Quick freezing method for large-scale freeze dryer
Fast-frozen raw materials can enter the freeze-drying stage after undergoing previous processes such as selection, rinsing, shaping, and putting on the plate. The first step of freeze-drying must be quick-freezing the freeze-dried materials. Put the tray with the materials in a large freeze-dryer for quick-freeze (some freeze-dryers do not have a quick-freeze function, and a quick-freezer is required). There are several quick freezing processes:
Soaking method: immerse the material in liquid nitrogen, liquid freon, low-temperature brine and other media for rapid freezing. This method freezes the fastest.
Air-cooled method: blowing low-temperature cold air on the material to freeze the material to a certain temperature within a specified time.
Direct cooling method: Put the refrigerant or refrigerant in the evaporation tube, and absorb the heat of the material to achieve the purpose of cooling.
Vacuum refrigeration method: Use the vacuum to evaporate the water in the material to take away the heat, and cool the material to achieve the purpose of refrigeration.
Regardless of the method adopted, the material should be frozen to a certain temperature within a specified period of time without substantial deformation.
In order to ensure that the ice in the material does not melt, it is directly vaporized (sublimated), and the vapor is continuously removed to ensure that the sublimation proceeds normally. The vacuum in the box must be lower than the saturated vapor pressure of ice at a certain temperature (below 610pa).
The cold trap is used to capture water vapor and reduce the load of the vacuum pump. A properly designed cold trap will not cause the vacuum pump to fail and consume low energy, but it has stricter requirements on the design of the refrigeration system and cold trap.
As the ice sublimates continuously, the steam also takes away the heat of the material, which keeps the temperature of the material (ice temperature) continuously lower, which reduces the saturated vapor pressure of the induced ice and slows the rate of sublimation. In order to maintain a certain rate of sublimation, it is necessary to constantly add heat to the material. Too little heat replenishment will reduce the rate of sublimation. Too fast heat replenishment will cause the melting of ice, which will cause the material to collapse and freeze dry.
Because the freeze-drying process of large-scale freeze dryers is more complicated and the range of working conditions is narrow, a little carelessness will cause the quality of freeze-drying to decrease, so it is required to use an automatic control system. The design of the automatic control system of the lyophilizer requires simple operation. It starts from the button of putting the material in, and it is quickly sublimated and dried in a vacuum. The relevant temperature and pressure control points should be carefully selected and reasonably controlled. The correlation between quick freezing, vacuum maintenance and heating should be properly controlled, and process curves should be set for different materials.
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