Flash dryer

These generally follow the same rule as used in the example above, however, one major difference is that one needs to know the fluidizing velocity of the wet cake or back mixed material to dry. The factors involved in determining the fluidizing velocity are particle size, particle density, particle shape, bulk density, and medium for fluidization. Since there are too many factors to place into a reliable equation, the most expedient method of 

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The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

Manufacture. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother Hquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide (341,342). In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkaH. The solution is cooled and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, eg, in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible. 

Following carbonation, the product can be further purified by screening. This screening, also used to control the maximum size of the product, is followed by dewatering (qv). Rotary vacuum filters, pressure filters, or centrifuges are used in the mechanical removal of water. Final drying is accompHshed as with natural calcium carbonate in either a rotary, spray, or flash dryer. Products having mean particle sizes from submicrometers (- O-OS fiTo) to several micrometers are available. 

Particle size distribution determines surface-to-mass ratios and the distance internal moisture must travel to reach the surface. Large pieces thus have higher critical moisture contents than fine particles of the same material dried under the same conditions. Pneumatic-conveyor flash dryers work because very fine particles are produced during initial dispersion and these have low critical moisture contents. 

FIG. 12-95ii Strong-Scott flash dryer with integral coarse-fraction classifier to separate undried particles for recycle. (Bepex Cotp.)... 

The essential difference between a conventional flash dryer and the ring diyer is the manifold centrifugal classifier. The manifold pro-... 

The agitated flash dryer (Fig. 12-106) consists of four major components feed system, drying chamber, heater, and exhaust air system. 

Design Methods The size of the agitated flash dryer is based on the evaporation rate required. The operating temperatures are product-specific. Once established, they determine the airflow requirements. The diyiug chamber is designed based on air velocity (approximately 3 m/s) and residence time (product-specific). 

Wiped film stills in place of continuous still pots —Centrifugal extractors in place of extraction columns —Flash dryers in place of tray dryers —Continuous reactors in place of batch —Plug flow reactors in place of CFSTRs —Continuous in-line mixers in place of mixing vessels... 

Pneumatic dryers, also called flash dryers, are similar in their operating principle to spray dryers. The product to be dried is dispersed into an upward-flowing stream of hot gas by a suitable feeder. The equipment acts as a pneumatic conveyor and dryer. Contact times are short, and this limits the size of particle that can be dried. Pneumatic dryers are suitable for materials that are too fine to be dried in a fluidised bed dryer but which are heat sensitive and must be dried rapidly. The thermal efficiency of this type is generally low. 

Pneumatic conveying dryer, in hazardous waste management, 25 816 Pneumatic-conveyor flash dryers, 9 107 Pneumatic conveyors, 9 124-126 Pneumatic flotation machines,... 

Direct Heating, Dilute Solids Bed, Flash Dryer... 

Although flash dryers may differ from one another in geometry, operating pressures, and other conditions, no flash dryer subclasses have been identified. 

Z. Mindziul, A. Kmiec, Modeling gas-solid flow in a pneumatic-flash dryer, Drying 96 Proc. 10th Int. Drying Symp. A (1996) 275-282. 

A relative comparison between flash dryer and IS contactor dealing with the same amount of... 

Type of equipment Impinging stream contactor Flash dryer... 

Flash drying Wet feed is dispersed and conveyed in high velocity stream of hot gas and dries almost instantaneously to granular product. Pneumatic conveying drying Flash dryers, conveying systems... 

Fig. 7.12. Principal components of a flash dryer with dry product recycle for sludges, cakes and pastes.

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