Rate of Drying Curves

If a material is dried by passing hot air over a surface which is initially wet, the rate of drying curve in its simplest form is represented by BCE, shown in Figure 16.3... 

Figure 16.3. The use of a rate of drying curve in estimating the time for drying...

Figure 9. Typical rate-of-drying curve, constant drying conditions.

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. 

B Rate of Drying Curves for Constant-Drying Conditions... 

Using the data calculated from Eq. (9.5-2), a plot of free moisture content X versus time t in h is made as in Fig. 9.5-la. To obtain the rate-of-drying curve from this plot, the slopes of the tangents drawn to the curve in Fig. 9.5-la can be measured, which give values ofdX/dt at given values of t. The rate R is calculated for each point by... 

Typical drying-rate curve for constant drying conditions (a) plot of data as free moisture versus time, (b) rate of drying curve as rate versus free moisture content. 

Plot of rate-of-drying curve. In Fig. 9.5-lb the rate-of-drying curve for constantdrying conditions is shown. At zero time the initial free moisture content is shown at point A. In the beginning the solid is usually at a colder temperature than its ultimate temperature, and the evaporation rate will increase. Eventually at point B the surface temperature rises to its equilibrium value. Alternatively, if the solid is quite hot to start with, the rate may start at point A. This initial unsteady-state adjustment period is usually quite short and it is often ignored in the analysis of times of drying. 

In fine pores in solids, the rate-of-drying curve in the second falling-rate period may conform to the diffusion law and the curve is concave upward, as shown in Fig. 9.5-2b. For very porous solids, such as a bed of sand, where the pores are large, the rate-of-drying curve in the second falling-rate period is often straight, and hence the diffusion equations do not apply. 

Method using rate-of-drying curve for constant-rate period. Instead of using the drying curve, the rate-of-drying curve can be used. The drying rate R is defined by Eq. (9.5-3) as... 

EXAMPLE 9.6-2. Drying Time from Rate-of-Drying Curve Repeat Example 9.6-1 but use Eq. (9.6-2) and Fig. 9.5-lb. 

Rate-of-drying curve From the data obtained, 4uring such a test, a curve of moisture content as a function of time (Fig. 12.9) can be plotted. This will be useful directly in determining the time required for drying larger batches under... 

The rate-of-drying curve is sometimes plotted with the ordinate expressed as mass moisture evaporated/(mass diy solid)(time), which in the present notation is -dX/dB,... 

Within limits, it is sometimes possible to estimate the appearance of a rate-of-drying curve such as Fig. 12.10 for conditions different from those used in the experiments. In order to determine the time for drying for such a curve, we proceed as follows. The rate of drying is, by defintion,... 

N as ordinate, X as abscissa, the data for which can be obtained from the rate-of-drying curve. 

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