关键词: 冷冻干燥, 结晶, 共溶剂, 传质, 非饱和

Abstract:

Freeze-drying is characterized by high quality product but long drying time and high energy consumption. Optimization of freezing stage in freeze-drying was reviewed in the present article from the viewpoint of process enhancement. Conventional optimization methods of freezing stage comprise freezing rate control, ice nucleation regulation and annealing, which can all attain large and uniform ice crystals. A common feature of these methods is that the sublimation drying rate can be increased due to the increased size of crystals, while the desorption drying rate would be decreased due to the decreased internal surface area. These conventional optimization methods can moderately improve the drying rate of less hygroscopic materials. Due to its higher vapor pressure, organic solvent as a cosolvent can increase the driving force of mass transfer. However, the requirement for a lower residual content of organic solvent restricts its wide application compared with pure water in food, pharmaceutical, and biological industries. The technical idea of “freeze-drying of initial unsaturated porous media” provides a new solution for optimization of freezing stage. The key point of the proposal is that liquid material to be dried is first prepared into frozen material with a certain initial porosity, and then freeze-dried. The prefabricated pore structure benefits the migration of sublimated vapor and the tenuous solid substrate promotes the desorption of bound moisture. This kind of frozen material can simultaneously enhance the sublimation drying stage and desorption drying stage. This novel method is a perfect combination of high product quality with low processing cost, and is able to solve the problem of long drying time and high energy consumption in the traditional freeze-drying process.

Key words: freeze-drying, crystallization, cosolvent, mass transfer, unsaturated