Drying pneumatic conveyor dryer

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 ... 

FlaSi Drying Special designs of pneumatic-conveyor dryers, described in Sec. 17 can handle niter and centrifuge cakes and other sticlty or pasty feeds to yield granular size-enlarged products. The dry product is recycled and mixed with fresh, cohesive feed, followed by disintegration and dispersion of the mixed feed in the drying-air stream. 

Free-flowing powders and granules may be dried while being conveyed in a high velocity air stream. A pneumatic-conveyor dryer consists of a vertical or inclined drying leg, a fan to propel... 

As the flow velocity of the desiccant is increased, the drying good may be transported pneumatically. This kind of flow is found in a continuous pneumatic conveyor dryer as shown in Fig. 10.1-9. Here, the drying good is trarrsported by the heated air. The solid as well as the gaseous phase may also be directed vertically downward in cocurrent flow. Figure 10.1-10 shows a spray dryer. The suspension to be dried is dispersed by nozzles at the top and moves downward with the heated air in cocurrent flow. 

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. 

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. 

Special designs of spray dryers may provide for cooling air to enter around the chamber, closed systems for the recoveiy of solvents, and air sweepers or mechanical rakes to remove dry product from the walls and bottom of the chamber. Some are followed by pneumatic conveyors as depicted in Fig. 12-101, in which drying air is diluted with cool air for product cooling before separation. Spray dryers may... 

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. 

Several typical products dried in pneumatic conveyors are described in Table 12-34. The air-stream type referred to is an ordinary single-stage dryer like the first stage of Fig. 12-94. 

Ring Dryers The ring dryer is a development of flash, or pneumatic-conveyor, drying technology, designed to increase the versatility of application of this technology and overcome many of its limitations. 

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