Flash drying is defined as the drying of particles that are suspended and conveyed in a hot air stream. Typical products that are suited for flash drying are polymers, e.g. s-PVC, PAN, and ABS/MBS.
Flash dryers are designed for the flash drying of wet particles with “surface water” from centrifuges or filters where the product can be dispersed by the high air velocity prevailing in the flash dryer. The product is dried to a free-flowing powder in a very short time. All flowsheets are available for download in the Animation Download area.
The short product retention time, especially when cyclones are used for product collection from the exhaust drying air, provides the possibility for faster grade charges. Bag filters can be added as a “police” filter or even by replacing the cyclone if the authorities require lower emission to the atmosphere.
The turbulent dispersion and mixing of the wet particles in the air allows high temperatures of the inlet drying air due to the cooling effect provided by the fast evaporation of the moisture from the particle surfaces.
Powders with a mean particles size (d50) from 50 micron to 500-800 micron are suitable when not being sticky at temperatures 50-100 °C. Drying economy depends very much upon source of energy and maximum allowable inlet temperatures (often 150 to 225 °C depending on the heat sensitivity of the product).
If sufficiently low residual moisture is not easily obtained in the simple flash dryer, special flash dryer designs can be considered or the final drying/cooling can take place in a second flash drying stage or more often in a fluid bed dryer/cooler where longer residence time at a lower and safer temperature is possible. Unwanted over-drying can then be avoided.