Summary

　　 Because China is located in the north temperate zone, the northeast corn harvest period is in the autumn with low temperature and high humidity. The moisture content of harvested corn is generally high, as high as 28% to 35%, while the national standard stipulates that the grain moisture content entering the warehouse cannot exceed 13.5%. When vitamin C exceeds 40 ° C, it decomposes and changes its original properties; protein denatures at high temperatures, changing the nutrients of the materials. In order to do a good job in the "four scattered" circulation of food, this paper discusses the research and design of low temperature vacuum dryer system. The system dries the material at temperatures below 40 °C. When vacuum drying, the temperature gradient inside and outside the material is small, and the water in the grain moves by the reverse osmosis, which overcomes the phenomenon of solute loss. The dryer enables the process parameters such as temperature, moisture and time to be adjusted and controlled during the low-temperature vacuum drying process to prevent over-drying or insufficient material drying, and to ensure the original color, fragrance, taste, nutrient composition and quality of the material after baking. Constant, easy to store, transport and sell. The drying device has the advantages of large processing capacity, high precipitation amplitude, automatic display, alarm, flexible operation and convenient use.
Key words corn low temperature vacuum drying green drying

The experiment and research on low temperature vacuum drying of corn is one of the topics of the “Tenth Five-Year” National Science and Technology Research Project – Key Technology Research Project for Grain and Oil Product Storage and Quality Inspection. It is based on the principle that the temperature of the boiling water of the grain is changing with the pressure of the environment. The steam jet vacuum technology is used to form a space with a certain degree of vacuum, and the grain with high water content is continuously dehydrated and dried under vacuum low temperature state. In accordance with the concept of “high quality, high nutrition, high efficiency, low loss, low pollution, low cost”, the company has studied the relevant drying technology and equipment through experiments.

Corn is one of the main food crops in China. Northeast China has high original moisture and uneven maturity, and is highly vulnerable to insecticides. Because the corn kernel has large embryos and high fat content, under the same conditions, it has stronger life activity and higher respiratory intensity than other grains, and is easily oxidized and rancid. Therefore, corn is less stable than other grains and is generally not suitable for long-term storage and transportation. In conventional storage, the moisture content of the grain is generally reduced by means of hot air drying.

Corn is one of the main varieties of grain drying in China. High-moisture corn is not conducive to safe storage. Therefore, it is generally necessary to dry before storage to reduce the moisture below the safe moisture. When the drying temperature is too high, the grain will lose its viability and its germination power will be greatly reduced. Below 40 ° C, the method used to dry the northeast high moisture corn in large quantities to ensure the original quality of the corn is basically unchanged, is the goal that science and technology workers are constantly pursuing. The low temperature vacuum dryer system is one of the best ways to solve high moisture corn.

Corn is one of the food varieties that are difficult to dry. The main reason is that its grain is large, the unit specific surface area is small, and the grain surface structure is tight and smooth, which is not conducive to the transfer of water from the inside to the outside of the grain. Especially under the action of high-temperature drying medium, due to the rapid vaporization of the surface moisture, the moisture under the epidermis of the grain can not be transferred in time, causing the pressure to rise, causing the epidermis to burst or the grain to swell and deform. Moreover, if the temperature of the drying medium is too high, the grain will be burnt and the grain of the machine will be broken. Rapid and economical reduction of corn moisture to maintain good quality has always been an important topic in food drying research.

Experiments have shown that when the temperature of the drying medium exceeds 150 ° C, when the temperature of the corn is higher than 60 ° C, a large number of striated lines are generated and the quality is degraded. For conventional hot air drying equipment, the temperature of hot air is generally 120~160°C, the natural temperature of drying season is generally -10~-30°C, and the temperature difference between hot air and corn kernel is as high as 130~190°C. During the process, high temperature hot air damages the internal structure of corn kernels, resulting in a higher crack rate of grain after drying, generally around 18%, and high to 35%, resulting in a decline in the quality of corn after baking. In the process, it is easy to break, and a large number of corn seriously reduces the grade of the commodity due to the problem of the level of drying technology, which affects the normal domestic and foreign trade of corn. The use of low-temperature vacuum drying technology to avoid the quality degradation caused by high temperature is the key to the research of this topic.

1 Test purpose, principle and content

1.1 Test purpose To determine whether the low temperature vacuum dryer meets the design requirements. The corn in Northeast China was dried by vacuum drying technology, and the vacuum degree, drying rate, drying time and drying characteristics of corn were studied under low temperature vacuum condition. The dried product conforms to the grain storage and transportation standard, making it easy to process, transport, store and ensure product quality.

1.2 Test principle The continuous drying effect of corn in the drying cylinder is determined under the premise of maintaining the predetermined vacuum degree and the heating temperature substantially unchanged. Determine the operation and air-tightness of the equipment, analyze the various problems occurring in the experiment in time, find out the reasons, study the dryer system, determine the next improvement measures, and accumulate practical experience and data for the next experiment.

1.3 Research content The key research contents are drying cylinder, feeding and discharging device, dynamic airtight technology, material anti-clogging technology, wear-resistant technology, etc.; research and determine the main technical parameters such as vacuum degree and evaporation temperature. Focus on solving the optimal heating design parameters of the drying chamber heating device, as well as the adaptability, stability, continuity and reliability of the dryer system, so as to achieve high efficiency, energy saving, environmental protection, and to ensure the quality of the product after baking. Solve a series of problems encountered in actual production, and provide basis for the design and test of production test equipment.

1.4 Determination of the vacuum and evaporation temperature in the drying cylinder; corn moisture and temperature into the machine; corn moisture and temperature; corn crushing rate increase; corn crack rate increase.

2 Field test

2.1 Basic conditions of field test

2.1.1 Test time April 13, 2004 to May 9, 2004.
2.1.2 Test site Zhengzhou Zhengzhou Company.
2.1.3 The weather is fine, the southeast wind is 2~3, and the temperature is 20~23°C.
2.2 Test materials 2.2.1 Supply of water, electricity and steam The existing infrastructure of Zhengping Company is utilized.
2.2.2 The corn that was naturally harvested in 2003 was 25t of high-moisture corn, which was produced in Jilin. The rating is level 2. The average water content is 24%, the water content unevenness is less than 3%, the impurity is 0.9%, and there is no mold change.
2.2.3 Main testing equipment steam pressure gauge -0~1.6MPa, YSG-3 vacuum pressure gauge, XMT-102 digital temperature sensor, LFX80-2 steam flowmeter, computer moisture analyzer 0%~40%, Semiconductor point thermometer 0~100°C, SL-401 sound level meter, FSF type pulverizer, TQ3288 (division value 0.1mg) analytical balance, thermometer 0~100°C, etc.
2.2.4 Main test equipment bucket elevator, belt conveyor, buffer balance bin, drying cylinder, vacuum system, circulating heating system, electrical control system, etc.

2.3 Test methods

2.3.1 Vacuum drying principle The vacuum drying process is to place the dried material in a closed drying chamber, and vacuum the vacuum system to continuously heat the material to be dried, so that the moisture inside the material diffuses to the surface through pressure difference or concentration difference. The water molecules obtain sufficient kinetic energy on the surface of the material to escape the low pressure space of the vacuum chamber after overcoming the mutual attraction between the molecules, thereby being evacuated by the vacuum pump. Vacuum drying makes it easy to recover useful and harmful substances, and it can be sealed well. In the sense of environmental protection, some people call vacuum drying "green dry".

Under vacuum, the boiling point of the material moisture decreases and the vaporization process accelerates. In the process of vaporization of corn, the boiling evaporation temperature is proportional to the ambient pressure. The high-moisture corn is placed in a closed drying cylinder, and the drying cylinder is formed and maintained with a certain degree of vacuum by steam jet vacuum technology. The corn inside is continuously heated, so that the moisture inside the corn diffuses to the surface through the pressure difference, and is evacuated by the vacuum pump after being vaporized, thereby continuously dehydrating and drying the high-moisture corn under the vacuum low temperature state. Vacuum drying reduces the boiling evaporation temperature of water from 100 ° C to below 40 ° C. Since the evaporation temperature is lower than the gelatinization temperature of starch, it does not damage corn kernels. It avoids the expansion and bursting of corn kernels and ensures the quality of the corn after drying. It is a low-temperature drying in the true sense, which is completely different from the general hot air drying.

2.3.2 Dry conditions are based on the original moisture content of the grain, the harvesting method, the maturity and the use of the grain. The greater the original moisture content of grain, the worse its thermal stability, ie temperature resistance. Incompletely mature grain, its temperature-tolerant grain is poor. The newly harvested high-moisture grain, due to the uneven maturity and moisture content of the grain, the surface layer of the grain is not fully hardened. Therefore, the drying condition of lower temperature is adopted; if the high-temperature drying condition is used, the grain is damaged. The surface of the grain is indurated, causing a large amount of damage to the capillary on the surface of the grain, which is not conducive to the drying process. For this reason, in the drying of newly harvested high-moisture foods, it is necessary to take into consideration its thermal stability and surface characteristics, and adopt mild drying conditions.

2.3.3 Test preparation The dryer system is commissioned to normal operation according to the test requirements, and the steam pressure is stable in the normal state. A portion of the pad machine is placed in the drying cylinder to prepare for continuous drying. The vacuum system is turned on to bring the degree of vacuum and hot water in the drying cylinder to a predetermined range. Normal continuous drying is carried out in accordance with the predetermined entry and exit procedures.

2.3.4 The steam pressure is read by the pressure gauge and recorded every 30 minutes.

2.3.5 Vacuum The pressure in the drying cylinder was read according to the vacuum pressure gauge and recorded every 30 minutes.

2.3.6 Temperature detection adopts 1 line, middle and bottom 3 points, which are read by digital temperature sensor and recorded every 30 minutes.

2.3.7 Steam consumption is recorded every 60 minutes according to the data displayed by the steam flow meter.

2.3.8 Grain moisture is carried out according to GB5497-85, 130 °C fixed temperature drying method. The difference of water content between corn and corn entering is the precipitation rate.

2.3.9 The breaking rate is determined according to the provisions of GB6970-86.

2.3.10 Noise is measured according to GB16769.

3 Results and analysis

3.1 Results

From the field test situation, the low-temperature vacuum dryer processing capacity is 40t / d; precipitation amplitude exceeds 8%; the corn crushing and increasing rate is less than 0.3%; the corn crack increase rate after drying is less than 5%; the corn color smell is normal, No heat damage particles, striated grain, good corn quality; noise below 85dB (A). During the whole test period, the material enters and exits the dryer smoothly, no blockage occurs, and there is no abnormal fluctuation in the test temperature and vacuum pressure, indicating that the low-temperature vacuum dryer device has good air tightness, vacuum system, heating system, Both the material handling system and the electrical control system are working properly.

3.2 Analysis

3.2.1 During the installation process of the system, it must be meticulous and not sloppy. Otherwise, despite the smooth installation, there may be a tight seal, resulting in a small amount of local leakage, which does not meet the vacuum requirements required for material drying.

3.2.2 From the perspective of material transportation, although the overall test requirements are met, if the transportation volume is increased, the working hours can be saved and the utilization efficiency of the dryer system can be increased.

3.2.3 From the vacuum system, although the pre-design requirements are met, increasing the pumping capacity is beneficial to improve efficiency and output.

3.2.4 From the perspective of the heating system, if recycling is used, it is conducive to saving energy and improving the utilization rate of hot water.

3.2.5 From the perspective of the control system, control points should be added to facilitate the detection of local evaporating temperatures and material levels in the system.

3.2.6 Due to the tight test time, the insulation of the drying cylinder and the heating pipeline was not taken, and the thermal energy was not fully utilized.

3.2.7 Due to the limited amount of corn to be tested, the whole test period is short, and the abrasion resistance test of the heating tube in the drying cylinder is not carried out.

3.2.8 From the field test conditions, the outdoor temperature is between 20 and 23 ° C, which is far from the actual situation in the northeast -10 to -30 ° C. It is subject to further experiments and research in the Northeast.

3.2.9 Due to the lack of detection means and equipment, no detection and analysis of germination rate, grain quality, dust concentration, etc., only the moisture content, the rate of increase of breakage and the sensory, etc., the drying effect is good, the color of corn is normal, no Coke paste, good quality.

4 Conclusion

4.1 Practice shows that the low-temperature vacuum dryer system is feasible for drying high-moisture corn. The dryer can ensure that the original color, aroma, taste, nutrient composition and shape of the material are basically unchanged after drying, which effectively delays the storage process. The aging of grain quality has enhanced the competitive advantage of domestic and foreign trade, and has increased considerable economic benefits for enterprises, laying a good foundation for “four scattered” circulation and “green” grain protection.

4.2 The low-temperature vacuum dryer is used to dry the northeast high-moisture corn, and its drying rate needs further exploration. The continuity, adaptability and stability of the dryer system are subject to further experimental research in the Northeast. Based on this, the processing capacity of 150t/d and 300t/d was studied to meet the needs of the production test, and technical support and conditions were provided.

4.3 The key to low temperature vacuum drying is the internal vacuum and evaporation temperature of the drying cylinder. Study the optimal vacuum drying process to minimize the heat supplied to the material, the fastest evaporation of water, the shortest drying time, the lowest energy consumption per unit of water, and the best quality after drying.

4.4 Low-temperature vacuum drying is adopted. The premise is that the sealing performance is good. The production and installation of the drying cylinder should be strictly controlled. It is especially important to ensure the air tightness inspection during the working period to ensure the vacuum degree in the drying cylinder.

4.5 For many years, the Zhengzhou Scientific Research and Design Institute of the National Grain Reserve Bureau has accumulated rich experience in grain drying. How to apply the low-temperature vacuum drying technology scientifically, reasonably and effectively in food drying, we will continue to carry out extensive and relevant research. Various experiments and research will actively explore green grain storage technology, make greater contributions to China's safe grain storage, and strive to make China's grain storage new technology, new equipment and new materials to a new level.

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