Views: 4 Author: Site Editor Publish Time: 2021-12-07 Origin: Site
Vacuum freeze dryer is also called freeze dryer. The basic principle of freeze vacuum drying is heat and mass transfer under low temperature and low pressure conditions. Due to the different properties of the freeze-dried materials, freeze-drying methods and quality requirements for freeze-dried products, the models describing the freeze-drying process and their solutions are also different. Generally, there are three major stages in the freeze-vacuum drying process: freezing stage, sublimation drying stage, and analytical drying stage. The whole process is actually a process in which heat and mass transfer are carried out at the same time. The heat and mass transfer rate together affect the drying rate, thereby affecting the entire freeze-drying cycle. All factors that affect heat and mass transfer will affect the drying rate. Therefore, the determinants of the time of vacuum freezing, drying, and freeze-drying samples are mainly as follows.
Freeze drying chamber pressure
The pressure in the freeze-drying chamber of the vacuum freeze dryer affects the rate of heat and mass transfer. For mass transfer, the lower the pressure, the better; for heat transfer, the higher the pressure, the better. The mass transfer rate is mainly determined by the temperature and pressure difference between the sublimation interface and the surface of the drying layer. To increase the escape rate of water vapor in the drying layer, the temperature of the sublimation interface can be increased to increase the water vapor pressure of the interface; the vacuum degree of the freeze-drying chamber can also be increased, and the vapor pressure on the surface of the drying layer can be reduced.
Heat transfer method
According to the traditional classification, it can be divided into: heat conduction (caused by particles with different temperatures in thermal motion, which can be generated in solid, liquid, and gas), thermal convection (convection is due to the relative occurrence of parts of fluids with different temperatures. Movement, intermixing with each other to transfer heat energy), thermal radiation (all objects with a temperature higher than absolute zero, regardless of their temperature, are continuously radiating electromagnetic waves of different wavelengths) and medium heating (microwave heating). Since the process of sublimation drying involves the transfer of heat and quality (water vapor), which heat transfer method is used to transfer heat to the material more effectively has a greater impact on the drying rate.
Concentration of organic solvent in the sample
The concentration of the organic solvent in the sample has a greater impact on the freeze-drying rate. The higher the concentration, the slower the freeze-drying rate, and the lower the concentration, the faster the freeze-drying rate. To
Crystal size
The size of crystals formed during pre-freezing largely affects the rate of freeze-drying and the rate of dissolution of the product after freeze-drying. There are the following differences between quick freezing and slow freezing: the ice crystals produced by quick freezing are smaller, and the ice crystals produced by slow freezing are larger. Large ice crystals are good for sublimation, while small ice crystals are not good for sublimation. Fast freezing will result in low sublimation rate and fast resolution rate; slow freezing will result in fast sublimation rate and slow resolution rate.
The number of samples
When the sample is freeze-dried, there is a certain surface area to material thickness ratio after being divided into the container, that is, the freeze-drying is related to the sample amount. Large surface area and small thickness are conducive to water sublimation, easy to freeze-dry and ideal quality. When drying, the dry load per unit area of the tray is an important factor in determining the drying time: in general, the thinner the thickness of the material accumulation, the faster the heat and mass transfer speed, and the shorter the drying time. However, the thinner the material, the fewer samples dried per batch per unit freeze-drying area, which is not good for improving the unit freeze-drying area and the output per unit time.
