Dryers, pilot plant sizes

The method described here is for a plant-size, direct rotary dryer although the technique is applicable to other dryers of granular media as well. In a plant scale, effects of various parameters cannot be studied in depth as one could in a laboratory or pilot-plant size equipment. Hence, such data are not generally applicable. 

In most cases, direct-heat rotary dryers are still sized on the basis of pilot-plant tests, because rarely is all the moisture to be removed truly Tree moisture, and residence time for diffusion is frequently needed. 

Batch fluidized bed dryers are made in quite small sizes, of the order of 100 lb/hr of feed as the data of Table 9.14(a) show, and are suitable for pilot plant work. 

The material comes off as flakes 1-3 mm or less thick. They are broken up to standard size of about g in. square. That process makes fines that are recycled to the dryer feed. Drying times fall in the range of 3-12 sec. Many laboratory investigations have been made of drying rates and heat transfer coefficients, but it appears that the only satisfactory basis for sizing plant equipment is pilot plant data obtained with a drum of a foot or more in diameter. Usually plant performance is superior to that of pilot plant units because of steadier long time operation. 

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). 

The design of spray dryers is based on experience and pilot plant determinations of residence time, air conditions, and air flow rate. Example 9.10 utilizes such data for the sizing of a commercial scale spray dryer. 

Sizing a Spray Dryer on the Basis of Pilot Plant Data... 

In some practical kinds of dryers, the flow patterns of gas and solid are so complex that the kind of rate equation discussed in this section cannot be applied readily. The sizing of such equipment is essentially a scale-up of pilot plant tests in similar equipment. Some manufacturers make such test equipment available. The tests may establish the residence time and the terminal conditions of the gas and solid. 

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