[[TitleIndustry]]

Analysis of factors affecting drying rate of vacuum freeze dryer

Date:Oct 18, 2019

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Freeze-drying process is a complex heat and mass transfer process, involving refrigeration, vacuum, electronics, chemistry, cryogenic medicine and other disciplines, with high technical content and complex freeze-drying process.In the traditional Chinese medicine, food and other industries, vacuum freeze dryer is one of the popular equipment.

Structurally, the vacuum freeze dryer is a new type of box, which combines the refrigeration system, the vacuum system, the heat conduction oil heating system and the moisture removal system. It can make use of the space in the box to store materials efficiently for freeze vacuum drying.At present, the vacuum freeze dryer is suitable for the drying of raw materials, Chinese medicines, drug intermediates, food and other materials.


Prefreezing rate of materials

It is understood that the pre-freezing of materials is the first thing to do when the lyophilizer is working.The pre-freezing can be divided into fast freezing and slow freezing. The size of the crystal formed during freezing greatly affects the whole drying rate and the dissolution rate of the product after drying.

Specifically, rapid freezing and slow freezing processes have the following differences: rapid freezing produces smaller ice crystals, slow freezing produces larger ice crystals.Large ice crystals are favorable to sublimation, while small ice crystals are unfavorable to sublimation. Rapid freezing results in low sublimation rate and fast desorption rate.Slow freezing results in fast sublimation rate and slow analytic rate.


Accordingly, the industry points out, the speed that prefreeze must hold according to material accurate.

Pressure in a drying room


The industry says that the amount of pressure in a drying room directly affects the rate of heat and mass transfer (water vapor).In terms of mass transfer, lower pressure is better, while higher pressure is better for heat transfer.


Since the mass transfer rate is mainly determined by the temperature and pressure of the sublimation interface and the surface of the dry layer, there are generally two operations to improve the rate of water vapor escape in the dry layer: raising the temperature of the sublimation interface and increasing the pressure of water vapor at the interface.Or to increase the vacuum of the drying chamber, reduce the vapor pressure on the surface of the drying layer.


Form of material

It is reported that in the freeze-drying process, freeze-dried materials are usually divided into solid and liquid form.The bulk density of solids and the composition density of liquids affect the freezing time of materials.


Wet reloading capacity

It is known that when the material is freeze-dried, there is a certain ratio of surface area to material thickness after being put into the container.In the case of drying, the wet reloading load on the drying tray per unit area is an important factor that determines the drying time.


Generally, the thinner the thickness of the material, the faster the heat and mass transfer, and the shorter the drying time.However, if the material is thin, there will be less dry materials per batch per unit freeze-dried area, which will be unfavorable to increase the yield per unit freeze-dried area and per unit time.So need to carry on comprehensive consideration according to material.


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