Heat and mass transfer in sublimation and drying of small freeze dryer

　　Small freeze dryer is a science that studies the characteristics and laws of heat and mass transfer process of lyophilized materials under vacuum and low temperature environment. It is an interdisciplinary comprehensive technology. The development of freeze-drying technology needs the support of vacuum, refrigeration, heating, mechanical transmission, automatic control and other technologies. The application range of vacuum freeze-drying technology is very wide, as long as it contains water, non-toxic and non-corrosive substances, it can be made into freeze-dried products using freeze-drying technology. Almost all agricultural and sideline products, such as meat, poultry, eggs, aquatic products, vegetables, fruits, etc. can be made into freeze-dried foods, seasonings, instant foods, etc. At present, the applications of freeze-drying technology are widely in the food industry, biopharmaceutical industry, medical industry, health care industry, nano-scale materials, etc. With the rapid development and application of freeze-dried foods, there are more and more domestic freeze-dried food manufacturers. How to use freeze-dried equipment to produce qualified products is well known to manufacturers. But how to apply the research results of freeze-drying theory and freeze-drying process to the actual production process to control the operation of freeze-drying equipment, so that the entire freeze-drying art can reach the best, shorten the freeze-drying time, increase productivity, and reduce costs. Hot topic in the field.
　　To maintain the continuous sublimation and drying, two basic conditions must be met, namely the continuous supply of heat and the continuous elimination of generated steam. At the beginning of drying, if the temperature of the material is relatively high, the latent heat required for sublimation can be taken from the sensible heat of the material itself. However, as the sublimation progresses, the temperature of the material quickly drops to a temperature that is balanced with the steam partial pressure of the drying chamber. At this time, uninterrupted external heat supply and sublimation and drying proceed smoothly. In the case of external heating, the steam generated by sublimation is removed in time, and the freeze-drying process is completed.
　　Vacuum freeze-drying technology of Small freeze dryer is used in the food industry, and the products processed by this technology are called freeze-dried food. It can process almost all agricultural and sideline products, such as meat, poultry, eggs, aquatic products, vegetables, fruits, etc. can be made into freeze-dried food. Freeze-drying technology is also used in the biopharmaceutical industry, medical industry, health care industry, etc. It can maintain the color, fragrance, taste, shape and nutrients of the original food to a great extent, and the effect of rehydration.
　　1. The ice crystals in the frozen condition are sublimated and dehydrated. The skeleton remains the prototype, and the space occupied by the original ice crystals forms sponge-like micropores that are easy to absorb water.
　　2. Dehydration at low temperature, not only the heat-sensitive components are not destroyed, but also the loss of pigments and aromatic substances is very small.
　　3. Dehydration in a vacuum environment is anoxic in a vacuum environment, food ingredients will not oxidize and deteriorate, and can inhibit the harmful effects of certain bacteria.
　　4. Sublimation starts from the surface and gradually penetrates into the interior. When the ice crystal sublimates, only water molecules are removed, and the substances dissolved in the water remain in place.