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Applying prilling towers as the finishing technology for new urea production plant is still a popular choice nowadays in some areas where the urea product is used domestically. On the other hand fluid bed granulation and Rotoformer technologies offer better quality products and lower emission figures.
In principle prilling towers can be divided into natural draft and forced (induced) draft prilling towers with each its advantages and disadvantages. The prilling devices can be shower head sprayers or rotating buckets, the latter are more efficient as these realize a better and more even prill distribution over the coverage area of the prilling tower.
Advantages of prilling towers are its simplicity and in the case of a natural draft tower its low power consumption. Disadvantage are the relative low quality of the prills and the emission of dust and ammonia present in a large volume of air.
The dust and ammonia emission in kg/hr have a proportional relation with the production rate. Many times governmental regulations require only a maximum concentration for the dust and ammonia emission. This is not sufficient as in the design phase the air flow and thus the concentration can be chosen freely. So in fact in this way one could simply dilute the emissions.
Towers with natural draft cooling are reported to have less dust emission than towers with forced/induced draft air cooling. Operational and maintenance procedures have a significant effect on the dust emission figures: Fouling of the prilling device, operation at non-optimal speed, high melt temperature, high prill temperature at the tower base, high air velocities and the air velocity distribution all influence dust emission figures.
Selection of the appropriate equipment for dust scrubbers can be a complex issue. Wet scrubbers seem to be more attractive than dry dust collectors and only few companies like for example BECO, USA have proven to be successful.