Conditions and speed of lyophilizer sublimation

When the saturated vapor pressure of ice at a certain temperature is greater than the partial pressure of water vapor in the environment, it can begin to sublime; the suction and capture of water vapor by a condenser lower than the product temperature is a necessary condition for maintenance. The distance a gas molecule travels between two consecutive collisions is called the mean free path, and it is inversely proportional to the pressure. Under normal pressure, its value is very small, and the sublimated water molecules can easily collide with the gas and return to the surface of the steam source, so the sublimation speed is very slow. As the pressure decreases below 13.3Pa, the mean free path increases by 105 times, which makes the sublimation speed significantly faster, and the flying water molecules rarely change their aspect, thus forming a directional steam flow. The vacuum pump functions in the freeze dryer to remove permanent gas to maintain the low pressure necessary for sublimation. 1g of water vapor is 1.25L under normal pressure and expands to 10,000L at 13.3Pa. It is impossible for ordinary vacuum pumps to remove such a large volume in unit time. The condenser actually forms a vacuum pump dedicated to trapping water vapor. The temperature of product and condensation is usually -25℃ and -50℃. The saturated vapor pressures of ice at this temperature are 63.3Pa and 1.1Pa respectively, so there is a considerable pressure difference between the sublimation surface and the condensation surface, if the partial pressure of the non-condensable gas in the system can be ignored at this time Regardless, it will promote the water vapor sublimated from the product to reach the surface of the condenser directionally at a certain flow rate to form frost. The sublimation heat of ice is about 2822J/g. If the sublimation process does not supply heat, the product can only reduce the internal energy to compensate for the sublimation heat, and the sublimation will stop until its temperature is balanced with the condenser temperature. In order to maintain the temperature difference between sublimation and condensation, sufficient heat must be provided to the product.