筒状固定床花生通风干燥性能指标模拟与分析

Performance index simulation and analysis of peanut ventilation drying in barrel-shaped fixed bed

为了解筒状固定床花生干燥机理、作业性能,确定合理的结构和通风参数,根据干燥过程花生荚果和介质空气间的热质传递关系,以PDE模型为理论基础,建立了适用于筒状固定床花生通风干燥计算机模拟的离散模型,该模型可计算花生干燥实时状态及批次干燥耗时、不均匀度、生产率、单位质量能耗等干燥指标。经试验验证,模型模拟结果与试验结果基本相符,料层花生平均含水率和温度模拟值和试验值的相关系数r>0.975,模型模拟可用于筒状床花生干燥过程分析。在此基础上,分析了单位面积通风量、筒状固定床外径、内径变化对上述指标的影响。结果表明:受介质空气温度降低和相对湿度增加影响,内层物料干燥起始时间和干燥速率存在一定的滞后性,但单位面积通风量沿通风方向逐渐增大的特性对内层物料的干燥滞后有较好改善;随着单位面积通风量增加,干燥不均匀度明显降低,生产率亦有显著提高,但单位质量能耗增幅较大;筒状床外径增加或内径减小都可增加生产率,降低单位质量干燥能耗,但干燥不均匀问题很难解决。为进一步确定最优的结构和通风参数,采用均匀设计模拟试验和综合加权评分法,得出筒状固定床外径2.75 m,内径0.935 m,外进风面单位面积通风量0.36 m^3/(m^2·s)时干燥效果最优,此时干燥时耗39.2 h、生产率86.55 kg/h、单位质量能耗5.87×10~6 J/kg、干燥不均匀度1.54%。该研究可为筒状固定床干燥设备设计优化提供技术支撑。

Mechanical drying become gradually used to reduce post-harvest losses of peanuts because the conventional sun-drying facilities cannot meet the requirements of peanuts drying in China. A barrel-shaped fixed bed drying, a kind of economical and practical dry method, which can be effectively suitable for the specific conditions in Chinese rural production. However, there are few reported on the parameters of structure and ventilation associated with the barrel-shaped fixed bed. In order to obtain the drying mechanism and operating performance, a set of partial differential equation models were proposed for peanut drying in the barrel-shaped fixed bed, indicating the heat-and mass transfer relationships between peanut pods and medium air during the processing of mechanical drying. Then, a fully discretized physical model was established for the computer simulation. This model could also be used to calculate the moisture content and temperature of the peanut material layers, as well as the humidity and temperature of air medium in the real-time state during the peanut drying, specifically including the time consumption, unevenness distribution, productivity and energy consumption in the per unit mass during operation simulation for each running of peanut drying. The simulation results were in good agreement with the experimental ones. Between test-and simulation values, the correlation coefficients of the average moisture content for the outer, middle and inner of peanut material layers were 0.997, 0.995 and 0.998, respectively, whereas the correlation coefficients of the temperature for the outer, middle and inner of peanut material layers were 0.995, 0.979 and 0.991, respectively. The changing mechanism was therefore established to analyze the moisture and temperature distribution on the peanut-pods bed during drying process. The ventilation volumes per unit area of the outer cylindrical surface, external diameter and inner diameter of the barrel-shaped fixed bed were also analyzed according the previous four drying indexes(drying time consumption, productivity, energy consumption per unit mass, drying unevenness). The calculated results showed that the starting time of drying and drying rate of the inner materials layers were lagged due to the decrease in the temperature of air medium, while the increase in the relative humidity, compared with the outer layers in the peanut drying. However, the drying delay of the inner materials was well relieved as the increase of the ventilation volume per unit area along the ventilating direction, whereas the drying evenness of the barrel-shaped fixed bed decreased significantly as the increase of the ventilation volume per unit area of the outer cylindrical surface, but the productivity and the energy consumption per unit mass increased significantly. The productivity of peanuts would be increased, while the energy consumption per unit mass of the batch drying would be reduced as the increase in the external diameter or decrease in the inner diameter of the barrel-shaped fixed bed, but the distribution of the peanut moisture contents after drying operation was still remained unclear during this time. Therefore, the uniform design simulation test and synthetical weighted mark method were used to obtain the optimal structure-and ventilation parameters. The simulated results demonstrated that the optimal drying condition was achieved, where the external diameter of the barrel-shaped fixed bed was 2.75 m, the internal diameter was 0.935 m, and the ventilation volume per unit area of the outer cylindrical surface was 0.36 m^3/(m^2 s). At this time, the drying time consumption was 39.2 h, the productivity was 86.55 kg/h, the energy consumption per unit mass was 5.87× 10~6 J/kg, and the drying unevenness was 1.54%. These findings can therefore provide systematically technical support to the design and optimization of the barrel-shaped fixed bed for peanuts drying.