Continuous modeling of superheated steam drying of single rice grains

Abstract

In this work, a transient macroscopic model is proposed for studying the conjugated heat and mass transfer phenomena that occur inside a single rice grain during the superheated steam drying process. The governing equations of this model are derived based on the volume-averaging approach. Since rice is a highly hygroscopic material, more than 80% of water is accumulated as bound water in rice grain. Both the bound water diffusive flow in the solid matrix and the convective flow of water vapor and liquid water in the void volume are taken into account in this model. Thermophysical properties of rice which serve as model inputs are measured, except the absolute permeability and the bound-water diffusivity coefficients. These two properties are determined by minimizing the square of residuals between simulation results and experimental data, which were obtained by means of a magnetic suspension balance system at a drying temperature of 160 °C. The validity of these two estimated parameters is reflected by a good agreement between the numerical and the experimental data at drying temperatures of 120 and 140 °C. Finally, a model-based sensitivity analysis is carried out, from which quantitative guidelines for the relationship between both the absolute permeability and the bound-water diffusivity and drying kinetics are proposed.