THE MECHANISM OF MOISTURE MOVEMENT DURING DRYING

۱۶٫۴٫۱٫ Diffusion theory of drying
In the general form of the curve for the rate of drying of a solid shown in Figure 16.2,
there are two and sometimes three distinct sections. During the constant-rate period,
moisture vaporises into the air stream and the controlling factor is the transfer coefficient
for diffusion across the gas film. It is important to understand how the moisture moves
to the drying surface during the falling-rate period, and two models have been used to
describe the physical nature of this process, the diffusion theory and the capillary theory.
In the diffusion theory, the rate of movement of water to the air interface is governed by
rate equations similar to those for heat transfer, whilst in the capillary theory the forces
controlling the movement of water are capillary in origin, arising from the minute pore
spaces between the individual particles.