The Modification of Solvents for Liquid-Liquid Extraction Processes

Abstract

The ability to modify the efficiency of selected solvents in liquid-liquid extraction by the addition of small amounts of additives would offer advantages in extraction, solvent recovery and inventory costs. Therefore, the literature relating to desirable solvent properties, variations in solvent use, methods of determining, representing, interpolating, extrapolating and predicting liquid-liquid equilibria and the fundamentals of mass transfer has been critically reviewed. The use of pro- and anti-solvents to modify the phase equilibria of ternary systems is well established but these compounds are added to influence the solubility curve. In a preliminary study of these effects, it was noticed that polar molecules affected the tie-lines more than the solubility. Therefore, in depth study of the ternary systems, namely: Acetone - Toluene - Water, Methanol - Xylene- Water and Isopropanol - Cyclohexane - Water were determined at 20°C. The Sulphuric Acid was added at 2% and 5% by weight. Significant tilting of the tie-lines was confirmed and the changes in the slopes of the distribution diagram favourably indicate solubility enhancement due to the salting effect. The mass transfer was studied on a pilot scale Rotating Disc Contactor. The hydrodynamic behaviour of the R.D.C. was reviewed and established. The evaluated overall mass transfer coefficients suggested that the acid generally increased the values.