CALCULATION OF CONCENTRATION-DEPENDENT MUTUAL DIFFUSION-COEFFICIENT IN DESORPTION OF FILM

For the isothermal desorption of film, the diffusion equation for a slab based on the dissolved solid coordinate is solved numerically with various types of concentration dependence of the diffusion coefficient. Based on the assumption of concentration distribution similarity, the relations between desorption rate and integral average diffusion coefficient are derived as functions of the ratios of average concentration to center concentration for both the penetration period and the regular regime, regardless of the type of concentration dependence of the diffusion coefficient. By means of these relations, Methods of calculating the diffusion coefficient as a function of solvent concentration are presented for both the initial rate in the penetration period and the rate in the regular regime, which covers the whole concentration range included in the desorption data. The method is usable within an error of 10% in the range of 0.5 < (the ratio of average concentration to center concentration) < 0.9, which covers a variety of concentration dependences of the diffusion coefficient. © 1990, The Society of Chemical Engineers, Japan. All rights reserved.