Heat And Mass Transfer Of Sewage Sludge Thin-layer During Hot Air Convective Drying

With the increasing population and industrialition, sewage sludge is largely generated from municipal waste water trratment. Sewage disposal is becoming an critical attention. As an important pretrement of sewage disposal, drying can reduce size of sewage sludge, increase heating value and decrease harmful material of sewage. Thin-layer drying behavior of the sewage sludge during hot air convective drying is investigated in a laboratory scale setup. The effects of hot air temperatures and speeds on the average temperature, surface temperature and surface heat transfer coefficients of the sewage sludge thin layer were estimated. The neural network models were developed to predict the moisture content and average temperature in the sewage sludge thin layer.For SSI and SS2sample, the thin layer drying processes of sewage sludge presented a combination of a short warm-up period, an interim constant rate period, and two distinguished falling rate periods (refer to the first falling rate period and the second falling rate period) and a collection of a warm-up period, a long constant rate period and falling rate period, respectively. Five common thin-layer drying models were applied to describe the drying behavior of sewage sludge, in which the Modified Page model was the best model to characterize the thin layer drying of sewage sludge. The average values of determination coefficient (R2) were between0.995and0.997. The thin layer drying kinetics was evaluated based on two falling rate stages. The effective moisture diffusivities of SS1and SS2thin layer were from7.46×10-9to2.47×10-8m2/s and4.92×10-9to3.53×10-8m2/s for the first falling rate period, and from1.76×10-8to4.61×10-8m2/s and1.22×10-8to2.17×10-7m2/s for the second falling rate period, respectively. The average apparent activation energies of SS1and SS2thin layer in the first falling rate period and the second falling rate period were17.78kJ/mol and25.26kJ/mol,13.75kJ/mol and23.34kJ/mol, respectively.For SS1sample, the chemical equilibrium temperature of the thin layer at hot air temperature range of100℃-160℃increased by about52%to55%when the wind speed rose from0.6m s-1up to2.0m s-1. The average surface heat transfer coefficients in the first falling rate period were about3～4times of those in the second falling rate period. The dimensionless surface heat transfer correlation of the sewage sludge thin-layer was obtained for the first falling rate period.The average temperature and moisture content were predicted by both BP and GRNN models, which technique would predict more accurately depending on the mean square error and the coefficient of determination. The BP model could predict more accurately the moisture content in the thin layer than the GRNN model. For prediction of average temperature, the performance of BP model and GRNN model were related to experimental operating condition.