Drying of porous materials at low moisture content

Drying experiments were conducted using packed beds of glass beads with initial moisture content near or slightly above the irreducible moisture content. The objective was to validate the vapor phase diffusion coefficient determined in earlier experiments and to quantify the effect of temperature gradient on vapor phase diffusion in the presence of irreducible saturation. The results indicate that for isothermal drying the enhancement factor, beta, which is the ratio of the diffusion coefficient in the packed bed to that in free space, is slightly less than unity. This is consistent with previous results. However, the results for non-isothermal drying show little effect of temperature gradient for temperature gradients either assisting the concentration gradient or opposing the concentration gradient. This is contrary to the traditional thinking about enhanced vapor diffusion originally proposed by Philip and deVries (1957).