高压电场下玉米的干燥特性

玉米刚收获时含水率一般超过18%,为了能够安全存储必需将含水率降低到14%以下,因此干燥是玉米生产加工中不可缺少的工序。该文利用高压电场干燥技术,以玉米为对象研究了物料厚度、针电极密度对干燥特性的影响,并与自然通风干燥对比分析了高压电场干燥玉米的脱水速率和含水率的变化。结果表明:高压电场干燥时,当针电极密度在113~283根/m2范围内时,脱水速率随针状电极的密度的增加呈先增大后减小的趋势,针电极存在一个较佳密度170根/m2;而物料堆积厚度对高压电场干燥的影响较小,当物料堆积较薄(d=0.8 cm),含水率为36.7%时,电场脱水速率是对应自然通风脱水速率的2倍,干燥时间是自然通风干燥的50%;但当物料的堆积厚度较大(d=1.9 cm),含水率为20%时,电场脱水速率是对应自然通风干燥的6.5倍,可节省85%的干燥时间,当物料的堆积厚度大于3.6 cm时,水分的自然蒸发对电场干燥的影响可忽略;高压电场干燥的能量利用率不低于9.3%,是热风干燥的10余倍。因此高压电场干燥适用于厚物料层的干燥,不仅节能而且还能有效减少晾晒场地和工作量,提高干燥效率。研究结果可为玉米等粮食作物的高效节能干燥提供参考。

Drying characteristics of corn in high voltage electric field

Abstract: The internal moisture in corn exists in the states of free water and bound water. It is generally beyond 18% (Wet basis) in the new corn harvested, however, in order to storage safely, the moisture content of corn should be kept below 14%. Therefore, drying becomes an indispensable process in corn's production and processing. The aim of this study was to investigate the dehydration rate of corn by changing the stacking thickness of corn, the density of needle electrode and the electric field intensity in a high voltage needle-disk electric field drying system, as well as to analyze the difference between the traditional natural air drying and hot air drying methods. The results indicated that the dehydration rate increased and then decreased with the increase of the density of needle electrode. When the voltage between the needle and the disk was 40 kV and the separation distance between them was 6.2 cm, the optimum needle electrode density was 170 pieces/m2, but its increasing was not significant. Perhaps the reason was that when the density of the electrode was small, effective electric field intensity was not able to cover all the material. However, when the density of the electrode became larger, the charge distributed on the needlepoint was reduced and the electric field intensity was weakened, and the interference increased between two adjacent needles, which led to the decrease of the dehydration rate. Furthermore, the corn stacking thickness had little effect on the dehydration rate under high voltage electric field. There was no obvious effect on the dehydration rate when the thickness was less than 4.6 cm, but the dehydration rate slightly slowed down with the increasing of the stacking thickness, due to the natural evaporation of moisture in corn. The natural evaporation of moisture occurs mainly in the surface layer of material and the thinner stacking thickness leads to the more natural evaporation and the relative faster dehydration rate. When the stacking thickness was thinner than 1 cm, the dehydration rate under the electric field was double to that by natural drying and the drying time reduced by 50%. On the other hand, when the thickness was equal to 1.9 cm, the electric dehydration rate was 6.5 times higher than the natural drying and the drying time reduced by 85%. Subsequently, when the thickness increased from 1.9 to 4.6 cm, the dehydration rate increased only by 28%. On the contrary, the drying time extended with the increase of the thickness in the natural drying, because the natural drying occurred mainly in the surface layer. However, in the high voltage electric field drying system, electric field was evenly distributed in corn material and drying happened in the interior and surface area of the material at the same time, therefore, the stacking thickness of corn showed a less effect in the high voltage electric field drying system. In conclusion, the high voltage electric field drying is suitable for drying the thicker materials which can effectively reduce the drying space and workload, as well as improve the efficiency of drying.