Wave-absorbing material aided microwave freeze-drying of vitamin C solution frozen with preformed pores

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.