Evaluating the heat and mass transfer effective coefficients during the convective drying process of paddy (Oryza sativa L.)

AbstractThis work investigates important heat and mass transfer parameters of a variety of paddy named Fajr, during a thin layer convective drying. The experiments on drying are conducted at air temperatures in the domain of 30–80°C with air velocities between 0.54 and 3.27 m s−1. Mathematical equations are developed to adjust the experimental data obtained for the thin layer drying. The drying results show that the Henderson and Pabis' model satisfactorily fit all the experimental data for the convective drying behavior of paddy. The effective diffusivity of moisture transfer, calculated by the Fick's second law of diffusion, varies from 1.109 × 10−11 to 5.858 × 10−10 m2 s−1 and the highest values of the Dincer, Biot, Reynolds, and Nusselt numbers obtained are 2.383 × 105; 0.2026; 1.833 × 107; and 8,434, respectively. The obtained results demonstrate that the coefficient of mass transfer (hm) for paddy lies between 5.118 × 10−8 and 4.807 × 10−6 m s−1, while the heat transfer coefficient (hc) ranges from 811 to 2,474 W m−2 K−1. Furthermore, the values of the energies of activation obtained for moisture diffusion (Ed) and convective mass transfer (Ec) are in the range of 50.538–61.825 kJ mol−1 and 72.325–87.386 kJ mol−1, respectively. Therefore, knowledge regarding to the determination process of heat and mass transfer characteristics can help to distinguish the suitable operating conditions for saving the maximum value of energy.Practical ApplicationsSome grains have a short shelf life that limits their commercialization as fresh products and increase postharvest losses. Drying is an interesting alternative to the development of new products of grains like paddy with added value. This research work involved the estimation of heat and mass transfer properties during convective drying by using two calculation methodologies, allowing estimating the diffusivity and the mass and heat transfer coefficients for paddy. Also, the activation energy for moisture diffusion and for convective mass transfer was determined, assuming that the temperature dependence of the diffusion coefficient and the mass transfer coefficient follows an Arrhenius type relationship. Therefore, also the study presents a simple method to greatly enhance the shelf life of wet paddy by convective drying and it can be applied for the better preservation of this product.