Power of flash drying

0f all the gas-suspension dryers, flash dryers are the simplest and require the least space. Often looked upon as an extreme case of fluid-bed drying, a flash dryer employs high velocity for the drying gas to convey all of the solids to a product separator. Because of the high gas velocities involved, product retention times are usually in the range of 0.5 to 3.0 s. For this reason, the particles must be reasonably small and only zero-order drying can be accomplished.

Because the particles cannot reach equilibrium, the outlet temperature from a flash dryer must be higher than that from a fluid bed. On the other hand, the short residence time permits higher inlet temperatures.

In a flash dryer, drying gas is forced through a heater and upward through a duct or flash tube. Feed is introduced into the flash tube just after the heater and must be instantly suspended by the gas stream, and conveyed to collection equipment. As with the other gas-suspension drying systems, cyclones or bag collectors are used to separate product from the spent drying gas.

The feed to a conventional flash dryer must be reasonably dry, crumbly and nonsticky. Often the feed is introduced into a cage mill along with the hot drying gas to break up the feed into sufficiently small particles. If the feed is too wet or pasty to be broken up in a cage mill, it is often recommended to backmix dry solids to create a suitable feed consistency. This can require as much as 100% recycle, adding little to the heat requirement, while doubling the solids-handling load and increasing the air-flow requirement.

Often, the feed may contain particles that are small enough to be conveyed, but still too large to be dried sufficiently in the time provided by a flash dryer. Here a fluid bed may provide an easy answer, but there are also flash dryer designs that allow recirculation of the larger particles. This gives both a longer time for drying as well as an opportunity for disintegration of the larger particles.

In order to handle very wet, sticky or paste-like feeds and at the same time avoid backmixing, agitated flash dryers are often used. In these dryers, the conventional cage mill is replaced with a high-speed disintegrating rotor. This rotor, in combination with high-velocity heated gas, keeps all particles moving until they are sufficiently disintegrated and dried to be carried from the drying chamber, through the ductwork, and on to the product-collection equipment. The high concentration of dust in the drying chamber prevents wet particles from sticking to the walls.

Although the gas-retention time in agitated flash dryers is on the same order as conventional flash dryers, the particle contact time is varied by the classifying action of the rotor and drying chamber. Thus, each particle remains in the system until it is sufficiently processed. The chamber containing the rotor is larger in diameter than a flash tube, but considerably shorter. In fact, the agitated flash dryer is the most compact of all gas-suspension drying systems.

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