Spray dryers pilot unit

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer characteristics and other operating variables on spray dryer performance are summarized in Table 9.18. A detailed survey of theory, design and performance of atomizers is made by Masters (1976), but the conclusion is that experience and pilot plant work still are essential guides to selection of atomizers. A clear choice between nozzles and spray wheels is rarely possible and may be arbitrary. Milk dryers in the United States, for example, are equipped with nozzles, but those in Europe usually with spray wheels. Pneumatic nozzles may be favored for polymeric solutions, although data for PVC emulsions in Table 9.16(a) show that spray wheels and pressure nozzles also are used. Both pressure nozzles and spray wheels are shown to be in use for several of the applications of Table 9.16(a). 

Spray dryers Surface moisture is removed in about 5 sec, and most drying is completed in less than 60 sec. Parallel flow of air and stock is most common. Atomizing nozzles have openings 0.012-0.15 in. and operate at pressures of 300-4000 psi. Atomizing spray wheels rotate at speeds to 20,000 rpm with peripheral speeds of 250-600 ft/sec. With nozzles, the length to diameter ratio of the dryer is 4-5 with spray wheels, the ratio is 0.5-1.0. For the final design, the experts say, pilot tests in a unit of 2 m dia should be made. 

FIGURE 23.6 A vertical pulse combustion spray drying installation and its pulse combustor developed by Pulse Combustion Systems, the United States, (a) Schematic of the PC spray dryer (1, air 2, unidirectional air valve 3, eombustion chamber 4, fuel 5, pilot 6, tailpipe 7, atomizer 8, quench air 9, liquid 10, drying chamber), (b) Photo of pulse combustor used in the PC spray dryer. 

Lab unit (Buchi B-290 Advance), pilot unit (Niro Mobile Minor) commercial unit (Niro PSD4) Fig. 8.1 Lab-, pQot-, and commercial-scale spray dryers at Hovione... 

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