Flash dryers are the most economical for drying solids that have been de-watered or inherently have a low moisture content. Also known as "pneumatic dryers," they are the simplest gas suspension dryers with the smallest footprint. A single operation provides mixing, heat transfer, and mass transfer to dry a solid. Residence time within the dryer is very short, usually less than three seconds, producing almost immediate surface drying.

Feeds are:

• Moist, powdery, granular, or crystallized
• Wet solids discharged from centrifuges, rotary filters, and filter presses
• Small in particle size
• Reasonably dry, friable, and not sticky

Due to the very rapid drying process, flash dryers are not suitable for diffusion-controlled drying processes. Figure 1 shows a typical drying curve as obtained in a fixed or fluidized bed; the constant drying-rate area where surface moisture is removed is ideal for the flash-drying operation. The process is strictly controlled by the heat input without a residence-time requirement; the drying occurs "in a flash".

Advantages

• Relatively simple in operation
• Take little space
• Generally require lower capital investment than other types of dryers
• Excellent choice for processing heat-sensitive or easily oxidized feed materials
• Product inventory in the flash-drying system is very low
• Easy to change product or product grade with minimum downtime
• Control of the flash-drying process is very simple
• Control system responds very quickly to operational changes

Design Considerations

Flash drying systems are designed based on feed and product characteristics, available or permissible heating source, and operational safety requirements. Such systems can be designed in a closed-cycle arrangement suitable for evaporation of organic solvents rather than water. The drying gas is inert (typically nitrogen), and the solvent evaporated in the flash dryer is subsequently condensed.

Pilot Testing

Although some theoretical basis exists, development of firm design data always requires pilot plant testing, unless there is operational experience in a production plant. The critical data collected through pilot-plant testing includes drying temperatures and product moisture content, but equally important is the experience gained in handling the feed material and evaluating how the feed material is properly dispersed in the drying gas.