Traditional tray freeze drying methods require a lot of time and energy and only produce ‘baked’ cakes that need milling and classification. The process is subject to contamination by handling throughout these stages. Active Freeze Drying (AFD) technology, introduced by Hosokawa, is able to eliminate all these disadvantages!

Tray based lyophilization – In a conventional tray freeze dryer system, the plates are filled with the liquid product. The product is then cooled and frozen. Once frozen (mostly in the vacuum chamber) the vacuum is applied and the sublimation process begins. Now the plates are heated up to T_max of the product to increase the drying process. After a serious amount of time (normally 12 – 48 h.!) the product in the plates has been dried and has been transformed into a kind of cake. This material needs to be unloaded, milled and classified. These subsequent processes can be carried out of course but will increase the contamination risk.

The new standard: Active Freeze Drying – In an Active Freeze Dryer System, the product is frozen quickly, requiring as little as 25% of the time needed to freeze the product using traditional methods. Due to the very high heat exchange coefficient of the AFD, the drying times will be much shorter (sometimes down to < 50% of the time needed in a tray lyophilisator). The major advantage is reduced handling and consequently the reduction of contamination risk: the AFD will produce a powdered product directly in the drying system.

How the Active Freeze Drying process works – In an Active Freeze Dryer, the product is first frozen dynamically by moving it in a specially designed chamber. Whether the material to be dried starts off as a liquid, granular solid or paste, the forced motion inside the drying chamber ensures that it freezes in the form of free-flowing solid granules.

Once the product is fully frozen, a deep vacuum is applied and the sublimation starts. Heat applied to the vessel jacket is effectively distributed throughout the product due to the dynamic environment. The initially coarse, frozen granules gradually reduce in size as the ice structure connecting the frozen material sublimes away, resulting in a loose powder consisting of fine, dry particles.

When most of the solvent is sublimated the product temperature starts to rise, until finally it equals the jacket temperature. This marks the end of the freeze drying process, at which point all the material has been transformed into a fine free-flowing powder. Once the vacuum has been released, the chamber is opened and the powdered product can be discharged.