1. Industrial application of microwave heating drying
In the 1970s, a subsidiary unit of China's electronics industry established a microwave application research institute to promote microwave heating technology. Some military enterprises have successfully developed 5kw, 10kw, and 20kw magnetrons and speed control tubes, and trial produced 20kw and 40kw industrial microwave ovens, which have been applied in industrial sectors such as wood, food, medicine, chemical industry, and ceramics, achieving high economic benefits.
(1) Microwave quick drying of food
Drying vegetables into "dried vegetables" with a moisture content of less than 20% using microwave heating is more than 10 times more efficient than traditional methods. Drying seaweed food with microwave heating has little difference in freshness compared to fresh seaweed. Microwave drying of fried foods not only saves fuel, 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 based fortifiers, salt, seasonings, and expansion agents, stirred, molded, and pre dried, and then microwave heated to produce carbon dioxide and water vapor to obtain loose textured food. Cut the apple into slices, pre dry it (with a moisture content of 92% to 20%), then microwave dry it and rapidly cool it to obtain apple crisps with the same delicious taste as fresh apples.
(1) Microwave cooking and processing of meat
Cooking meat with microwave heating saves an average of 63% energy, and the nutritional content of meat is not affected. If a 2.2kw microwave oven with a frequency of 2450MHz is used for grilling meat, it only takes 2.8 minutes, and the meat bones can reflect microwaves, so the interior of the meat is easy to cook through.
(2) Microwave sterilization and preservation of food
80% of urban households in our country have household microwave ovens, and few people know that heating leftovers in a microwave for 1-2 minutes will usually kill off bacteria.
(3) Microwave heating, drying, and sterilization of medicinal herbs
Chinese herbal medicine is a national treasure of China, but whether it is decoction pieces, water pills, or honey pills, their export for foreign exchange is restricted due to their high content of miscellaneous bacteria. Everyone knows that traditional Chinese medicinal materials are the roots, stems, and leaves of plants. After harvesting, it is sun dried, washed and dried in an old-fashioned drying room, resulting in a high amount of miscellaneous bacteria, reaching 50000 to 100000 per gram. The amount of miscellaneous bacteria irradiated with cobalt-60 in traditional Chinese medicine factories has decreased to around 10000. Drying with microwave heating not only shortens the time significantly (for example, traditional honey pill drying rooms require 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), without affecting the composition of traditional Chinese medicine. Pills have good color and low shrinkage, especially herbs.
(4) Microwave heating for drying ceramic embryos
Using microwave two-stage continuous drying of ceramic bodies, the drying time is reduced by 2/3 compared to the old method (from 1.5 hours to 0.5 hours), and the moisture content of each part of the ceramic body is uniform, with no wrinkles on the surface. High yield and good quality of glazed and fired products.
(5) Microwave heating for drying wood
A high-tech enterprise in Nanjing provided many sets of microwave heating and drying equipment for timber enterprises in Northeast China. A 915MHz, 60kw tunnel type microwave dryer was used to heat and dry redwood, birch and other materials, 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 vivid 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 secondary dust collection, which often uses bag filters or wet scrubbing dust collectors. The washing water should be recycled and not directly discharged, otherwise it will cause secondary pollution to the environment. If pre drying is used in the early stage (controlling the moisture content at around 15%) for granulation (particle diameter according to the product size), and then microwave heating is used to dry the particle dye, uniform drying products 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, thus obtaining the required crystal form for the product.
2. Microwave heating dryer
The microwave heating dryer is mainly composed 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 be high-power speed regulating tubes, magnetrons (5kw, 10kw, 20kw) or low-power microwave tubes (1kw, 2kw). By adopting a multi-source energy feeding method, both high-power and low-power microwave tubes can be designed and manufactured to produce high-power microwave dryers. If two 20kw magnetrons are used to feed microwave energy from two coupling holes to the drying chamber, a 40kw dryer can be obtained. If 20 2kw microwave tubes are divided into 20 coupling holes to feed energy to the drying chamber, a 40kw microwave heating dryer can also be obtained. Both methods have their own advantages and disadvantages for obtaining high-power dryers, but using low-power tubes with multiple ports for energy feeding has the following advantages: low-power tubes are inexpensive, easy to repair, and single tube damage can be replaced without stopping the machine.
The drying chamber (or microwave resonant cavity) of microwave heating dryer can be made into various types, mainly including multi-mode microwave oven (rectangular, circular standing wave cavity), single-mode resonant cavity, traveling wave cavity, etc. Design different types of drying chambers based on the shape of the heated and dried material, such as traveling wave cavities for filamentous, fibrous, and paper materials, electromagnetic waves propagating straight or tortuous in the heater, flat plate, V-shaped waveguide, ridge arch waveguide, etc. The drying chamber for materials such as powder, paste, and particles can be designed as a box or tunnel microwave dryer. Regardless of the type of microwave chamber, its advantages are measured by its inherent quality.
After passing the characteristics
In the formula, V is the volume inside the cavity, m3; S is the surface area inside the cavity, m2; δ， Skin depth, m, δ=/π f μ σ, μ magnetic permeability, H/m。 This formula tells us that the Qo value of the resonant cavity can be estimated based on the material, shape, and size used. Qo is a fundamental parameter for measuring cavity loss, while the resonant frequency fo is a fundamental parameter for the resonant characteristics of the cavity. The characteristic impedance ξ o is a fundamental parameter for measuring the electric field strength on a reference surface inside a cavity.
It should be emphasized here that fo, Qo, and ξ o are all related to 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 chamber, 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, microwave heating drying devices have small volume, simple structure, high efficiency, and easy operation and maintenance. Why is the one-time investment for microwave heating dryers relatively high? The main reason is the high investment in microwave sources, such as a 5kW microwave source and control cabinet that requires (30000 to 50000 yuan), and a 10kW microwave source that requires (50000 to 70000 yuan). With the development of the electronics 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 dryers with low-power microwave tubes for multi-source energy feeding will reasonably decrease.
3. Conclusion
This article proposes a new process route for powder processing, which involves pre drying the powder in the early stage, controlling the final moisture content at 10%~15% - then rolling and granulating this loose wet material - and drying the granular material using microwave heating method. Through the analysis of the entire text, this process route has the following characteristics:
(1) The entire process did not result in severe dust flying or 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 the constant speed drying stage, which is controlled by external factors. Therefore, simply changing external variables such as hot air consumption, medium temperature, strong heat transfer, etc. can accelerate drying (of course, to prevent material defects). In the later stage of particle drying, microwave heating method is used. Heat is generated simultaneously inside and on the surface of the material particles, and the internal moisture evaporates 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 kinetic conditions of heat and mass transfer. That is, the internal moisture of the material is rapidly migrated to the surface and transferred to the air by the 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 into granules, 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 as a one-stop production process, facilitating automated production and management. It is suggested that relevant national units consider promoting the application.
