توصيفگر ها :
استراحتدهي , الگوريتم كنترلگر , دياگرام و تابع حالت , شبيهسازي عددي , گراديانهاي محتواي رطوبتي و حرارتي
چكيده انگليسي :
During the drying of paddy, according to the changes in temperature and moisture content, rice starch shows different physical and thermal properties and can change from rubbery state to glassy state and vice versa. These changes have a significant effect on cracking and breakage in the next milling stages. Intermittent drying is an approach to overcome this issue. The operating parameters of the intermittent drying method are very important. Determining these parameters by laboratory methods is very time-consuming and costly due to the many levels existed for them. Through considering the glass transition concept and the behavior of the rice layers, drying process could be controlled and optimized in such a way that the process is done at the least duration and energy consumption. This also helps to prevent from the qualitative production loss and the creation of initial cracks during drying. The purpose of this study was to determine the optimal operating parameters of the intermittent drying method using simulation and numerical modeling. So that, the dynamic behaviors of different layers could be investigated. To do this, a new mathematical model for simulating and prediction the intermittent drying process of rice was developed in COMSOL Multiphysics based on the finite element method, and taking the concept of glass transition into account. In this model, the real 3D body fitted geometry of rice grain was used for simulation. In order to determine the required coefficients in the simulation and validation of the model, continuous and intermittent drying experiments were performed at temperatures of 40, 50, 60, and 70ºC. A correlation coefficient of 99% was observed between the experimental and simulated results for the kinetics of the average moisture content. According to the glass transition concept, a quantitative function state (S function) was defined to determine the state change of different layers of rice grain at each time. In addition, the spatial and temporal temperature and moisture content distribution inside the rice kernel were simulated. In the next step, with the aim of performing the drying operation within the rubbery state, preventing the growth of thermal and moisture content gradients, and minimizing the duration and energy consumption, an optimization controller algorithm was designed and implemented in Matlab software. By coupling this algorithm to the developed model, the optimal values of the number of stages, and drying and tempering duration at each stage were determined and calculated. Furthermore, the energy consumption under each drying condition was determined according to the calculated drying durations. The results showed that with increasing drying temperature, the number of drying stages decreased. Also, with an increase in the drying temperature, the total duration of the operation decreased considerably. Under drying conditions at 40 and 50ºC, drying in the third stage caused the surface and middle layers transit into the glassy state, which could lead the possibility of cracking. However, at the temperature of 60 and 70ºC, the drying was completely done in the rubbery state. The maximum and minimum amount of moisture content gradients (MCGs) were equal to be 11.3% d.b. and 6.9% d.b. at temperatures of 70 and 40ºC, respectively. The percentage of the rubbery state area at the end of the drying process was also determined to be 0%, 9.2%, 66.3%, and 85% at temperatures of 40, 50, 60, and 70ºC, respectively. Finally, comparison of the optimization results revealed that a two-stage intermittent drying method at 60ºC including the first drying stage for 43 min, the first tempering stage for 167 min, the second drying stage for 8 min, and the second tempering stage for 129 min, will result in the best performance in terms of the state changes, total drying duration, moisture content gradient, and energy consumption.
