Vacuum drying characteristics
2017-05-10
Vacuum drying process is to be dried material placed in a closed drying room, vacuum system with the vacuum at the same time the dry material is heated, so that the water inside the material through the pressure difference or concentration difference spread to the surface, water molecules in the material The surface to obtain enough kinetic energy, to overcome the mutual attraction between molecules, the escape to the vacuum chamber of low pressure space, which was vacuum pump pumped away the process.
In the vacuum drying process, the drying chamber pressure is always lower than the atmospheric pressure, gas molecules, low density, low oxygen content, and thus can easily oxidize the deterioration of the material, flammable and explosive dangerous goods. Drugs, food and biological products can play a certain role in disinfection and sterilization, can reduce the chance of bacteria infection or inhibit the growth of certain bacteria.
Because the temperature in the vaporization process is proportional to the vapor pressure. So the vacuum drying material in the water at low temperatures can be vaporized, can achieve low temperature drying. This is beneficial for the drying of heat-sensitive materials in certain pharmaceuticals, foodstuffs and agricultural and sideline products. For example, more than 70 ~ C part of the sugar ingredients will become brown, reduce the product value of goods; vitamin C over 40 ℃ on the decomposition, change the original performance; protein denatured at high temperatures, changing the nutritional content of materials. In addition, drying at low temperatures, the utilization of heat energy is reasonable.
Vacuum drying eliminates the need for surface hardening in the case of atmospheric pressure drying. Atmospheric pressure hot air drying, in the dry material surface to form a fluid boundary layer, heated vaporization of water vapor through the fluid boundary layer to the air diffusion, dry moisture inside the surface to move. If the speed of its movement can not catch up with the evaporation velocity of the boundary layer, the boundary layer water film will break, the dry material surface will appear local dry crack phenomenon, and then extended to the entire outer surface, the formation of surface hardening. Vacuum drying between the material and the pressure difference between the larger, under the action of pressure gradient, the water quickly moved to the surface, no surface hardening. At the same time can improve the drying rate, shorten the drying time, reduce equipment operating costs.
Vacuum drying can overcome the solute dispersion caused by hot air drying. Hot air drying causes the inside of the dried material to form a large temperature gradient, causing some of the ingredients in the dried material to emerge. Especially food, will lose the aroma, affecting its taste. Vacuum drying of the material inside and outside the temperature gradient is small, by the reverse osmosis of the water as a solvent to move alone, to overcome the solute loss phenomenon. Some of the dry material contains valuable or useful material composition, need to be recycled after drying; and some dry materials contain harmful to human health, toxic and harmful substances, dry exhaust does not allow direct discharge to the space environment In the need to focus on processing. Only vacuum drying can easily recover these useful and harmful substances, and can do a good seal. From the sense of environmental protection, it was said that vacuum drying for the "green dry."
Vacuum drying is easy to achieve product diversification. To food drying, for example, you can control the temperature and vacuum, the product foam, puffing, to produce crisp, instant food and other food.
Vacuum freeze-drying is the vacuum before drying the material to the eutectic point of the material below the temperature, until the material is completely frozen and then vacuum drying. Therefore, vacuum freeze-drying in addition to the characteristics of the vacuum drying, but also because of the pre-frozen when the material fixed shape, the formation of a solid skeleton, after drying to maintain the shape of the basic unchanged, the material composition within the material distribution, Material structure, easy to absorb water. For the food materials, vacuum freeze-drying (hereinafter referred to as freeze-dried), to maintain the material color, smell, taste, nutrients, shape basically unchanged; for pharmaceutical materials, to ensure that the same effect, quantitative and accurate, Biological products, live bacteria, live poison, in vitro biological tissue materials, freeze-dried to maintain its activity, in order to facilitate the application. The moisture content of the freeze-dried material can be very low, to facilitate long-distance transport and long-term storage.
Although the vacuum drying equipment has many advantages, but it needs to configure the vacuum system, large-scale continuous production equipment also need to set the import and export parts of the vacuum dynamic seal structure. As a result of this structural components, prone to leakage, the use of the process requires regular inspection, timely replacement, to add trouble to the user. Vacuum drying equipment also has a series of shortcomings such as high manufacturing cost, high operating cost, high price of dried products, especially higher prices of freeze-dried products. However, due to its unique drying characteristics, a lot of materials have to use vacuum drying process.
--------- Excerpt from "vacuum drying" Author Xu Chenghai Zhang Shiwei Guan Kuizhi
