Salt rejection, intrinsic

A summary of the properties of some of these materials is presented in Tables 8 and 9. Diffusion coefficients and NaCl partition coefficients are presented in Table 8 for cellulose triacetate and for Nomex K>lyamide. In most cases, however, intrinsic transport properties are not known, and what is reported in the literature is the water flux and salt rejection of RO membranes under given test conditions. Under comparable test conditions (0.5-1% NaCl, 27—68 atm) the water fluxes of these newer membranes are comparable (0.4-0.8 m /m -day). Salt rejection data are presented in Table 9. To put these data in perspective, the salt rejection of cellulose 2.5-acetate membranes is typically <99%. However, for various reasons none of these new membranes has yet become commercially important. 

The true measure of the dynami c layer s selectivity, is the intrinsic salt rejection, Rint However, due to concentration polarization effects caused by the large volume flow through the membrane, one actually measures Robs> He apparent salt rejection, given by the relation... 

An examination of the data for dynamic membranes formed under various conditions suggests that a combination of higher pressure and cross-flow rate yields both the highest product flux and lowest oil extraction values. However, the overall impression is that the TOC or extractables level of the product is far less sensitive to formation conditions than the hydraulic permeability or even the intrinsic salt rejection toward 10 3m Na2S0ij solutions. Further evi-... 

A = 4.05 X lO " cm/(s-kPa)(4.1 X 10 cm/(s-atm)) and = 1.3 x 10 cm/s (4)//= 1 mPa-s(=cP), NaCl diffusivity in water = 1.6 x 10 cm /s, and solution density = 1 g/cm . Figure 4 shows typical results of this type of simulation of salt water permeation through an RO membrane. Increasing the Reynolds number in Figure 4a decreases the effect of concentration polarization. The effect of feed flow rate on NaCl rejection is shown in Figure 4b. Because the intrinsic rejection, R = 1 — Cp / defined in terms of the wall concentration, theoretically R should be independent of the Reynolds... 

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