Mathematical modeling of rough rice dehydration with dehumidified air in a fluidized bed drying system

Abstract. Highlights Verma and Modified Henderson models were best fit for fluidized bed rice drying with and without dehumidification. Coefficient of determination for Verma et al. and Modified Henderson and Pabis models ranged from 0.9760 to 0.9996. The moisture diffusivity increased with the increase in air temperature and with air dehumidification. Activation energy under dehumidification conditions was lower than under non-dehumidification conditions. Abstract. There are numerous well-developed empirical models for rice drying, but none have been fitted to check their relevance with ambient air dehumidification of rough rice and fluidized bed drying systems. Consequently, the goals of this research were (1) to determine the best-fit model or models for rough rice dehydration in a fluidized bed dryer system using ambient air or dehumidified air, and (2) to assess the effective moisture diffusivity and the activation energy of rough rice drying. Nine mathematical models were selected and fitted to the experimental data of rough rice fluidized bed drying at 40°C, 45°C, and 50°C using ambient air or partially dehumidified air. Ambient air dehumidification was introduced as a novel technology to partially dehumidify humid ambient air to maintain the drying process running mainly in damp conditions. Nonlinear regression analysis instituted that Verma et al. model and Modified Henderson and Pabis model were the two best models for describing fluidized bed drying characteristics of rough rice with and without dehumidification conditions. The goodness of fit was determined using coefficient of determination (R2), root mean square error (RMSE), and reduced chi-square (χ2), with values for the two best models ranging from 0.9760 to 0.9996, 0.0000 to 0.0022, and 0.0040 to 0.0180, respectively. Verma et al. model was the best fit of 66.67% of the studied cases. Modified Henderson and Pabis model was the best fit for 33.33% of the studied cases. The results revealed that the evaluated adequate moisture diffusivity increased with the drying air temperature and ambient air dehumidification. It varied from 2.19x10-9 to 2.44x10-9 m2/s with R2 higher than 0.9556. The activation energy under the dehumidification conditions (13.720 kJ/mol) was lower than the activation energy under the non-dehumidification conditions (19.756 kJ/mol). Pre-exponential factor values were 4.908 x 10-7 and 4.940 x 10-6 with and without dehumidification, respectively. Future grain drying research can utilize from the modeling results about the fluidization drying technique and external ambient air dehumidification.