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Data from bench-scale tests can be extrapolated for design, modeling and scaleup of drying operations
A combination of heat, gas and agitation are essential for process drying. The location and level of these key inputs are what distinguish direct and indirect drying systems. However, the choice of one drying method over another is largely determined by the material to be dried. With good design and modeling techniques, and sound judgment in equipment selection, the drying process can be optimized. And, performance objectives - ranging from reduction of system complexity, thermal degradation and byproduct formation, to improvement of system efficiency, safety, and, if necessary, recovery of volatiles - may be achieved.
Direct drying is established by direct contact between the product and the heat-transfer medium. For constantrate drying, the material reaches steady state near the gas wet-bulb temperature. By comparison, indirect drying is established by a heated surface in contact with the product. The heating medium and product are separated by a wall. For constant-rate drying, the material normally reaches steady state near the liquid boiling point. Some dryers can accommodate direct and indirect drying processes (Figure 1).
System modeling typically involves the determination of multiple coefficients to accomplish those goals. Data from bench-scale tests can be used to simulate performance in productionscale processes. Direct and or either indirect drying equipment can be selected to provide a system that accurately controls product temperature, residence time, heat-transfer rate and gas contact temperature, gas introduction location, and quantity and degree of agitation.
MODELING THE PROCESS
Using the equations provided for drying solids, computer modeling programs can be developed that predict the performance of a dryer for a given solid and volatile substance. The drying equipment is broken down into a series of smaller units, with heat and mass transfer for each calculated separately, and the interaction between units determined.
It is often difficult to obtain samples for pilot-scale tests on drying equipment. The solid may be a newly developed material that is limited in quantity, or the material may be hazardous and difficult to remove from the process and transport. For these reasons, reliable bench-scale analysis must be available to determine the constants in Equations 1-6.
For constant-rate drying, the equipment manufacturer may know the U value,...