Drying of porous materials at low moisture content

Drying experiments were conducted using packed beds of glass beads with ini tial moisture content near or slightly above the irreducible moisture conte nt. The objective was to validate the vapor phase diffusion coefficient det ermined in earlier experiments and to quantify the effect of temperature gr adient on vapor phase diffusion in the presence of irreducible saturation. The results indicate that for isothermal drying the enhancement factor, bet a, which is the ratio of the diffusion coefficient in the packed bed to tha t in free space, is slightly less than unity. This is consistent with previ ous results. However, the results for non-isothermal drying show little eff ect of temperature gradient for temperature gradients either assisting the concentration gradient or opposing the concentration gradient. This is cont rary to the traditional thinking about enhanced vapor diffusion originally proposed by Philip and deVries (1957).