Drying Theories: Diffusion, Capillarity, and Pressure Gradient

Theories of drying- Various theories are proposed to explain the movement of moisture. They are- A. Diffusion theory B. Capillarity theory C. Pressure gradient theory D. Gravity flow theory E. Vapourisation and condensation mechanism Diffusion theory Rate of flow of water is proporti...

Theories of drying- Various theories are proposed to explain the movement of moisture. They are- A. Diffusion theory B. Capillarity theory C. Pressure gradient theory D. Gravity flow theory E. Vapourisation and condensation mechanism Diffusion theory Rate of flow of water is proportional to moisture gradient. Movement of moisture takes place by two ways- 1) Water diffuses to the surface and evaporates. 2) Evaporation of water takes place in the intermediate zone(below solid surface), vapours diffuses through solids to surface and goes to atmosphere. Limitation- Diffusivity decreases as moisture content and temperature decreases while increases with pressure. Capillarity theory Applicable to porous granular solids. Porous material contains network of interconnected pores and channels which are not circular or straight which produces capillary like structure. As drying starts, moisture from capillaries comes to the surface, capillary force is greater in small pores than large pores. Therefore small pores pull more water than large pores so large pores gets emptied first. Air gets entrapped into large pores(moisture at surface is more). Applicable for hygroscopic material. Pressure gradient theory Applicable to drying of solids by using radiations. Radiation is source for generation of internal heat. Radiations interact with polarized molecules and ions of the material. This aligns the molecules in order. It gives random kinetic energy or heat to the inside surface of solids. Therefore liquid inside the solid gets vapourised. As a result vapour pressure gradient is developed which is the driving force for movement of vapours to the surface.