Microwave Freeze-drying Of Initially Unsaturated Frozen Material Performed With Porosity

Freeze-drying is the process of removing moisture from frozen materials by sublimation/desorption at a relatively low temperature and pressure condition.Compared to other drying processes,freeze-drying could effectively avoid the inactivation of heat-sensitive components due to its moderate operation condition.Although freeze-drying has the highest product quality,the problem of long drying time and high energy consumption limits its industrial applications.Given freeze-drying is a typical process involving conjugate mass and heat transfer,the main process enhancement ways in freeze-drying can therefore be summed up as two aspects,i.e.,the enhancement of mass and heat transfer.Conventional methods of the enhancement for mass transfer include freezing rate control,ice nucleation regulation,and annealing.It should be pointed out that these approaches of the freezing stage are all to freeze liquids directly into frozen materials without initial porosity.To solve this problem fundamentally,soft ice technology was used to prepare the frozen material with a certain initial porosity,which would offer easy access to transfer vapor.As for the enhancement of heat transfer,the advantage of the microwave volumetric heating could act directly on the sublimation interface within the material in a short time,effectively decreasing the heat transfer resistance.In this paper,a lab-scale multifunctional microwave freeze-drying and eutectic point test experimental system were designed and assembled.To achieve the simultaneous enhancement of heat and mass transfer,initially unsaturated frozen samples were prepared by using soft ice technology using instant coffee as the solute;solid state microwave source and wave-absorbing material were used for offering and absorbing microwave energy.The strengthening mechanism could be explored by drying curves and SEM morphology of the drying products.The experimental results showed that the resistivity of the instant coffee solution increased as the temperature decreased during the freezing process.The resistivity reached the highest point at the temperature of-17.5~oC.Therefore this temperature is the eutectic temperature of the solution.It can be seen in the drying curves that the unsaturated samples saved 14.9%of drying time compared with the saturated one.The SEM morphological characterization of the materials showed that the unsaturated material presents a homogenous and loose porous spheroidal structure,which larger pore size and the solid substrate can improve the rate of the mass transfer effectively.When microwave power was feed in at 1W,19.1%and 12.5%of drying times were saved,respectively,for the initially saturated and unsaturated frozen samples.With the chamber surface temperature increasing from 25?C to35?C,the saturated samples collapsed,which proves the superiority of the unsaturated one again.In order to further enhance heat transfer and improve energy utilization,wave-absorbing materials—silicon carbide(SiC)was adopted as the supporting pad of sample.The experimental results showed that microwave assisted freeze-drying with the SiC pad can greatly enhance the drying process.Under the same operating conditions,the drying time for microwave freeze-drying of the unsaturated sample can be 32.5%shorter than that for conventional freeze-drying,and 42.6%shorter than that for conventional freeze-drying of the saturated one.This demonstrated that 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.