Falling rate drying

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.

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 ... 

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

Falling needle viscometer, 27 738 Falling rate drying, 9 109-110, 113-115 Falling rate period, 9 97 Falling rod viscometer, 27 738 False Irish moss, common and scientific names, 3 188t... 

Product moisture at start of falling-rate drying 10 percent... 

Special designs of direct rotary dryers, such as the Renneburg DehydrO-Mat (Edward Renneburg Sons Co.), are constructed especially to provide lower retention during the falling-rate drying period for the escape of internal moisture from the solids. The DehydrO-Mat is a cocurrent dryer employing a small-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. 

Since the contact time is short, heat-sensitive materials with good drying characteristics are particularly suited to this kind of dryer, but sticky materials obviously are not. Moreover, since attrition may be severe, fragile granules cannot be handled safely. Other kinds of dryers should be considered for materials that have substantial falling rate drying periods. 

A presentation for a theoretical model of constant rate and falling rate drying is available in work by Yang et al. (19) in a study of a vibro-separator. Major factors discussed include the role of vacuum level, air bleeding rate, the critical moisture content, vibratory energy transfer, and particle size. The modeled drying rates are compared to experimental values. 

Note that tfs > tcR. Likewise, to convert to a two-stage drying process with constant-rate drying down to X,r and first-order falling-rate drying beyond, the equation is... 

The mass and heat balance equations are the same for any type of dryer, but the particle transport equation is completely different, and the heat- and mass-transfer correlations are also somewhat different as they depend on the environment of the particle in the gas (i.e., single isolated particles, agglomerates, clusters, layers, fluidized beds, or packed beds The mass-transfer rate from the particle is regulated by the drying kinetics and is thus obviously material-dependent (at least in falling-rate drying). 

For falling-rate drying throughout, time tpn is given by Eq. (12-63) the multiplying factor for drying time is 1.2 In 6 = 2.15 for all dryer i es. 

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