Pneumatic dryer

Particle property Spray dryer Pneumatic dryer Fluidised-bed dryer... 

Pneumatic Conveyor Dryers Pneumatic conveyor dryers, often also referred to as flash dryers, comprise a long tube or duct carrying a gas at high velocity, a fan to propel the gas, a suitable feeder for addition and dispersion of particulate solids in the gas stream, and a cyclone collector or other separation equipment for final recovery of solids from the gas. 

The short residence time category comprises spray dryers, pneumatic dryers, and thin-film dryers in which the residence time may be of the order of several seconds. 

Spray dryer, fluid dryer, pneumatic dryer... 

It establishes more stringent standards for emissions of particulate matter and opacity included in the existing guidelines for thermal dryers, pneumatic coal-cleaning equipment and coal-handling equipment. 

Minimum ignition temperature MIT T = MIT-50°C Spray dryer, fluid dryer, pneumatic dryer... 

Coal handling and storage Hot air sweep pulverizer/dryer Pneumatic coal feeding... 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Dryers. Drying, another type of evaporation technique, is suited for waste streams of very high soHds content. Several common types of dryers are vacuum rotary dryers, dmm dryers, tray and compartment dryers, and pneumatic conveying dryers. 

A continuously operating pneumatic conveying dryer is used for appHcations similar to the tray and compartment dryer. In this case, however, drying is performed in conjunction with grinding, as the soHds are conveyed and dried within the unit. 

The cmde diatomite, which may contain up to 60% moisture, is first milled in a method that preserves the intricate stmcture of the diatomite. This material is fed to dryers operating at relatively low temperatures, where virtually all of the moisture is removed (see Drying). Coarse and gritty nondiatomaceous earth material is removed in separators and preliminary particle si2e separation is made in cyclones. For many producers, all of the manufacturing processes, with the exception of the calcination step, take place while the material is being pneumatically conveyed. The resultant material is termed natural product. This is the only type of diatomite made by some producers. 

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. Material residence time profiles in A, a pneumatic conveyor, B, a spray, and C, a rotary or fluid-bed dryer.

Fig. 16. Single-stage pneumatic conveyor dryer using a venturi for material acceleration and gas—soflds mixing.

The principal appHcations of pneumatic conveyors are for materials that are nonsticky and readily dispersible in the gas stream as drying must be entirely constant rate. Many are employed as predryers ahead of longer residence time fluid-bed and rotary dryers in polymer drying operations. 

FIG. 12-94 Two -stage air-stream and cage miU, pneumatic-conveyor dryer. (ABB Raymond. )... 

FIG. 12-95a Air-lift pneumatic-conveyor dryer includes partial recycle of dry product and expanding tube and cone sections to provide longer holdup for coarse particles. (Bepex Coip. )... 

TABLE 12-34 Typical Products Dried in Pneumatic-Conveyor Dryers ... 

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