Pneumatic conveying dryers

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. 

Pneumatic conveying dryers normally take particles 1-3 mm dia but up to 10 mm when the moisture is mostly on the surface. Air velocities are 10-30 m/sec. Single pass residence times are 0.5-3.0 sec but with normal recycling the average residence time is brought up to 60 sec. Units in use range from... 

Pneumatic Conveying from the Dryers (DR-401) to the Extruder Feed Tanks (D-510)... 

Rotary valves have been specified for the receivers of the vacuum pneumatic conveying systems (RV-601 and RV-602) and for the feeders (RV-40 1) and receivers (RV-501) of the positive-pressure system transferring powder from the dryers to the extruders. Perry72 gives some recommended sizes for rotary valves. The power required by all these valves will probably not exceed 2 kw and will be ignored. 

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

Such pilot plant data are cited for the rotary dryer of Example 9.6. For the pneumatic conveying dryer of Example 9.8, the tests establish heat and mass transfer coefficients which can be used to calculate residence time under full scale operation. 

Figure 9.4. Types of dryers cited in Tables 9.1 and 9.2. (a) Tray or compartment, (b) Vacuum tray, (c) Vertical agitated batch vacuum drier, (d) Continuous agitated tray vertical turbo, (e) Continuous through circulation, (f) Direct rotary, (g) Indirect rotary, (h) Agitated batch rotary (atmos or vacuum), (i) Horizontal agitated batch vacuum drier, (j) Tumble batch dryer, (k) Splash dryer. (I) Single drum, (m) Spray, (n) Fluidized bed dryer, (o) Pneumatic conveying (mostly after Nonhebel and Moss, 1971).

TABLE 9.13. Performance Data of Pneumatic Conveying Dryers (Sketches in Fig. 9.12)... 

The process of sublimation is analogous to the drying of solids so much the same kind of equipment is usable, including tray dryers (Fig. 9.6), rotary tray dryers (Fig. 9.8), drum dryers [Fig. 9.11(b)], pneumatic conveying dryers (Fig. 9.12), and fluidized beds (Fig. 9.13). The last of these requires the subliming material to be deposited on an inert carrier which is the fluidized material proper. 

After a review of the customary calculation methods [97] for both essential classes of dryers (convective and contact dryers), the dimensioning methods for spray dryers [98, 99], fluidized and spouted bed dryers [100, 101, 102], cascading rotary dryers [103], pneumatic conveying dryers [104], conductive-heating agitated dryers [105] and layer dryers [106] were presented. They all confirmed the initially made conclusion that the scaling up of dryers is still made today without dimensional analysis and the model theory based thereupon. 

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... 

R.J. Aldrich, Pneumatic conveying dryers, in W.J. Mead (Ed.), The Encyclopedia of Chemical Process Equipment, Reinhold, New York, 1964, pp. 241—251. 

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