Membranes versus osmosis

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. 

A reverse osmosis membrane is commercially available, named NF-40 (FilmTec Corporation), which is closely based on the NS-300 membrane technology. Typical solute rejection data for this membrane are as follows sodium chloride, 45% calcium chloride, 70% magnesium sulfate, 93% sucrose, 98%. Water flux of the membrane averages about 23 gfd at 225 psi and 25°C. As already noted for NS-300, the sulfate anion is associated with high rejection the chloride anion, low rejection. Partial discrimination between monovalent and divalent cations (sodium versus calcium) has also been observed for NF-40. The membrane can be operated at temperatures to 45°C and in a pH range from 3 to 11. In this respect, it would find probable use in industrial separations where... 

GagHatdo P., Adham S., Tmssell R. (1997), Water purification using reverse osmosis thin film composite versus cellulose acetate membranes, Proc. AWW A Membrane Technolog Conference, New Orleans, Feb. 97, 597-608. 

Plots of the product permeability versus time on a log-log coordinate system are often linear over relatively long time periods, as shown in Fig. 20.5-2. Similar behavior is observed in asymmetric reverse osmosis membranes. The log-log plotting approach provides a simple and reasonably satisfactory means of predicting the performance change of fibers under long-term operation by exuapolation of short-term data. Mechanical creep and volume recovery in glassy polymers aftw an initi perturbation also ate known to be reasonably represented on such log-log plots. ... 

After a prestress, to take up all excess membrane area, a small tension is applied ( 0.3 mN m ) and the vesicle is transferred into an adjacent microchamber where the solution is at 10 % higher osmolarity (hyperosmotic) than that in the vesicle interior. Water then leaves the vesicle by osmosis. The subsequent change and rate of change of vesicle volume due to water efflux, at constant vesicle membrane area, is thus measured from the increase in the projection length of the membrane in the pipet. The result is a plot of relative volume change V/V versus time (Figure 9.14) from which the permeability coefficient is derived. 

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