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 solv ents using saturated salt solutions circulating in silicone tubing sub merged in the solvent was studied. The mass transfer could be regarded as a flux through a cylindrical geometry. Integration over the barrie r gave diffusion coefficients of water (D-aw) which were similar for t he different tubing sizes used. The driving force for the transfer was shown to be the difference in water activity and not the water concen tration 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 swellin g behavior of the tubing and the solubility of water. The water transf er was studied in a water ''producing'' system consisting of 2.5 m sil icone tubing submerged into a 250-ml tank reactor with a constant infl ux of water-saturated solvent. Different steady-state levels were obta ined at different flow rates and the corresponding D-aw values were ca lculated. The data obtained can be used to predict the required amount of tubing necessary to achieve a desired water transfer in a new appl ication. (C) 1997 Elsevier Science Inc.