Thermodynamic and sensitivity analyses on drying subprocesses of various evaporative dryers: A comparative study

Drying is an energy intensive process, which takes a big portion of global energy consumption. The innovation of energy-saving drying techniques are crucial for the sustainable development. To achieve energy saving of evaporative drying, exergy destructions and losses in the dryers should be located. In this study, the drying process is divided into three subprocesses, i.e., heat transfer, moisture evaporation and diffusion, and drying exhaust emission. Thermodynamic models of dryers are developed, and performance of evaporative dryers, including rotary, solar, spouted bed, etc. Are analyzed and compared. The exergy consumption rate, i.e., the exergy consumption rate to dehydrate 1 kg water, is defined and used to evaluate the energy utilization of dryers. The exergy consumption rate of dryers ranges from 330 kJ (kg H₂O)⁻¹ to 10,490 kJ (kg H₂O)⁻¹, which is mainly related to the energy consumption rate and energy grade. The irreversibility of the three subprocesses of dryers are compared, e.g., for the rotary dryer with hot air at 300 °C as heat source, the exergy loss and destruction of the subprocesses are 415.7, 451.3, and 293.4 kJ (kg H₂O) ⁻¹, respectively. The results of this study can provide guidance to enhance the energy utilization efficiency of evaporation dryers.