Citric acid extraction with microporous hollow fibers

Nondispersive equilibrium separation of citric acid from an aqueous solution by reversible chemical complexation with trioctyl amine in a diluent of methyl isobutyl ketone is achieved in a microporous hydrophobic hollow fiber extractor. The extractor had a shell-and-tube configuration : a total of 102 hydrophobic polypropylene (Celgard X-20, 240 pm id, 290 pm od, Hoechst Celanese) hollow fibers potted in epoxy in a stainless steel tube of 0.46 cm id. The effective fiber length was 18.5 cm and surface area/volume was 46.8 cm. Theoretical models have been developed to predictthe rate of extraction of citric acid in such systems. It essentially considers diffusion of solute molecules through the boundary layers of the two phases, in addition to the diffusion through the organic liquid occupying the membrane pores, and reversible chemical equilibrium at the phase interface. Conventionally in an aqueous-organic system having a high distribution coefficient for small solutes with a hydrophobic fiber and organic in pores, the aqueous boundary layer resistance controls. Here that is true also except when the membrane resistance of the large citric acid-amine complex diffusing through the organic-filled pores is higher than the aqueous phase boundary layer resistance. Experimental observations show further that the back extraction rate of citric acid present in the solvent (amine-organic diluent) into aqueous sodium hydroxide is much lower than the extraction rate for such a system in the same hollow fiber extractor.