Theoretical And Experimental Research On Heat And Mass Transfer In Vacuum Freezedrying Of Pitaya

As an agricultural country,there are many kinds of fruits and vegetables in China.The output of fruits and vegetables ranks among the top in the world.However,fresh fruits and vegetables are easy to rot during transportation and have a short shelf life,resulting in a lot of waste.However,traditional drying methods have some defects such as material oxidation,denaturation of heat-sensitive components in materials,inactivation of microorganisms,hard texture,and volume shrinkage.Low-temperature drying technology is to freeze the material completely below its eutectic point and heat it in a vacuum so that the ice crystal in the material is heated and sublimated into water vapor,to achieve the purpose of drying.This drying method can maintain the original structure to the greatest extent and reduce the loss of nutrients.On the one hand,the freeze-drying equipment includes a refrigeration system,vacuum system,heating system,and control system.It contributes to more expensive equipment prices and large initial investments.On the other hand,low enough cold trap temperatures and high vacuum are demanded before the ice crystals of the material can be sublimated by heating and coagulating on the surface of the cold trap.Only the freezedrying process can operate normally.The drying stage takes a long time,resulting in high energy consumption of freeze-drying.Therefore,it is important to explore the heat and mass transfer mechanism in the freeze-drying process,optimize the freeze-drying process and shorten the freeze-drying time.The vacuum freeze-drying process is affected by many factors,such as material thickness,final freezing temperature,freezing rate,sublimation temperature,analytical temperature,and so on.At the same time,the thermophysical parameters of materials change with temperature in the freeze-drying process.It is time-consuming and laborious to carry out experimental research only,and there are some obstacles that the research parameters cannot measure.Therefore we use the simulation software to simulate the temperature field distribution,sublimation interface,and moisture content change of materials in the freeze-drying process.It is very convenient to predict the freeze-drying cycle and improve the research efficiency.Pitaya has high moisture content and is rich in vitamins and water-soluble dietary fiber.It is very perishable during transportation after picking.In the paper,we studied the red pitayas from Vietnam.After measuring its thermophysical parameters,we simulate the sublimation drying stage of Pitaya slices by COMSOL multi-physical field simulation software and predict the sublimation cycle.Based on experimental verification of the effectiveness,we explored the influence of Pitaya thickness and sublimation temperature on the sublimation drying process.And we can also obtain the best freeze-drying working condition of Pitaya.The main research contents of this paper are as follows:(1)Through the measurement of experimental instruments and the calculation of empirical formula,the basic thermophysical parameters of pitayas are obtained,which provides essential data for the freeze-drying experiment and Simulation of pitayas.It mainly includes: the eutectic temperature of pitaya fruit is-16 ℃,the eutectic point temperature is-1.5 ℃,the freezing point temperature is-3.5 ℃ and the specific heat value that corresponding to different temperatures is determined by differential scanning calorimetry(DSC),to provide basic parameters for the determination of pitayas freezing temperature and sublimation temperature in drying stage;The initial moisture content of Pitaya determined by rapid moisture detector is 80.36%,which can preliminarily determine the weight of Pitaya slice at the end of sublimation drying stage,to prepare for the determination of sublimation and analytical drying endpoint;The density of Pitaya is determined by weighing method and sick density calculation formula,to provide data for sublimation drying simulation;Finally,we obtain the thermal conductivity and surface heat transfer coefficient of Pitaya by the empirical formula.(2)After obtaining the basic thermophysical parameters of pitayas,we build the geometric model of pitaya、add the "Darcy’s law" interface,"porous medium heat transfer" interface,and "deformation geometry" interface in COMSOL software.And we use the "deformation geometry" interface to track the ice surface in pitaya fruit,set boundary conditions,and establish the physical model of heat and mass transfer in the sublimation drying stage.A fully coupled transient solver is used to simulate the sublimation drying stage of Pitayas.We carried out vacuum freeze-drying experiments on pitaya slices of the same size as the simulation.We control the freezing temperature,partition temperature,and vacuum degree in the sublimation stage are the same as the simulated values.Therefore,we can verify the effectiveness of the simulation by the experimental results.By comparing the simulated and measured values of the sublimation drying cycle,pitaya temperature,and its internal moisture content.It is concluded that the established simulation model can accurately simulate the sublimation and drying process of pitaya fruit.(3)We systematically analyzed the influencing factors of Pitaya sublimation drying.Through the combination of numerical simulation and experiment,we investigated the effects of different thickness and sublimation partition temperature on heat and mass transfer and sublimation cycle of sublimation drying.Firstly,the model is used to simulate the Pitaya slices with the thickness of 8,10,12,14,and 16 mm respectively.We can verify the simulation by the freeze-drying experiment under the same working conditions.And the thickness range of Pitaya applicable to this model is determined.Combining the sublimation rate and loading capacity,it can conclude that the best freeze-drying thickness of Pitaya is 12 mm.On this basis,we remain other working conditions,only change the sublimation partition temperature to-10 ℃,-5 ℃,and 0 ℃ respectively,simulate and experiment with the sublimation drying process of Pitaya slices.Consider the sublimation rate and the quality of freeze-dried products Comprehensively,we can come to the conclusion that when the 12 mm thick pitaya slices are freeze-dried,the partition temperature in the sublimation stage is better to be set to-5 ℃.