Efficient recovery of food ingredients after a dryer can be quite challenging, especially for finer particles. The same problem occurs in many milling and micronization applications.
Usual spray drying arrangements (single point, two point and closed cycle) include a high efficiency cyclone and a bag filter.
The cyclone serves the purpose of separating and collecting the dried powder originated in the spray drying chamber. Often, the fraction of product escaping to the bag filter is not considered as first grade product, and thus seen as losses. The main reasons for this are contamination with filter fibers, product cross contamination and risk of heat degradation.
For fine powders with a median diameter in volume (MVD) of less than 5µm, losses due to low cyclone efficiency can rise to more than 25%.
The preferred way to recover these sensitive and expensive products is with cyclone collectors due to their direct and sanitary powder capture. Unlike bag filters or wet scrubbers, cyclones minimize filter or product contamination and product cross contamination and can easily handle a wide range of operating temperature, pressure and moisture conditions.
Product losses due to cyclones' low efficiency represent a high cost. Cyclones need to be complemented with expensive second stage filters that may not allow for product recovery. Even for a few applications where the losses may not be so costly (less expensive products), they are not environmentally and socially responsible, with waste or side stream management being an additional problem to tackle.