Powder agglomeration during the spray-drying process: measurements of air properties

Instant food powders are usually produced by spray drying of formulated liquid and powder agglomeration, either into the drying chamber or in an external fluid bed. During spray drying, according to the liquid composition, the viscosity of the initial liquid drops increases more or less rapidly, until the drop surface reaches a rubbery state considered as sticky, before further drying. This sticky property may be used to produce agglomerates inside the chamber by adding dry solid particles, as fines. The present study deals with a methodology to identify regions where the drying drops are sticky in relation to drying conditions (inlet air temperature 144-159-174 degrees C, liquid flow rate 15 to 75 mL center dot min(-1)), using a co-current pilot spray dryer with rotary atomizer. Assuming that drying air properties' (temperature and humidity) evolution is representative of drop drying (drying rate, amount of evaporated water), we realized a map of temperatures and relative humidity for drying air inside the drying chamber. Trials were performed with water (feasibility) and a maltodextrin aqueous solution (40% w/w, DE12) as a model product with sticky properties during drying. The results showed that drying behavior was different for water and aqueous maltodextrin solution. For both, in the tested conditions, the percentage of evaporated water was found to vary from 57 to 89% in the top part of the chamber, close to the atomizer. That represents a possible region where dry fines could be introduced for agglomeration.