Numerical modeling of water uptake in white rice (Oryza sativa L.) using variable diffusivity approach

Abstract

Soaking and drying of rice kernels are two very important processes in parboiling. Since diffusion of water through the rice kernels plays a crucial role in the design and operation of parboiling plants, it becomes essential to develop models to predict accurately the process of water uptake or removal under various operating conditions. In this study, moisture absorption by white rice (Oryza sativa L.) has been studied using a nonlinear model of Fickian diffusion. The moisture diffusion coefficient has been varied as a function of both moisture content and temperature. The proposed model has been tried with two different boundary conditions: (i) immediate surface saturation and (ii) two layers with different diffusion properties, using finite difference approach, and fitted into previously published experimental data for a temperature range of 25–80 °C. The model, using variable diffusivity and a fixed grid step method, explains the absorption process and the phenomenological effect of swelling (expansion) due to absorption comparatively better than a model which uses effective (lumped) diffusivity. An optimal set of process parameter values are determined for this temperature range. The proposed model is validated with hydration and swelling characteristics of chickpeas (Cicer arietinum L.) using experimental results obtained from literature.