Drying process first falling rate period

The solution of Equations 37.43 and 37.44 for a first-order drying process (linear falling rate period) is given in detail by Keey [17]. The drying can be divided into three stages ... 

To obtain the moisture profile in the machine direction, Nissan et al. [90] have made several assumptions about the drying process, principally that the drying was a first-order process (linear falling rate period) and that the pressing felt in phase 2 of the drying cycle reduced the evaporation rate to one tenth that in the sheet s free traverse between cylinders under similar temperature driving forces. Over each of the periods when the sheet touches the cylinder, it is assumed that the sheet temperature is constant (or linearly varying about an arithmetic mean), both in the plane of the sheet and normal to it. These concepts lead to a relatively simple relationship for the sheet temperature after a time interval t ... 

As shown in Fig. 5.17, the drying process can be divided into two major stages (1) a constant rate period (CRP) where the evaporation rate is nearly constant and (2) a falling rate period (FRP) where the evaporation rate decreases with time or the amount of liquid remaining in the body. In some materials it is possible to further separate the FRP into two parts. In the first falling rate period (FRPl), the evaporation rate decreases approximately linearly with time, while in the second falling rate period (FRP2), the rate decreases in a curvilinear manner. 

When drying continues the liquid film near the surface first breaks up into isolated pockets (pendular state) and finally this state spreads over the complete thickness of the gel. This situation is called the second falling rate period (FRP2) where evaporation takes place inside the gel body and the principal transport process is expected to be Knudsen diffusion of vapour. 

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