Multiphysics Analysis On Microwave Freeze-drying Of Porous Media Assisted By Absorbing Material

Freeze-dried products are of the highest quality among various drying methods.Therefore,it plays an irreplaceable role in food,pharmaceutical and other fields.However,the long drying time and high energy consumption limit the application of freeze-drying.Freeze-drying is a typical process coupling heat and mass transfer.Single factor enhancement of either heat transfer or mass transfer could moderately be effective,but the remaining one would inevitably become the drying rate-controlling factor.In order to reduce freeze-drying time and improve the energy utilization efficiency,wave-absorbing material assisted microwave freeze-drying was proposed by author's team.That is to use wave-absorbing material assisted microwave heating to enhance heat transfer,at the same time to use initially unsaturated frozen material with prefabricated porosity to enhance mass transfer,thus achieving simultaneous enhancement of heat and mass transfer.The enhanced effect of wave-absorbing material assisted microwave heating on traditional freeze-drying was investigated theoretically and experimentally.Theoretically,a multiphase transport mathematical model coupling temperature field,concentration field and electromagnetic field was developed and numerically solved on COMSOL Multiphysics software platform based on finite element method.Experimentally,microwave freeze-drying experiments of mannitol aqueous solution were carried out,and dielectric properties of mannitol solid powder were measured.Sintered silicon carbide(SiC)was used as the wave-absorbing material.Both numerical and experimental results show that using wave-absorbing material as the supporting pad of sample can effectively enhance the microwave freeze-drying process.Under the typical operating conditions,microwave freeze-drying time of the initially unsaturated material can be 18 % and 30 %,respectively,shorter than the traditional freeze-drying process of the initially unsaturated and conventionally saturated samples.Excellent agreements were received between experimental measurements and model predictions.This indicates that the proposed freeze-drying method can achieve the simultaneous enhancement of heat and mass transfer.Based on the profiles of temperature,ice saturation and electrical field strength,mechanisms of heat and mass transfer,as well as electromagnetic wave propagation and dissipation inside a sample were analyzed during drying.Unlike the traditional freeze-drying process,the average temperature of the sample mold at the end of microwave freeze-drying was significantly higher than the ambient temperature,and the "frozen core" was located in the upper part of the sample.The distribution of electric field in the sample is closely related to the distribution of temperature and saturation.As the electromagnetic wave passed through the sample,its intensity decreased,and some of the electromagnetic energy was converted into heat energy.During microwave freeze-drying,the accumulatively absorbed energy of radiation and microwave was almost the same as those of traditional freeze-drying of the initially saturated and unsaturated samples.This demonstrates that the proposed method only increases the energy utilization so as to largely reduce the freeze-drying time.Finally,influencing factors of microwave freeze-drying were discussed.Increasing microwave power and operating temperature can accelerate the drying process.However,excessive microwave energy input or too high operating temperature could cause frozen sample to burn and even melt.Therefore,the appropriate microwave power and operating temperature should be carefully determined.The position of sample affects the microwave absorption rate.Hence it is important to determine the best sample position so as to improve the utilization of microwave energy.The enhanced effects of initially unsaturated material and microwave were both greatly influenced by material geometry.Therefore,the thickness of filling layer must be determined reasonably.The dielectric loss factor of material greatly affects the microwave freeze-drying process.When the electric field intensity is constant,the larger the dielectric loss factor is,the more microwave energy absorbed.For material with large loss factor,there is no need to use wave-absorbing material.Nevertheless,for material with small loss factor and can hardly absorb microwave,it is very necessary to use wave-absorbing material to enhance heat transfer.