In some cases the product flow through the dryer is more or less nonuniform. This is true for the continuous fluidized-bed dryer as described in Section 334, class II(c). As a consequence of the perfect product back mixing, there is a residence time distribution of the individual particles between zero and infinity with a most probable residence time t̄_R = of the order of

\[\label{eq1} \bar{t}_{R}=\frac{{V}_{s}}{\dot{V}_{s}}\tag{1}\]

where V_s is the solid holdup of the fluid bed and V̇_s is the solid throughput. Some particles will leave the dryer still entirely wet and some others will already be overdried. There is a moisture distribution among the particles leaving the dryer according to the residence time distribution. The nonuniform drying occurring as a result of the nonuniform residence time is a principal disadvantage of those types of dryers. One method to reduce this nonuniformity is to split the whole dryer into a number of stages.

In a spray dryer, as near as possible, uniform droplet size should be arranged, whereas in a rotary dryer, back mixing should be avoided as far as possible. In general, each type of dryer has its own residence time distribution, which must be determined in individual large-scale experiments.