Abstract: Through the process analysis and key technology research of vacuum low temperature continuous drying production line, a 300t/d high moisture corn vacuum low temperature continuous drying industrial automation production line was established, and the production line system was tested for productivity and key technology verification. It proves the new technology of vacuum low temperature continuous drying, energy saving, high efficiency, green environmental protection and good drying quality. A new way to solve the problem of food drying has been explored, which has pointed out the direction for the development of green drying technology in China.
Key words: vacuum low temperature continuous drying green drying new way of energy saving
introduction
At present, improving the drying capacity and dry quality of grain is an urgent problem for China's agriculture. The seasonality of corn drying is very strong, and only mechanized and efficient operation can ensure the storage and circulation requirements in an appropriate time. In view of the characteristics of high-moisture corn, research and design of drying process and production line, development of a high-efficiency, energy-saving, environmentally friendly and easy-to-use grain drying device with industrial automation production has become an urgent need for food production development.
Due to the breakage rate and crack rate of corn, the safe storage and operation of the “four-distribution†circulation system have been seriously affected. At present, corn drying faces a new era and new tasks of vigorous development. Drying equipment must adapt to new market requirements. It is urgent to adopt high-tech upgrading to meet the needs of the new era. "High scientific and technological content, good economic returns, low resource consumption, and low environmental pollution. "Product demand. Solve the problem of corn drying, reduce the post-harvest quantity and quality loss of corn, realize the optimization and upgrading of the industry, and upgrade the product. For the stabilization of food production, the realization of the "four scattered" circulation of food, and the protection of domestic and foreign trade have multiple effects, vacuum low temperature continuous drying new technology is in line with such development direction and requirements.
Vacuum low temperature drying is suitable for the drying of heat sensitive materials, which can reduce the crack rate and breakage rate of corn, avoid the destruction of nutrients and vitamins under high temperature, ensure the color, fragrance, taste, shape and nutrient composition of material quality, and improve at the same time. The drying speed is the true low temperature drying. In the environmental sense, vacuum drying is called "green drying."
The three-state temperature of water is directly related to the pressure. As the pressure decreases, the freezing point does not change much, and the boiling point becomes lower and lower, gradually approaching the freezing point. When the pressure is low to a certain value, the boiling point coincides with the freezing point, and the solid ice can be directly converted into a gaseous state without the liquid state. The pressure at this time is called the triple point pressure, and the corresponding temperature is called the triple point temperature. Continuous low-temperature vacuum drying reduces the vaporization temperature of water from 100 ° C to below 43 ° C. Below the gelatinization temperature of starch, it can avoid the destruction of nutrients, proteins, vitamins, etc. at high temperatures, and will not damage corn kernels. The realization of low-temperature drying under vacuum is mainly to solve the problem that the dried material can be uniformly obtained to obtain the required heat under a certain vacuum pressure and vacuum condition. Through the conduction heating and temperature control system, the dried material in the vacuum chamber is heated to obtain the heat of vaporization required for uniform heating, and then the water vapor generated in the vacuum drying chamber is removed in time by the vacuuming system to maintain the required vacuum degree. The purpose of vacuum drying of the material. Tests have shown that vacuum drying eliminates the surface hardening that is likely to occur under normal pressure drying. The temperature gradient of the vacuum drying material is small, the pressure gradient between the material and the surface is large, and the moisture quickly moves to the surface without surface hardening. At the same time, it can increase the drying rate, shorten the drying time and reduce the operating cost of the equipment.
1 Vacuum low temperature continuous drying tower structure and parameters
1.1 Vacuum low temperature continuous drying tower
The vacuum low-temperature continuous drying tower adopts the integral assembly structure, adopts modular design, standardized production, easy transportation and on-site installation. It is one of the key technologies for drying tower design to ensure that the high moisture grain falls at the same speed and evenly to maintain the same drying time and precipitation amplitude. The high-moisture corn enters the vacuum drying chamber through the lifting conveyor system and the feeding device. The production process requires that the grain in the vacuum drying tower maintains a certain level of material, and plays the role of uniform constant velocity drop and buffer. This process relies on the gravity of the grain itself. The grain-discharging organization is implemented. The grain-discharging mechanism of the vacuum drying tower adopts an integral sealing structure to ensure the airtightness and vacuum requirements of the drying chamber. Through the control and adjustment of the frequency converter, the filling coefficient of the grain-receiving wheel is satisfied and the grain-ground vibration effect is achieved, and the grain in the tower is uniformly flowed as a whole.
1.2 Main technical parameters of vacuum low temperature continuous drying tower
Treatment volume (corn): 300t/d;
Precipitation range: 10%~15%;
Dryness unevenness: ≤1%;
Unit heat consumption: <5000kJ/kg·H2O;
Increase rate of corn crack in the tower: ≤ 5%;
Increase rate of corn broken in the tower: ≤1%;
The color and smell of corn are normal, and there is no coke paste or popcorn.
2 High moisture corn vacuum low temperature continuous drying production line and process
Production line system consists of vacuum low temperature continuous drying tower system, conveying cleaning system, vacuum system, heating system, condensing system, electrical control system, insulation system, wet granary, dry granary and so on.
Process: high moisture corn→belt conveyor→cylinder cleaning screen→bucket elevator→wet granary→belt conveyor→bucket hoist→vacuum low temperature continuous drying tower→belt conveyor→bucket hoist→dry granary → Belt conveyor → bulk grain car or warehouse (Figure 1).
Figure 1 300t/d high moisture corn vacuum low temperature continuous drying industrial automation production line
3 Key production technology of high moisture corn vacuum low temperature continuous drying
3.1 The main process parameters of corn vacuum low temperature continuous drying system include vacuum degree, vaporization temperature, heating temperature, drying rate, drying strength, precipitation amplitude, unevenness and energy consumption. In order to ensure these properties, on the one hand, optimization design is carried out on the vacuum drying tower; on the other hand, the drying process parameters are optimized, and both can not be despised on either side. According to the thermal stability and fracture sensitivity characteristics of corn, considering the allowable temperature of corn does not exceed 43 ° C (below the starch gelatinization temperature), the effective volume of the vacuum drying chamber should be increased during the design to ensure that the material can be pre-treated in the drying tower. Hot, continuous feeding and discharging and uniform grain discharge requirements.
3.2 The conveying and cleaning system mainly consists of a belt conveyor, a bucket elevator, a cylinder cleaning sieve, a magnetic separator, an inlet and outlet device, and a slide pipe. The whole logistics process is to remove large impurities, small impurities, dust, dust, cotton and linen preparations, ferromagnetic substances, etc. into the vacuum drying to avoid uneven drying.
3.3 The vacuum system consists of a vacuum pump, a vacuum drying chamber, a vacuum valve, a pipe, a vacuum instrument, etc. Since the controllability and consistency of the material temperature in the vacuum drying tower are not affected by the temperature of the heating medium, it is only determined by the degree of vacuum, and the overall requirement is pumping. Gas capacity and pressure stability are good. In order to reduce the vaporization evaporation temperature, it is mainly achieved by increasing the system vacuum.
3.4 The heating system is heated by closed high temperature hot water circulation, and the heat energy utilization rate is high. It mainly includes heat supply, temperature, adjustability and stability. The stability of the heating system is an important condition for ensuring normal drying.
3.5 Condensation system, in terms of vacuum condensing system, the water consumption of the condensing system is relatively large, and a large amount of cold water must be used to condense the low-temperature steam in the vacuum drying chamber to obtain a high and stable vacuum, which is maintained. Vacuum drying is necessary for normal production.
3.6 The control system consists of control cabinet, control instrument, measuring instrument, regulating instrument, safety alarm device, PLC programmable controller, touch screen display, pressure sensor, material level sensor, temperature sensor and various switching circuits, which effectively monitor and control Dry process operation and product quality.
3.7 When the material is under vacuum, it is critical to ensure continuity, uniformity, constant heating, no dead angle, and no local overheating. The material is required to have good passability in the drying chamber and can be uniformly heated, and at the same time ensure that the material can be discharged in time after vaporization.
3.8 Because the equipment as a whole is working outdoors, it is especially important to take good insulation measures and ensure the required heat.
4 Analysis of production test results
The vacuum low-temperature continuous drying automatic production line passed the production test and the application effect was good. The production data of ten days shows that the actual production capacity of the unit can reach 300~360t/d, and the total dry corn is more than 6000t. The heat and dry low temperature continuous drying tower was tested by the Drying Machinery Equipment Quality Supervision and Testing Center of the Ministry of Agriculture of Heilongjiang Province. The initial moisture content was 24%, the grain temperature was -10 °C, and the precipitation rate was 10%. The unit heat consumption was less than 5000kJ/kg·H2O. The average unit heat consumption of the local 75 hot air dryers is 7380 kJ/kg·H2O, which is much lower than the local hot air drying tower heat consumption index under the same initial conditions, saving about 30% and noise less than 85dB (A). . The quality of the corn is good, which can guarantee the color, fragrance, taste, shape and nutrients of the corn quality. For the vacuum drying tower, the application of the variable frequency speed control technology, the system energy saving effect is more obvious. During the whole production cycle, the material enters and exits the drying tower smoothly, no blockage occurs, and there is no abnormal fluctuation in the test temperature, vacuum degree and pressure, indicating that the vacuum low temperature continuous drying system has good air tightness, vacuuming system, heating system, The material handling system and the electric temperature control system are all working normally. The continuity, adaptability and stability of the vacuum low-temperature continuous drying system have been further verified by experiments.
The vacuum low temperature continuous drying system is suitable for heat sensitive materials, high initial moisture content and materials with high final moisture content requirements. Practice has proved that the application of vacuum technology to corn drying has explored a new drying route. Its successful application provides experience for the dehydration and drying of similar products in the future, while providing new ideas for drying other granular materials. Any wet grain that can be directly dried by a hot air drying tower, but with a higher final moisture content and quality requirements, can be dehydrated and dried by the above vacuum low temperature continuous drying system. The drying requirements of the product are achieved by adjusting the effective volume, heating temperature, vacuum degree and residence time of the low temperature drying chamber. The design of the vacuum low-temperature continuous drying system should be based on the physical and chemical characteristics of the material to reasonably determine the precipitation range that should be achieved by the vacuum low-temperature continuous drying system, and according to the allowable working environment pressure, drying ambient temperature, treatment volume and final moisture content. Determine the model and specifications of the system unit. The heating system, condensing system, vacuum system, control system and logistics system are designed and configured to have the highest efficiency, the lowest equipment investment and the lowest operating cost.
5 Conclusion
High-moisture corn vacuum low-temperature continuous drying new technology research and development with industrialized continuous production as the goal, user-centered, for the purpose of productive testing. Focusing on the research content of the subject, research, design, manufacturing, installation, commissioning, production and testing are carried out. Practice has proved that the vacuum low temperature continuous drying system has the following characteristics:
(1) The production line and process parameters are designed reasonably, reaching the 300t/d northeast high moisture corn industry automation production design requirements.
(2) The adjustability and stability of the vacuum system, heating system, etc. are conducive to giving full play to the production potential of the equipment.
(3) The use of moisture online detection and microcomputer automatic control is conducive to improving work efficiency.
(4) The production operation of vacuum low-temperature continuous drying shows strong vitality in terms of quality, energy saving and environmental protection. The quality of grain and the quality of natural drying are basically the same; compared with the local 75 hot air dryers, the heat consumption is obviously lower than the atmospheric hot air drying at the same temperature, saving about 30%; the drying temperature is low and the drying speed is fast. The quality of the color, fragrance, taste, shape and nutrients, while reducing the emissions of dust and volatile substances.
In this paper, the new technology of vacuum low-temperature continuous drying provides experience for the drying of similar performance products and the development of large-scale vacuum low-temperature continuous drying production line above 300t/d. It has a multi-purpose and unique environmental protection and energy-saving effect and innovation. Leading the way in drying equipment for similar food products. The successful development of the vacuum low-temperature continuous drying device has been highly valued by the national grain, food, environmental protection, chemical and other related industries, and has great market development value. China's drying industry has huge market potential. How to apply vacuum technology scientifically, reasonably and effectively in the grain drying industry, science and technology workers should give full play to their professional advantages and new theoretical research, and vigorously develop new and practical new technologies and processes. The new device makes its due contribution to China's food industry.
The vacuum drying technology has developed rapidly and involves a wide range of contents. This article only briefly discusses the typical production examples of vacuum technology in the application of food drying industry. There are still many unrelated contents, please understand.
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Fund Project: “Eleventh Five-Year†National Science and Technology Support Program “Research and Development and Demonstration of New Technology Equipment and Facilities for Grain Drying†(2006BAD08B06).
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