Factors Affecting Freeze-drying Efficiency

The freeze-dried solid material presents a porous structure due to the sublimation of tiny ice crystals,and maintains the original frozen volume.It is very easy to dissolve and restore its original properties after adding water.The freeze-dried sample can remove more than 95% to 99% of the water,which is conducive to the long-term preservation of the product.With the rapid development of biochemical drugs and biological preparations,the importance and superiority of freeze-drying technology have become increasingly prominent.The entire freeze-drying process is actually a process in which heat transfer and mass transfer (water vapor) are carried out simultaneously.The transfer rates of heat and mass jointly affect the drying rate,thereby affecting the entire freeze-drying cycle.All factors that affect heat and mass transfer will affect the drying rate.

In summary, the factors that affect heat and mass transfer are mainly:

1. Material form and grouping

According to the form of freeze-dried materials, they are usually divided into solids and liquids. The form of solids and the concentration of liquids have a greater impact on the freeze-drying rate.

2. Pre-freezing rate

The size of the crystals formed during freezing greatly affects the drying rate and the dissolution rate of the dried product. The quick freezing and slow freezing processes have the following differences: quick freezing produces smaller ice crystals, while slow freezing produces larger ice crystals. 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 desorption rate.

3. Loading amount

When the material is freeze-dried, there is a certain ratio of surface area to material thickness after being divided into containers, that is, freeze-drying is related to loading amount. Small surface area and thickness are conducive to water sublimation, easy freeze-drying and ideal quality. During drying, the wet weight load per unit area of the tray is an important factor in 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, the material dried per batch per unit freeze-drying area is less, which is not conducive to increasing the unit freeze-drying area and unit time output.

4. Pressure in the drying chamber

The pressure in the drying chamber affects the rate of heat and mass transfer. As far as mass transfer is concerned, the lower the pressure, the better, while for heat transfer, the higher the pressure, the better. The size of the mass transfer rate is mainly determined by the temperature and pressure of the sublimation interface and the surface of the drying layer. To increase the escape rate of water vapor in the drying layer, one is to increase the temperature of the sublimation interface to increase the interface water vapor pressure; the other is to increase the vacuum degree of the drying chamber and reduce the vapor pressure on the surface of the drying layer.

5. Heat transfer method

According to the traditional classification method, it can be divided into: conduction, convection, thermal radiation and medium heating (microwave heating). Since the sublimation drying process involves the transfer of heat and mass (water vapor). Therefore, the heat transfer method used to transfer heat to the material more efficiently has a greater impact on the drying rate.