I microwave drying Application of dehydration technology in industry and agriculture:

(1) Heating and temperature rising of physical properties of materials to dry and dehydrate materials, including temperature recovery and unfreezing of materials.

⑵ Interactions on biological properties of organisms, such as killing bacteria, fungi, insects and eggs, inactivating or activating enzyme activity, and medical or medical surgery.

⑶ The effect on the chemical catalytic properties of materials can accelerate the chemical reaction process, shorten the chemical reaction time, digest, extract, and produce materials such as diamond films by plasma chemical vapor deposition, microwave plasma processing, etc.

It can be said that the microwave drying function is the earliest recognition of microwave application. because microwave heating Unique performance, Microwave heating equipment For domestic cooking Microwave Oven It is gradually expanded to manufacturing industrial microwave heating equipment for industrial production to meet the needs of food cooking, processing, sterilization and fresh-keeping in food processing industries; There are also rubber vulcanization and reclaimed rubber desulfurization; Textile dyeing and fixing; Chemical fiber products shaping, drying and dehydration; Drying and sterilization of leather, paper (including sanitary napkins) and cosmetics; Drying and dehydration of film and other coatings; Drying of tobacco leaves and puffing of tobacco stems; Drying and sterilization of Chinese medicinal materials or products; Sand mold solid drying, graphite materials, ceramics heating and other industries need.

2、 Characteristics and Mechanism Analysis of Microwave Drying

Materials need to be dried and dehydrated for daily storage, transportation or other purposes. Since ancient times, there have been many drying methods for materials. Ideal drying methods are expected to keep or at least not change the original quality of the dried materials. Microwave drying technology has developed rapidly in pursuit of this expectation. Its vigorous vitality lies in its drying mechanism and characteristics different from those of general conventional drying technology.

During the drying process of materials, the drying layer first forms in the inner layer of materials, and then extends outward from the inner layer. The reason is that the microwave energy penetrates into the material and is absorbed, and its energy is instantly converted into heat energy to raise the overall temperature of the material (including the temperature of the moisture contained in the inner material). At this time, the sudden rise of water vapor pressure in the inner layer drives the water vapor to discharge to the material surface. The drying layer first appears in the inner layer of the material and gradually expands to the outer layer.

Conventional heating and drying materials are easy to form dry hard shells on the surface, which not only reduces the quality and appearance of the dried materials (especially food), but also slows the drying process of the inner layer of the materials, extending the overall drying time of the materials. The reason for the formation of hard shell is that the heat transferred from the hot medium to the material causes excessive evaporation of the surface moisture due to the influence of the material's heat conduction capacity, or that the surface is not continuously coordinated with other parts. There are only two solutions: one is to consider the material's heat conduction capacity and not excessively heat the material, and the other is to abandon the heating method that causes the material's surface to form hard shells.

From the point of view of heat transfer dynamics, the temperature gradient of conventional heating for material heating is from the inner layer of material to the outside, while the direction of heat conduction is opposite to the direction of steam migration, which indicates that the temperature difference between the inner and outer layers of conventional heating materials is large, the overall temperature analysis is uneven, and the transfer of heat is blocked by the mass migration, so the overall temperature rise process is delayed, which is commonly known as long heating time. For microwave heating, the material is heated as a whole. The temperature of the material surface is lower than that of the inner layer due to water evaporation. The temperature gradient is from the outer layer of the material to the inner layer, and the direction of heat conduction is the same as that of steam migration. Compared with conventional heating, the overall temperature distribution of the material is more uniform, and the transfer direction of heat and steam is the same, without affecting each other. That is to say, microwave heating has two major advantages: small overall temperature difference and short heating time of materials, thus avoiding the problem of crusting on the surface of materials.

Why is the energy utilization rate of microwave heating high? Improving the energy utilization rate is the long-term goal of the heating equipment technical designers. The general approach is to increase the energy supply - increase the heat supply or increase the temperature of the heating medium. Increasing the heating area of materials can increase the total heat supply, but it may also increase the total surface area of equipment due to the increase of heating area, resulting in the disadvantage of increasing the heat dissipation area of equipment. To increase the temperature of the hot medium, it can only work within a certain limit. If it exceeds this limit, the heat accumulation on the material heating interface will exceed the possible amount of heat conduction to the material inside the interface, which will make the material heated interface hot and burnt, forming a hard shell layer, greatly reducing the drying quality of the material, and also increasing the heat loss of the interface layer to the surrounding environment, Invalid energy consumption increases. This is the inevitable defect of conventional heating methods, including infrared heating, due to the restriction of surface heating conduction mechanism of materials.

If we change a method that is different from conventional heating and can increase the heating area of materials, we can avoid the above disadvantages and meet the requirements of improving energy utilization. This is the charm of microwave heating.

The high energy utilization rate of microwave heating is due to the fact that microwave energy is absorbed by materials, and it is also a feature of penetrating materials. The characteristics of microwave penetration and absorption of materials lead to two results when materials are heated:

(1) Forming the overall heating of materials is equal to increasing the heating area of materials, not limited to the surface interface of materials.

⑵ According to the electromagnetic field theory, when infrared heating, the material has a selective absorption peak for infrared (including far-infrared) spectral band absorption. The so-called absorption peak is that the material absorbs infrared wave strongly at the peak, that is, it absorbs much more energy than other places. For example, the infrared absorption peak wavelength of starch material is 3.9 μ m. Sugar is 3.7 μ M. Their wavelengths fall in the far outer band of the infrared spectrum. These facts are as follows: only the infrared energy at the absorption peak of the infrared spectrum emitted by infrared devices is used. Others are not absorbed and utilized by materials. Therefore, the selection of infrared heating elements in the infrared heating equipment with infrared heating method not only needs to consider the type and object of materials, but also needs to select the infrared heating elements that generate the infrared spectrum bands that can be absorbed by the materials. Even so, most of the energy has not been well used because it is the heat generation property of the surface heat conduction of materials.

However, this disadvantage does not exist when microwave heating materials. Because the material fully absorbs microwave energy. No wave frequency in the microwave spectrum is limited; There is also no problem of lagging of materials in the inner layer. Therefore, microwave heating is an energy-saving heating mode with high energy utilization and rapid material temperature rise.

It must be pointed out that shortening the heating time of materials will create favorable conditions for energy conservation. As we all know, the total energy consumption of equipment is equal to the product of energy consumption per unit time and time. Therefore, under the same condition of energy consumption per unit time, we expect to select equipment that can make the working time shorter to reduce the total energy consumption, and microwave drying can really save energy and be efficient.