1, capillary force (surface tension) induced fluid migration in capillary tubes.

2. The migration of moisture content in porous media under pressure gradient.

3, due to the temperature gradient inside the material, the moisture diffusion heat transfer.

4. Wet separation caused by evaporation and condensation in capillary channels.

5. Diffusion and migration of wet fraction (liquid and steam) under the action of concentration gradient.

2.6.1 liquid diffusion theory

Spray dryerMoisture migration in solid materials during drying is driven by liquid concentration gradient and carried out in the form of liquid diffusion. They assume that the diffusion coefficient is constant or linearly related to temperature or concentration, and describe it by Fick equation on the basis of isotropy in the material. This theory holds that liquid diffusion is the only way of water migration.

The diffusion equation can be used to dry clay, starch, flour, fabric, paper and wood when the equilibrium moisture content of the material is lower than the atmospheric saturation point.

(2) for solid and liquid mutually dissolved single-phase solid systems, the diffusion equation can be used for drying materials such as soap.