Freeze drying, also called lyophilization, is a particularly gentle drying process and is therefore, suitable for sensitive products. In the food industry, high quality fruits and coffee, for example, are freeze-dried as bulk material, largely preserving aroma and color. Due to the high throughput rates, the largest freeze drying systems are used in this area. A special feature here are the continuous drying systems often used in coffee drying, which are equipped with vacuum load locks.
- High water vapor tolerance
- Long maintenance intervals
- High reliability
How does it work?
The material to be dried is initially frozen, and then dried in a vacuum chamber. In contrast to conventional vacuum drying, the primary drying takes place by sublimation. This requires a much deeper vacuum. The water or solvent is transitioned directly from the solid to the gaseous state. Meanwhile, the heat necessary for the phase change is provided, for example, via heating plates. Followed by optional secondary drying to further reduce the residual moisture content of the product. This is done by desorption of bound moisture. A low pressure and an increased heat supply promote the subsequent drying. The vapor is frozen by a cold desublimation trap, often called a condenser.
For the freeze drying process, achieving a pressure below the triple point of the solvent used (e.g. water) is imperative. In addition, during evacuation, unwanted substances such as oxygen are removed from the drying chamber. While the working pressure for the duration of the primary drying is typically between 0.5 hPa and 1*10-2 hPa, it can be up to 1*10-3 hPa during secondary drying or also for the conditioning of the chamber.
Pfeiffer vacuum offers a comprehensive portfolio for freeze drying with vacuum pumps for evacuation, pressure gauges and calibration pumping stations, mass spectrometer for gas analysis for process monitoring, as well as leak detectors for the localization of leaks.