Energy of activation, instantaneous energy consumption, and coupled heat and mass transfer modeling in drying of sorghum grains

Abstract

Sorghum [ Sorghum bicolor (L.) Moench] has attracted much attention due to its promising approach for ethanol production. One main issue of industrial-scale sorghum processing is drying, which is a highly energy-cost operation. On the other hand, researchers have been suggesting potentials for energy savings by analyzing simultaneous heat and mass transfer phenomena by mathematical modeling. The current study aims to perform mathematical modeling of the drying process of sorghum grains considering the coupling between heat and mass transfer and analyze its energy performance. A mathematical model was developed consisting of two differential equations encompassing a transient macroscopic energy equation and a differential form of the analytical solution of the diffusion model. The model parameters were estimated from experiments conducted in a laboratory-scale fixed bed dryer under air temperatures ranging from 40 to 75 °C and air velocity of 1.8 m s −1 . It has shown that moisture diffusion is the limiting transport mechanism due to the approach to the isothermal drying condition, leading to a good correlation between predicted and observed data. The estimated value of activation energy was 14.32 kJ mol −1 , which is in the range for grains and seeds. Instantaneous values of the specific energy consumption decreased exponentially over time, and the calculated mean values varied from a minimum of 7358.5 kJ kg −1 to a maximum of 8927.1 kJ kg −1 from 40 to 75 °C. The findings will be used in future optimization studies of sorghum drying to balance process costs with system energy consumption. • Experimental analysis of convective air-drying of sorghum grains. • Mathematical modeling considering analytical solution of the diffusive model. • Mathematical modeling of the heat and mass transfer. • Analysis of the coupling between the heat and mass transfer. • Determination of instantaneous indices of specific energy consumption.