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

摘要：

冷冻干燥产品质量高，但时间长、能耗高。本文综述了冷冻干燥过程强化中冷冻阶段的优化方法，控制冷冻速率、调节冰晶成核和退火处理可以获得大而均匀的冰晶从而提高升华干燥阶段速率，但物料内部比表面积的减小会降低解吸干燥阶段速率，这类常规的冷冻阶段优化方法对弱吸湿性的物料有一定的强化效果。有机溶剂具有较高的蒸气压，作为共溶剂时可以增加传质推动力，但较低的有机溶剂残留量要求阻碍了其进一步应用。“初始非饱和多孔介质冷冻干燥”的技术思想是将液体物料首先制备成具有一定初始孔隙的冷冻物料，然后再进行冷冻干燥。物料具有的初始孔隙为水蒸气的迁移提供了便捷的通道，而且纤薄的固体基质也有利于结合水的解吸，可以同时强化升华干燥阶段和解吸干燥阶段。该技术思想是过程低消耗和产品高质量的完美结合，为解决冷冻干燥过程速率低的问题提供了新的方案。

关键词: 冷冻干燥, 结晶, 共溶剂, 传质, 非饱和