膨化颗粒饲料穿流干燥试验

Experimental study of cross flow drying of expanded feed particles

为了了解颗粒物料在穿流干燥过程中的干燥特性,利用自制的干燥试验台,在不同的干燥条件下以膨化饲料颗粒为例进行了穿流干燥试验。分析了通风方式(单向通风和换向通风)、热风温度(90和100℃)、床层厚度(5、7.5、10、15、20 cm)对其干燥动力学及干燥均匀性的影响。研究结果表明,床层厚度为15 cm,干燥至30 min时,换向通风的干燥不均匀度为10%,而单向通风的干燥不均匀度为23%,因此,换向通风比单向通风物料的水分均匀性提高,但不能提高干燥速率。当床层厚度为20cm,干燥将要结束时,100℃干燥的干燥不均匀度比90℃干燥要高6%,即温度越高,物料的干燥不均匀度越大。单向通风试验在床层厚度为10cm、热风温度为100℃时,会出现整体床层的"表观恒速干燥"现象;当床层厚度大于10cm时,会出现短暂的干燥速率增加现象。

Extrusion process is a widely used processing technology in feed industry. Expanded feed is popularly applied in aquaculture and animal cultivation. However, the moisture content of the freshly expanded feed is big for safe storage. Mostly, the feed is dried by cross flow drying method. Therefore, in order to study the cross flow drying behavior of expanded feed particles in fixed bed, experiments were performed in a laboratorial scale dryer. Effect of ventilation method, hot air temperature and bed thickness on drying kinetics and drying uniformity was discussed. Thin-layer drying, one-way drying and reversing airflow direction drying were investigated in digital tunnel dryer and domestic drying test bed respectively. Hot air temperature was set at 90 and 100℃,while the bed thickness was set at 5, 7.5, 10, 15 and 20 cm. The moisture content and temperature of the feed, air temperature and relative humidity at outlet were determined by oven （DHG-9140A, Jinghong, Shanghai, China）, electronic balance （ FB224, Hengping, Shanghai, China）, infrared thermometer （Raynger ST6L, Reytek, USA） and humidity and temperature meter（HM70, Vaisala, Finland） respectively and the moisture content and moisture uniformity curves were plotted with the obtained data. The results showed that the reversing ventilation drying couldn’t increase the drying rate, but it could balance the moisture distribution during cross flow drying, because the expanded feed at the bottom layer of the bed was dried faster at the beginning, but moisture absorption emerged at the bottom layer after airflow direction was first reversed. The moisture content increased and the relative humidity decreased from 95%to 60%at the first reversal at 100℃, and the reduced moisture was detained in the feed. Thus, local over-drying was avoided and the feed could be dried uniformly by changing the airflow directions constantly. Higher hot air temperature could generate local over-drying easier, so the difference of moisture content between the top layer and the bottom layer would be bigger, which could lead to the larger moisture nonuniformity. A superficial constant drying rate stage was observed at a critical bed thickness （10 cm） in one-way drying, but for a bigger bed thickness, an increasing drying rate stage was observed. The reason was that moisture absorption was observed at the top layer at the beginning of drying at bed thickness of 15 and 20cm and then moisture content increased slightly. When the drying front reached the top layer, feed particles with bigger moisture content were dried and the drying rate was bigger. But the superficial constant drying rate was smaller than thin layer constant drying rate. The thin layer drying rate was 0.034kg/（kg min） at the temperature of 60℃, while the superficial constant drying rate was 0.0275 kg/（kg min） at the temperature of 100℃.