Granules heat equation

Prior to a discussion on the impact of processing air dew point and temperature on the drying rate behavior of a product, it is necessary to consider heat and mass transfer. Water will move from the granule to air in an attempt to reach an equilibrium, or saturated condition, determined by thermodynamics, which can be read from a phase diagram or psychrometric chart. The rate at which water will move from liquid in the granule to vapor in the air increases the further away the system is from equilibrium. When the water evaporates, it requires an amount of energy, the heat of vaporization, in order to change from liquid to vapor. Because of this, we must also consider transfer of heat as well as movement of material. These concepts can be described by equations shown in Table 5. 

Table 5 Equations for Heat and Mass Transfer in a Wet Granule...

The evaporation rate of liquid film surrounding the granule being dried is related to the rate of heat transfer by the equation... 

Figure 4-24 For Tapioca Starch Dispersions Two Linear Equations or a Single Exponential Equation (not shown) Provided a Better Fit of the Data between cQ and (A / o (A is the diameter of the heated starch granule and A is that of the unheated granule) (Rao and Tattiyakul, 1998).

Scale-up with vacuum dryers is relatively easy and straightforward. Each vessel has a characteristic surface to volume ratio that identifies the quantity of heated surface available and the working volume of the granules in the vessel. The drying time in a laboratory vessel may be used to determine drying time in a larger vessel through use of Equation (28) ... 

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