The pressure difference between the freeze-drying chamber and the cold trap is not necessarily large. When the pressure difference reaches half of the saturated vapor pressure of ice corresponding to the sublimation product temperature, the sublimation rate has reached the maximum. When the pressure difference increases again, the sublimation rate does not increase. For example, if the temperature of sublimated product is - 30 ℃, the saturated vapor pressure corresponding to www.bjsyhx.com is 0.37 mbar, half of which is 0.18 mbar, and the temperature corresponding to 0.18 mbar is - 37 ℃, then the temperature of cold trap is lower than - 37 ℃. Considering the heat transfer temperature difference after freezing of cold trap, the actual temperature of cold trap needs to be lower.
In addition, from the temperature pressure curve between 0 ℃ and - 80 ℃ of ice, it can be seen that the pressure difference changes greatly from - 20 ℃ to - 40 ℃, the pressure difference between - 40 ℃ and - 80 ℃ changes very little, the chemical cost is great, the cold trap temperature is reduced very low, but the water vapor pressure difference increases very little. Generally, the temperature of the cold trap is lower than 20 ℃ of the product.
During sublimation, the product absorbs heat and turns into steam. During condensation, the steam releases heat and turns into frost. Therefore, the temperature of the cold well will rise due to the heat. This is a normal phenomenon, but the temperature of the cold well cannot rise excessively, otherwise there will be chain reaction. Firstly, the pressure difference between the freeze-drying box and the cold trap is reduced, which affects the sublimation rate; Secondly, the pressure in the system increases, so the sublimation slows down and the product temperature increases. Finally, the product temperature exceeds the eutectic point or disintegration point temperature, the product melts and foams, and freeze-drying fails.
According to the eutectic point temperature of products, some scholars divide the freezer of lyophilizer into several grades: for products with eutectic point temperature of - 15 ℃ and - 30 ℃, the cold trap temperature is - 30 ℃ and - 40 ℃ respectively, and the single-stage compression refrigeration cycle is enough; For products with eutectic point of - 40 ℃, the cold trap temperature is - 60 ℃, and the refrigerator needs two-stage compression refrigeration cycle; For products with eutectic point of - 60 ℃, the cold trap temperature needs to be - 80 ℃, and the refrigerator needs cascade refrigeration cycle.
