Abstract
Diffusion characteristics of chlorferon and diethylthiophosphate (DETP) in Caโalginate gel beads were studied to assist in designing and operating bioreactor systems. Diffusion coefficients for chlorferon and DETP in Caโalginate gel beads determined at conditions suitable for biodegradation studies were 2.70โรโ10^โ11^ m^2^/s and 4.28โรโ10^โ11^ m^2^/s, respectively. Diffusivities of chlorferon and DETP were influenced by several factors, including viscosity of the bulk solution, agitation speed, and the concentrations of diffusing substrate and immobilized cells. Diffusion coefficients increased with increasing agitation speed, probably due to poor mixing at low speed and some attrition of beads at high speeds. Diffusion coefficients also increased with decreasing substrate concentration. Increased cell concentration in the gel beads caused lower diffusivity. Theoretical models to predict diffusivities as a function of cell weight fraction overestimated the effective diffusivities for both chlorferon and DETP, but linear relations between effective diffusivity and cell weight fraction were derived from experimental data. Calciumโalginate gel beads with radii of 1.65โ1.70 mm used in this study were not subject to diffusional limitations: external mass transfer resistances were negligible based on Biot number calculations and effectiveness factors indicated that internal mass transfer resistance was negligible. Therefore, the degradation rates of chlorferon and DETP inside Caโalginate gel beads were reactionโlimited. Biotechnol. Bioeng. 2008;100: 698โ706. ยฉ 2007 Wiley Periodicals, Inc.