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1 Test Purposes, Principles and Contents 1.1 Test Objectives To assess whether the low-temperature vacuum dryer has reached the design feast. The vacuum drying technology was used to dry the corn in Northeast China. The vacuum degree, drying rate, drying time and drying characteristics of the corn were studied under low temperature and vacuum conditions. The dried product meets grain storage and transportation standards, making it easy to process, transport, store, and ensure product quality.
1.2 Test principle Under the premise of maintaining the predetermined vacuum degree and the heating temperature basically unchanged, the continuous drying effect of corn in the drying cylinder is determined. Determine the operation status and airtightness of the equipment. Analyze the problems occurred in the experiment in a timely manner, find out the reasons, study the dryer system, determine the next improvement measures, and accumulate practical experience and data for the next test.
1.3 Research Contents The key research contents are drying cylinders, feeding and discharging devices, dynamic airtight technologies, material anti-clogging technologies, wear-resistant technologies, etc.; research and determination of the main technical parameters such as vacuum degree and evaporation temperature. The focus is on solving the optimum 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 baked product. Solve a series of problems encountered in actual production, provide the basis for the design and test of production test equipment.
1.4 Determination Contents The degree of vacuum and evaporation temperature in the drying cylinder; corn man-machine water content and temperature; corn out of the water content and temperature; corn crushing rate of value;
2 Field tests 2.1 Basic conditions of field tests 2.2 Test materials 2.2.1 Supply of water, electricity and steam Use the existing infrastructure of Zhengping Company.
2.2.2 Test maize The high-moisture maize naturally harvested in 2003 was 25t, and was produced in Jilin. Level 2 The average moisture content was 24%, the non-uniformity of the water content was less than 3%, the impurity was 0.9%, and no mildew was observed.
2.2.3 Main Inspection Instruments Steam Pressure Gauge - 0~1.6MPa, YSG-3 Vacuum Pressure Gauge, XMT-102 Digital Display Temperature Sensor, LFX80-2 Steam Flowmeter, Computer Moisture Analyzer 0%~40%, Semiconductor point thermometer O-lOOtSLOl type sound level meter, FSF type grinder, TQ3288 (divisional value O.lmg) analytical balance, thermometer 0 ~ 100 and so on.
2.2.4 The main experimental equipment bucket elevator, belt conveyor, buffer balance warehouse, drying cylinder, vacuum pumping system, circulating heating system, electrical control system and so on.
2.3 Test method 2.3.1 Principle of vacuum drying The vacuum drying process is to place the material to be dried in a closed and dry room. Vacuum is applied to the material to be dried while the vacuum is used to continuously heat the inside of the material through the pressure difference or concentration difference. Diffusion to the surface, the water molecules get enough kinetic energy on the surface of the material, after overcoming the mutual attraction of molecules, escape to the low pressure space of the vacuum chamber, which is then pumped away by the vacuum pump. Vacuum drying makes it easy to recover useful and harmful substances and seals well. In the sense of environmental protection, some people claim that vacuum drying is “green dryingâ€.
In the vacuum state, the boiling point of the moisture of the material is reduced and the vaporization process is accelerated. In the process of vaporization of corn moisture, the boiling evaporation temperature is proportional to the ambient pressure, and the high moisture corn is placed in a closed drying cylinder. The steam jet vacuum technology is used to make the drying cylinder to form and maintain a certain degree of vacuum while drying the cylinder. The inner corn is continuously heated, so that the moisture inside the corn spreads to the surface through the pressure difference, and is vaporized and then pumped away by the vacuum chestnut to achieve continuous dehydration and drying of the high moisture corn under vacuum and low temperature. Vacuum drying causes the boiling evaporation temperature of the water to decrease from 100* to below 40T. Since the evaporation temperature is lower than the gelatinization temperature of the starch, corn kernels are not damaged.
The expansion of the corn grain and the explosion of the corn are avoided, and the quality of the corn after drying can be guaranteed. It is a true low-temperature drying, completely different from the general hot-air and low-temperature drying.
2.3.2 The drying 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 is its thermal stability, ie its temperature resistance. Incompletely mature food, its temperature-resistant mature grain is poor. For freshly harvested high-moisture foods, due to uneven grain and moisture content, the surface layer of grains is not fully hardened, therefore, lower temperature drying conditions should be adopted; if high temperature drying conditions are used, grain grains will be damaged. As a result, the surface of the grain is indurated and the capillaries on the surface of the grain are destroyed, 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 account its thermal stability and the characteristics of the surface, and to take gentle dry conditions.
2.3.3 Test preparation According to the test requirements, the dryer system is debugged to the normal working state, and the steam pressure is stabilized in the normal state.
In the drying cylinder, a part of pad grain is installed to prepare for continuous drying. The vacuum pumping system is turned on so that the vacuum degree and hot water temperature in the drying cylinder reach a predetermined range. The normal continuous drying operation is performed according to a predetermined entry procedure and exit procedure.
2.3.5 Vacuum degree Read the pressure inside the drying cylinder according to the vacuum pressure gauge and record it every 30 minutes.
2.3.6 temperature detection using 1 line, middle and lower 3 points, read by the digital temperature sensor, record once every 30min.
2.3.7 Steam Consumption According to the steam flow meter display data, it is recorded every 60 minutes.
2.3.8 The grain moisture is executed according to GB5497-85, 130t constant temperature drying method. The difference between the moisture content of the corn and the incoming corn is the precipitation rate.
3 Results and Analysis 3.1 Results From the field test situation, the processing capacity of the low-temperature vacuum dryer is 40t/d; the precipitation amplitude exceeds 8%; the added value of the broken corn is less than 0.3%; and the increase rate of the cracked corn after drying is less than 5%; The machine has a normal color and smell of corn, no heat-damaged grains and scarlet grains, and the quality of corn is good; the noise is less than 85dB (A). During the entire test period, the material flowed in and out of the dryer smoothly, no blocking occurred, and there were no abnormal fluctuations in the test temperature and vacuum pressure, indicating that the low-temperature vacuum dryer has a good air tightness, a vacuum system, a heating system, The material handling system and electrical control system are working properly.
3.2 Analysis 3.2.1 During the installation of the system, it must be meticulous and not sloppy. Otherwise, despite the smooth installation, there may be loose seals, resulting in a small amount of local leakage and failing to meet the vacuum requirements for material drying.
3.2.2 From the perspective of material transportation, although the overall test requirements are satisfied, if the delivery volume is increased, the man-hours can be saved and the utilization efficiency of the dryer can be increased.
3.2.3 From the perspective of the vacuum pumping system, if the pre-design requirements are met, increasing the pumping volume will improve the efficiency and output.
3.2.4 From the perspective of the heating system, the use of recycling will help save energy and increase the utilization of hot water.
3.2.5 From the control system point of view, control points should be added to facilitate the detection of local evaporation temperatures and material levels in the system.
3.2.6 Due to the tight test time, no heat preservation measures have been taken for drying cylinders and heating pipes, and the heat energy has not been fully utilized.
3.2.7 Due to the limited number of corns tested, the entire test period was short, and the detection of the wear resistance of the heating tubes in the drying cylinders was not carried out.
3.2.8 From the field test conditions, the outdoor temperature is 20~23t, and there is a big gap with the actual situation in the northeast from -10 to -30 foot. It needs to be further tested and studied in the northeast.
3.2.9 Due to the lack of detection means and equipment, no detection, analysis of germination rate, grain quality, dust concentration, etc. was performed. Only the moisture content, crushing increase rate, and sensory evaluation were good for the drying effect. The color and smell of corn was normal. Charpy paste, good quality.
4 Conclusions 4.1 Practice has shown that a low-temperature vacuum dryer system is feasible for drying high-moisture corn. Using the dryer can ensure that the original color, aroma, taste, nutrient composition and shape of the material after baking are basically unchanged, and effectively in the storage process. Delayed the aging of grain quality, enhanced the competitive advantages of domestic and foreign trade, increased the considerable economic benefits for enterprises, and laid a good foundation for the “four scattered†circulation and “green†grain conservation.
4.2 The use of a low-temperature vacuum dryer to dry high-moisture corn in Northeast China, the drying rate needs further exploration. The continuity, adaptability and stability of the dryer system need to be further tested in the Northeast. On this basis, 150 t/d and 300 t/d throughput were 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. The optimum vacuum drying process was studied to minimize the amount of heat supplied to the material, the fastest evaporation of water, the shortest drying time, and the lowest amount of energy consumed by water from the unit, and the best quality after drying.
4.4 The use of low-temperature vacuum drying, the premise is a good sealing performance, production and installation of the drying cylinder, should be strictly controlled, to strengthen the air tightness inspection during the work to ensure that the degree of vacuum in the drying cylinder is particularly important.
For more than 4.5 years, the National Grain Reserve Bureau Zhengzhou Research Institute of Science and Technology has accumulated rich experience in grain drying. How to apply cryogenic vacuum drying technology scientifically, reasonably and effectively in grain drying, we will continue to extensively develop relevant The various experiments and studies will actively explore the technology of green grain storage, make greater contributions to China's safe grain storage, and strive to make China's new grain storage technology and equipment new materials to a new level.
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