Identification of Appropriate Size and Operating Parameters of Recirculating Paddy Dryer for Major and Husking Rice Mill of Bangladesh

Automatic and semi-automatic rice mills have large-scale separate dryer for parboiled and aromatic paddy whereas major and husking mill require medium-scale dryer. Hence, the objective of the study was to identify the appropriate size of medium-scale dryer and controlling conditions to dry both parboiled and aromatic paddy in a dryer. Technical data of 22 rice mills of Mymensingh, Sherpur, and Netrokona districts of Bangladesh were collected during winter season of 2020 using instruments and semi-structured questionnaire. Capacity utilization and huller capacity of major, husking, automatic, and semi-automatic rice mills were found 29.4% and 1.9 ton/hour, 24% and 2.7 ton/hour, 72.6% and 4.8 ton/hour, and 64.3% and 2.6 ton/hour, respectively. The capacity utilization of both major and husking mill can be optimized to 72.5% with the condition of adopting 12 ton dryer and huller capacity of no less than 2.5 ton/hour. Hot air temperature and exposure time per ton parboiled and aromatic paddy were 85°C and 2.43 minute; 70°C and 2.83 minute; respectively. Synchronizing air temperature with exposure time, both type of paddy can be dried in one dryer. Initially, 29 minute exposure of 12 ton parboiled paddy with ambient air will equalize the previous hydro-thermal stress. Slowly passing of 60°C hot air will be enough for removing exterior lose (>26%) moisture. Elevated temperature of 80°C can be used to reduce the kernel moisture to 16%. Finally, to ensure uniform drying lowering of hot air flow rate and temperature by 15°C could be an energy efficient option.