Modeling Heat and Mass Transfer of Long-Grain Hybrid Rice in a Chilled Environment

Abstract

HighlightsThis study determined adsorption and desorption isotherms of long-grain hybrid rough rice in chilled environments.Parameters of the sorption models were evaluated by fitting experimental data in a thin layer drying equation.The moisture diffusivity and activation energy values of rough rice subjected to chilled environments were determined.The finite element method was used to simulate the heat and mass transfer of long-grain hybrid rice at low temperatures. Abstract. While chilling aeration technology for grains has found use in other countries, application in the U.S. for rice has not yet been widely adapted by the industry. The aim of this study was to determine the moisture sorption behavior of rough rice in chilled environments. Such data is vital for automated grain conditioning monitoring and control of aeration systems at cool ambient conditions. Adsorption and desorption isotherms of hybrid long-grain rough rice (XL745) subjected to temperature environments which are typical in U.S. storage conditions (5°C to 15°C) were measured. A dynamic vapor sorption analyzer, the IGAsorp, which consists of an ultrasensitive microbalance within a controlled environment was used to accurately measure water sorption in rice. Changes in sample weight were continuously monitored across a range of equilibrium relative humidity (10% to 70%) until a steady-state mass was attained. Nonlinear regression analysis was used to estimate empirical constants of five models used for describing grain sorption isotherms. The appropriateness of each model in describing the equilibrium data was evaluated to minimize the root mean square error (RMSE). The modified Chung-Pfost equation best described the experimental data with RMSEs of 0.557 and 0.912 for adsorption and desorption data, respectively. The equilibrium moisture content (EMC) of rice measured at 5°C with 10% relative humidity (RH) was 12.5% (dry basis), which was higher than the EMC value predicted from other studies by extrapolation at 30°C (9.1% d.b.). All five prediction models (modified Chung-Pfost, modified Henderson, modified Oswin, modified Halsey, and GAB) were in good agreement for experimental data at moderate temperature conditions (e.g., at 30°C). The results indicated that it is more accurate to use newly developed constants for EMC models of rice in chilled environments. A three-dimensional ellipsoid shape was used to describe rough rice geometry. Mathematical models were developed to predict the moisture and temperature distribution during the drying processes at 5°C and 15°C. Models were solved by the finite element method using Comsol Multiphysics® simulation program. Increasing the drying temperature from 5°C to 15°C increased the effective moisture diffusivity from 9.0×10-13 to 1.1×10-12 m2/s. The drying activation energy value was 25.1 kJ/mol in the chilled environments investigated. Keywords: Activation energy, Chilled environment, Equilibrium moisture content, Mathematical modeling, Moisture diffusivity, Rough rice.