As a complex heat and mass transfer process integrating refrigeration, vacuum, electronics, chemistry and other disciplines, vacuum freeze drying technology has a wide range of applications in the pharmaceutical and food industries. With the continuous advancement of technology, vacuum freeze dryers have become ideal equipment for drying raw materials, Chinese medicinal materials, drug intermediates and food. The industry said that when the vacuum freeze dryer is working, the drying rate is a point of great concern to users. However, in actual operation, it is often found that the drying rate is not satisfactory.
Which Factors Influence the Drying Rate in Vacuum Freeze Drying?
Prefreezing Rate of Materials
Pre-freezing is the first step in the vacuum freeze-drying process, which is mainly divided into two methods: rapid freezing and slow freezing. Rapid freezing forms smaller ice crystals, which are not conducive to the sublimation process; while slow freezing forms larger ice crystals, which is conducive to the increase of sublimation rate. Therefore, the selection of pre-freezing rate needs to be precisely controlled according to the material characteristics to achieve the best drying effect.
Pressure in the Drying Chamber
The industry says that the amount of pressure in a drying room directly affects the rate of heat and mass transfer (water vapor).In terms of mass transfer, lower pressure is better, while higher pressure is better for heat transfer.
Since the mass transfer rate is mainly determined by the temperature and pressure of the sublimation interface and the surface of the dry layer, there are generally two operations to improve the rate of water vapor escape in the dry layer: raising the temperature of the sublimation interface and increasing the pressure of water vapor at the interface.Or to increase the vacuum of the drying chamber, reduce the vapor pressure on the surface of the drying layer.
Form of Material
It is reported that in the freeze-drying process, freeze-dried materials are usually divided into solid and liquid form.The bulk density of solids and the composition density of liquids affect the freezing time of materials.
Wet Reloading Capacity
It is known that when the material is freeze-dried, there is a certain ratio of surface area to material thickness after being put into the container.In the case of drying, the wet reloading load on the drying tray per unit area is an important factor that determines the drying time.
Generally, the thinner the thickness of the material, the faster the heat and mass transfer, and the shorter the drying time.However, if the material is thin, there will be less dry materials per batch per unit freeze-dried area, which will be unfavorable to increase the yield per unit freeze-dried area and per unit time.So need to carry on comprehensive consideration according to material.
Conclusion
Achieving optimal drying efficiency in vacuum freeze dryers requires precise control of multiple factors, including prefreezing rate, chamber pressure, material form, and loading capacity. By fine-tuning these parameters, manufacturers can enhance product quality, improve drying speed, and maximize output.
