MOISTURE TRANSPORT IN SILICA GEL PARTICLE BEDS

A theoretical and experimental study of the performance of silica gel packed particle beds is described. A bench-scale test rig was used to obtain data for parameter values pertinent to solar air-conditioning applications. Both adsorption and desorption experiments were performed for Regular Density (RD) silica gel for a wide range of particle size. Adsorption data was also obtained for Intermediate Density (ID) gel. A model of heat and mass transfer in the bed was developed with special attention paid to the modeling of solid side resistance. For this latter purpose an extensive review of the available literature on moisture adsorption and moisture transport in silica gel was made. Both Knudsen and surface diffusion are found to be important mechanisms of moisture transport in Intermediate Density gels (mean pore radius 68 (ANGSTROM)) while surface diffusion is dominant in Regular Density gels (mean pore radius 11 (ANGSTROM)). A general equation for moisture transport in a spherical silica gel particle was developed and was incorporated into the model equations governing heat and mass transfer between the gel particles and air flowing through a packed particle bed. A computer code DESICCANT was written to solve the coupled set of partial differential equations using a finite difference numerical method.;The agreement between theory and experiment for adsorption on RD gel is satisfactory, and is somewhat better for the outlet water vapor concentration than for the outlet air temperature. The predictions are superior to those of the pseudo gas-side controlled model developed by previous investigators, especially for outlet concentration, and particularly immediately after a step change in inlet conditions. The agreement between theory and experiment for desorption from RD gel and adsorption on ID gel is satisfactory, but not as good as for adsorption on RD gel. The important parameters of the models, i.e., heat of adsorption, adsorption isotherm, and surface diffusion coefficient were investigated with particular attention given to the surface diffusion coefficient.