PREDICTION OF DRYING RATE CURVES ON SINTERED SPHERES OF GLASS-BEADS IN SUPERHEATED STEAM UNDER VACUUM

The superheated steam drying of sintered spheres of coarse glass beads was investigated under vacuum (48-305 mmHg). The drying rate curves for the constant-rate period and the critical moisture contents were estimated on the basis of an evaporation zone model for superheated steam drying at atmospheric pressure, and the drying rate curves for the falling rate period were estimated from an evaporation front model. The predicted results were in good agreement with the experimental data which revealed that dry spots were present in the thin surface layer of the sintered sphere where evaporation took place during the constant-rate period, resulting in lower critical moisture contents in superheated steam under vacuum than those in air under vacuum, and that for the falling rate period the drying rates were higher than those in air under vacuum. These facts indicated that the evaporation zone model was applicable to superheated steam drying over a wide range of pressures (from 48 mmHg to atmospheric pressure) and that superheated steam drying under vacuum is superior to air drying under vacuum. © 1990, American Chemical Society. All rights reserved.