Early experiments at Hosokawa Micron BV in Doetinchem, starting about 10 years ago, with a conventional vacuum dryer operated at low temperatures and ultra-low pressures showed that it was possible to operate a freeze dryer under active conditions. The result was a lump-free, free-flowing product.
Nowadays the Active Freeze Dryer consists of a dedicated designed drying chamber and collecting filter. In the chamber the material to be dried is frozen very fast with the aid of a freezing medium or jacket cooling. Typically a frozen granular structure is obtained after freezing in this way. Once the freezing step is completed the drying chamber is further evacuated. After evacuation of the eventual freezing agent the sublimation process will start. During sublimation the heat is supplied through the jacket and efficiently distributed throughout the product by the stirrer. The initially coarse granules will gradually reduce in size due to the sublimation of the connecting ice structure in between the frozen material. The released dried particles will make up a loose powder moving to the filter. Towards the end of the drying process when most of the frozen solvent is sublimated the product temperature will start to rise. Finally the product temperature will equalize the wall temperature, indicating that the drying process is finished. By then all material is transformed into a fine and loose powder. After breaking the vacuum the dryer can be discharged easily from the filter or dryer vessel.
Simplified process
Active Freeze Dryers exhibit a better heat transfer rate due to the continuous mixing of the product. Overall, the freeze drying process is simplified because all steps are done in a single processing unit instead of handling trays filled with product between freezing units, drying chambers and crushers. This results in minimal risk for contamination.
With the introduction of the Active Freeze Drying technology a new type of production technology has been made available. Active Freeze Dryer batch volumes can range from a few liters for lab-scale and small scale production applications to bulk drying of hundreds of liters. In all sizes the advantages are obvious: rapid drying, simple product handling and an unique product quality
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