Industrial application of microwave heating and drying
In the 1970s, a subsidiary unit of China's electronic industry established the Microwave Application Research Institute to promote microwave heating technology. Some military enterprises have successfully developed 5kw, 10kw, 20kw magnetrons and speed regulators, and trial produced 20kw and 40kw industrial microwave ovens, which have been applied in industrial sectors such as wood, food, medicine, chemical, and ceramics, achieving high economic benefits.
(1) Microwave Rapid Drying of Food
Drying vegetables into dried vegetables with a moisture content below 20% using microwave heating is more than 10 times more efficient than traditional methods. Drying seaweed food using microwave heating has little difference in taste compared to fresh seaweed. Deep fried foods are finally dried using microwaves, which not only saves oil but also produces light and delicious foods with low oil content, such as instant noodles.
(2) Microwave puffing and drying of food and fruits
Protein based raw materials are mixed with starch fortifiers, salts, seasonings, and expansion agents, stirred, formed, pre dried, and then heated by microwave to produce carbon dioxide and water vapor to obtain loose texture food. Cut the apples into slices, pre dry them (from 92% moisture to 20% moisture), then microwave dry them, and then rapidly cool them to obtain crispy apples with the same fresh taste as fresh apples.
(1) Microwave cooking and processing of meat
Using microwave heating to cook meat can save an average of 63% energy, and the nutritional content of the meat is not affected. If you use a 2.2kw microwave oven with a frequency of 2450MHz to roast meat, it only takes 2.8 minutes. The meat bones can reflect microwaves, so the inside of the meat is easy to ripen.
(2) Microwave sterilization and preservation of food
80% of urban residents in our country have household microwave ovens, and few people know that heating leftover food in the microwave for 1-2 minutes can generally kill bacteria.
(3) Microwave heating, drying and sterilization of medicinal herbs
Chinese herbal medicine is a national treasure of China, but whether it is herbal medicine, water pills, or honey pills, their export for foreign exchange is restricted due to their high content of miscellaneous bacteria. As we all know, Chinese medicinal materials are the roots, stems, and leaves of plants. After picking, it is sun dried, washed, and dried in an old-fashioned drying room, resulting in a higher amount of miscellaneous bacteria, ranging from 50000 to 100000 per gram. The amount of miscellaneous bacteria irradiated with cobalt 60 by the traditional Chinese medicine factory decreased to around 10000. Using microwave heating for drying not only shortens the time significantly (for example, the old style honey pill drying room takes 6 hours, while microwave continuous drying only takes 22 minutes), but also reduces the bacterial count by an order of magnitude (10000 bacteria can be reduced to about 1000), and the traditional Chinese medicine ingredients are not affected in any way. Pills have good color and low shrinkage rate, especially for herbs.
(4) Microwave heating and drying of ceramic embryos
Using microwave two-stage continuous drying of ceramic bodies, the drying time is shortened by 2/3 compared to the old method (reduced from 1.5 hours to 0.5 hours), and the moisture content in each part of the ceramic body is uniform, with no surface wrinkles. Glazed and fired with high yield and good quality.
(5) Microwave heating and drying of wood
A high-tech enterprise in Nanjing provides many sets of microwave heating and drying equipment for Northeast timber enterprises. The 915MHz, 60kw tunnel microwave dryer is used to heat and dry redwood, birch, and other woods, achieving satisfactory results.
(6) In the field of drying materials such as dyes and polymers, microwave heating drying technology has great potential
According to data analysis, environmental protection has become a prominent issue for some dyes and their additives during the drying process. These materials have small particle sizes (<100 μ m) and have bright colors, and even a small amount of dust can cause significant pollution to the environment. Therefore, the collection of dust in the exhaust during the drying process should be meticulous, usually requiring a second level of dust collection. The second level often uses bag filters or wet scrubbing dust collectors, and the washing water should be recycled without direct discharge, otherwise it will cause secondary pollution to the environment. If pre drying is used in the early stage (controlling the water content to be around 15%) for granulation (particle diameter according to the commercial particle size), and then microwave heating drying is carried out on the particle dye, a uniformly dried product can be obtained, and physical properties such as color, solubility, diffusion, and adsorption can be improved. Because microwave heating drying can strictly control the drying temperature and time, thereby obtaining the required crystal form for the product.
Microwave heating dryer
The microwave heating dryer mainly consists of a microwave source control cabinet, a resonant cavity (drying chamber), a transmission device, a dehumidification fan, etc. Various forms of materials can be heated and dried intermittently or continuously. Microwave sources can use high-power speed regulators, magnetrons (5kw, 10kw, 20kw), or low-power microwave tubes (1kw, 2kw). By using a multi-source energy feeding method, high-power microwave dryers can be designed and manufactured for both high-power and low-power microwave tubes. If two 20kw magnetrons are used to feed microwave energy from two coupling holes to the drying chamber, a 40kw dryer can be obtained. For example, 20 2kw microwave tubes can be divided into 20 coupling holes to feed energy to the drying chamber, or a 40kw microwave heating dryer can be obtained. These two methods have their own advantages and disadvantages, but using low-power tubes with multiple feeding ports has the following advantages: low-power tubes are inexpensive, easy to maintain, and single tube damage can be replaced without stopping the machine.
The drying chamber (or microwave resonant cavity) of a microwave heating dryer can be made into various types, mainly including multi-mode microwave ovens (rectangular, circular standing wave cavities), single-mode resonant cavities, and traveling wave types. Design different types of drying chambers based on the shape of the heated and dried materials, such as filamentous, fibrous, and paper. Traveling wave chambers can be used, and electromagnetic waves can propagate in a straight or tortuous manner in the heater. Flat plate type, V-shaped waveguide, ridge shaped waveguide, etc. The drying chamber for materials such as powder, paste, and particles can be designed as a box type or tunnel type microwave dryer. Regardless of the type of microwave chamber, its inherent quality is used to measure its advantages.
After passing the characteristic
In the formula, V represents the volume of the cavity, M3; surface area of S cavity m2; δ， Skin depth, m. δ=/π f μ σ, μ magnetic permeability, H/m。 This equation tells us that the Qo value of the resonant cavity can be estimated based on the material, shape, and size used. Qo is the fundamental parameter for measuring cavity losses, while the resonant frequency fo is the fundamental parameter for cavity resonance characteristics. The characteristic impedance ε o is a fundamental parameter for measuring the electric field strength on a reference plane within a cavity.
What needs to be emphasized here is fo Qo and zeta are both for certain oscillation modes, and the values of their basic parameters are generally different for different modes. The resonant cavity requires metal conductor materials, such as stainless steel, aluminum, etc.
The role of the transmission device in the microwave heating dryer is mainly to transport materials into and out of the drying room, such as conveyor belts, medium cylinders, etc., which need to be made of insulating materials, such as polytetrafluoroethylene, quartz glass, 95 porcelain, etc.
The microwave heating dryer is equipped with a yoke flow device for microwave leakage at the inlet and outlet of the material entering and exiting the drying chamber, with a leakage rate of less than 2mw/cm2.
In short, the microwave heating and drying device has a small volume, simple structure, high efficiency, and convenient operation and maintenance. Why is the one-time investment of microwave heating dryers higher? The main reason is that the investment in microwave sources is large, such as a 5kW microwave source and control cabinet costing 30000 to 50000 yuan, and a 10kW microwave source costing 50000 to 70000 yuan. With the development of the electronic industry, low-power microwave sources will achieve the goals of high output, advanced manufacturing processes, reduced prices, high efficiency, and long lifespan. The price of microwave heating and drying equipment using low-power microwave tubes with multiple energy sources will be reasonably reduced.
conclusion
This article proposes a new process route for powder treatment, which involves pre drying the powder in the early stage and controlling the final moisture content between 10% and 15%. Then, the loose and wet material is subjected to rolling granulation, and the granular material is dried using microwave heating method. Through full text analysis, this process route has the following characteristics:
(1) The entire process is free from severe dust flying and environmental pollution.
(2) The drying rate is high, and the moisture removed in the early stage of drying is free water. The drying is in a constant rate drying stage, which is controlled by external factors. Therefore, it is only necessary to change external variables such as the amount of hot air, medium temperature, strong heat transfer, etc. to accelerate drying (of course, to prevent material defects). In the later stage of particle drying, microwave heating method is used, which generates heat both inside and on the surface of the material particles. The internal moisture vaporizes to form a pressure gradient, which promotes the migration of internal moisture to the surface and accelerates drying. Although the process is controlled by internal conditions, the characteristics of microwave heating change the heat and mass transfer dynamics conditions. That is, the internal moisture of the material quickly migrates to the surface and transfers to the air under the joint promotion of pressure gradient, positive temperature gradient, and humidity gradient, and is discharged by the dehumidification fan.
(3) Energy saving and loose wet materials are granulated using a roller press, which is easy to form, uniform in size, high in strength, and low in energy consumption. The pre drying and post drying methods are scientifically reasonable, and energy is efficiently utilized.
(4) Drying granulation drying can be arranged in a one-stop production process, facilitating automated production and management. Suggest relevant national units to consider promoting its application.
