Grain Microwave Drying Machine
In recent decades, China has made great progress in the research and development of grain drying technology and equipment. However, with the increase of energy shortage, the disadvantages of energy consumption in China's grain drying equipment are becoming more and more prominent, exploring new types of low-energy grain drying. Technology seems imperative. At present, research on biomass fuel, solar energy drying, and infrared drying are actively carried out. Microwave drying equipment is a new type of drying equipment developed in recent decades. It has been widely used in food, chemical, metallurgical and other fields, but research in the field of domestic grain drying is still in its infancy. Grain microwave drying machine has the advantages of high speed, high efficiency and low energy consumption. Therefore, actively exploring the microwave drying of grain and in-depth study of the application form of microwave in grain drying will have broad prospects and practical significance.
1 Introduction to microwave drying mechanism
The microwave is a high-frequency electromagnetic wave having a wavelength of 0.001 m to 1 m and a frequency of between 3.0 x 102 and 3.0 x 105 MHz. In reality, the grain material is mainly composed of polar molecules. In the microwave field, due to the charge “same-rejection, opposite-sex attraction”, the molecules will produce corresponding movements, and its vigorous movement promotes interaction between molecules. Thus, the thermal motion of the molecule is intensified, and the macroscopically appears to be an increase in the temperature of the material. As the external microwave frequency increases and the electric field strength increases, the thermal motion of the molecules will also increase and the temperature of the material will become higher. For a certain medium or material, the power absorbed by the medium is proportional to the power frequency and electric field strength. The relationship is: where pa is the microwave power of the medium in a microwave field, and the unit is W/cm3; E is the electric field strength. Valid value, the unit is W/cm; f is the frequency, the unit is Hz; ! is the dielectric constant of the medium; tan" is the loss tangent of the medium.Since the material is internally heated in the microwave field, it is easy to form a temperature gradient and a humidity gradient in the same direction, so that the two moisture driving forces are in the same direction, which is favorable for the material moisture to be discharged outward.
2 Characteristics of microwave drying
2.1 Strong penetration
For cereal materials, microwaves have a strong penetrating power. Part of the electromagnetic wave acting on the material is absorbed by the material, and a part of it is deep into the material. The microwave penetration depth D can be calculated by the following equation: where λ is the wavelength; in part, for food, it can be taken as 1, so D≈λ. Therefore, the larger the wavelength λ value, the larger the microwave penetration depth D. The microwave wavelength is between 0.001m and 1m, and the wavelength of the infrared radiation is between 0.75 and 1000μm. Therefore, the microwave has a ratio of infrared to infrared.Much greater penetration.
2.2 Selective heating.
The absorption of microwaves by materials is related to the nature of the materials. The loss factors of moisture, protein, fat and carbohydrates in grains are different from each other, so their absorption capacity for microwaves is also different, that is, different substances are in the same microwave field. Absorbed energy
The amount is different. Among them, water is most sensitive to the absorption of microwaves, and the absorption of microwaves by fats, proteins and carbohydrates is much worse. Therefore, microwave heating and drying have a strong selective absorption.
2.3 Thermal inertia is small
Microwave is a kind of electromagnetic wave, which has the same direct and reflection law as electromagnetic waves. Therefore, the microwave emission and power change can be completed in an instant. Microwave energy can be directly absorbed by the material into heat energy. It does not need any intermediate medium, and the reflection speed is fast. The microwave control in the drying process is convenient and fast. These are all due to the heat habit of microwave heating and drying.
2.4 Fast drying speed and short time
Since the microwave has strong penetrability to the grain material, the whole heating can be achieved, and the surface moisture evaporation of the material takes away the heat, so that the surface temperature is lowered, and the internal temperature of the material is higher than the surface temperature, and the moisture transfer caused by the temperature gradient at this time The driving force is from the inside to the outside. On the other hand, during the drying process, the surface moisture of the material evaporates continuously, the surface humidity is low, the internal humidity is large, and the direction of the moisture transfer driving force caused by the humidity gradient is also from the inside to the outside, and the two moisture driving forces are superimposed to make the water easier to discharge. The temperature gradient of hot air drying is often reversed from the direction of the humidity gradient, which is not conducive to drying.
2.5 Easy to implement automatic control
The microwave radiation power is adjustable, the response is rapid, the thermal inertia is small, and the automatic control is convenient. It can be applied to the change of different process parameters, which is beneficial to improve product quality.
3 Microwave heating system
The microwave heating system is mainly composed of a DC power supply, a microwave tube, a transmission line or a waveguide, a dryer, and the like, and its structural block diagram is shown in FIG. 2 . Microwave tubes and microwave power supplies form a microwave generator, which is a key part of the microwave heating system. The microwave tubes currently used for heating and drying are mainly magnetrons. The function of the waveguide is to transfer the microwave generated by the microwave generator to the microwave dryer. The dryer is a space for material and microwave action. After the material absorbs the microwave energy, it converts into internal energy, and the temperature rises and the drying is realized. Commonly used microwave drying equipment is in the form of tunnel type, flat type, box type and the like.
4 Research progress in microwave drying of cereals
At present, microwave-dried cereals are still being researched and developed at home and abroad, and relevant reports are constantly appearing. Dong Tieyou of Henan University of Science and Technology studied the microwave drying problem of thick brown rice under downstream ventilation. The experimental results show that with the increase of microwave power, the temperature and drying speed of brown rice increase. If the power of the microwave is controlled within the range of 0.05 to 0.09 kW/kg and the wind speed is controlled within the range of 0.12 to 0.20 m/s, quality problems such as bursting and germination rate reduction can be ensured. And the effective dry thickness of brown rice.The degree is about 0.130 m, which is greater than the effective dry thickness of the rice under the same conditions. The efficiency of drying brown rice under microwave heating is higher than that of rice drying.
5 Conclusion
After several decades of development, microwave drying technology has been used more and more in the production field, but microwave drying grain technology is still in the stage of continuous exploration, and there are many technical and economic problems to be solved. Microwave drying technology will be combined with other drying technologies while improving its own technical methods and equipment, such as microwave hot air combined dry grain, microwave infrared dry grain, using the advantages of various drying processes to different in different drying stages of materials. Drying process to achieve high efficiency, low consumption and high quality drying effect.