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

Abstract

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 °C, and the nonisothermal drying experiments were performed at a heating rate of 10 °C/min from 30 to 140 °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 × 10 −8 to 5.13 × 10 −8 m 2 /s, and the drying activation energy values were 5.3 and 6.5 kJ/mol under isothermal drying conditions and nonisothermal drying conditions, respectively.

A comparison of isothermal with nonisothermal drying kinetics of municipal sewage sludge

“…In addition, the apparent activation energy of the sewage sludge in the isothermal drying was less than that in the nonisothermal drying. Similar result was found in the drying of biomass [13].…”

“…Chen et al [13] revealed the isothermal drying kinetics of powdered rice straw particles at 50-80°C, indicating that the logarithmic model was the best model for describing isothermal drying.…”

“…Chen et al [13] also obtained the nonisothermal drying of powdered rice straw particles at a heating rate of 10°C min -1 from 30 to 140°C and derived that the Henderson model had the best fit for the drying.…”

“…The drying of the sewage sludge is a complicated process as it mainly depends on drying method, operating conditions and also the status of the water in the sludge [10,11]. Kinetics analysis on drying is a significant approach for better elucidating of the moisture migration mechanisms and giving available information for designing and improving a drier [13].…”

“…Comparison of the kinetics of data obtained under isothermal and nonisothermal conditions is a source of information about the mechanism of the process [26]. A thermogravimetric approach is also applied to reveal the drying kinetics of rice straw particles under isothermal and nonisothermal conditions [13]. Cai et al [27] indicate that the Page model is the best for describing the nonisothermal drying characteristics of wheat straw and corn stalk within the range of room temperature to 170°C at a heating rate of 30°C min -1 .…”