Analysis of Low Temperature Vacuum Drying of Maize

Analysis of Low-temperature Vacuum Drying of Maize Ding Xianyu (National Institute of Grain Reserves Zhengzhou Institute of Scientific Research and Design, Zhengzhou, Henan Province 450053 billion t, of which maize production accounts for 25%, corn production in China's northeast and Inner Mongolia 4 provinces and regions account for 30 of the country's corn production %, that is, about 30 billion tons. Most corn in the northeastern region is a late-maturing variety of autumn grain. The moisture content at the time of corn harvest is generally 28% to 35%, and the corn moisture content at the time of acquisition is still 23% on average -30% due to regional natural conditions. Due to the climate characteristics, the freezing time is early, and these high-moisture corns have no time to dry naturally, relying on mechanical drying methods to reduce the moisture content of corn to below 14%, so as to achieve the requirements of safe storage.Because the mechanical drying precipitation method is convenient and fast, and saves a lot of manpower. , Reduce post-harvest loss of food, free from climate constraints, and make grain dryers widely used and developed throughout the country, especially in the three eastern provinces and the Inner Mongolia Autonomous Region. There are about 2 000 grain dryers in four provinces and regions in the northeast, according to At present, the average life expectancy of China's grain dryers is 10 - and the annual demand for grain dryers in China is around 200 units. At present, these grain dryers have some problems in the production and application to varying degrees. The processes vary from manufacturer to manufacturer, such as poor performance of the dryer and deterioration of grain quality after baking. The obvious performance of these grain dryers is the rate of grain cracking after baking. High, generally about 18%, high to reach 35%, these high-ripple corn in the subsequent delivery process is very easy to break through, and some corn after drying after the delivery of the broken rate of 20%. The main cause of cracking is the hot air dryer system currently used extensively in the dryer market in China.

For conventional hot air dryers, the temperature of the hot air coming into contact with the dryer and corn is between 120-180. The ambient temperature during the drying season is generally between -40-10. The temperature difference between hot blast and corn is as high as 130-. 200. During the drying process of hot air, high-temperature hot air warms up the surface temperature of corn rapidly, which leads to the rapid vaporization of moisture on the surface of corn kernels. The process of the transfer of tissue water inside corn kernels to the surface is an infiltration process. This process is relatively slow. In this way, a moisture gradient is formed between the surface and internal structure of the corn kernels, the water film in the boundary layer will be broken, and local dry cracking will occur on the surface of the corn kernels, and the surface will harden. This is the reason that the high temperature dry corn has a high creep rate. s reason.

Low-temperature vacuum drying reduces this gradient.

The low-temperature vacuum drying technology of corn is based on the principle that the moisture evaporation temperature of the surface of the grain varies with the environmental pressure. The vacuum jet vacuum technology is used to form a space with a certain degree of vacuum, and the maize with high moisture content is continuously subjected to vacuum at a low temperature. Dehydration drying process.

To achieve low-temperature vacuum drying, must have two conditions: 1 under vacuum conditions; 2 a certain temperature environment. These two conditions are mutually restrictive. If the degree of vacuum does not meet the requirements, the corresponding drying temperature requirement is high. Under one atmospheric pressure, it is equivalent to hot air drying; the lower the vacuum degree, the lower the required temperature is in the drying process. During the process, the destruction of the internal structure of the corn kernel is small, that is to say, the quality after baking is good.

So how can we achieve this vacuum? At present, there are two ways to create vacuum technology either internationally or domestically: %1 mechanical pumping vacuum, 2 using jet pumping vacuum technology. Both methods have their own advantages and disadvantages. The advantage of mechanical pumps is that they only require power. However, at present, there is a relative shortage of electricity in our country. The electricity price is relatively high. Therefore, using electricity as the energy source of mechanical pumps, the drying cost makes it difficult for the grain warehouses to accept them. Due to the fact that the current technology of mechanical pumping in China is not closed, most of them need to use imported equipment, and the one-time investment is relatively high. Therefore, mechanical pumping vacuum technology is difficult to promote.

So at present we can only use jet pumping vacuum technology, its advantages are reliable performance, low cost, low energy consumption, but the inadequacies are based on coal as energy, equipped with the appropriate boiler and environmental protection equipment. Vacuum jet media are usually mercury and water vapor or non-volatile organic oils. Here's a brief introduction to the working principle of steam vacuum jet pump.

The steam-type vacuum jet pump consists of two conical tubes, one of which is constricted and the other is diffused. The middle throat portion is intermittent and is connected to a vertical tube, as shown. The working principle of the pump: Steam flows through the contraction section of the T tube to generate a large flow rate, so that most of the pressure of the air flow can be converted into kinetic energy, so that the pressure at the outlet of the nozzle is reduced to form a vacuum, and the flow surface at the inlet is taken as 1 -1, the steam at the injection nozzle is 2-2. The steam to be taken is analyzed according to conservation of energy. If the unit mass of steam source is when the steam flows to 2-2 the degree of vacuum pz is P11 is the pressure on section 1-1 And flow rate; P22 is the pressure and flow rate at section 2-2; steam density; the amount of steam per unit time.

From this, it can be seen that the ejection of steam from the nozzle is related to the diameter of the nozzle, and the smaller the diameter d2 of the nozzle is, the higher the degree of vacuum generated is.

The economic benefits of low-temperature vacuum drying are mainly the recovery losses from corn drying. The low-temperature vacuum dryer is popularized in the northern regions and can replace the high-temperature hot air dryer. Each set of high-temperature hot air dryer output 300t / d, according to the average operation 120d / a, dry corn after drying 36 corn from the grain warehouse shipped to the port, the crushing rate is generally about 10%, each set of drying facilities every year caused by broken corn 3 Loss of corn and other losses 0.02 yuan / kg, the loss is = 36000x2000x0.02) = 1.44 million yuan. On the other hand, 5% of the crushed corn flour was lost with dust, and the price per ton of corn was calculated at 900 yuan, and a set of drying equipment lost 36,000 x 5% x 900 = 1.62 million yuan. The two losses are: 144+162=306 million yuan/year. In the northeast region, there are 2,000 dryers of various types, and about 970 sets of 300t/d dryers are used. The total economic loss is about 2.968 billion RMB/a. If the low-temperature vacuum drying technology and equipment are widely applied, these losses will be recovered. From this it can be seen that it is possible and very necessary to promote vacuum low temperature corn dryers.

(Continued from page 7) Building roof openings, stairways, hanging holes, and other reserved openings shall be made of cement mortar, masonry, cement mortar, and a 120mm high retaining line to prevent subsequent process construction. Contamination of ceilings, beams, columns, etc.

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8 Quality Assurance System Establish a project quality assurance system. In order to ensure the continuous and effective operation of the project quality assurance system and ensure the achievement of project quality objectives, we must establish and improve the quality organization of the project department according to the actual situation of the project, clarify its quality responsibilities, establish a rewards and punishments reward mechanism, and give full play to all employees. Participate in the initiative of project quality management, continuously improve and improve the quality of the project, in order to facilitate the implementation of quality objectives and the full-scale management of quality. In addition, the project department and professional construction team shall set up full-time quality inspectors to work in class, check at any time, and implement internal quality control.

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