Simulation and process optimization of upward and downward reversing ventilating drying by fixed bed

Abstract: In recent years, with the rapid development of harvest mechanization in China, the whole level of wheat harvest mechanization has been more than 90%. The current sun-drying facilities in Chinese rural areas are far from being able to meet the requirements of post-harvest drying of wet wheat. Mechanical drying is gradually considered to reduce post-harvest losses of crops in Chinese rural areas. As an economical and practical grain dryer, bin-ventilation dryer is considerably fitting for Chinese rural production practice. However, such devices are always without recommended work parameters for different crops under different ambient conditions and different initial moisture contents of material, so a large sum of energy is wasted in drying process. In order to obtain the best work parameters for a self-developed bin-ventilation dryer whose ventilation direction could be changed by manual operation, a set of partial differential equation models for wheat drying of fixed bed were developed, which reflected the relationships of heat and mass transfer between material and air during the drying process. On this basis, for the unique characteristics of upward and downward reversing ventilating, the partial differential equation models were discretized by forward finite difference method. Besides, the simulation program of upward and downward reversing ventilating drying of fixed-bed wheat was written by software. Through the simulation program, real-time drying status of wheat, drying time consumption and economic cost could be calculated by inputting ambient temperature, ambient relative humidity, air temperature, air volume, and initial moisture of wheat. Then the optimal operating parameters, which caused the lowest economic cost, would be obtained under different ambient conditions and initial moistures of wheat by further analysis and comparison. In order to verify the accuracy of simulation results, experimental studies of post-harvest wheat drying were performed with the self-developed 5H-2.0A box-typed static bed reversing ventilation dryer, which involved in testing and analysis of the average moisture distribution of materials in the thickness direction of wheat bed and the energy dissipation. The wheat dry bed was 40 cm in depth, with an initial average moisture content of 19.2% (wet basis). The air temperature and volume were set as 40℃ and 625 m3/(m2·h) respectively, and the ambient average temperature and relative humidity were 26.4℃ and 62.3% respectively during the experiment. The area of drying region on horizontal plane was evenly divided into 9 units for testing moisture content of the wheat dry bed, and the dry bed in 40 cm deep on vertical plane was evenly divided into upper, middle and bottom layer. It was concluded that the partial differential equation models could be used to simulate the actual drying process of wheat reliably. The model simulation results were in consistent with the experimental results, and the correlation coefficient of average moisture content for the whole wheat bed between test value and analog value was 0.995, among which correlation coefficients for the upper, middle and bottom layer were 0.994, 0.973 and 0.998, respectively. Based on this, the change rule and mechanism of moisture and temperature distribution on wheat bed during reversing ventilating drying were analyzed. Finally, when the ambient temperature ranged from 20 to 35℃, the ambient relative humidity varied from 20% to 85%, and the average initial moisture content was 16%, 18% and 20%, respectively, the best ventilation temperature and volume, which caused the lowest cost of energy consumption, were analyzed and then determined by comparing the cost of energy consumption at different air temperature (from 40 to 50℃) and air volume (from 500 to 1000 m3/(m2·h)).