Analysis of the principle of in-situ Freeze Dryer
The in-situ Freeze Dryer technology is actually a drying technique in which the ice in the pre-frozen sample is directly sublimated into a water vapor discharge sample under certain vacuum and low temperature conditions, so that the sample can be dried by water. , often referred to as freeze-drying technology. As with other drying methods, in order to maintain this sublimation drying continuously, two basic conditions must be met, namely the continuous supply of heat and the continuous elimination of the generated steam.
First talk about the supply of heat. Why do you need heat supply? This is because the lyophilization actually carries out the sublimation process, the moisture in the sample is separated from the sample by sublimation, and the 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, and the loss of heat is a manifestation of temperature drop, which is why the sample will remain cold during lyophilization. However, as the sublimation progresses, the temperature of the sample quickly drops to a temperature equilibrium with the vapor partial pressure of the drying chamber. At this time, if no external heat is supplied, the sublimation process will stop. Therefore, during lyophilization, the sample must be heated by ambient air or artificially designed to provide sufficient heat to the sample to maintain 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 condensation will occur. Decondensation is an exothermic process, and the sample absorbs heat and temperature. It will also increase. When the eutectic point of the material is reached, the ice crystals in the material will melt and freeze-drying will not proceed. Therefore, in the in-situ Freeze Dryer system, a vacuum pump is required to provide the pumping power to concentrate the steam to the cold trap, and the low temperature of the cold trap causes the steam to condense into ice, thereby completing a freeze-drying process.
The process of heat supply is a heat transfer process, and the process of steam removal is a mass transfer process. Therefore, the sublimation drying process of lyophilization is essentially a process of simultaneous heat transfer and mass transfer. Any process occurring in nature has a driving force. The heat transfer driving force in sublimation drying is the temperature difference between the heat source and the sublimation interface, and the mass transfer driving force is the vapor partial pressure difference between the sublimation interface and the cold trap. The greater the temperature difference, the faster the heat transfer rate; the greater the vapor 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 an external heat transfer process, and then transferred to the actual occurrence of ice sublimation in the sample through an internal heat transfer process. The water vapor produced must pass through the internal mass transfer process to the surface of the sample and then transferred to the cold trap through an external mass transfer process. Any process or processes together can become a "bottleneck" in the drying process, depending on the design of the lyophilization equipment, the operating conditions, and the characteristics of the sample being dried. Only by simultaneously increasing heat transfer, mass transfer efficiency, and increasing the surface area per unit volume of lyophilized material can a faster drying rate be achieved.
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