As with other drying methods, two basic conditions must be met in order to maintain the continuous sublimation drying, namely, the continuous supply of heat and the continuous elimination of steam generated.
In the initial stage, if the temperature of the material is relatively high, the latent heat required for sublimation can be obtained from the sensible heat of the material itself.
However, with the sublimation process, the temperature of the material will soon fall to the temperature that is in equilibrium with the partial pressure phase of the steam in the drying chamber. At this point, if there is no external heating, the sublimation drying will stop.
Under the condition of external heating, if the steam generated by sublimation is not removed in a timely manner, the partial pressure of steam will rise, and the temperature of the material will also rise accordingly. When the freezing point of the material is reached, the ice crystals in the material will melt, and freeze drying cannot be carried out.
The process of supplying heat is a heat transfer process, while the process of removing steam is a mass transfer process. Therefore, the sublimation drying process is essentially a process of simultaneous heat 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, while the mass transfer driving force is the steam partial pressure difference between the sublimation interface and the steam trap (or cold trap).
The greater the temperature difference, the faster the heat transfer rate.
The greater the partial pressure difference of steam, the faster the mass transfer rate.
When freeze-drying, it is necessary to maintain the good quality of the product and achieve a faster drying rate.
The latent heat needed for sublimation must be transferred from the heat source to the surface of the material to be dried through the external heat transfer process, and then to the actual place of ice sublimation within the material through the internal heat transfer process.
The generated water vapor must reach the surface of the material through the internal mass transfer process, and then be transferred to the steam trap (cold trap) through the external mass transfer process.
Any one or several processes together may be the "bottleneck" of the drying process, depending on the design of the freeze-drying equipment, the operating conditions and the characteristics of the material to be dried.
Only by improving the efficiency of heat and mass transfer and increasing the surface area of freeze-dried material per unit volume can the drying rate be faster.
Water has three phases: solid, liquid and gas.
According to the phase equilibrium theory in thermodynamics, as the pressure decreases, the freezing point of water does not change much, while the boiling point gets lower and lower and closer to the freezing point.
When the pressure drops to a certain vacuum and the boiling point of water coincides with the freezing point, ice can vaporize directly into a gas without being liquid. This process is called sublimation.
Vacuum freeze-drying of food is to sublimate the frozen water in food under the condition of low temperature and low pressure below the triple point of water.