Separation of linear hydrocarbons and carboxylic acids from ethanol and hexane solutions by reverse osmosis

In this study, asymmetric cellulose acetate membranes with moderate NaCl re jection (85.5%) were prepared and used to study the influence of the chemic al nature of organic solutes in different organic solvents. The solute reje ction and the solvent flux of linear hydrocarbons (M-w = 226-563 g/mol) and linear carboxylic acids (M-w = 228-340 g/mol) in ethanol and hexane were s tudied as a function of the molecular weight, the feed concentration and th e transmembrane pressure. The ethanol flux was three times higher than the hexane Aux. The rejection coefficients for both types of solute were quire acceptable (R = 60-90%), w hen ethanol was the solvent. In hexane the linear hydrocarbons showed a rej ection of 40-60%, while all carboxylic acids reached a negative rejection o f -40 to -20%. This negative "observed" rejection can be attributed to accu mulation of carboxylic acid at the membrane; the solute concentration at th e membrane becomes much higher than in the bulk solution, due to a higher a ffinity of the solute with the membrane in hexane than in ethanol. Sorption experiments support this hypothesis. Furthermore, it was found that the rejection increases with increasing mole cular weight and the rejection and Aux are hardly affected by the feed conc entration. (C) 2001 Elsevier Science B.V. All rights reserved.