High Temperature Spouted Bed Paddy Drying with Varied Downcomer Air Flows and Moisture Contents: Effects on Drying Kinetics, Critical Moisture Content, and Milling Quality

Abstract

This research is an experimental study on paddy drying conducted in a two-dimensional spouted bed batch dryer to investigate the effects of downcomer airflow, drying temperature, and initial moisture content on drying kinetics, milling quality, and thermal energy consumption. The system of spouted bed drying and a comparison of spouted bed drying in the present study, and fluidized bed drying in the related literature, are also discussed. Downcomer air flows of 0, 20, and 30% ([defined as mass flow rate of downcomer air/total mass flow rate of air] × 100%), inlet air temperatures of 110, 130, and 150°C, and initial moisture contents of 18–35% d.b. were used. It was found that moisture transfer did not only occur in the spout region but also took place in the downcomer region. The moisture content and temperature of the paddy dropped as grain moved downward in the downcomer, which resulted from the presence of an evaporative cooling phenomenon. The characterization of drying curves regardless of any drying condition could be described by nearly linear relationships between moisture content and time. Although high downcomer air flow and drying temperature could enhance effective moisture reduction, they caused an adverse decrease in critical moisture content and HRY (head rice yield). Critical moisture content and HRY could also increase with an increase in initial moisture content. The difference in moisture content between the initial and critical moisture contents varied between 4.5 and 8.0%d.b., depending upon drying conditions. No significant effect on color was evident. From the point of view of HRY, the correct management of a two-stage spouted bed drying system could be a suitable and attractive alternative for rice mills. Finally, a comparison between the spouted bed drying and fluidized bed drying showed that the spouted bed had advantages over the fluidized bed in terms of product quality. The specific drying rates (kg water evaporated h⁻¹ m⁻³) of both techniques were comparable. With respect to energy consumption, spouted bed drying was not as efficient as fluidized bed drying for intermediate initial moisture content, but a contrary result was obtained for low initial moisture content.

Keywords:

• Dehydration
• Dryer
• Fluidized bed
• Head rice yield
• Product quality
• Rice mill
• Rough rice
• Spouting
• Thermal energy consumption
• Whiteness

ACKNOWLEDGMENTS

This research was supported by a grant from Thailand Research Fund (TRF). We would like to gratefully acknowledge the contribution of Viboon Thepent (Post Harvest Engineering Research Group, Agricultural Engineering Institute, Department of Agricultural) for milling quality test equipments. Thanks also are due to graduate students of mechanical engineering program (Mahanakorn University of Technology): Karn Karnjanabat, Siam Buntaowong, Arnut Suksa-add, Niwat Yimyoo, Arrun Maneenun, Arkom Siamol, Vacharapong Rukthongloh, Ubol Somjai, Apisit Prasertvit, Pakpoom Srilian, Itipon Yatoonkroh, and Nipol Chaitui for their assistance in experiments.

Notes

^a DC = downcomer.

^b Difference in moisture content between initial and critical moisture contents.

^a Air recycled fraction ranged between 0.7 and 0.8.

^b Using diesel oil with 97% combustion efficiency instead of electrical heaters was an assumption for these SEC_th calculations.