Spray drying is widely used to convert a liquid phase into particles by spraying it into a hot gas stream. Usually the drying gas is ambient air with varying humidity. But climatic effects such as temperature, season and weather strongly influence the level of humidity, making constant and reproducible results difficult to achieve.
An efficient way to obtain constant parameters is by conditioning the inlet air, so Swiss company Buechi Labortechnik has developed the Dehumidifier B-296 for use with its lab-scale spray dryer. By incorporating a refrigeration unit, the inlet air is cooled down to 2-5°C. The humidity is condensed and dry air, independent of the ambient humidity, is generated for the spray drying process.
If the substances to be spray dried are oxygen-sensitive, operation under nitrogen is the typical approach. Nitrogen is also used when working with inflammable solvents. The Dehumidifier B-296 minimises nitrogen consumption.
Nitrogen is circulated in a closed loop and evaporated solvent is condensed in the dehumidifier. The cleaned nitrogen is re-used in the spray dryer. Having a heat exchanger in between the two significantly increases efficiency and enables operation even at high temperatures, with condensation temperatures of below 6°C.
When using mixtures of water and organic solvents (max. 40% concentration), working in closed mode is a suitable method.