The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content

Highly 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 at all salt rejection levels for a 0.5 wt‐% NaCl feed at 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 at any given salt rejection level below 96% salt rejection. Addition of small amounts of ZnCl₂ 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 ZnCl₂ 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 Kimura‐Sourirajan‐type membrane performance plot indicates that for a 0.5 wt‐% NaCl feed at 600 psig, membranes of the present work perform as well as the best performing membranes reported in the literature for conversion of brackish water.