Numerical study of convective drying process in a sludge-packed bed: Effect of particle size distributions

The belt sludge dryer is a promising sludge drying device to address the growing challenges of sludge treatment and disposal. The belt sludge dryer essentially functions as a packed bed reactor, where particle size distribution (PSD) strongly influences the reactor performance. The computational fluid dynamics-discrete element method (CFD-DEM) model is employed to simulate the convective drying process in a sludge particle packed bed. The accuracy of the CFD-DEM model has been validated by the previously published data. The current study focuses on the comprehensive effects of particle size and PSD on the pressure drop, heat transfer, and moisture removal in the reactor. The modeling considers five different cylindrical particle sizes with different length-to-diameter ratios (L/D), as well as four PSD including Mono-type, Binary-type, Mean-type, and Gaussian-type, respectively. The numerical results show that the Mono-type PSD with a larger L/D has a lower pressure drop and less heat absorption, which is beneficial for reducing energy consumption. The drying rate also increases with the increase of L/D. When the L/D is greater than 7, the increase in L/D no longer significantly promotes drying. Given a certain average L/D, the drying rate of the Binary-type with high dispersion, the Flat-type, and the Gaussian-type increase sequentially compared with the Mono-type PSD, while the energy consumption for increasing pressure and heat increases. The Gaussian-type dries the fastest, saving 7.6% drying time compared to the Mono-type PSD. Increasing the average L/D for the Gaussian-type PSD can accelerate the drying efficiency and thus reduce the energy consumption. Moreover, compared to the Mono-type PSD with the same L/D, the drying time can be shortened by up to 14.2%. Therefore, the Gaussian-type PSD with a larger average L/D is the optimal one for the packed-bed reactor. In addition, a mathematical model of the drying process related to PSD is developed. Belt sludge dryers and similar wet granular packed beds can draw inspiration from this study on how to operate efficiently.