Moisture transport and adsorption on silica gel-calcium chloride composite adsorbents

A measurement of moisture uptake curves in silica gel-calcium composite adsorbents is performed by using the thermal gravimetrical method in a relative vapor pressure range from 0.5 to 0.9 at different temperature levels of 25, 35 and 45 degrees C, respectively. Also, dynamic adsorption curves individually corresponding to the temperature and relative pressure conditions of (25 degrees C, 0.7), (35 degrees C, 0.7) and (35 degrees C, 0.5) have been measured. This experimental study shows that the sorption rate is highest for non-impregnated macroporous silica gel; however, the same silica gel impregnated with calcium chloride has a much greater final water loading. This confirms that composite adsorbents result in a better adsorption performance compared with non-impregnated silica gel, leading to a trade off in finalizing the content of CaCl2 in the composite. Through theoretical analysis on the experimental data, it is found that the saturated adsorption data can be well fitted by the FHH model and the dynamic adsorption data well fitted by the Crank diffusion model, simultaneously yielding the effective solid side mass diffusivities. A comparison with these obtained diffusion coefficients further supports that the mass diffusivity in the composite adsorbent also increases with an increase in temperature, as well as in humidity, but drastically decreases due to the presence of salt inside the matrix pores compared with the non-impregnated host matrix. (c) 2006 Elsevier Ltd. All rights reserved.