具有预制孔隙的维生素C水溶液微波冷冻干燥

设计和组装了一套实验室规模的多功能微波冷冻干燥装置,探究了具有初始孔隙的非饱和物料微波冷冻干燥过程。以维生素C为溶质,采用“软冰”冷冻技术制备了初始饱和与非饱和的冷冻样品。结果表明,软冰冷冻制备的样品能够避免崩塌。在35℃和20 Pa条件下,初始非饱和物料的干燥时间比饱和物料缩短了30.4%。SEM表征显示,非饱和物料具有疏松的球状孔隙结构、连通性好,有利于水蒸气的迁移。采用吸波材料碳化硅辅助的微波加热能够进一步强化冷冻干燥过程。在相同条件下,非饱和物料的微波冷冻干燥(5 W功率)时间比常规冷冻干燥(0 W功率)缩短了28.1%,比饱和物料的常规冷冻干燥缩短了50.0%。吸波材料辅助的初始非饱和物料微波冷冻干燥实现了传热传质的同时强化。

Microwave freeze-drying of vitamin C solution frozen with preformed pores

A lab-scale multifunctional microwave freeze-dryer was designed and assembled to explore the microwave freeze-drying process of initially unsaturated frozen material with preformed pores. Initially saturated and unsaturated frozen samples were prepared by using“soft ice”freezing technique using vitamin C as the solute. The results showed that the saturated sample by soft ice freezing can effectively avoid structural collapse during freeze-drying, and the initially unsaturated frozen material can significantly enhance the freeze-drying process. Under 35℃ of the ambient radiation temperature and 20 Pa of the drying chamber pressure, the initially unsaturated sample saved 30.4% of drying time compared with the saturated one. SEM images of dried products revealed that the initial unsaturated material had a loose solid substrate and a spherical pore structure, which is favorable to the migration of sublimated/desorbed vapor. Appropriate increasing the radiation temperature was conducive to reducing the drying time and changing the chamber pressure had insignificant effect on freeze-drying. With a wave-absorbing material―silicon carbide (SiC) as the supporting pad of sample, microwave heating can further enhance the freeze-drying process. Under the same operating conditions, the drying time for microwave freeze-drying (5 W input) of the unsaturated sample can be 28.1% shorter than that for conventional freeze-drying (0 W input), and 50.0% shorter than that for conventional freeze-drying of the saturated sample. SiC assisted microwave freeze-drying of the initially unsaturated frozen material is a feasible way to achieve the simultaneous enhancement of heat and mass transfer in freeze-drying.