The flash dryer is an economical choice for drying pastes and filter cakes.
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 certain 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 short residence time of one to three seconds requires that the moisture that shold evaporate 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 a more extensive drying time than the smaller particles in order to reach the desired moisture content.
Extended residence time flash dryer
The top part of the flash tube has an expanded cross-sectional area in order to reduce the flow velocity. The expanded area typically includes an internal baffle arrangement. This concept allows extended residence time at the outlet temperature level and has been utilized for some polymer materials with great success.
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For U.S flash dryer inquiries, please visit the Niro Inc website: www.niroinc.com.
Two-stage flash drying system
The initial feed material is dried in a flash dryer using the spent drying air from the second stage. The 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. The surface of the semi-dried material that was collected in the first stage of the flash dryer must be sufficiently dry in order to avoid cogging in the cyclone collector. This concept is often used for drying of cellulose pulp.
A more cost-effective option is to use partial recycle of the spent drying air, where part of the exhaust airflow is re-circulated back into the air heater. This way the bleed-off air stream can be reduced to 25-35% of the total exhaust airflow, reducing the heat consumption and resulting in a size reduction 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 sludge are often processed in such systems.
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