Research On Process And System Optimization Of Maize Under Hot-air Drying

China is a country which consumes a huge amount of food every year.Maize is the main crop which plays a pivotal role in the field of food security.After the harvest of mature maize,its moisture content is still high and there would be vigorous respiration inside the kernels.This not only consume some nutrients,but also produces a large amount of heat which result in the rapid deterioration.Therefore,the moisture content of harvested maize must be reduced to a safe range for storage.In this process,the large and rapid drying of maize become one of the important parts after harvest.At present,there are various drying methods,including hot-air drying,microwave drying and freeze drying,etc.Microwave drying and freeze drying have high cost but low output.So,in large-scale production,hot-air drying is mainly used.Various technology parameters(hot-air temperature,hot-air speed and tempering ratio,etc.)in hot-air drying would have an important impact on the drying effect(comprehensively considering the drying rate,energy consumption and quality with dried maize).Therefore,it is necessary to find out the optimum parameters to maximize the drying effect,explore the process method to improve the quality with dried maize and develop a control algorithm to achieve efficient and automated drying.Last but not least,the structure of hot-air drying oven is optimized to maximize the drying uniformity.The specific research are as follows:(1)Analysis of technology parameters under hot-air drying.The technology parameters of hot-air drying mainly include hot-air temperature,hot-air humidity,hot-air speed and tempering ratio.In the actual production,the demand for hot-air is very large.It is impractical to change hot-air humidity.Therefore,its impact on drying effect would not be studied.First of all,the influence of each technology parameter on drying effect is explored through single factor experiment.Then their optimal range is obtained.On this basis,the orthogonal experiment is carried out to obtain the drying effect under different combination of technology parameters.With the help of data handling,the optimal parameter group is found to reach the maximum when the drying rate,energy consumption and quality with dried maize are comprehensively considered.(2)Study on the stress characteristics of maize during hot-air drying.There is a large stress in the process of hot-air drying of kernels,which is an important factor affecting the quality of dried kernels.According to average geometric size,a three-dimensional body-fitted model of maize is established.Based on heat and mass transfer equation,stress-strain theory and the law of conservation of heat and mass transfer,a hot-air drying model is constructed.The heat and mass transfer simulated calculation is carried out to obtain the temperature gradient and moisture gradient,which coupled with the stress model to reveal the variation of drying stress.And the correctness and accuracy of hot-air drying model is verified experimentally.The results show that the thermal stress and moisture gradient stress are larger on surface but smaller in interior,and both increased rapidly at the initial stage and then decreased slowly.Maize is mainly affected by moisture gradient stress,which causes cracks due to excessive it at the early stage of drying.Properly increase air humidity can significantly reduce the stress level.The maximum error between the simulated value and experimental value of maize’s temperature is 8.5 %.(3)Research on temperature control algorithm of hot-air drying.It is evident that the hot-air temperature affects drying effect by affecting drying rate,energy consumption and the quality with dried maize.Through the analysis of technology parameters,the optimal temperature to maximize the drying effect has been obtained.Taking the temperature in the optimal parameter as the expected value,the transfer function of the temperature control system of hot-air drying oven is derived.Combining immune algorithm and fuzzy algorithm,the proportional,integral and differential coefficients of PID controller are adjusted online.A fuzzy immune PID controller based on classical PID controller is designed to accurately control the temperature during drying.With the help of simulated software,the temperature response curves of three control algorithms of classical PID,fuzzy PID and fuzzy immune PID are compared and analyzed.The results show that the rise time,settling time and peak overshoot of fuzzy immune PID controller are better than classical PID controller,and its antiinterference ability is also significantly better than fuzzy PID controller.Through comprehensive comparison,it is concluded that among the three algorithms,the fuzzy immune PID algorithm is the best method to realize automatic temperature control under hot-air drying of maize.(4)Improvement and optimization of structure with hot-air drying oven.Based on the analysis of hot-air drying of maize and stress simulated calculation,the structure of the drying oven is optimized and redesigned.Based on the flow field visualization,with the temperature uniformity and velocity uniformity as evaluation indicators,the number and relative position of the hot-air inlet are redesigned.The distribution uniformity of the flow field in the drying oven before and after the optimization is analyzed.The goal is to find the optimal structure,facilitate the drying of maize kernels,and realize the maximization of drying uniformity.