Drying through circulation

Manufacture. Phosphoms pentachloride is manufactured by either batch or continuous processing. In the former, the phosphoms trichloride usually dissolves in carbon tetrachloride before being treated with chlorine. A mixture of ca one part of phosphoms trichloride to one part of carbon tetrachloride is introduced to a water-jacketed vessel that contains an efficient stirrer and a tight cover with a redux condenser. The chlorine is passed into the vessel below the Hquid level, and crystals of phosphoms pentachloride form in the Hquid. When the reaction is completed, the suspension of crystals of phosphoms pentachloride in the carbon tetrachloride is drawn out of the vessel and the crystals are filtered and then dried by circulating hot water through the jacket of the filter. The clarified carbon tetrachloride is returned to the reaction vessel. 

A relationship for through-circulation drying analogous to equation 30 employs equation 9. 

Values of critical moisture contents for some representative materials are given in Table 12-7 for diying by cross circulation and in Table 12-15 for diying by through circulation. The tabulated values are only approximate, since critic moisture content depends on the drying histoiy. It appears that the constant-rate period ends when the moisture content at the surface reaches a specific value. Since the critical... 

Further, because of the nature of sohds-gas contacting, which is usually by parallel flow and rarely by through circulation, heat transfer and mass transfer are comparatively inefficient. For this reason, use of tray and compartment equipment is restricted primarily to ordinaiy drying and heat-treating operations. Despite these harsh hmitations, when the listed situations do exist, economical alternatives are difficult to develop. 

TABLE 12-14 Methods of Preforming Some Materials for Through-Circulation Drying... 

TABLE 12-15 Experimental Through-Circulation Drying Data for Miscellaneous Materials... 

Drying under vacuum is commonly practiced for sensitive materials. Figure 9.6 shows cross and through circulation tray... 

Through circulation dryers employ perforated or open screen bottom tray construction and have baffles that force the air through the bed. Superficial velocities of 150 ft/min are usual, with pressure drops of 1 in. or so of water. If it is not naturally granular, the material may be preformed by extrusion, pelleting, or briquetting so that it can be dried in this way. Drying rates are greater than in cross flow. Rates of 0.2-2 lb/(hr)(sqft tray area) and thermal efficiencies of 50% are realized. Table 9.7(d) has performance data. 

The kind of data desirable in the design of through-circulation drying are presented for a particular case by Nonhebel and Moss (1971, p. 147). They report on effects of extrusion diameters of the original paste, the bed depth, air linear velocity, and air inlet humidity, and apply these data to a design problem. 

Figure 9.8. Rotary tray, through-circulation belt conveyor, and heated screw conveyor dryers, (a) Rotary tray dryer (Wyssmont Co.), (b) Action of a rotating tray and wiper assembly (Wyssmont Co.), (c) A single conveyor belt with air upflow in wet zone and downflow in dry (Proctor and Schwartz Inc.), (d) A two-stage straight-through belt conveyor dryer, (e) A three-belt conveyor dryer as the material becomes dryer, the loading becomes deeper and the belt longer (Proctor and Schwartz Inc.), (f) Screw conveyor dryer with heated hollow screw (Bepex Corp.).

Batch through-circulation. Direct type, batch operation Not applicable Not applicable Suitable only if material can be preformed. Suited to batch operation. Shorter drying time than tray dryers Not applicable... 

Thixotropic filter cakes from rotary vacuum filters that cannot be preformed by any of the above methods can often be scored by knives on the filter, the scored cake discharging in pieces suitable for through-circulation drying. 

Design Methods for Continuous Tunnel Dryers In actual practice, design of a continuous through-circulation dryer is best based upon data taken in pilot-plant tests. Loading and distribution of solids on the screen are rarely as nearly uniform in commercial installations as in test dryers 50 to 100 percent may be added to the test drying time for commercial design. 

More than one kind of dryer may be applicable to a particular product, or the shape and size may be altered to facilitate handling in a preferred kind of machine. Thus, application of through-circulation drying on tray or belt conveyors may require prior extrusion, pelleting, or briquetting. Equipment manufacturers know the capabilities of their equipment, but they are not always reliable... 

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