Spray dryers performance

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). 

Theoretical correlations of spray-dryer performance published by... 

Fletcher, D., Guo, B., Harvie, D. et al. 2003. What is important in the simulation of spray dryer performance and how do current CFD models perform In Proceedings of the Third International Conference on CFD in the Minerals and Process Industries, Melbourne, Victoria, Australia, www.cfd.com.au. 

Goldberg, J. E. 1987. Prediction of spray dryer performance, PhD thesis. University of Oxford, Oxford, U.K. 

Effect of selection of atomizer on spray dryer performance a comparison between different atomizers. 

Spray Drying of Emulsion-PVC. Effect of Selection of Atomizer on Spray Dryer Performance A Comparison between Different Atomizers... 

Disinfection processes, as advanced wastewater treatment, 25 909-910 Disjoining pressure, 12 6-7 Disk agglomeration, 19 8 Disk atomizer spray dryer, 9 127 Disk bowl centrifuge, theory of performance, 5 518 Disk centrifuges, 18 144... 

The performance of a spray dryer or reaction system is critically dependent on the drop size produced by the atomiser and the manner in which the gaseous medium mixes with the drops. In this context an atomiser is defined as a device which causes liquid to be disintegrated into drops lying within a specified size range, and which controls their spatial distribution. 

TABLE 9.16. Performance Data of Spray Dryers (a) Data of Kroll (1978)... 

TABLE 9.19. Product Numbers and Performance of a 30 X 29 in. Pilot Plant Spray Dryer... 

Some of the fluidized bed dryers of Figure 9.13 could be equipped with sprays and adapted to granulation. The dryer performance data of Tables 9.14 and 9.15 may afford some concept of the sizes and capacities of suitable granulators, particularly when the sprays are somewhat dilute and evaporation is a substantial aspect of the process. 

The above information is all that is required to perform a preliminary sizing on a spray dryer. The temperature limit of the product is given at 170°F. Using 160 F as the outlet temperature of the dryer should allow us the safety necessary so as not to exceed the temperature limit of the dry powder (product). The fact that we get evaporative cooling while drying the spherically shaped droplets to a similarly shaped powder allows us to use an inlet temperature of about 320°F. Thus the temperature difference (AT) is 160°F. This is the primary driving force in motivating the water to leave the product due to the enormous difference in saturation equilibrium between the wet droplet/dry powder and the very hot dry air. 

Both of these alternative separation techniques could be evaluated in a spray dryer MgO pilot plant. If either or both of these tests yield satisfactory solutions to the MgO/ash separation problem, it might be possible to reduce the investment and revenue requirements significantly by eliminating the 95 efficient ESP and replacing it with a mechanical collector. Therefore it may also be necessary to perform some test work in a pilot plant to determine the relative densities and particle sizes of the MgO and the ash in the calciner off-gas. 

Many different typas of spray device have been developed and no generalized correlations are available for predicting performance. They can be categorized into two basic types (I) preformed spray, which includes countercurrent, cocurrent, and crosscurrent spray lowers, spray dryers, cyclonic spray devices, and injector venturis and (2) gas atomized spray, which consists primarily of venturi scrubbers and related designs. 

One of the primary reasons that the spray dryer-scrubber is able to achieve excellent sulfur dioxide removal with such low liquid-to-gas ratios is the small size of the droplets produced by the high speed centrifugal atomizer. This type of atomizer also has an easily controlled turndown capability which is a desirable feature that has been demonstrated in the pilot tests. As gas flow decreases, the amount of sodium carbonate solution can be decreased in direct proportion without interfering with sulfur dioxide removal efliciency. The atomizer actually produces finer droplets at the lower liquid flow rates. This appears to compensate for any gas-liquid mixing problems that could impair performance. 

The proper operation of a spray dryer-scrubber also requires that a dry product be formed and subsequently removed from the gas stream. Pilot tests have shown that the product salts will be dry and collectable if the gas temperature at the dryer outlet is maintained about 20 °F above its dewpoint. This also tends to minimize plume formation. The cyclone collectors used in the pilot tests removed 89-99% of the product. Although this was excellent performance by mechanical collectors, particulate emission standards will require either replacement of the cyclones or additional collection devices in series with the cyclones. The system design presently favored involves using cyclones to remove the bulk of the product and adding a small electrostatic precipitator for final particulate removal. The sodium salts produced in the spray dryer-scrubber... 

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