Error in the estimation of effective diffusion coefficients from sorption measurements

A gravimetric sorption method was employed for the estimation of effective diffusion coefficients of water in silica gel particles. Air with a certain concentration of water passed through an adsorber with a fixed bed of silica gel particles with a height equal to the diameter of a single particle. The adsorber was weighed at chosen time intervals. This method is based on the assumption of zero mass transfer resistance in the fluid phase surrounding the adsorbent particles. From the two-film theory it follows that for the measurement conditions employed here, the maximum resistance in the fluid phase is 12.5%. Hence, the resistance in the solid phase is loaded by a maximal error of 12.5%. The total error of the estimated values of the effective diffusion coefficients is not greater than 40%. The values of the effective diffusion coefficients obtained by the measurements of the stepwise adsorption and the values of the effective diffusivity obtained by the measurements of the stepwise desorption are within the range 0.90 x 10(-10)-2.1 x 10(-1)0 m(2) s(-1). Data obtained in this way are in agreement with data published in the literature. Comparison of the estimated values of the effective diffusion coefficients with those calculated under the assumption that the water transport in silica gel particles takes place solely by molecular diffusion or by Knudsen diffusion demonstrates that the latter does not occur under the conditions investigated.