Flash Drying Equipment

The heart of a flash drying system is a vertical duct, or flash tube, in which the drying process takes place. A fan forces drying gas (frequently air, occasionally an inert gas such as nitrogen) through a heater and upwards through the flash tube. The feed enters into the drying-gas stream, which instantly suspends it and carries it to the collection equipment, usually a cyclone or a bag collector.

Simple Flash Drying System

Proper dispersion of the wet feed material into the drying gas is crucial. Most commonly a venturi section is incorporated in the flash tube, and the feed is introduced in the venturi throat where the high gas velocity will assist in dispersing the solid in the gas stream. For fine, free-flowing powders a metering device, such as a screw feeder, is adequate. Materials containing soft agglomerates that are easily broken up may require a feed injector or cascading screen, both configured as a fast-moving rotary valve. Products with hard agglomerates require a disintegrator or hammer mill, this can result in combined drying and milling as in a pulverizing system. There are additional ways to ensure proper dispersion of the solid in the gas stream, including back mixing of dry solids, when dealing with a sticky, non-friable feed material.

When air is the drying gas, a single induced draft fan can be used; alternatively, both a supply air fan and an exhaust air fan can be provided in order to obtain close to ambient pressure at the feed entry point. For heating of the drying air, direct-fired gas heaters or steam heat exchangers are commonly used, but indirect-fired heaters can be incorporated, if required.

Cyclone collectors are the least costly means of product collection and will capture the bulk of the solid. However, cyclones often fail to meet the required emission limit, so bag filters are often used instead of, or in addition to, the cyclone collector. In cases where bag collectors are unsuitable for the application the cyclone collector can be followed by a wet scrubber system.