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Spray dryer - movement of moisture in materials
2018/9/17 11:09:01
The spray drying process of material is actually a process of heat and mass transfer. On the one hand, the material absorbs heat from the drying medium, that is, heat is transferred from outside to inside. On the other hand, the moisture in the material is transferred from inside to outside, and the porous medium containing moisture is reduced to achieve the purpose of dryness. Most of the dried solid materials in ceramic industry belong to porous media.
Internal heat and mass transfer problems exist in chemical, metallurgical, architectural, food, geological and hydrological fields. Therefore, the guest medium has become one of the main contents of drying research.
The results show that there are five possible mechanisms of moisture migration in the drying process of wet porous materials.

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.

Babbit believes that the power of diffusion in a body should not be a concentration gradient of moisture content, but a pressure gradient. Moreover, due to the complexity of adsorption and desorption, there is no linear relationship between concentration and pressure. Diffusion theory does not consider shrinkage, surface hardening and other factors, in addition to the concentration and temperature, other factors related to the diffusion coefficient are ignored in the application. Therefore, its physical validity is doubtful. However, it can not be denied that liquid diffusion can cause moisture migration, and because of its simple form and convenient use, it has been used by many people so far.



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