The effect of concentration driving force on liquid-liquid mass transfer

Abstract

The enhancement of the rate of mass transfer due to concentration driving force-induced interfacial turbulence has been investigated in several typical liquid-liquid systems. The three solvent pairs studied were: isobutanol-water, 85 volume % tributyl phosphate (TBP) in n-hexane-water, and 85 volume % tributyl phosphate in carbon tetrachloride-water. Nitric acid was the solute in all three systems. The experiments were conducted in an unbaffled stirred-vessel extractor operated as a steady state continuous flow device. The stirring speed was 72 rev/min for all experiments. For all the systems transfer from the organic to the aqueous phase was unstable. The over-all mass transfer coefficient increased sharply by factors of 1·5–4 as the driving force was increased. Transfer from aqueous to organic was stable, except for the TBP-n-hexane system where the mass transfer coefficient increased at high driving forces. The transfer coefficient was found to depend only on the concentration driving force, except for the TBP-hexane system where it appeared to be a function of concentration level as well as driving force. The results were consistent with the qualitative features of Marangoni instability, although several aspects of the data could not be explained on this basis.