Flash Drying - Process Concepts and Variations

The simple flash dryer is suitable for drying a wide range of products, from inorganic chemicals like sodium bicarbonate, gypsum and alumina to organic products ranging from native starch to some polymer materials. However, process considerations for a flash drying system invariably lead to a discussion of the limitations inherent in the flash drying concept and design modifications that expand the practical application area of the technology. This creates a family of drying systems, all operating within the same basic constraints.

The very short residence time of one to three seconds requires that the moisture to be evaporated is truly surface water and that the particles in the feed cake are completely dispersed in the drying air. Otherwise, the larger particles in the feed material may require longer drying time than the smaller ones in order to reach the desired moisture content.

Extended Residence Time Flash Dryer

The upper part of the flash tube has an expanded cross-sectional area in order to reduce the flow velocity. Such an expanded area typically includes an internal baffle arrangement. This concept allows an extended residence time at the outlet temperature level and has been successfully used for some polymer materials.

Extended Residence Time Flash Dryer

Ring Dryer

Another and more elegant solution is the ring dryer, which incorporates a centrifugal classifier (manifold) in the flash dryer loop. The manifold has a series of adjustable deflector blades to control the amount of recirculation within the dryer. The manifold provides selective classification of the larger, wetter particles back to the drying duct for extended residence time. The addition of the manifold results in lower product moisture using lower outlet temperature than in a simple flash dryer. The full ring dryer exposes the recirculated coarser particles to the inlet temperature level; the diagram also shows the incorporation of a disintegrator. This concept is very suitable for thermally stable products such as silica, zeolites, and precipitated calcium carbonate.

Full Ring Dryer

If the material to be dried is very heat sensitive a P-type ring dryer can be used. Here the semi-dried particles discharged from the manifold are exposed to the outlet temperature level. This design also provides for the extended residence time required for products such as modified starches and some polymer materials.

P Type Ring Dryer

Two-stage flash drying system

The initial feed material is being dried in a flash dryer using the spent drying air from the second stage. This semi-dried product is then introduced into the second stage of the flash dryer, where the material is dried by contact with the hottest air. This arrangement is obviously only feasible if the semi-dried material collected in the cyclone from the first flash dryer stage is dry enough on the surface to avoid clogging of this cyclone collector. This concept is often used for drying of cellulose pulp.

Counter Current Two Stage Flash Drying System

Flash Drying System With Back Mixing

Proper Dispersion of Feed

A more cost-effective possibility is to use partial recycle of the spent drying air. Here a part of the exhaust airflow is recirculated back to the air heater. The bleed-off air stream can this way be reduced to 25-35% of the total exhaust airflow, reducing the heat consumption and resulting in a reduction in size of the ultimate air-pollution-control equipment. Another feature of this concept is self-inertization of the drying gas (less than 10% oxygen) if a direct-fired heater is used for heating of the drying air. Spent grains and waste sludges are often processed in such systems.

Agitated Flash Dryer

To avoid having part of the product exposed to multiple passes through the dryer, the feed material is transferred from an agitated feed tank through a screw feed into a flash-drying chamber provided with a rotor/disintegrator. The rotor continuously exposes wet surfaces from the feed material to accommodate the short residence time available for the drying process. Niro's unique design is the Swirl Fluidizer™. This system is particularly useful for inorganic and organic pigments as well as inorganic products like kaolin and alumina.

Partial Recycle Concept

Flash Fluid Bed Drying System

Combination Systems

The final application area for flash drying is in combination systems with flash drying as the first stage. Again referring to the drying curve, flash drying can be used for removal of the surface moisture while a drying system providing longer residence time would be used for removal of bound moisture. For example, a fluid bed dryer can be used as the second drying stage. These multiple-stage drying systems are used for a wide range of polymer products.