SynopsisHighly productive cellulose acetate membranes were cast under conditions of very short air exposure periods from cellulose acetate-acetone-formamide casting solutions having a high cellulose acetate (CA) content and lying close to the phase boundary. Air exposure periods as short as 0.05 sec were used with CA content up to 32 wt-%. Membranes from a casting solution containing 30 wt-% cellulose acetate (E-398-3), 45 wt-% acetone, and 25 wt-% formamide perform as well as membranes from other compositions a t all salt rejection levels for a 0.5 wt-% NaCl feed a t 600 psig. Partial replacement of acetone by dioxane in the casting solution substantially increases the water flux from membranes cast with short air exposure periods a t any given salt rejection level below 96% salt rejection. Addition of small amounts of ZnC12 to nondioxane casting solutions with 32 wt-% CA improves membrane performances remarkably for lower salt rejection levels, while the improvement in performance of membranes from 30 wt-% CA casting solutions with dioxane due to ZnClz addition is marginal. Variation in air exposure from 0.05 to 2 sec results in minor performance variations in the membranes having any of these compositions. With air exposure periods beyond 2-3 sec, membrane fluxes drop drastically. The concept of a thinner skin satisfactorily explains the improvement in mixed solvent systems, whereas ZnCl2 acts as a swelling salt. A KimuraSourirajan-type membrane performance plot indicates that for a 0.5 wt-% NaCl feed a t 600 psig, membranes of the present work perform as well as the best performing membranes reported in the literature for conversion of brackish water.
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