Drying Kinetics of Rice Straw under Isothermal and Nonisothermal Conditions: A Comparative Study by Thermogravimetric Analysis

A thermogravimetric approach was applied to study the drying of powdered rice straw particles to determine the drying kinetics under isothermal and nonisothermal drying conditions. The isothermal drying experiments were performed at 50, 60, 70, and 80 degrees C, and the nonisothermal drying experiments were performed at a heating rate of 10 degrees C/min from 30 to 140 degrees C. The thermogravimetric approach was suitable for determining the effective moisture diffusivity and drying kinetics due to its online weight recording and minimal material requirements as well as its capacity for precise temperature control. The drying process under both conditions mainly occurred during the falling rate period. However, the trend of water loss in rice straw under nonisothermal conditions was different from that under isothermal conditions, especially in terms of the drying rate and the final moisture content. Four widely used models were chosen to describe the isothermal drying process, and their corresponding nonisothermal models were obtained for describing the observed nonisothermal drying behavior. The results of model fitting showed that the logarithmic model was the best model for describing isothermal drying, whereas the nonisothermal Henderson model had the best fit for results obtained under nonisothermal conditions. The effective moisture diffusivity varied from 4.35 x 10(-8) to 5.13 x 10(-8) m(2)/s, and the drying activation energy values were 5.3 and 6.5 kJ/mol under isothermal drying conditions and nonisothermai drying conditions, respectively.