Northeast China is the main corn producing area in China and its output accounts for 30% to 40% of the country's total. Affected by natural conditions, the moisture harvested from corn often reaches 22% to 30%; the maturity and moisture content are not uniform. According to the thermal properties of drying corn heat-sensitive materials and market requirements, taking into account its thermal stability and breaking sensitivity Characteristics, adopting the vacuum low-temperature drying technology for production test methods, analyzing the drying process and methods, designing a 300t/d high-moisture maize vacuum vacuum continuous drying industrial automation production line, and analyzing and researching the key technologies of the production test. . Based on the experimental results, the relationships and interactions between parameters such as corn moisture content, drying medium temperature, vacuum degree, corn temperature, drying time, vaporization vaporization temperature, etc. were analyzed. The production test shows that when the moisture content of corn is 24%, the degree of vacuum in the drying tower is maintained at -0.096 MPa, and the high temperature water at 95 to 100°C is used for heating. The vaporization evaporation temperature is 40 to 43°C and the amplitude of one precipitation reaches 10% to 15%. It guarantees the color, aroma, taste, shape and nutrient composition of corn quality, and has the advantages of good drying quality, fast precipitation speed, high output, low energy consumption, convenient operation, and high cost-effectiveness.
Key words high moisture maize vacuum low temperature drying productive test energy saving high efficiency green environmental protection
Keywords: Corn with High-Moisture Content, Low-Temperature Vacuum Drying, Production Test, Energy-Saving, Environmental Protection
0 Introduction The research and development of high-temperature corn vacuum low-temperature drying technology in Northeast China is one of the topics of the “Tenth Five-Year Plan†national science and technology research project “Research on key technologies for grain and oil product storage and quality inspectionâ€, which is based on evaporation temperature of grain moisture vaporization. The principle of changing the height with the environmental pressure is to use a drying tower and a vacuum pump to form a space with a certain vacuum, and to continuously dehydrate and dehydrate the high-moisture corn in a vacuum at a low temperature. According to the concept of “high quality, high nutrition, high efficiency, low loss, low pollution, low cost†and green environmental protection, relevant drying technology and equipment are studied through production tests.
So far, there have been few studies on the production of vacuum high-temperature drying technologies and processes for high-moisture maize at home and abroad. In this paper, the high-moisture corn has a large range of precipitation and short drying cycle, and the vacuum low-temperature drying process production test, and the actual experimental structure was analyzed and discussed, in order to improve the high moisture corn drying technology and research and development of cost-effective drying The device provides a practical reference.
1 Test purpose, principle and content
1.1 Test Objectives Test and verify the adaptability, stability, continuity, and reliability of the vacuum low-temperature drying device system so that it can achieve energy-saving, high-efficiency, environmental protection, and ensure the quality of the dried product. Solve a series of problems encountered in the actual production test, and provide reference for the design and research of complete sets of equipment for the construction of industrial automation demonstration production lines.
1.2 Principle of Vacuum Drying
It can be seen that during vaporization of water, the vaporization temperature is in a logarithmic relationship with the pressure to which it is subjected, that is, the vaporization temperature of water increases with the increase in the pressure it withstands, decreases with the decrease, and is achieved under vacuum conditions. Low-temperature vaporization; in the case of constant pressure ps, heating of the system, in order to maintain the balance of the formula, there must be more liquid vaporization, so that the drying speed is faster; if you maintain the same T, reduce the ps, there will be more liquid Conversion to steam is one of the ways to speed up vacuum drying.
1.3 Contents of the measurement Vacuum degree of the vacuum drying chamber and vaporization evaporation temperature; water content and temperature of the corn entering the machine; water content and temperature of the corn; heating temperature; corn crushing rate, crack rate increase; water unevenness, etc.
2 Test food and methods
2.1 Experimental food
Naturally harvested high-moisture maize in 2005, with a continuous treatment capacity of 15 t/h, originating in Jilin. The average moisture content is 24%~30%, the non-uniformity of moisture content is less than 3%, the impurity is less than 0.9%, and there is no mildew.
2.2 Test method In the corn vacuum low-temperature drying device into the grain mouth and row of food section of the full-section randomly selected raw grains or drying process samples taken once every 60min, each time to take 5kg, a total of 100 times, and then mixed. Use sampling method, weighing method, pressure rise method, temperature contrast method, etc. to judge whether the drying meets the requirements. Sample moisture content, water non-uniformity, crack rate, broken rate and other determination according to GB6970 method.
2.3 The experimental equipment and location adopts 5ZHCY15 corn vacuum cryogenic drying device and cleaning sieve, bucket elevator, belt conveyor, vacuum pumping system, heating system, cooling system and electric air temperature manufactured by Zhengzhou Institute of Scientific Research and Design, State Grain Reserve Bureau. Control system and other equipment. Computer moisture analyzer 0%~40%, YSG-3 vacuum pressure gauge, XMT-102 digital temperature sensor, semiconductor point thermometer 0~100°C, SL-401 sound level meter, FSF crusher, TQ3288 ( Indexing value 0.1mg) Analytical balance, thermometer 0 ~ 100 °C and other instruments as test equipment, the vacuum drying device is a continuous model. Under the action of gravity, the continuous tumble during the falling of the material is conducive to the migration of moisture inside the dried material and the movement of water molecules, so that the material can be dried. The material is discharged from the top to the bottom as a drying cycle, and a single precipitation meets the production requirements. The test site was in Jilin Province in Northeast China, from November 2005 to March 2006.
3 Results and Analysis
3.1 Test results The vacuum low-temperature drying automated continuous production line has been put into use in the Dehui National Grain Reserve in Jilin Province, with good application results. Ten days of production data show that the unit's actual production capacity can reach 360t/d, with a total of more than 6000t of dried corn. The vacuum drying machine was inspected by the Quality Supervision, Inspection and Testing Center of Dry Machinery of Heilongjiang Province, with a power of less than 200kW, a working vacuum of -0.096MPa, a vaporization evaporation temperature of less than 45°C, and a crack increase rate of less than 5%. The rate is less than 1%, the drying unevenness is less than 1%, the dust concentration is 3.2mg/m3, the initial moisture content is 24%, the average grain temperature is -10°C, and when the precipitation amplitude is 10%, the unit heat loss is less than 5000kJ/kg. H2O; and the local 75 corn hot air dryer unit average heat consumption of 7380 kJ/kg • H2O, under the same initial conditions far below the local hot air dryer heat consumption index, energy saving about 30%, the noise is lower than 85dB ( A) High environmental performance. The corn quality is excellent, which can guarantee the color, aroma, taste, shape and nutrition of corn quality. During the entire production cycle, the material flowed in and out of the dryer smoothly, no blocking phenomenon occurred, and there were no abnormal fluctuations in the test temperature, vacuum, and pressure, indicating that the vacuum low-temperature drying system had good air tightness, vacuum pumping system, heating system, and materials Transmission system and electrical temperature control system are all working properly. The continuity, adaptability and stability of the vacuum cryogenic drying system have been further tested and verified.
3.2 Drying medium temperature and corn moisture
The empirical formula is suitable for all kinds of cereals, the grain moisture is between 3.2% and 35% (wet basis); the drying time is between 0.02 and 2 hours. When the grain variety is different, the coefficients a, b, and c change. For the same species, the two critical temperatures have different coefficients a, but the coefficients b and c are the same. The initial moisture and drying time of corn are the main factors that determine the thermal sensitivity of corn. In order to obtain a safe drying temperature for corn and increase the drying speed, a vacuum low-temperature drying method is adopted. 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 determined only by the degree of vacuum. The hot water of t=95~100°C is used as the heating medium for circulating heating. The properly designed heating system and vacuum drying chamber can make the drying temperature reach 40~43°C. This is the proper drying temperature range for corn. Hot water has high heat content, stable temperature, easy control of process, and guaranteed drying quality. High-temperature hot water boilers are low in cost, safe and reliable, and easy to operate. Hot water circulation heating saves both water and waste water, and simplifies soft water treatment equipment. The high-temperature water circulation heating uses high-temperature hot water boilers, and the hot water circulation system operates at a gauge pressure of P=0.3 MPa, and controls the water temperature t≤115°C. The key to this method lies in the pressure-keeping measures of the system. It is necessary to ensure that the saturation temperature of hot water is about 30°C higher than the actual temperature in order to avoid cavitation in the circulating hot water pump.
3.3 The relationship between vacuum degree, drying speed and moisture content The vacuum degree is high, which is conducive to moisture vaporization at lower temperature, but high vacuum degree is not conducive to heat conduction and affects the heating effect of the material. Drying speed is the main index reflecting the moisture migration rate in corn. Under the premise of ensuring the drying quality, the drying speed should be increased as much as possible. In the initial stage of drying, the moisture content of the material is high, and the heat exchange process is ideal. The higher the degree of vacuum is, the faster the water evaporates. With the weakening of convection and the increase of thermal resistance, the water content of corn decreases, if the vacuum pumping is maintained. With constant speed, the degree of vacuum is getting larger, convection is getting weaker, and the drying efficiency is getting lower and lower. The use of pulsating vacuum, rational deployment of vacuum and convection heat transfer to the best combination, is more efficient.
When the vacuum is high, the moisture in the microcapillaries of the material is first extracted. At this time, the microcapillaries gradually become turbid from filling, that is, the microcapillary pores become unobstructed, the deep moisture is not easily discharged, and then the vacuum degree Reduce the micro hair pores and gradually become full and smooth. When the degree of vacuum is increased, the moisture in the inner layer is easily discharged. Repeatedly, moisture can be discharged faster. Compared with drawing vacuum continuously, the repeated effect is much better, and the process of lowering the vacuum degree is also an aeration process. The added gas increases the air in the drying room, improves the convection effect, and accelerates the heating of the material itself. Therefore, vacuum pulsation drying is a good balance between the use of rapid temperature increase and pressure reduction, two favorable drying conditions. Next, as the deep temperature of the material continues to increase, the moisture content of the material is accelerated, the internal pressure increases, and the degree of external vacuum gradually increases, the pressure gradually decreases, the internal and external pressure difference gradually increases, and the deep moisture further spreads and overflows over time. Extend the amount of water in the material to less and less, to achieve a more thorough drying.
3.4 Relationship between vacuum degree and evaporating temperature The temperature of water in the process of vaporization is directly proportional to the vapor pressure, and the pressure and temperature of saturated water vapor have a one-to-one correspondence. In order to allow the moisture in the material to be discharged faster and dry, one of the commonly used methods is to increase the degree of vacuum and the second is to increase the temperature. The purpose of raising the vacuum degree is mainly to reduce the pressure of the surrounding environment of the material, which is beneficial for the moisture in the micro hair pores of the material to become vapor diffusion, migration and discharge. The purpose of raising the temperature is mainly to increase the partial pressure of water vapor in the material. In the case of the same moisture content, the higher the temperature, the better the diffusion effect, thereby increasing the drying speed of the material. The purpose of increasing the degree of vacuum to reduce the space pressure around the material is also to reduce the saturated steam partial pressure of the water. The higher the vacuum degree, the lower the saturated steam partial pressure of water, the easier the water evaporation, and the faster the material is dried. It can be seen that increasing the temperature and increasing the degree of vacuum are all for accelerating the drying of the material and improving the drying efficiency.
According to the characteristics of the material, vitamin C decomposes at more than 40°C, changing the original performance; the protein denatures at high temperature, changes the nutrient content of the material, the evaporation temperature should be lower than the starch gelatinization temperature 43°C, considering the evaporation temperature and The effect of vacuum. 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, which can eliminate the solute loss and surface hardening that are easily generated under normal pressure drying conditions, and at the same time can improve the drying rate and shorten drying. Time, reduce equipment operation costs.
4 Conclusions Prove by production test:
(1) The design of the drying process and production line is reasonable to meet the high-moisture corn industry automation 300t/d production design requirements.
(2) The vacuum drying quality is obviously better than the traditional hot air drying quality, and the original quality of corn is better guaranteed.
(3) Compared with the local 75 sets of corn hot air dryers, the coal consumption is significantly lower than normal atmospheric hot air drying with the same temperature, saving about 30%.
(4) Low drying temperature, fast drying speed, obviously faster than conventional hot air drying, showing strong vitality.
This vacuum low-temperature drying technology provides experience for the drying of similar performance products and the development of large-scale vacuum low-temperature drying production lines above 500t/d. It has a multi-purpose and unique environment-friendly energy-saving effect and innovation. Leading position in drying equipment for cereals of the same type. The successful development of the vacuum cryogenic drying device has been highly valued by the national food, feed, food, chemical, environmental protection and other related industries, and has great market development value.
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