The drying of sintered spheres of coarse glass beads in superheated steam was studied to elucidate the mechanism leading to the critical moisture content. The experiments were performed at superheated steam temperatures of 110-170 °C under atmospheric pressure, and the porosities of samples were 0.254-0.367 depending on the sintering conditions. A simple model for superheated steam drying in which evaporation takes place not only on the surface of the sample but also in the thin layer below the surface for the constant-rate period is proposed. The critical moisture content in superheated steam drying is lower than that in air drying for similar drying rates during a constant-rate period and can be estimated from the moisture content at the end of the first falling rate period in air drying. The predicted critical moisture contents and drying rate curves were in good agreement with the experimental results.
An earlier paper by us dealt with superheated steam drying only at atmospheric pressure. In this work, an apparatus for steam drying under vacuum was developed, and drying experiments of sintered, coarse, glass bead spheres were performed in steam and in air over a wide pressure range (7.9-760 mmHg). It was found that the critical moisture content in steam drying under vacuum was independent of the pressure and was lower than that in air drying under vacuum and that the drying rate during the falling rate period in steam drying under vacuum was higher than that in air drying under vacuum. Therefore, it is expected that steam drying under vacuum is a very useful process.The mechanism for the advantages of steam drying is not well-known. We reported on drying rate curves in the vicinity of the critical moisture content in superheated steam drying at atmospheric pressure in a previous paper (Shibata et al., 1988), and it was theoretically and experimentally found that in superheated steam drying the lack of diffusion resistance decreased the critical moisture content and increased the drying rate during the falling rate period in comparison with those for air drying. This fact indicates that the drying time is considerably decreased. However, during the constant-rate period, the temperature of the samples is a high boiling point, about 100 °C at atmospheric pressure. Therefore, superheated
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