Drying first falling rate period

At the conclusion of the first falling rate period it may be assumed that the surface is dry and that the plane of separation has moved into the solid. In this case, evaporation takes place from within the solid and the vapour reaches the surface by molecular diffusion through the material. The forces controlling the vapour diffusion determine the final rate of drying, and these are largely independent of the conditions outside the material. 

Toei et al. (T5, T6) believe that when the moisture content on the surface becomes less than the critical moisture content, the first falling-rate period starts and the evaporation occurs at the interior of the solid. The second falling-rate period starts when the moisture content at the surface reaches the equilibrium value. The evaporating plane retreats into the solid and dried-up zone begins to grow from the surface into the solid. The dried zone retains the equilibrium moisture content. 

The first, termed the Constant Rate Period, is the initial drying phase in which surface moisture exceeds a critical level and rate is controlled by surface area. When the level of moisture falls below the critical level, it begins to be controlled by mass transfer from inside the solid mass this is called the First Falling Rate Period. As drying proceeds, mass transfer is not able to supply moisture to the surface of the solid mass at a rate equal to the drying rate, and the free water content at the surface goes to zero. At this time, the surface temperature rises rapidly, and a receding evaporation... 

Texture modifications occur mainly during the first step of drying (the constant rate period) and are related to the visco-elastic properties of the gel network [6]. During the second step of drying (the falling rate period), liquid water leaves the capillaries and the pore walls can be damaged by forces linked to the existence of liquid-gas meniscus [6, 7],... 

The first section of the falling-rate period is shown by line BC in Fig. 24.8. The rate of drying during this period depends on the same factors that are active during the constant-rate period, since the mechanism of evaporation is unchanged and the vaporization zone is at or near the surface. The water in the pores is the continuous phase and the air the dispersed phase. In the first falling-rate period, the rate-of-drying curve is usually linear. 

The first falling rate period—corresponding to the critical moisture content for the fiber, associated with the occurrence of dry patches on the fiber surface and a falling evaporation rate. 

FIGURE 15.2 Rates of dr5dng. Curve A is characteristic for materials such as sand curve B is characteristic for materials such as soap and catalyst supports. Curve A initially exhibits a constant rate and, subsequently, a linear drop in drying rate (the first falling-rate period), and finally, a more slowly falling rate (the second falling-rate period). 

At point C on both plots, the drying rate starts to decrease in the falling-rate period until it reaches point D. In this first falling-rate period, the rate shown as line CD in Fig. 

Point C in Fig. 9.5-lb is at the critical free moisture content X. At this point there is insufficient water on the surface to maintain a continuous film of water. The entire surface is no longer wetted, and the wetted area continually decreases in this first falling-rate period until the surface is completely dry at point D in Fig. 9.5-lb. 

The shape of a diffusion-controlled curve in the falling-rate period is similar to Fig. 9.5-2a. If the initial constant-rate drying is quite high, the first falling-rate period of... 

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 shrinkage stops, further evaporation forces the liquid meniscus into the pores, and the evaporation rate decreases (Fig. 5.18c). This stage is called the falling rate period. As outlined earlier, the FRP can be divided into two parts. In the first falling rate period (FRPl), most of the evaporation is still occurring at the exterior surface. The liquid in the pores near the surface exists in channels that are continuous with the rest of the liquid. (The liquid is said to be in the funicular state.) These continuous channels provide pathways for liquid flow to the surface (Fig. 5.22a). At the same time, some liquid evaporates in the pores and the vapor diffuses to the surface. In this stage of drying, as air enters the pores, the surface of the gel may lose its transparency. 

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. 

Segment CD is the first falling-rate drying period. It is the period between the appearance of the first dry area on the material surface and the disappearance of the last liquid-wet area drying occurs at a gradually reduced rate. At point D, there is no significant area of liquid saturated surface. 

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