Abstract
Multiple-effect membrane distillation (MEMD) process for enriching semi-volatile organic acids from their individual aqueous solutions was performed by using a hollow fiber-based air gap membrane distillation (AGMD)module with the function of internal heat recovery. Aqueous solutions of glyoxylic acid, glycolic acid, lactic acid, pyruvic acid, malonic acid and glutaric acid were used as model feed. For a feed of 1% (mass fraction), each acid could be enriched for 8–20 times, which depended on the surface tension of the concentrate. The operation performance of MEMD process was characterized by permeation flux J, performance ratio PR and acid rejection rate R. The effects of cold feed-in temperature, heated feed-in temperature, feed-in volumetric flow rate and feed-in concentration on MEMD performance were experimentally evaluated. Maximum values of J, PR and R were 4.8 L/(h·m2), 9.84 and 99.93%, respectively. Moreover, MEMD process demonstrated a fairly good stability in a long-term experiment lasting for 30 d when aqueous solution of 4% (mass fraction) lactic acid was used as a feed.
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