Views: 237 Author: Site Editor Publish Time: 2024-07-14 Origin: Site
Factors Affecting Freeze-Drying Efficiency Of Yingtai Freeze Dryer
Freeze-dried solid substances exhibit a porous structure due to the sublimation of tiny ice crystals, maintaining their original frozen volume. When rehydrated, they dissolve easily and return to their original state. Freeze-dried samples can remove over 95%-99% of water, facilitating long-term preservation of the products. With the rapid development of biochemical drugs and biological agents, the importance and advantages of freeze-drying technology have become increasingly prominent.
The entire freeze-drying process involves simultaneous heat transfer and mass transfer (water vapor), with the rates of these transfers collectively influencing the drying rate and thus the overall freeze-drying cycle. All factors affecting heat and mass transfer will impact the drying rate. The main factors affecting heat and mass transfer include:
1. Form and Composition of Materials:
- Freeze-dried materials can be categorized into solids and liquids. The form of solids and the concentration of liquids significantly impact the freeze-drying rate.
2. Pre-freezing Rate:
- The size of the crystals formed during freezing greatly affects the drying rate and the dissolution speed of the dried product. There are notable differences between rapid freezing and slow freezing: rapid freezing produces smaller ice crystals, while slow freezing produces larger ice crystals. Larger ice crystals facilitate sublimation, while smaller ice crystals hinder it. Rapid freezing results in a lower sublimation rate but a faster desorption rate; slow freezing results in a faster sublimation rate but a slower desorption rate.
3. Loading Quantity:
- During freeze-drying, the material is distributed in containers, resulting in a certain ratio of surface area to material thickness, which relates to the loading quantity. A larger surface area and smaller thickness favor moisture sublimation, making freeze-drying easier and yielding ideal quality. The wet load on the unit area of the drying tray is a crucial factor determining drying time. Generally, the thinner the material layer, the faster the heat and mass transfer, and the shorter the drying time. However, thinner layers mean fewer materials dried per batch per unit freeze-drying area, which is not beneficial for increasing the yield per unit freeze-drying area and time.
4. Vacuum Degree:
- The pressure inside the drying chamber affects the rates of heat and mass transfer. For mass transfer, the lower the pressure, the better, whereas for heat transfer, higher pressure is preferable. The rate of mass transfer is mainly determined by the temperature and pressure of the sublimation interface and the drying layer surface. To enhance the escape rate of water vapor from the drying layer, the sublimation interface temperature should be increased to raise the vapor pressure at the interface, and the vacuum degree of the drying chamber should be increased to lower the vapor pressure at the drying layer surface.
5. Heat Transfer Method:
- Traditional classifications of heat transfer methods include conduction, convection, thermal radiation, and dielectric heating (microwave heating). Since the sublimation drying process involves heat and mass (water vapor) transfer, the method of heat transfer used to deliver heat more effectively to the material significantly impacts the drying rate.