Research on Sludge Drying Characteristics in a Double-Axis Paddle Dryer by Coupling Markov Chain and Penetration Theory

Sewage sludge drying is a critical step in the process of achieving final sludge treatment and disposal for its minimization and stabilization. This paper developed the drying model by coupling penetration theory and two dimensional Markov chain to simulate the sludge drying kinetics in a double-axis paddle dryer. A series of sludge drying experiments were carried out in a double-axis continuous paddle dryer so as to study the influences of sludge flowrate, stirrer speed, tilt angle, overflow height and dryer wall temperature on the drying mechanism. During the drying process, sludge crusting occurred in some experiments according to different operating conditions. It was a phenomenon that sludge in some cells adhered and then formed crust on the heat transfer surface with moisture content decreasing leading a decrease in sludge drying rate in the cells. Thus, in order to more accurately explain the sludge crusting in partial operating conditions, the viscous zone was introduced. The crusting drastically increases the heat transfer resistance which occurs easily in high wall temperature, high overflow height, high sludge flowrate and low stirrer speed condition while the tilt angle of the dryer has little effect on crusting. [GRAPHICS]