Exergy analysis and optimisation of an industrial-scale circulation counter-flow paddy drying process

Drying is a process of removing water from products down to a specific moisture content. It is an energy-intensive process widely-used for grain and biomass processing. Because of the complicated thermodynamics and transport phenomena involved in drying, it is challenging to analyse and optimise drying operations. In this study, computer simulations using a transient model are performed to simulate the real-time paddy drying, and analyse the exergy components, exergy efficiency, exergy loss and destruction of industrial-scale drying process under changing drying air temperature (60–80 °C) and flow velocity (0.56–0.83 m/s) and initial moisture content (0.22–0.3 g water/g wet matter) on a circulation counter-flow paddy dryer. Results show our model can predict paddy moisture content and temperature with mean relative deviations of 5.5% and 1.42%, respectively. Exergy efficiency of drying process is specified to be in the range of 28.75–35.68%. Main factor affecting drying rate and energy consumption in high moisture content regions of drying process is drying air flow velocity while that in low moisture content regions is drying air temperature. We also propose control strategy based on two-stage variable ventilation parameters that achieves high exergy utilization level, high drying rate, and high product quality for grain drying.