Falling rate

Fig. 2. Time—tempeiatuie piofile foi kettle calcination. Points A—B lepiesent the fill period B—C, the boil or drag C—D, falling rate or cook-off D,...

Critical moisture content is that obtained when the constant rate period ends and the falling rate periods begin. Second critical moisture content specifies that remaining in a porous material when capillary flow dominance is replaced by vapor diffusion. 

For materials that foUow equation 30 the drying rate is inversely proportional to material thickness falling rate drying time is estimated,... 

Fig. 8. Drying time and rate profiles for leather pasted on glass plates and dried in two temperature stages. Gas velocity = 5 m/s in parallel flow, 71°C in the first stage, 57°C in the second. The falling rate, drying rate is proportional to residual moisture content.

Falling-rate period is a drying period during which the instantaneous diying rate continually decreases. 

The diffusion equation for the falling-rate drying period for a slab can be derived from the diffusion equation if one assumes that the surface is diy or at an equilibrium moisture content and that the initial moisture distribution is uniform. For these conditions, the following equation is obtained ... 

Estimates of both the constant-rate and the falling-rate periods are needed to estimate the total drying time for a given diying operation. If estimates for these periods are available, the total diying time is estimated by summing as... 

Special designs of direct rotaiy dryers, such as the Renneburg DehydrO-Mat (Edward Renneburg Sons Co.), are constructed especially to provide lower retention during the falling-rate diy-ing period for the escape of internal moisture from the solids. The DehydrO-Mat is a cocurrent diyer employing a smaU-diameter shell at the feed end, where rapid evaporation of surface moisture in the stream of initially hot gas is accomplished with low holdup. At the solids- and gas-exit end, the shell diameter is increased to reduce gas velocities and provide increased holdup for the solids while they are exposed to the partially cooled gas stream. 

Effectiveness As a reac tant diffuses into a pore, it undergoes a falling concentration gradient and a falling rate of reaction. The concentration depends on the radial position in the pores of a spherical pellet according to... 

The total drying time is the sum of the durations of the eonstant rate and falling rate periods and is often simplified into two linear portions (Figure 4.27). The period taken for X to reaeh Xc is known as the eonstant rate time, Oqr, whilst that to go from Xc to X is the falling rate time, 0pR. The sum of these two quantities gives the total drying time 6i. 

Rise Rate Fall Rate Maximum Quantum... 

When diffusion becomes the rate-limiting step, the time required for falling-rate drying can be estimated using an approximate equation ... 

This equation can be used to relate the drying time of the material to the moisture content wt in the falling rate region. 

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