The drying of solids may be simplified into two categories, constant-rate and falling-rate drying. In constant-rate drying, the volatile material is usually on the surface of the solid particle and the limiting factor for removal is the supply of heat. In falling-rate drying, the volatile is typically bound within the particle and the limiting factor for removal can be either the rate of mass transfer (time) or the equilibrium between the solid and the gas phase.
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. The selection of one drying approach over the other is largely determined by the material to be dried. With good design and modeling techniques, testing, and sound judgment in process selection, optimization of performance objectives may be achieved. These objectives may include reduction of system complexity, thermal degradation reduction, byproduct formation, efficiency improvements, safety, or recovery of volatiles.