The whole process of freeze drying is actually the process of heat and mass transfer (water vapor) at the same time, and the transfer rate of heat and mass jointly affects the drying rate, thus affecting the whole freeze drying cycle. All the elements affecting the heat and mass transfer will affect the drying rate. In summary, the elements that affect heat and mass transfer are mainly as follows.
1、Shape and components of the material
According to the shape of lyophilized materials, they are usually divided into solid and liquid. The shape of the solid and the concentration of the liquid have a greater impact on the freeze-drying rate.
2、Pre-freezing rate
The crystal size formed during freezing largely affects the drying rate and the dissolution rate of the dried product. Quick-freeze and slow-freeze processes have the following differences: quick-freeze occurs with smaller ice crystals, and slow-freeze occurs with larger ice crystals. Large ice crystals are conducive to improvement. Small ice crystals are not conducive to improvement. Fast freezing leads to a low rate of improvement and a fast rate of desorption; slow freezing leads to a fast rate of improvement as well as a slow rate of resolution.
3、Volume
Materials in freeze-drying are divided into containers after the existence of a certain ratio between the external surface area and material thickness, that is, freeze-drying and loading volume related. The external surface area and thickness are small and are conducive to the increase of moisture, simple freeze-drying, and ideal quality. During drying, the unit area of the material plate is dried. Wet weight loading is an important element of the resolution of the drying time. In general, the thinner the thickness of the material pile, the faster the heat and mass transfer rate, the shorter the drying time. However, the material thickness is thin, the unit freeze-drying area of each batch of dry material is less, and the unit time production value is obscure.
4、Drying room pressure
The pressure inside the drying room affects the rate of heat and mass transfer. As far as mass transfer is concerned, the lower the pressure, the better, and as far as heat transfer is concerned, the higher the pressure, the better. The magnitude of the mass transfer rate is mainly resolved by raising the temperature and pressure of the interface and the exterior of the drying layer. One way to improve the escape rate of water vapor in the drying layer is to raise the temperature of the interface so that the interface water vapor pressure increases; the other is to improve the vacuum of the drying room and reduce the vapor pressure on the exterior of the drying layer.
5、Heat transfer method: Heat transfer method
Accordingly, the traditional classification can be divided into: conduction, convection, thermal radiation, and medium heating. The process of improving drying involves the transfer of heat and mass (water vapor). Therefore, the heat is transferred to the material more effectively through which heat transfer method has a greater impact on the drying rate.
