ion pairs Involved have been generated for the above mend>rane material-solution systems. These data offer a means of predicting the performance of cellulose acetate membranes in the reverse osmosis treatment of methanol solutions involving the above ions from only a single set of experimental data. 

Typical data for asymmetric fibers for reverse osmosis applications are reported in Table 20.5-1. The ranges of these variables for as-spun and post reared cellulose acetate and polysulfone membranes currently used in gas separation are proprietary. Nevertheless, the surfnee porosity for such membranes is undoubtedly lower than for those described in Table 20,5-1, since, as indicated in Table 20,1-2, in their posttreated forms such membranes have seleclivities approaching the values or dense films. Porosities as high as those shown in Table 20.5-1 weuld produce unacceptably low seleclivities as a result of nondiscrirafimat pore flow,... 

Fabrication of a thin film composite membrane is typically a more expensive route to reverse osmosis membranes because it involves a two-step process versus the one-step nature of the phase inversion film casting method. However, it offers the possibility of each individual layer being tailor-made for maximum performance. The semipermeable coating can be optimized for water flux and solute rejection characteristics. The microporous sublayer can be optimized for porosity, compression resistance and strength. Both layers can be optimized for chemical resistance. In nearly all thin film composite reverse osmosis membranes, the chemical composition of the surface barrier layer is radically different from the chemical composition of the microporous sublayer. This is a common result of the thin film composite approach. 

It should be noted for simplicity reasons that a two-layer model has been assumed for the HR95 reverse osmosis membrane, but a more complex structure, including an intermediate layer with gradual changes in the pore radii/porosity from one layer to another (three-layer model), could be more realistic (Zholkovskij 1995). In this context, the compaction or partial inclusion of the intermediate layer due to membrane aging determined by IS measurements for nanofiltration membranes shows the utility of this technique for membrane modification characterization (Benavente and Vazquez 2004). 

Composite membranes constitute the second type of structure frequently used in reverse osmosis while most of the nanofiltration membranes are in fact composite membranes. In such membranes the toplayer and sublayer are composed of different polymeric materials so that each layer can be optimised separately. The first stage in manufacturing a composite membrane is the preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer ... 

Ultrafiltration, using synthetic membranes of controlled porosity may be very efficient for effluents in which the radioactive components are fixed on insoluble or colloidal particles electrodialysis and reverse osmosis may be highly efficient for effluents with high salt concentration. 

Kosutic, K., L. Kastelan-Kunst, and B. Kunst. 2000. Porosity of some commercial reverse osmosis and nanofiltration polyamide thin-fikn composite membranes. J. Memb. ScL 168 101-108. 

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