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Introduction of the main drying stage of a mini vacuum freez
Open the vacuum pump, creating a vacuum environment, triggering sublimation.
Acid or other volatile solvents containing corrosive substances can not be dried with this machine.
Special attention should be given to the use of azide compounds because they can explode with nodules of copper or other non-ferrous metals.
Once the water of the frozen material begins to sublimate, it will absorb the heat and cause the temperature of the material to drop.
The pressure in the drying chamber rises with sublimation and must be reduced by the condenser.
The duration of the main drying process mainly depends on:
(1) the thickness of the material
(2) The amount of heat delivered to the substance during the drying process
(3) The pressure in the drying chamber during drying
Increase the pressure inside the drying chamber, you can increase the rate of sublimation, shorten the drying cycle.
The residual moisture in the dried material depends mainly on:
(1) the material temperature of the final drying stage
(2) the degree of vacuum that can be achieved in the last drying stage
(3) When the temperature of the material is almost the same as the temperature of the separator (the temperature of the separator and the material is about 3K to 5K), the main stage of drying is over. As the adsorbed water of the material begins to be expelled, the final stage of drying begins with analytical drying.
During the freeze-drying process, the area where the material sublimates gradually shrinks from the surface to the inside, so that the water vapor generated by the sublimation needs to pass through the dried material layer to reach the outside of the material. So in the drying process, the sublimation resistance is increasing. The drying curve therefore depends mainly on the latent heat of sublimation and on the rate of water vapor transport. In order to increase the thermal conductivity of the object to be dried and to minimize the volume of water vapor, the drying conditions should be as close as possible to the freezing point (eutectic point).
The drying time depends very much on the degree of vacuum in the drying process. Along the vapor pressure curve on ice, the closer the vacuum is to the freezing point, the shorter the drying time.