Discussion on the new process route of powder drying granulation microwave heating (2)

Industrial application of microwave heating drying

Microwave heating in the 1970s, China's electronic industry subordinate units set up the microwave application research institute to promote microwave heating technology. Some military enterprises have successfully developed 5kW, 10kW, 20kW magnetrons and speed regulating tubes, and trial produced 20kW and 40kW industrial microwave ovens, which have been applied in wood, food, medicine, chemical industry, ceramics and other industrial sectors, and achieved higher economic benefits.

(1) Microwave quick drying of food

Using microwave heating method to dry vegetables into "dried vegetables" with water content less than 20%, which is more than 10 times higher than the traditional method. There is little difference between the fresh seaweed and the fresh seaweed when the seaweed food is dried by microwave heating. The final drying of fried food by microwave can not only save oil, but also obtain light and delicious food with low oil content, such as instant noodles.

(2) Microwave puffing drying of food and fruit

Protein based raw materials are mixed with starch fortifier, salt, seasoning and swelling agent, stirring, molding, pre drying, and then microwave heating to produce carbon dioxide and steam to obtain loose texture food. The apple was cut into slices, pre dried (the moisture content was from 92% to 20%), then dried by microwave heating, and then quenched, the crisp apple with the same taste as fresh apple was obtained.

(1) Microwave cooking of meat

Using microwave heating to cook meat, the average energy saving is 63%, and the nutrition of meat is not affected. For example, it takes only 2.8 minutes to roast meat in a 2.2kW microwave oven with 2450MHz frequency. The meat bone can reflect microwave, so the meat is easy to be cooked.

(2) Microwave sterilization and preservation of food

80% of the families of urban residents in China have microwave ovens. Few people know that if the leftovers are heated in the microwave oven for 1 ~ 2min, the common bacteria will be killed.

(3) Drying and sterilization of medicinal materials by microwave heating

Chinese herbal medicine is China's national treasure, but whether it is decoction pieces, water pills, honey pills, because of the high content of miscellaneous bacteria, so the export of foreign exchange is restricted. As we all know, Chinese herbal medicines are roots, stems and leaves of plants. After picking, they are sun dried, washed and dried in the old style drying room, so the number of miscellaneous bacteria is high, reaching 50000-100000 per gram. The amount of bacteria irradiated by cobalt 60 in Chinese medicine factory decreased to about 10000. Microwave heating drying not only shortens the time (for example, it takes 6 hours in the old style drying room, but also takes only 22 minutes for continuous microwave drying), but also reduces the number of bacteria by an order of magnitude (10000 bacteria can be reduced to about 1000), and the components of traditional Chinese medicine are not affected. Pills have good color and small shrinkage, especially herbal medicine.

(4) Microwave drying of ceramic embryo

The results show that the drying time is shortened by 2 / 3 (from 1.5h to 0.5h) by using microwave two-stage continuous drying, and the moisture content of each part of the ceramic body is uniform, and there is no crease on the surface. Glaze, firing high yield, good quality.

(5) Microwave drying of wood

A high-tech enterprise in Nanjing provided many sets of microwave heating and drying equipment for Northeast Wood enterprises. The 915MHz, 60kW tunnel type microwave dryer was used to heat and dry mahogany and birch, and the gratifying results were obtained.

(6) In the field of drying dyes, polymers and other materials, microwave heating drying technology is very useful

According to the data analysis, some dyes and their auxiliaries in the drying process environmental protection has become a prominent problem, this kind of material particle size is small (< 100 μ m) and has a bright color, even if a small amount of dust flying, will also cause obvious pollution to the environment, so the dust collection in the exhaust gas of the drying process should be careful, generally requires secondary dust collection, and the second stage often uses bag filter Or wet scrubber, washing water should be recycled, not directly discharged, otherwise it will cause secondary pollution to the environment. If we use pre drying (control the moisture content at about 15%) to granulation (the particle diameter is based on the commercial particle size) and then microwave heating drying the granular dyes, we can get uniform drying products, and improve the physical properties such as color, solubility, diffusion and adsorption. Because microwave heating drying can strictly control the drying temperature and time, so as to obtain the crystal form needed by the commodity.

Microwave heating dryer

The microwave heating dryer is mainly composed of microwave source control cabinet, resonant cavity (drying chamber), transmission device, dehumidification fan, etc. Various forms of materials can be heated and dried intermittently or continuously. Microwave source can be high-power speed regulator, magnetron (5kW, 10kW, 20kW) or low-power microwave tube (1kW, 2kW). A high power microwave dryer can be designed and manufactured by means of multi-source energy feedback. If two 20kW magnetrons are used to feed microwave energy from two coupling holes to the drying chamber, a 40 kW dryer can be obtained. For example, 20 2kW microwave tubes are divided into 20 coupling holes to feed energy to the drying chamber, or 40 kW microwave heating dryer can be obtained. These two methods have advantages and disadvantages, but the use of multi port energy feedback of small power tube has the following advantages: low price of small power tube, easy maintenance, single tube damage can be replaced without shutdown.

The drying chamber (or microwave cavity) of microwave heating dryer can be made into various types, mainly including multi-mode microwave oven (rectangular, circular standing wave cavity), single-mode resonator and traveling wave type. According to the shape of the materials to be heated and dried, different types of drying chambers are designed, such as filament, fiber and paper, traveling wave cavity, electromagnetic wave propagating straight or zigzag in heater, flat plate waveguide, V-shaped waveguide, ridged waveguide, etc. The drying chamber of powder, paste and granule materials can be designed into box type and tunnel type microwave dryer. No matter what kind of microwave cavity, its advantages are measured by its inherent quality.

Where V is the cavity volume, m3; s is the cavity surface area m2; δ is the skin depth, m, δ = / π f μ σ, μ permeability, H / m. The formula tells us that the Qo value can be estimated according to the material, shape and size of the resonator. Qo is the basic parameter to measure the cavity loss. Another parameter, resonance frequency fo, is the basic parameter of cavity resonance characteristics. The characteristic impedance ξ o is a basic parameter to measure the electric field intensity on a reference plane in the cavity.

What should be emphasized here is that fo, Qo and ξ o are all for certain oscillation modes, and the values of basic parameters of different modes are generally different. Metal conductor materials, such as stainless steel and aluminum, are required for the resonant cavity.

Transmission device in the role of microwave heating dryer, mainly conveying materials in and out of the drying chamber, such as conveyor belt, medium cylinder, etc., all need to be made of insulator materials, such as PTFE, quartz glass, 95 porcelain, etc.

The microwave heating dryer is equipped with a yoke at the inlet and outlet of the material entering and discharging the drying chamber, and the leakage amount is less than 2MW / cm2.

In a word, the microwave heating drying device has the advantages of small volume, simple structure, high efficiency and convenient operation and maintenance. Why is the one-time investment of microwave heating dryer higher? It is mainly due to the large investment in microwave sources, such as (30000-50000 yuan) for 5kW microwave source and control cabinet, and (50000-70000 yuan) for 10kW microwave source. With the development of electronic industry, small power microwave source will achieve the purpose of large output, advanced manufacturing technology, low price, high efficiency and long service life. The price of microwave heating dryer with multi-source energy fed by low-power microwave tube will decrease reasonably.

 Conclusion

In this paper, a new process for powder treatment is put forward, that is, the powder is pre dried in the early stage, and the final moisture content is controlled at 10% ~ 15% -- then the loose wet material is granulated by roller pressing and the granular material is dried by microwave heating method. Through the full text analysis, this process has the following characteristics:

(1) There is no serious dust flying and environmental pollution in the whole process.

(2) The drying rate is high, the moisture removed in the early stage is free water, and the drying is in the constant speed drying stage, that is, the drying stage controlled by external factors. Therefore, the drying can be accelerated only by changing the external variables, such as the amount of hot air, medium temperature, strong heat transfer, etc. In the later stage, the microwave heating method is used to heat the particles and the surface at the same time, and the internal moisture vaporization forms a pressure gradient, which promotes the internal moisture to migrate to the surface and accelerates the drying. Although the process is controlled by the internal conditions, the characteristics of microwave heating change the dynamic conditions of heat and mass transfer, that is, the moisture in the material in the pressure gradient, positive temperature gradient and humidity gradient are jointly pushed forward Under the action, it migrates to the surface rapidly, and transfers to the air, and is discharged by the dehumidification fan.

(3) The energy-saving and loose wet material is granulated by roller press, which is easy to granulation, uniform, high strength and low energy consumption. The methods of early drying and later drying are scientific and reasonable, and the energy is efficiently utilized.

(4) Drying granulation drying can arrange one-stop production, which is convenient to realize automatic production and management. It is suggested that the relevant units of the state should consider the popularization and application.