腐殖酸肥料射频真空干燥特性

Radio frequency vacuum drying characteristics of humic acid fertilizer

为研究腐殖酸肥料射频真空干燥特性及品质,该研究探讨了不同极板间距(160、165 mm)、不同初始湿基含水率(10%、15%、20%)下腐殖酸肥料的升温速率和加热均匀性,在此基础上进行射频真空干燥试验,并以热风干燥试验为对照,采用Weibull函数拟合腐殖酸肥料射频真空干燥特性曲线,对比射频真空干燥与热风干燥对腐殖酸肥料颜色、总氮、总磷、总钾及有机质的影响。结果表明:真空度为0.085 MPa,极板间距为160 mm、初始湿基含水率为15%时,干燥速率和升温速率较快,加热均匀性较好;升温曲线在53℃出现拐点,53℃前升温较快,53℃后升温较慢;除含水率为20%的物料外,极板间距对加热均匀性影响不显著(P>0.05),但初始湿基含水率对其有显著影响(P<0.05);射频真空干燥所需时间较热风干燥缩短约208 min,射频真空干燥可显著提高干燥速率(P<0.05);Weibull函数能较好的拟合腐殖酸肥料射频真空干燥过程,尺度参数和形状参数随极板间距的减小而减小,随初始湿基含水率的增加呈先减小后增大的趋势;水分有效扩散系数,随极板间距的减小而增大,随初始湿基含水率的增加,呈先增大后减小的趋势;极板间距与初始湿基含水率总体上对腐殖酸肥料的色泽高度L*、红绿值a*、蓝黄值b*、总氮、总磷、总钾、有机质无显著影响(P>0.05);65℃热风干燥后样品的色差值显著高于射频真空干燥(P<0.05),总氮和有机质含量显著低于射频真空干燥(P<0.05)。在设置的试验条件下,腐殖酸肥料的较佳干燥参数为真空度0.085 MPa、极板间距160 mm、初始湿基含水率15%。研究结果可为腐殖酸肥料射频真空干燥应用提供参考。

Humic acid fertilizer has been one of the most important multifunctional fertilizers,particularly for the recyclable organic resource.There is an outstanding emulsifying and dispersing effect of humic acid fertilizer as protein feed on pesticides for the wide application prospect.Drying has widely been a necessary step for the processing of humic acid fertilizer for the high quality products.However,the slow heat transfer,low heating uniformity,and low efficiency often occur in the commonly-used drying for the humic acid fertilizer.Fortunately,radio frequency(RF)vacuum drying can be widely expected to combine RF heating and vacuum.An RF vacuum drying can be used to keep the material in a high frequency alternating electromagnetic and vacuum field,where the electromagnetic wave between electrodes can produce the heat inside the material through the friction caused by the charged ion shift and dipole rotation,leading to the significantly improved heating speed,heating uniformity,moisture migration rate,and penetration depth for the high drying efficiency.The vacuum environment can also be beneficial to reducing the color changes and the loss of important components during drying.This study aims to investigate the RF vacuum drying characteristics and the quality of humic acid fertilizer.The heating rate and heating uniformity of humic acid fertilizer were recorded at different electrode gaps(160,and 165 mm)and initial moisture content(10%,15%,and 20%w.b.).An RF vacuum drying experiment was carried out,where the hot air drying was treated as the controlled group.A Weibull model was selected to fit the drying characteristic curves of a humic acid fertilizer.A comparison was made on the performances of RF vacuum and hot-air drying on the color,total nitrogen,total phosphorus,total potassium,and organic matter in the humic acid fertilizer.The results indicated that the better heating uniformity,the higher heating,and drying rate were achieved under 0.085 MPa vacuum degree,160 mm electrode gap,and 15%initial moisture content.Temperature-time curves of humic acid fertilizer presented the inflection point at 53℃,where the heating rate before 53℃was higher than after 53℃.Except for the material with 20%moisture content,there was no significant effect of the electrode gap on the heating uniformity in the top layer of humic acid fertilizer(P>0.05),but a significant influence of initial moisture content was found(P<0.05).The time required for the RF vacuum drying was about 178 min shorter than that of hot air drying,indicating that the RF vacuum drying could significantly improve the drying rate(P<0.05).The Weibull model was also used to accurately simulate the RF vacuum drying process.The scale parameters and shape parameters decreased with the decrease of electrode gap,but first decreased and then increased with the increase of initial moisture content.Effective diffusion coefficients Deff increased with the decrease of electrode gap,and first increased and then decreased with the increase of initial moisture content.Meanwhile,in general,neither electrode gap nor initial moisture content had a significant influence on the color,total nitrogen,total phosphorus,total potassium,and organic matter of humic acid fertilizer(P>0.05).By contrast,the humic acid fertilizer that was dehydrated by hot air drying at 65℃presented a significantly higherΔE value,but a lower total nitrogen and organic matter content,compared with RF vacuum drying(P<0.05).An optimal combination of RF vacuum drying parameters was achieved,where a 160 mm electrode gap,and 15%initial wet basis moisture content.This finding can provide the theoretical basis for the application of RF vacuum drying of a humic acid fertilizer.