Separation of Fermentation Products by Membrane Techniques. I. Separation of Lactic Acid/Lactates by Diffusion Dialysis

Abstract

The results of the separation of lactic acid from sodium lactate by diffusion dialysis are presented in this paper. Neosepta AFN-7 and Selemion DSV membranes were used for the separation. The dialytic separation factor for lactic acid/sodium lactate was found to be ∼20 for Neosepta AFN-7 and ∼30 for Selemion DSV membranes. The fluxes inherent in the separation were up to ∼1 mol/m2·h for the acid and only up to ∼0.07 mol/m2·h for the salt. The effects that enable the separation, i.e., the differences in solubility and diffusivity of the acid and the salt, were estimated (single solute experiments). The partition coefficients for lactic acid were found close to or above unity whereas those for sodium lactate were much lower. On the contrary, the apparent diffusion coefficients of both solutes were comparable, being of the order of 10−11 m2/s. It results that the separation of the weak acid and salt is based on differences in solubilities rather than in diffusivities, i.e., the phenomenon is different from those which enable separation of strong acids and salts. The results prove the diffusion dialysis technique is effective for separating lactic acid from lactates.