Kinetics of water sorption on a CaCl2-in-silica-gel-pores sorbent:: The effects of the pellet size and temperature

Kinetics of water vapor sorption on the CaCl2-in-KSK-pores composite (SWS-1L) have been studied at T = 33-69 degrees C and vapor pressures of 8-70 mbar for pellet sizes of 2R(pel) = 0.355-0.425, 0.71-0.85, and 1.2-1.4 mm. Sorption has been measured under isothermal conditions on a thermobalance by abruptly raising the vapor pressure in the measurement cell by a small value and then maintaining the new pressure. In the initial portion of the kinetic curves, the amount of sorbed water (Delta m) increases in proportion to the sorption time (t) to the power 1/2. From the slope of the Delta m versus t(1/2) curve, it is possible to derive the sorption rate constant k(D) = D-eff/R-pel(2) and the effective diffusivity D-eff. The latter is independent of R-pel for 2R(pel) >= 0.71 mm. The rate of water sorption on smaller (0.355- to 0.425-mm) pellets grows less rapidly, apparently because of the effect of the heat of sorption. The effective diffusivity is determined by the local slope of the water vapor sorption isotherm for SWS-1L. Applying an appropriate correction enables one to calculate the effective diffusivity for water vapor in the sorbent pores, which appears to be D-e = (0.35 +/- 0.17) x 10(-6) m(2)/s. This value is approximately 10 times smaller than the Knudsen water diffusion coefficient calculated for a single cylindrical pore with a size equal to the average pore size of the composite. Two possible causes of this discrepancy are discussed, specifically, an increase in the pore tortuosity because of the presence of the salt and the interaction between water and the salt.