Dryers for Solutions and Slurries

Residence time depends on the nature of the material and mechanical features of the dryer. The performance data of Table 9.10 show a range of 7-90 min. A formula cited by Williams-Gardner (1971, p. 133) for the geometrical residence time 9, is 

The only safe way of designing a rotary dryer is based on pilot plant tests or by comparison with known performance of similar operations. Example 9.5 utilizes pilot plant data for upscaling a dryer. The design of Example 9.6 also is based on residence time and terminal conditions of solid and air established in a pilot plant. 

A formula for the power required to rotate the shell is given by Wentz and Thygeson (1979)  

In a countercurrent dryer the exit temperature of the solid approaches that of the inlet gas. In a parallel current dryer, the exit gas is 10-20°C above that of the solid. For design purposes the temperature of the exit solid in parallel flow may be taken as 100°C. 

Solutions, slurries and pastes may be spread as thin films and dried on steam-heated rotating drums. Some of the usual arrangements 

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Figure 9.11. Drum dryers for solutions and thin slurries (Buflovak Equip. Div., Blow Knox Co.), (a) Single drum dryer with dip feed and spreader, (b) Double drum dryer with splash feed, (c) Double drum dryer with top feed, vapor hood, knives and conveyor, (d) Double drum dryer with pendulum feed, enclosed for vacuum operation.

The first pilot scrubber tests were conducted using simulated flue gas to establish the feasibility of sulfur dioxide s reacting with sodium carbonate solutions and slurries in a spray dryer. Subsequent tests were conducted at the Mohave generating station, where a 5-ft diameter modified spray dryer was used to test sulfur dioxide removal from a side stream of flue gas from this coal-fired power plant (Figure 4). The spray dryer had been in operation for over 20 yr in various drying applications prior to modification to a sulfur dioxide scrubber. It was used in over 100 tests at Mohave without a single operational problem. 

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. 

The flow chart of full-scale production is shown in Figure 6. CET slurry at a scale of about 100 L per batch containing 28 kg of CET is turned into a supersaturated solution by the above-mentioned equipment. The solution is filtered by ultrafiltration to remove endotoxins and sterilized by a membrane filter. Seed crystal is added in the sealed condition and then subdivided into vials. All of these procedures are carried out at around 5°C. The vials are frozen in the chamber of the freeze-dryer at 0°C and freeze-dried. Three batches are achieved per day. The mean times required are 25 min (at the longest) from the preparation of the supersaturated solution and 15 min (at the longest) from seeding to freezing, respectively. In our preliminary experiment adequate products were obtained even when the samples were left for 2 h before addition of seed crystal and for 2 h after seeding at standstill condition. No product defect occurred in the first batch of full-scale production. 

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