Generally, we need to dry the materials under normal pressure by heating. When the materials are heated, their color, fragrance, taste and nutrition will be lost to some extent. If vacuum pump is used for vacuum drying, some materials that are prone to oxidation and other chemical changes in the drying process can better maintain the original characteristics and reduce the loss of quality due to the isolation of air under negative pressure.
The purpose of drying is to place the dried material in a closed drying chamber, and then vacuum the dried material. At the same time, heat the dried material properly and continuously to make the moisture in the material diffuse to the surface through the pressure difference or concentration difference. The moisture gets enough kinetic energy on the surface of the material. After overcoming the attraction between molecules, it escapes to the vacuum The low-pressure air in the chamber is removed by the vacuum pump.
There are various ways of vacuum drying, which can be divided into boiling point and melting point.
In the process of vacuum drying, the pressure in the drying chamber is always lower than the atmospheric pressure, the number of gas molecules is small, the density is low, and the oxygen content is low, so it can dry materials that are easy to oxidize and deteriorate, inflammable and explosive dangerous goods, etc. It can play a certain role in disinfection and sterilization of drugs, food and biological products, and can reduce the chance of material contamination or inhibit the growth of some bacteria.
In the process of vaporization, the temperature of water is directly proportional to the vapor pressure, so the water in the material can vaporize at low temperature during vacuum drying, which can realize low temperature drying. This is beneficial to the drying of heat sensitive materials in some drugs, foods and agricultural by-products.
The main factors affecting vacuum drying are: relative density of concentrate, vacuum drying temperature, vacuum degree of vacuum pump, vacuum drying time, etc. The relative density of natural material extraction concentrate should be controlled as 1.30-1.35 (60 thermometric) thick paste; the vacuum drying temperature is generally not higher than 70, usually controlled at about 60; the vacuum degree of vacuum drying is generally controlled at about 0.08mpa, the drying speed will be very slow when the vacuum degree is too low, and the material with too high vacuum degree is prone to splash; the vacuum drying time is generally based on the degree of complete drying, or water content When the content is controlled, the drying time of different materials varies greatly.
In addition, when the thick materials such as extract are dried, the amount of trays should not be too much, so as to avoid bubbles overflowing out of the tray, polluting the dryer, wasting materials and affecting the drying effect; during operation, the valve on the vacuum pipeline of the vacuum pump should be opened slowly, otherwise, bubbles overflowing may occur easily.
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