The vacuum drying process is to place the dried wet material in a closed drying chamber, and vacuum the vacuum system to continuously heat the material to be dried, so that the moisture inside the material is vaporized and diffused to the surface through a pressure difference or a concentration difference. The molecules obtain sufficient kinetic energy on the surface of the material to escape the low pressure space of the vacuum chamber after overcoming the mutual attraction between the molecules, thereby being evacuated by the vacuum pump.
In the vacuum drying process, 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 that materials which are easily oxidized or browned, and flammable and explosive dangerous goods can be dried. It can play a certain role in disinfection and sterilization of medicines, foods and biological products, which can reduce the chance of material contamination or inhibit the growth of certain bacteria.
Because the temperature of water during vaporization is proportional to the pressure or saturated vapor pressure (vaporization boiling), for example, the atmospheric pressure of 0.1 MPa in the plain area is vaporized at 100 ° C, and the atmospheric pressure of 0.03 MPa at the Everest atmosphere is boiling at 70 ° C. Therefore, when the vacuum is dried, the moisture in the material can be boiled at a low temperature, and low temperature drying can be achieved. This is advantageous for the drying of heat sensitive materials in pharmaceuticals, foods, agricultural by-products, and chemical products. For example, when the sugar liquid exceeds 70 ° C, some components will become brown, which will reduce the commercial value of the product: when the vitamin C exceeds 40 ° C, it will decompose and change the original properties; when the starch exceeds 54 ° C, it will begin to gelatinize; the protein will be at a high temperature. The lower denaturation changes the nutrient content of the material, and the beneficial bacteria in the biological product are inactivated when it exceeds 43 °C.
Vacuum drying can eliminate surface hardening and dry cracking which are easy to occur under normal pressure hot air convection drying. Atmospheric hot air convection drying forms a fluid boundary layer on the surface of the material to be dried. The vaporized water vapor is diffused through the fluid boundary layer and the water inside the dry material is moved to the surface. If the moving speed cannot keep up with the evaporation speed of the boundary layer surface, the water film in the boundary layer will be broken, and the surface of the dried material will be locally dry and cracked, and then spread to the entire surface to form a surface hardening and dry cracking. The pressure difference between the inside and the surface of the vacuum drying material is large. Under the pressure gradient, the moisture quickly moves to the surface without surface hardening and dry cracking. At the same time, it can increase the drying rate, shorten the drying time and reduce the operating cost of the equipment. It can also be summarized that the internal and external surfaces of the atmospheric hot air convection dry material have large temperature gradient, large water gradient, small pressure gradient, and surface hardening and dry cracking during drying. The temperature gradient of the inner and outer surfaces of the vacuum dried material is small and the water gradient is Small, large pressure gradient, no surface hardening, dry cracking.
Vacuum drying can overcome the solute loss caused by hot air drying. The convection drying of atmospheric hot air causes a large temperature gradient between the inner and outer surfaces of the material, which promotes some dispersion in the interior of the material to be dried. Especially aromatic materials and foods will lose their aroma and affect the taste of the product. When vacuum drying, the temperature gradient of the inner and outer surfaces of the material is small, and the water as a solvent is moved by the osmosis alone, thereby overcoming the solute loss phenomenon. Some of the materials to be dried contain valuable or useful substances, which need to be recycled after drying. Some of the materials to be dried contain toxic and harmful substances that are harmful to human health. After drying, the exhaust gas is not allowed to be directly discharged into the atmosphere. Need to be processed centrally. Only vacuum drying can easily recover these useful or harmful substances, and it can be sealed well without leakage. In the sense of environmental protection, it should be called green dry!
Vacuum drying can achieve a very low moisture content of <0.03% dry. Atmospheric hot air convection drying makes the internal temperature of the material low and the external surface temperature high, forming a large temperature gradient, the hot air pressure makes the internal pressure of the material low and the external pressure is high, forming a pressure gradient difference, which makes the moisture in the inner center of the material difficult to remove. High temperature, humid environment, especially large particles or multi-layered materials can not achieve low limit water <2% drying process. The inner and outer surfaces of the vacuum dried material have small temperature gradient, small water gradient and large pressure gradient. The moisture in the inner center of the material is quickly removed, and the water vapor density in the vacuum environment is low, which can achieve a very low moisture content of <0.03%.
The heat consumption of vacuum drying is low. Atmospheric hot air convection drying is based on dry hot air as the medium to directly heat the material. The high internal gradient of the material continuously escapes the surface to form high humidity gas. The surface of the high humidity gas and the heat medium dry hot air have a large humidity difference. When the heat medium is removed, a large amount of heat energy cannot be utilized, and it is not possible to recycle and recycle, so that the heat energy consumption is high. Vacuum drying is the heat medium through the heating pipe, the plate indirectly conducts thermal energy, and the far infrared radiant heat energy. The vacuum drying material heats up and boils, and the internal and surface pressure difference is large. Under the pressure gradient, the moisture quickly moves out to form saturated water. The vapor is pumped out of the chamber by a vacuum pump. The vacuum-dried heat medium is cooled and recycled by the heating pipe and the plate to heat the material, so that the heat energy consumption is low.
The inner and outer surfaces of the vacuum dried material have small temperature gradient, small water gradient and large pressure gradient. The moisture in the material is quickly removed under the pressure gradient, and the drying equipment has high efficiency and large production capacity.
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