Kinetics of water adsorption on silica Fuji Davison RD

The kinetics of water adsorption on loose grains of Fuji Davison RD silica gel was studied by a TG differential step method in the temperature range 29-64 degrees C and in the pressure range 6.5-34 mbar. Three grain sizes were selected, 0.3-0.325, 0.355-0.425 min and 0.8-1.0 mm. Furthermore, adsorption isobars at P-H2O = 9, 18 and 48 mbar were measured over the temperature range of 30-150 degrees C by a TG technique to determine pertinent equilibrium parameters which are used to calculate the coefficients of diffusion. The equilibrium uptake was described as a linear function of the Dubinin-Polanyi adsorption potential. It was found that the influence of particle size, temperature and pressure on the adsorption kinetics can be well described in terms of the Fickian diffusion model. The apparent water diffusivity D,P was found to be an Arrhenius function of temperature with the apparent activation energy E-a = 41.5 U/mol and the pre-exponential factor D-ap0 = 2.9 x 10(-4) m(2)/s. The apparent diffusivity of water in silica pores was measured to be D-e = (3.7-4.7) x 10(-7) m(2)/s and possessed a slight increase with temperature. This value is close to the Knudsen diffusivity, calculated for a cylindrical pore of radius r(p) = 1.0 nm. For smaller grains at T > 39 degrees C the contribution of thermal effects was revealed, which decreases the rate of water sorption. In this case, application of non-isothermal kinetic model of Lee and Ruthven allowed good description of experimental uptake curves as well as the estimation of parameters which determine simultaneous heat and mass transfer.