1. The condition of total sublimation in the freeze-drying process of the freeze dryer is the condition necessary to maintain the lift. When the saturated vapor pressure of ice at a certain temperature is greater than the partial pressure of water vapor of the environment, sublimation can be started; the condenser of the lower temperature than the temperature of the product sucks and captures the water vapor.
It is inversely proportional to pressure. Under normal pressure, the distance a gas molecule travels between two consecutive collisions is called the mean free path. The value is small, and the sublimated water molecules easily collide with the gas and return to the surface of the steam source, so the sublimation speed is very long. As the pressure drops below 13.3 Pa, the mean free path increases by 105 times, causing the sublimation speed to increase significantly, and the water molecules that fly away rarely change their aspects, thus forming a directed steam flow.
To maintain the low pressure required for sublimation. 1g of water vapor is 1.25L at normal pressure and expands to 10,000 liters at 13.3Pa. The vacuum pump acts as a permanent gas in the lyophilizer. It is impossible for a conventional vacuum pump to remove such a large volume per unit time. The condenser actually forms a vacuum pump that specifically collects water vapor.
Thus, a considerable pressure difference is created between the sublimation surface and the condensation surface, and the temperature of the product and condensation is usually -25 ° C and -50 ° C. The saturated vapor pressure of ice at this temperature is 63.3 Pa and 1.1 Pa. If the partial pressure of the non-condensable gas in the system is negligible, the water vapor which will promote the sublimation of the product will be directed to the condensation at a certain flow rate. The surface of the device is frosted.
If the sublimation process does not supply heat, the sublimation heat of the ice is about 2822 J/g. Then the product only reduces the internal energy to compensate for the sublimation heat, and after the temperature is balanced with the condenser temperature, the sublimation stops. In order to maintain the temperature difference between sublimation and condensation, it is necessary to provide sufficient heat to the product.
Temperature measurement during freeze-drying How is the temperature measured during freeze-drying, and what problems are often encountered during the process?
Temperature measurement during freeze-drying In the process of cold freeze-drying, the temperature parameters that are usually mainly monitored and detected include the following aspects.
(1) Temperature parameters in the cold system. It includes the condenser temperature of the freeze-drying system, the cooling water inlet and outlet temperature of the refrigeration system, and the compressor discharge temperature.
(2) Temperature parameters in the heating system. Such as shelf or radiant panel temperature, heating medium temperature, etc.
(3) Temperature parameters of cleaning, disinfection and sterilization systems.
(4) Sample temperature. Includes temperature distribution and dynamic changes in the sample.
The most important purpose of temperature parameter adjustment during cold freeze drying is to make the temperature of the sample change in time according to the process requirements. Therefore, accurate measurement of the temperature of the sample is of great significance for the research and control of the cold freeze drying process. In the freeze-drying process, in order to ensure the quality of the freeze-dried sample, the monitoring of the sample temperature is very important.

2. Problems with temperature measurement in freeze-dried samples During the measurement and control of freeze-dried samples, there are some special problems.
(1) Contact temperature measurement can only reflect the temperature condition of the local location of the temperature measurement component, and cannot represent the overall temperature condition of the freeze-dried article.
(2) When the temperature measuring element enters the inside of the freeze-dried article, the structure of the freeze-dried article near the temperature measuring element is damaged to some extent; during the cooling and freezing process of the material, the inserted thermocouple or thermistor will The action of the nucleus makes it easy for the solution at this position to form ice crystals first; in the vicinity of the temperature measuring element, the volume of ice crystals is relatively large.
(3) The temperature measuring element itself or its lead will heat the sample nearby; the sample with the temperature measuring element will dry faster than other samples without the temperature measuring element.
(4) The inserted thermocouples or thermistors are fixed, they cannot measure the temperature of the moving sublimation interface, and the freeze-drying sublimation interface temperature is a very important temperature parameter for the freeze-drying process.
(5) Limited temperature measuring components can only be placed in a limited sample. In mass production, a batch of samples often consists of a large number of samples, and the temperature changes measured by a limited number of samples are difficult to reflect the temperature conditions of the entire batch.
(6) Contact temperature measurement method brings inconvenience to aseptic operation and automatic continuous production.