具有初始孔隙的多孔物料冷冻干燥

实验研究了具有一定孔隙的非饱和多孔物料对液体物料冷冻干燥过程的影响。以甘露醇为主要溶质的待干料液采用“液氮制冰激凌法”制备非饱和物料进行冷冻干燥,并与常规饱和的冷冻物料相比较。结果表明,非饱和冷冻物料确实能够显著地强化液体物料的冷冻干燥过程。干燥产品SEM形貌分析显示,初始非饱和冷冻物料具有连续均匀的固体骨架和孔隙,固体基质更加纤细,孔隙空间更大,可以大大减小传质阻力。考察物料内部各点的温度变化发现,初始非饱和物料内部冰晶确实发生整体升华,但仍然存在主要升华区域;非饱和多孔物料的冷冻干燥过程主要是传热控制,而常规饱和物料冷冻干燥主要是传质控制。操作压力对过程的影响可以忽略。采用辐射/导热组合加热方式可改善初始非饱和多孔物料冷冻干燥过程的传热,进一步缩短干燥时间。

Freeze-drying of porous frozen material with initial porosity

Freeze-drying of porous frozen material with initial porosity was experimentally investigated. Mannitol was selected as the primary solute in aqueous solution to be dried. The liquid nitrogen ice-cream making method was used to prepare frozen materials with initial porosity. Freeze-drying experiments were performed using two kinds of frozen materials, the initially unsaturated one and the conventionally saturated one for comparison. Freeze-drying could be significantly enhanced with the initially unsaturated frozen material. SEM images of dried products showed that such prepared initially unsaturated material had larger void space and more tenuous solid matrix that was continuous and uniform than those with the conventionally saturated one. This would be beneficial to migration of sublimed vapor and reduction of mass transfer resistance. There was indeed an overall sublimation for the initially unsaturated frozen material, and a primary sublimation region still existed through examining temperature variations at different locations inside the material. Predominant drying rate-controlling factor was heat transfer for the initially unsaturated frozen material, and mass transfer for the conventionally saturated one. Operating pressure had little influence on the freeze-drying process. Combination of radiation heating with conduction heating was able to improve heat transfer in freeze-drying of initially unsaturated porous materials, further shortening drying time.