Research On Property And Model Of The Corn Low Temperature Vacuum Drying

Grain is the substance basic for man to live. It plays a key role in country economy and other areas. And grain drying is the basis that grain can be fully used. At present, grain drying adopts the traditional hot-air drying generally, but this drying method has some disadvantages such as slowly drying speed, quality damage after drying. Therefore, we should search one kind of new drying technology to resolve these questions. The vacuum drying technology can lower the evaporated temperature of water so it is likely to reach good drying effect under low temperature condition. And because of the existence of pressure grads, it can accelerate the drying speed as well as shorten the drying time. Besides, the low temperature vacuum drying technology has characteristics such as energy conservation, environmental protection, economy. And it is a new developing direction of corn drying technology and equipment in the future.Taking low temperature vacuum drying of corn for example, this paper mainly studies the property and model of low temperature vacuum drying in following aspects including theory analysis and experiment. The following research works have been done in this thesis:1)The vacuum drying technology already has being studied and applied broadly in chemical industry, medicine, and food areas, but the research and developmentin the continuous corn low temperature vacuum drying equipment takes the lead in getting a breakthrough just in recent years in our country, and this technology is short of pertinent research into basic theories at present. Aimed at this current situation, research technology route of the corn low temperature vacuum drying have been designed in the thesis.2)With two parameters of vacuum degree and the drying temperature as the experiment factor, this paper carried out dry experiment of corn low temperature vacuum research, and analyzed corn vacuum drying characteristic property. The test indicate that corn low temperature vacuum drying exists three drying stages as same as hot-air drying method: heating up stage, constant rate speed drying stage and deceleration drying stage; Vacuum degree and drying temperature have obvious infections to drying characteristic property curve, and the lower is the vacuum degree or the higher is drying temperature, the more rapid is the drying rate. But the vacuum drying rate is higher than hot-air drying rate at the same drying temperature.3)By the concept of the water potential introduced, this paper analyzed low temperature vacuum drying mechanism of corn, and thought that moisture content remove force come from the difference of water potential because of the temperature grad, pressure grad and moisture content grad. Moisture content migrates from inside of higher water potential corn pellet to the lower water potential pellet outside, boils away within an inch of the corn pellet outside and spill over the corn pellet surface. Comparing with hot-air drying, in the course of the corn low temperature vacuum drying, moisture content can be eliminate from the corn under the lower temperature with less energy consumption, because the inside of pellet exists bigger water potential difference.4)Based on the water potential theory, this paper built the model that moisture content removes during corn low temperature vacuum drying process in the inside of corn pellet. Under four hypotheses conditions: The corn pellet is a sphere penetration system; Initial material moisture content scatters homogeneously, and outer layer moisture content value of the corn seed reaches the balance moisture content when drying process just begins, but corn centre moisture content value always keeps initial value; In drying process, corn volume has no change; Moisture content diffuses always from corn centre to outside, moisture content removed process happening only in corn outside. Two mathematic models of corn low temperature vacuum drying are educed. Formula 3.20 which is the water distribution difference model in the inside of corn pellet, could calculate moisture content at different time and positions can under low temperature vacuum drying condition ; Formula 3.21 is an average moisture content model of corn pellet, can calculate out the average quantity of moisture content sometime. Verified the two model simulation results by the testing data, the value of simulating average moisture content is as consistent as the trial value, the correlation coefficient is more than 0.99.5)Judged by laboratory scale corn low temperature vacuum drying test data, estimated that unit energy consumption of corn low temperature vacuum drying is about 4000~5500 KJ/kgH2O, unit energy consumption saves 30%~60% compared with hot-air drying, and on this basis economic effect was forecasted. The result indicated a suit of corn low temperature vacuum drying machine with 300t/d may save 4979~6407.5 Yuan each day. For long-term interests, economic effect is better than hot-air drying. In addition, corn's broken value added is small during vacuum drying process, which resulted in the little dust produced. Because vacuum drying belongs to the low temperature drying, the quantity of heat from granary outsidewall released to the atmosphere also less than hot-air drying, therefore, low temperature vacuum drying technology has the merit of energy conservation and the environmental protection.The innovation points of the thesis:1)Based on the water potential concept and this paper pointed out the driving force of moisture content transfer originates from water potential difference,which contains the temperature grad, pressure grad and moisture content grad;2 ) Based on water potential theory analysis, this paper has developed mathematical model which can express the corn seed granule in low temperature vacuum drying at different time, the moisture content at different location, and be a model's turn to adopt differences numerical value method to carry out analysis; Besides having built a model which in the course of the low temperature vacuum drying, the moisture content of moment corn seed granule at different time. To sum up, the thesis studied the mechanism of corn low temperature vacuum and drying characteristic property based on water potential theory, this paper analyzed moisture transfer process of corn pellet inside, brought forward dry mathematic model of low temperature vacuum, provided theoretical analysis and the experiment basis for the developing of corn low temperature vacuum drying equipment. By estimating the budget with the economic effect and dry energy consumption of corn low temperature vacuum, this paper indicated corn low temperature vacuum drying has some merits such as energy conservation and environmental protection, the corn low temperature vacuum drying technology and equipment will represent the new developing direction of grain processing and storage in the future. The extension and application of the corn low temperature vacuum drying technology and equipment will produce the notable society and economic effect.