Abstract

To explore the effect of wave-absorbing material aided microwave heating on freeze-drying of initially porous frozen material, a lab-scale microwave freeze-dryer was designed with solid-state source selected as the microwave source instead of the traditionally used magnetron. Vitamin C was employed as the solute in aqueous solution. “Soft-ice” freezing technique was adopted to prepare solid and porous frozen materials. Experimental results showed that the solid sample by soft-ice freezing can successfully avoid structural collapse during freeze-drying, and the porous frozen material can significantly intensify mass transfer of the freeze-drying process. Under the operating conditions of 35 °C and 20 Pa, the drying time of the porous sample with the initial saturation of 0.25 decreased by 30.4% compared with the solid one, and the residual moisture content of dried product decreased with the initial saturation reducing. SEM images of the dried products revealed that the porous material had a loose pore structure and a tenuous solid skeleton, which is favorable to the migration of sublimated/desorbed vapor and the desorption of bound moisture. Evidences also showed that appropriately increasing radiation temperature led to reducing the drying time, while changing chamber pressure had an insignificant effect on freeze-drying. With sintered silicon carbide (SiC), which is a kind of wave-absorbing material, as the supporting pad of sample, microwave heating can greatly improve heat transfer of the freeze-drying process. Under the same operating conditions, the drying time of the porous sample with the microwave input power of 5 W was 28.1% shorter than that of the same sample without microwave heating, and 50.0% shorter than that of the solid sample for conventional freeze-drying. Wave-absorbing material aided microwave freeze-drying of the porous material frozen with preformed pores is a feasible and effective way to achieve the simultaneous intensification of heat and mass transfer in freeze-drying.

摘要

为了探索吸波材料辅助微波加热对初始多孔冷冻材料冷冻干燥的影响，设计了一种实验室规模的微波冷冻干燥机，选择固态源作为微波源，而不是传统使用的磁控管。维生素C用作水溶液中的溶质。采用“软冰”冷冻技术制备固体和多孔冷冻材料。实验结果表明，软冰冷冻固体样品可以成功避免冷冻干燥过程中的结构坍塌，多孔冷冻材料可以显着加强冷冻干燥过程的传质。在35℃和20Pa的操作条件下，初始饱和度为0.25的多孔样品的干燥时间比固体样品缩短了30.4%，随着初始饱和度的降低，干燥产品的残余水分含量也随之降低。干燥产物的SEM图像表明多孔材料具有松散的孔结构和细小的固体骨架，有利于升华/解吸蒸汽的迁移和结合水分的解吸。证据还表明，适当提高辐射温度可以缩短干燥时间，而改变腔室压力对冷冻干燥的影响不显着。用吸波材料烧结碳化硅（SiC）作为样品的支撑垫，微波加热可以大大改善冷冻干燥过程的传热。在相同的操作条件下，微波输入功率为 5 W 的多孔样品的干燥时间为 28。比没有微波加热的相同样品缩短1%，比常规冷冻干燥的固体样品缩短50.0%。吸波材料辅助微波冷冻干燥带有预制孔的多孔材料是实现冷冻干燥传热传质同时强化的一种可行且有效的方法。