Dryer Types

Several types of thermal dryers used by the chemical process industry can be applied to sludge drying. The sludge is always dewatered prior to drying, regardless of the type of dryer selected.

Flash dryers operate by promoting contact between the wet sludge and a hot gas stream (see Figure 7.49.5). Drying takes place in less than 10 sec of violent action, either in a vertical tube or in a cage mill. A cyclone, with a bag filter or wet scrubber, if necessary, separates the solids from the gas phase. The vapors are returned through preheaters to the furnace, minimizing odor problems. A portion of the solid product is often returned to precondition the wet sludge.

The screw conveyor dryer uses a hollow shaft and blades through which hot gas or water is pumped (see

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