A kinetic study of the transfer of acetone between toluene and water phases

Abstract

The purpose of this investigation was to apply the techniques of chemical kinetics to the transfer of acetone between water and toluene phases, and thereby attempt to develop mechanisms and evaluate resistances to such transfer.

An extraction cell was constructed which consisted of a 20-liter glass battery jar with independently driven impellers in each phase and a stationary phase divider at the interface. Extraction tests were conducted on the toluene-acetone-water system in the cell at 4, 15, and 30℃ at initial acetone concentrations ranging from 0.008 to 0.035 gram acetone per gram solvent. In all tests the stirrer bars were in contra-rotation at approximately 60 revolutions per minute.

Initial unidirectional transfer rates were correlated with initial acetone concentrations and the correlations were used to predict the net rate of mass transfer at other than initial conditions.

The results of this investigation led to the following conclusions:

The flux of acetone from toluene solution to pure water, gm/min, sq cm, at 30℃ (F_t) may be represented by the equation

F_t = 0.0403 C_t

where C_t is gm acetone/gm toluene.

The flux of acetone from an aqueous solution to pure toluene at 30℃ may be represented by the equation

F_w = 10 (- 0.0333 + 43.34 C_w) / 10⁴

for aqueous acetone concentrations between 0.010 and 0.035 gram acetone per gram water, and by

F_w = 0.0251 C_w

for aqueous acetone concentrations below 0.010 gram acetone per gram water.

For the range of concentration and driving force studied, the transfer of acetone between solutions in toluene and in water at 30℃ may be evaluated as the difference in the unidirectional fluxes.

A mechanism for the transfer of acetone between toluene and water is proposed which involves, for transfer in each direction, (1) eddy diffusion of the acetone to the region of the interface, (2) transfer of a small amount of solvent from the opposite side of the interface, (3) a change in solvation of the acetone involving solvent from the opposite phase dissolved at the interface, (4) molecular diffusion into the opposite phase, and (5) eddy diffusion of the solvated acetone away from the region of the interface into the bulk of the receiving phase.

Neither accepted theories, which predict that the coefficient of mass transfer will vary as a simple function of molecular diffusivity, nor correlations based on the rate of stirring and physical properties of the phases accounted for the large increase in the flux of acetone from water to toluene with increasing initial acetone concentrations in the aqueous phase.