Analysis on the principle of vacuum freeze-drying

Vacuum freeze-drying technology is actually a drying technology. Under certain vacuum and low temperature conditions, the ice in the pre-frozen sample is directly sublimated into water vapor and discharged from the sample, so that the sample can be dried by dehydration. Referred to as freeze-drying technology. As with other drying methods, to maintain this continuous sublimation and drying, two basic conditions must be met, namely the continuous supply of heat and the continuous elimination of generated steam.
First talk about the supply of heat. Why do we need heat supply? This is because freeze-drying is actually a sublimation process. The water in the sample is separated from the sample by sublimation. Sublimation is an endothermic process, and heat supply is a necessary condition. At the beginning of drying, if the sample temperature is relatively high, the heat required for sublimation can be taken from the sample itself. The loss of heat is a manifestation of the temperature decrease, which is why the sample will maintain a low temperature during the freeze-drying process. But as the sublimation progresses, the sample temperature quickly drops to a temperature that is in equilibrium with the partial pressure of the steam in the drying chamber. At this time, if there is no external heat, the sublimation process will stop. Therefore, during freeze-drying, the sample must be heated by ambient air or artificially designed to provide sufficient heat to maintain the sublimation.
In the case of external heating, more and more steam is generated by sublimation. If it is not eliminated in time, the partial pressure of steam will increase, and sublimation will occur. Sublimation is an exothermic process, the sample absorbs heat, temperature It will also rise accordingly. When the eutectic point of the material is reached, the ice crystals in the material will melt, and freeze-drying will no longer be possible. Therefore, in the freeze dryer system, a vacuum pump must be used to provide suction power to make the steam gather in the cold trap, and the low temperature of the cold trap will cause the steam to condense into ice, thus completing a freeze-drying process.
The process of heat supply is a process of heat transfer, and the process of steam removal is a process of mass transfer. Therefore, the sublimation drying process of freeze-drying is essentially a process of simultaneous heat and mass transfer. Any process occurring in nature has a driving force. The driving force for heat transfer in sublimation and drying is the temperature difference between the heat source and the sublimation interface, and the driving force for mass transfer is the difference in vapor partial pressure between the sublimation interface and the cold trap. The greater the temperature difference, the faster the heat transfer rate; the greater the steam partial pressure difference, the faster the mass transfer rate.
The heat required for sublimation must be transferred from the heat source to the surface of the dried sample through the external heat transfer process, and then transferred to the actual place where ice sublimation occurs in the sample through the internal heat transfer process. The water vapor generated must reach the surface of the sample through the internal mass transfer process and then be transferred to the cold trap through the external mass transfer process. Any one process or several processes together may become the "bottleneck" of the drying process, which depends on the design of the freeze-drying equipment, operating conditions and the characteristics of the sample being dried. Only by increasing the heat and mass transfer efficiency and increasing the surface area of ​​the freeze-dried material per unit volume can a faster drying rate be achieved.

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