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Mass transfer between moisture in a packed bed of porous aluminium silicate particles and vapour flowing through it is investigated theoretically and experimentally as well. The vapour and moisture both consist of isopropanol and water. If the inlet composition of the vapour is different from the vapour composition at the particle surface the more volatile component is enriched in the vapour while the latter passes through the bed. By variation of mixture composition, particle size, bed height and flow rate the extent of enrichment can be shown to depend on vapour-liquid phase equilibrium, liquid-phase diffusion inside the particles and gas-phase mass transfer.

The selectivity during the drying of porous materials containing binary mixtures depends on the thermodynamic equilibrium, the gas-side mass transfer and the liquid-side mass transfer. Very often the drying process is nonselective owing to the controlling liquid-side mass transfer. However, in this case at the early stages a selectivity might be expected, because the liquid-side mass transfer resistance is negligible at the beginning. This so-called initial selectivity can be extended to the whole drying process by intermittent drying. For this purpose the time required for the development of the steady state concentration profile at the beginning (transient time t T ) and the time required for the degraation of the steady state concentration profile during recovery (relaxation time t R ) had been estimated theoretically and experimentally in this work.

For vacuum contact drying of mechanically agitated, coarse, hygroscopic bulk material the drying rate and final moisture content can be controlled by a proper choice of drying conditions, mixing intensity and granular bed properties. This is demonstrated by means of theoretical and experimental investigations.