Water transfer kinetics in a water activity control system designed for biocatalysis in organic media

The performance of a water activity-controlling system in organic solvents using saturated salt solutions circulating in silicone tubing submerged in the solvent was studied. The mass transfer could be regarded as a flux through a cylindrical geometry. Integration over the barrier gave diffusion coefficients of water (D-aw) which were similar for the different tubing sizes used. The driving force for the transfer was shown to be the difference in water activity and not the water concentration across the membrane. Hydrophilic solvents (ethyl acetate) gave higher transfer rates than more hydrophobic ones (diisopropyl ether). The D-aw obtained in different solvents was influenced by the swelling behavior of the tubing and the solubility of water. The water transfer was studied in a water ''producing'' system consisting of 2.5 m silicone tubing submerged into a 250-ml tank reactor with a constant influx of water-saturated solvent. Different steady-state levels were obtained at different flow rates and the corresponding D-aw values were calculated. The data obtained can be used to predict the required amount of tubing necessary to achieve a desired water transfer in a new application. (C) 1997 Elsevier Science Inc.