Theoretical And Experimental Study On Heat And Mass Transfer Of Firedrake Fruit's Vacuum Freeze-Drying

Vacuum freeze-drying is a kind of technology that chill the materiel to its eutectic temperature or glass transition temperature before vacuum drying it.It contains many subjects such as vacuum,refrigeration,fluid,heat and mass transfer,dynamic engineering and control engineering.Since it can keep the material's basic characteristics,it has been used in food engineering,medicine engineering,biological product engineering and super imperceptibility material engineering.Though vacuum freeze-drying technology has widely development foreground it has many disadvantages.First,the drying course needs much time and much energy as its drying velocity is too low.Second,it is too hard to measure the physic property parameters of the materials.Third,as technology works in low temperature and high vacuum condition it needs much investment fee and process fee.The basic principle of vacuum freeze-drying and its four steps were expatiated in the paper. The model of dynamic pressure measurement was founded and has been used to calculate the moving sublimation front temperature.So the vacuum freeze-drying technology was learned through the measurement method.The drying process is subdivided to three steps namely primary drying stabilization step,primary drying anaphase step and the secondary drying step. During the primary drying stabilization step the temperature of sublimation front is relatively stable,the regression coefficient F decreases first and then increases gradually,the pressure curve contains two parts one is a gliding curve the other is a nearly straight line and the slop coefficient of the straight line increases gradually.During the primary drying anaphase step the regression coefficient K decreases rapidly,the regression coefficient F increases first and then decreases,the slop coefficient of the straight line part of the pressure curve is relatively big. During the secondary drying step the regression coefficient K decreases quickly,the regression coefficient F decreases more slowly than it is during the primary drying anaphase step.The pressure curve is almost a straight line and its slop coefficient is very small.The fluid compute software named FLUENT was used to simulate the temperature field of the space between the heating plate and the material upper surface in the drying chamber.If the material is nearer to the heating plate the temperature of its upper surface is larger and can get more energy.As the temperature grads of the fringe is larger than that of the center,the material on the edge of the tray can get more energy and be dried more quickly.The initial freeze point temperature,glass transition temperature and specific heat of firedrake fruit was measured by the differential scanning calorimetry.The results are as follows.The initial freeze point temperature is -2.5℃,the glass transition temperature is between -40℃and -47℃. The collapse temperature is between -20℃and -27℃and the eutectic temperature is about -30℃.The firedrake fruit's vacuum freeze-drying technology was learned.When the thickness of the materials is the same,the influence of the primary drying temperature is larger than that of the secondary drying to the total drying time.When the primary drying temperature of the material is the same,the thickness of the material is larger,the drying time is longer and the energy that was used is more.The drying temperature is higher,the probability of the dry shrinkage of the material will be greater.The thickness of the material is thinner,the probability of the dry shrinkage of the material will be bigger.So the best vacuum freeze-drying process is as follows.The thickness is 11mm,the primary drying temperature is 25℃and the secondary drying temperature is 50℃.