EFFECTS OF DRYING CONDITIONS OF FIXED BED LONGAN DRYING ON OPTIMAL BED THICKNESS

摘要

In this work, it is presented a study of the effects of drying conditions on the optimal bed thickness of the whole longan. The criteria for evaluation of the drying process are specific energy consumption and drying time which the difference of moisture between top and bottom of drying chamber is less than 10% dry basis. The mathematical model is developed for finding the effects of the drying conditions on the optimal bed thickness. The drying conditions are drying air temperature, specific air flow rate, and fraction of recycled air. Experimental data were compared with the simulated results to verify the model. Furthermore, the sensitivity analysis of the fraction of air recycled, drying air temperature, specific airflow rate, initial moisture content, and bed thickness of longan are study. The results showed that there was good agreement between the simulated drying rate and those experimentally observed. In addition, there was a well agreement with respect to the shapes of the drying air temperature and product temperature profiles. From the simulated results, the optimal bed thickness of 40 cm, the specific energy consumption of 10.56 MJ/kg-water and drying time of 64.2 h were found. The responsive conditions were drying air temperature of 75°C, the fraction of recycled air of 90%, and the specific air flow rate of 73 kg-dry air/h-kg dry longan.