Freeze Drying Of Initially Unsaturated Material Frozen From Liquid And Process Enhancement

Freeze-drying refers to the process of removing moisture from frozen materials at low temperature and low pressure.Although the quality of freeze-drying products is good,the problem of long drying time and high energy consumption always exists.How to shorten drying time and enhance freeze-drying process has become an important direction in the field of freeze-drying.For freeze-drying of conventional liquid materials,there is no porosity in the material after freezing.The sublimation of the ice crystals causes the sublimation interface to retreat internally,but the vapor migration only exists in the pores left by the ice crystals sublimation.The study found that some naturally formed materials,such as vegetables,fruits and wood,had initial pores inside.The initial porosity of materials can reduce the resistance of steam migration and improve the rate of freeze-drying.If the dry liquids are made into unsaturated refrigerated materials with a certain pore size,the drying rate can be greatly improved.In this paper,the initially saturated and initially unsaturated materials were prepared by using vitamin C as the solute.The freeze drying process of two kinds of materials was explored,and the drying process was enhanced by different ways.The experimental results show that the saturated vitamin C prepared by soft ice technology has a more stable freeze drying process.The dry product presents a homogeneous porous structure and avoids the collapse of vitamin C;the porous frozen material with a certain initial pore has a significant strengthening effect on the freeze drying process of liquid materials.The smaller the initial saturation of the material is,the shorter the drying time is,and the drying time of the unsaturated frozen material?S₀=0.25?can be reduced by 30.4%than that of the conventional refrigerated material?S₀=1.00?.The SEM morphology of the internal structure of the material shows that there is a large uniform spherical pore structure inside the unsaturated material,which reduces the migration resistance of water vapor.Drying time of the two kinds of frozen materials decreases with the increase of the ambient temperature.Pressure has little effect on the drying time.The drying rate of the saturated material is three stages of rising speed,constant speed and reducing speed,while the unsaturated material is always at the stage of decreasing speed,and the drying rate of unsaturated material is higher than that of the saturated material.Under the same moisture content,the solid content had little effect on the drying process.The microwave cavity of microwave freeze drying device should be a microwave vacuum integrated structure,and the microwave cavity with double layer structure has attenuation effect on microwave power.Microwave assisted freeze-drying of microwave absorbing materials SiC can enhance the freeze-drying process.Combined heating significantly shortened the drying time of the two materials.Under the condition of combined heating,the drying time of unsaturated materials is still shorter than that of saturated materials.The prefabricated pore structure of unsaturated materials combined with the saturated vapor pressure of ethanol cosolvent,thus the ethanol cosolvent can significantly strengthen the drying process of unsaturated materials.