The whole process of freeze-drying is actually a process in which heat and mass transfer (water vapor) are carried out at the same time. The heat and mass transfer rate jointly affect the drying rate, thus affecting the whole freeze-drying cycle. All factors affecting heat and mass transfer will affect the drying rate. To sum up, the main factors affecting heat and mass transfer are:
1. Form and composition of materials
According to the form of freeze-dried materials, they are usually divided into solid and liquid. The shape of solid and the concentration of liquid have a great influence on the freeze-drying rate.
2. Pre freezing rate
The crystal size formed during freezing greatly affects the drying rate and the dissolution rate of dried products. There are the following differences between quick freezing and slow freezing: the ice crystals produced by quick freezing are smaller and those produced by slow freezing are larger. Large ice crystals are conducive to sublimation, while small ice crystals are not conducive to sublimation. Rapid freezing leads to low sublimation rate and fast desorption rate; Slow freezing leads to fast sublimation rate and slow resolution rate.
3. Loading capacity
When materials are freeze-dried, there is a certain ratio between surface area and material thickness after sub loading into containers, that is, freeze-drying is related to loading capacity. Small surface area and thickness are conducive to moisture sublimation, easy freeze-drying and ideal quality. During drying, the dry wet weight loading on the material tray per unit area is an important factor determining the drying time. Generally, the thinner the material stacking thickness, the faster the heat and mass transfer speed, and the shorter the drying time. However, if the material thickness is thin, there are less materials dried in each batch per unit freeze-drying area, which is unfavorable to improving the output per unit freeze-drying area and unit time.
4. Pressure in drying chamber
The pressure in the drying chamber affects the rate of heat and mass transfer. In terms of mass transfer, the lower the pressure, the better, while in terms of heat transfer, the higher the pressure, the better. The mass transfer rate is mainly determined by the temperature and pressure of the sublimation interface and the surface of the drying layer. To improve the escape rate of water vapor in the drying layer, first, increase the temperature of the sublimation interface and increase the water vapor pressure at the interface; The second is to improve the vacuum degree of the drying chamber and reduce the steam pressure on the surface of the drying layer.
5. Heat transfer mode
According to the traditional classification, it can be divided into conduction, convection, thermal radiation and medium heating (microwave heating). Because the sublimation drying process involves the transfer of heat and mass (water vapor), the way of heat transfer to transfer heat to materials more effectively has a great impact on the drying rate.
