Experimental study of cross flow drying of expanded feed particles

Abstract: 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℃.