Freeze-drying, also known as lyophilization or sublimation drying is a method for the careful drying of high quality products. In freeze-drying the ice crystals sublimate directly without passing into liquid state. The end product of freeze-drying is known as lyophilization. Lyophilization in the pharmaceuticals industry is undertaken as a batch process.
As a rule the freeze-drying system is comprised of two containers. Chamber and condenser vessel are connected to each other via a valve (intermediate valve). The product to be dried stands on heatable and coolable surfaces and is frozen under atmospheric pressure. Coiled pipes are built into the condenser with a flow-through directly via injection of refrigeration agent or through a freezing solution.
In the primary drying phase the water contained in the material is sublimated. This is based on the principle that also when frozen, water has sufficiently high vapor pressure to pass from frozen directly to gaseous state. For this a vacuum is created.
Energy is taken up in sublimation. This energy is drawn from the surrounding temperature and so in the course of the process the temperature of the set-up surface would drop. In order to keep the temperature constant the set-up surfaces are subjected to the same amount of heat as is taken up by the water as sublimation energy. In the course of the drying process the atmosphere in the chambers consists almost exclusively of water vapor that precipitates as ice on the cold coiled pipes of the condenser.
In the further process comes the secondary drying, whereby through further heating, more strongly bonded water is removed from the product. Cooling of the set-up surfaces is via heat exchangers connected to the primary refrigeration system, the condenser can be directly charged with refrigeration agent. Temperatures of the condenser are typically between -75 °C and −90 °C. Defrosting the condenser is undertaken with steam in vacuum.