Dryers, classification spray

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

The majority of spray dryers in commercial use employ cocurrent flow of gas and solids. Countercurrent-flow diyers are used primarily for diying soaps and detergents. Their classif ng ability is useful in these applications. Air flow is upward, cariying entrained fines from the top of the chamber. The coarse product settles and is removed... 

The choice of dryer is more difficult when the drying step is also used to shape the product. Typical designs here are the spray dryer and the sprayed fluidized bed. If necessary, drying can be followed by a classification step in which undesired particles, for example, fines, are reyded to the feed stream for the dryer. When the desired product properties, for example, freedom from dust, pourabUity, rate of dissolution, and bulk density, are not attainable in the drying step, additional steps such as compaction and granulation must be used. 

Schliinder [23] has given a classification of dryers that encompasses the physical state of the product as well as the dwell time of the product in the dryer. For very short drying times (<1 min), flash, spray, or drum dryers are used. For very long drying times (>1 h), only tunnel, truck, or conveyor dryers are appropriate. Most dryers operate in the intermediate range, for which a very wide assortment of dryers is available [21]. 

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