Reinforced contact between sludge and hot wall for enhancing conductive drying by applying external load: Heat and mass transfer analysis

Abstract

Conductive drying is widely used in sludge management for emission reduction and energy recovery. However, interfacial vapor film between sludge and hot wall inhibits drying efficiency. This study employed external load to reinforce the contact of sludge with hot wall for facilitating interfacial heat transfer. Subsequently, a drying model was developed for heat and mass transfer mechanism analysis. The influence of external loads was explored on conductive drying behavior at 180 °C and 0, 25, 100 kPa. The drying rates were increased with external load. The drying process consisted of the warm-up, constant rate, and two falling rate periods. Without external load, moisture migration was dominated by liquid water diffusion induced by capillary force in the first falling rate period. However, at 100 kPa, the convective flow of water and vapor dominated the moisture migration in the first falling rate period due to increased interfacial heat transfer, evaporation rate, and gas pressure. Then vapor diffusion was predominant owing to enhanced gas diffusivity. Besides, this study provided essential information on design optimization of industrial applications, such as moisture content prediction for searching for equilibrium points between energy consumption and resource recovery and the critical factors for enhancing moisture migration.