Effect of Geometry of Rice Kernels on Drying Modeling Results

Geometry of rice grain is commonly represented by sphere, spheroid, or ellipsoid shapes in the drying models. Models using simpler shapes are easy to solve mathematically; however, deviation from the true grain shape might lead to large errors in predictions of drying characteristics such as moisture content (MC) and moisture gradients (MG). This research was undertaken to determine the impact of such shape considerations on prediction of drying characteristics. Impact of shrinkage of grains caused by drying was also investigated. Three separate mathematical models, each representing rice grain by sphere, spheroid, and ellipsoid shapes, were developed to describe the drying process. These models were solved by the finite element method using Comsol Multiphysics® simulation program. Drying simulations showed important differences in predictions of MC and MG in these three models. The sphere-shaped model predicted a slower drying than the spheroid- and ellipsoid-shaped models, whose MC predictions were similar. In all three models, maximum moisture gradients (MMG) were observed along the shortest axis in the bran region. During drying, MMG increases, reaches a peak, and then decreases. Magnitude and onset of peak of MMG were different in the three models. These differences in drying predictions among the three models make it important to use the appropriate shape to represent the rice grain in mathematical models. Ellipsoid shape, which closely resembles geometry of the rice grain, was found to be the most suitable. Reliable MG predictions from such ellipsoid-shaped models could be correlated to grain fissuring, which thereafter can be employed to optimize the drying process. The impact of shrinkage of rice grains during drying on model predictions is very small. In any drying simulation, maximum error due to neglecting shrinkage would be less than 5% of total moisture loss value.