Mass Transfer Resistance Analysis of L-Tryptophan Extraction in an Emulsion Liquid Membrane System

Abstract

The extraction of L-tryptophan with an emulsion liquid membrane containing di-2-ethylhexy phosphoric acid as a carrier, Span 80 as a surfactant, kerosene as a solvent, and hydrochloric acid solution as an internal phase stripping reagent, was studied. The effects of external phase pH, carrier concentration, internal stripping reagent concentration, and external initial solute concentration on the mass transfer flux were examined. The fractional resistances of external phase diffusion and emulsion globule diffusion to the overall process were defined and calculated by the proposed model. Thus, the rate-controlling steps for the overall process were quantitatively identified. For higher external phase pH and/or higher carrier concentration as well as lower external initial solute concentration, the overall process is mainly determined by the external phase diffusion. On the other hand, for lower external pH and/or lower carrier concentration, the emulsion diffusion is a rate-controlling step. However, in the majority of cases, the overall process is governed by the combined effects of both the external phase diffusion and the emulsion globule diffusion. Compared with the external phase pH and carrier concentration, the concentrations of internal stripping reagent and external initial solute are unimportant factors determining the overall process.