Salt permeability

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Chlorine is the oldest and most widespread method of water disinfection. In reverse osmosis systems, chlorine may be added to feedwater for control of micro-organisms and, in addition, to prevent membrane fouling by microbiological growth. According to Vos et al. [i,2], chlorine will attack cellulose diacetate membranes at concentrations above 50 ppm. Membranes were found to show a sharp increase in salt permeability and a decrease in strength after one week of continuous exposure. Under milder conditions (10 ppm chlorine for 15 days) no detectable change in performance was observed. Spatz and Friedlander [3] have also found cellulose acetate membranes to be resistant to chlorine when exposed to 1.5 ppm for three weeks. 

An exact mathematical relationship is obtained between the salt rejection and total volume flux in reverse osmosis based on a purely phenomenological theory assuming constant salt permeability. This approach does not require a specific membrane model ... 

Radiation Crossllnked PVA Meinbranes. The water and salt permeability coefficients of the radiation crossllnked PVA membranes, obtained by using equations 1 and 2 were found to decrease with increasing the applied pressure. A linear correlation was found between the reciprocal of the water permeability coefficient and the pressure, as shown in Figures 1 and 2, at various temperatures. This linear correlation can be expressed by the following equation ... 

Diffusive Salt Permeability Through PVA Membranes. In order to verify the Interdependence between the water and salt transport through PVA, the diffusive permeability of salt through an un-pressurlzed PVA membrane was measured. The diffusion system employed In this experiment Is described schematically In Figure 8. The diffusion cell consists of an upper cell and a lower cell, separated by the tested membrane, a 150 micron thick, untreated PVA membrane. Initially, the lower cell contalnes distilled water, which Is circulated through a conductivity cell, and the... 

Diffusive salt permeability through u treated PVA membranes measured at atmospheric pressure ( car Is x 10 )... 

Heat Treated PVA Membranes. The concept of independent water and salt transport through PVA Is further supported by the permeability properties of heat treated PVA membranes. It was found that by subjecting the PVA membranes to heat treatment, a sharp decrease of the water and salt permeabilities Is caused. The... 

Study of the temperature dependence of the salt and water permeabilities through heat treated PVA membranes indicates that the heat treatment effect is expressed mainly in changes in permeability activation energies. The activation parameters for water and salt permeability of PVA membranes following various treatments are presented in Table IV. It is evident that the salt permeability activation energy is increasing about 2-3 times as... 

Temperature dependence of water and salt permeabilities of heat treated PVA membranes -E /RT P=P e ... 

Heat treatment Water permeability Salt permeability Feed concentration... 

Figure 9. Effect of the duration of heat treatment at I75°C on salt and water permeabilities of PVA at 30°C (A) salt permeability coefficients (O) water permeability coefficients. Open symbols refer to 100% hydrolyzed, 86,000 mol wt PVA solid symbols refer to ]00%> hydrolyzed, 115,000 mol wt PVA.

The industry is extremely competitive, with the manufacturers producing similar products and competing mostly on price. Many incremental improvements have been made to membrane and module performance over the past 20 years, resulting in steadily decreasing water desalination costs in inflation-adjusted dollars. Some performance values taken from a paper by Furukawa are shown in Table 5.5. Since 1980, just after the introduction of the first interfacial composite membranes, the cost of spiral-wound membrane modules on a per square meter basis has decreased seven-fold. At the same time the water flux has doubled, and the salt permeability has decreased seven-fold. Taking these improvements into account, today s membranes are almost 100 times better than those of the 1980s. This type of incremental improvement is likely to continue for some time. 

Figure 13.5 Charge mosaic membranes, consisting of finely dispersed domains containing fixed negatively and fixed positively charged groups, are salt permeable [15]...

Cosenza, P., Ghoreychi, M., Bazargan-Sabet, B. and de Marsily, G. (1999) In situ rock salt permeability measurement for long term safety assessment of storage, Int. J. Rock Mech. Min. 36, 509-526... 

K = salt permeability coefficient (a function of the salt diffusivity through the membrane)... 

One other form of pressure-driven membrane process should be mentioned piezodialysis In this process, a selectiwly salt-permeable membrane is used... 

Table 3 Results from the history matches cases A-G and M and N using diffusion and Darcy flow of methane gas balances, diffusion constants and rock salt permeabilities ...

Additionally a scale of temperature with depth is given using the mean regional temperature gradient of 33°C/1000 m. The temperatures of 115 and 151°C have been marked in Fig. 7. They confine an anomaly of higher rock salt permeabilities. 

Schirg and Widmer [52] published mathematical models for the calculation of retention and selectivity for nanofiltration of aqueous dye-salt solutions. A modification of Eqs. (12.11) and (12.12) has been proposed in which the integral salt permeability co could be described by the introduction of an exponential function... 

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