Moisture Online Analysis And Energy Efficiency Evaluation Of Grain Drying

Grain is the most important strategic material for national economic and social development. Its energy consumption for drying, quality and efficiency are much concerned. There are various factors causing and influencing the grain drying. The medium state, the physical property & texture property of grain as well as the configuration, flow state and location of the cluster take the real-time change during the drying process. In turn, it leads to the essential change of the heat and mass transfer, drying exergy and its transfer direction. In the process of transient transmission, the interactive mechanism of powers among phases is quite complicated. Simply depending on the physical detection, it is hard to get quite reliable measurement result which can represent the system characteristics. One of expectant and significant methods is the model analysis which contributes to revealing the change rule and achieving objective and true moisture online analytical solution during the drying process, so as to realize self-adaptation control while drying. Hence, aiming to high-quality, efficient, low-consumption, safe and environmental grain drying, this paper regards the development of grain moisture online analysis theory and reasonable drying energy matching as major tasks. Based on study on the qualitative characteristic parameters of the drying system and dimensionless analysis, aiming to the specific grain drying system, it applies the theoretical analysis and test research to indicate the inter-relation and interaction mechanism among system features, technology and process disturbance as well as explain the process analytical method. Meanwhile, it makes the energy efficiency evaluation on the drying system. The contents and the innovation points of this research can be seen in following aspects.1) The dimensionless parameters of drying system was deduced. Meanwhile, Based on the moisture diffusion model of thin bed drying process and the mass conservation equation of deep bed drying process, as well as the state function and irreversible thermodynamics analysis method, the basic function of deep bed grain drying was set up and solved, and the analytical formula of the distribution of grain moisture content and drying rate in the drying methods of concurrent flow and counter flow, cross flow and standing drying were obtained. The result show that the drying rate experienced the continuously decreasing process inside the concurrent flow deep bed drying, and the maximum point occurred at the beginning of the drying, that said the moisture content decreased quickly, and the late changed extremely smooth in the hot air inlet position.In counter flow deep bed drying, the drying rate had an extreme value point, and the maximum drying rate did not necessarily in the position of hot air inlet and outlet of drying layer. Indeed, the drying rate of counter flow was significantly higher than concurrent flow drying under the same drying conditions of ventilating temperature, humidity and air volume, which indicates that the energy utilization effect in the counter flow layer was better than that of the concurrent flow layer. The grain drying characteristics of cross-flow layer and standing layer was the same,the drying rate in the position of hot air inlet and outlet had a big difference, which means the drying uniformity and quality was poor, and the drying efficiency was low.2) Based on the argumentation of the similar dehydration characteristics between the single grain and the single grain in different directions, the theoretical expression of the characteristics of deep drying was deduced, and the theoretical analysis and quantitative evaluation were made on the qualitative characteristic parameters of the deep drying system.3) Through studying on volume shrinkage during drying, the volume expansion coefficient was introduced, and the theoretical expression of drying time and average flow rate of grain with different moisture content in the drying layer was proposed. Moreover, by taking the paddy drying as an example, the research of the influence of the paddy morphological characteristics, impurity rate, vessel geometry size and shape on the volume expansion coefficient was conducted. The result showed that the paddy morphological characteristics and impurity rate had significant impact on the expansion coefficient, but the vessel geometry size and shape were not significant. The expansion coefficient in the given drying system was the certain constant.4) By referring to 5HNH-15 paddy counter-flow hot air dryer, the multi-stage counter-flow tempering drying analytical model of grain was built. And then, according to the field condition of the test, the model analysis of the practical drying process was conducted. In this way, it found that the analysis result had good consistency with the field data measured. The deviation of the moisture content in the process was not beyond ±0.3%w.b.. The deviation of the outlet grain moisture content was only-0.14%w.b.. The reliability of the model was verified.5) The energy efficiency evaluation method of the drying system based on the exergy theory, the energy consumption of the low-temperature circulating dryer and the continuous dryer were respectively analyzed. The result showed that the main reason causing low average thermal efficiency and exergy efficiency of the low-temperature circulating dryer was due to quite poor energy matching in the drying process. However, its unit heat consumption of the drying system was 5200 k J/kg. Comparing with the national standard 7400 k J/kg, it still had 29.7% energy conservation. For the continuous grain dryer, its average heat efficiency and exergy efficiency were relatively ideal. Its unit heat consumption was 2944.6k J/kg. Comparing with the national standard 7400 k J/kg, the energy conservation reached 60.2%, with the significant energy-saving effect. The result showed that the reasonable matching of the drying process parameter and design parameter was helpful to realize the high-quality, efficient and energy-saving of grain drying.6) By demonstrating the existing irreversible heat losses in the system including grain warming, evaporative moisture warming, inertial flow and heat dissipation of the drying room as well as converting these losses to the grain warming, the heat capacity theoretical expression of the representation system heat loss was got. Meanwhile, a method for evaluating the performance of the drying system was proposed, that was the maximum thermal efficiency, and the method of determination was described. The maximum thermal efficiency of the circulating dryer and continuous dryer were all tested, and the value of the heat capacity of the system was obtained.