Pervaporation of aqueous organic acid solutions by porous ceramic membranes

Porous silica, silica-zirconia and silica-titania membranes were prepared by the sol-gel techniques for separation of aqueous solutions of organic acids (acetic acid and propionic acid) by pervaporation at 50-100 degrees C and at normal boiling points. Applying the hot coating procedures with the colloidal sols prepared in different concentrations (different particle sizes), a quite thin porous silica layer active for separation was formed on a porous a-alumina cylindrical substrate. A fresh porous silica membrane showed a long time-dependency of several hours in the pervaporation performance before reaching a steady state due to the formation of silanol groups on the pore surface. Because of its small thickness less than about 0.5 mu m quite large water fluxes of about 300 mol(.)m(-2.)h(-1) were observed with a separation factor higher than several hundreds at around 73 mol% of acetic acid (10 wt% water) at 100 degrees C, for example. Stability tests of the porous ceramic membranes were performed in aqueous organic acid solutions to find that silica and silica-titania membranes were quite stable in an aqueous solution of organic acid of concentration higher than about 20 mol% acid. Silica-zirconia membranes, on the other hand, were not stable in aqueous organic acid solutions of high concentration. The membrane stability is discussed on the observed solubility of inorganic membrane materials. Furthermore, separation and permeation mechanisms were discussed based on a simple pore model for pervaporation proposed previously to give good coincidence between observed and simulated results.