Outlet gas conditions and recycle of fines

When the spraying system is located above the bed surface, considerable drying of the spray may occur before it contacts the bed particles. The extent of this drying depends, among other factors, upon the difference between the outlet gas temperature and its dew point. The drying potential 

Similarly, the degree of grinding of oversize product and its recycle back to the bed together with fines strongly influences the creation of new granule centres. Control of the fineness and amount of recycled solids is the most satisfactory way to control particle size and stabilize granulator operation [12,22]. 

Detailed design of fluid spray dryers must, however, be accompanied by experimental work. It has been noted [22] that even quite similar materials may behave very differently in a spray granulator. Experimental work will indicate such information as the natural mechanism of particle growth (coalescence vs layering), attainable product sizes, the preferred temperature and moisture conditions for the bed material, suitable ranges of bed voidage 

Characteristics of batch fluid bed spray granulators to produce tablet granulations in the pharmaceutical industry. Flowsheet given in Fig. 7.9. 

Type of sludge Incinerator size Bed temper- ature Capacity Granular product composition 

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