Development and Experimental Evaluation of a 1D Distributed Model of Transport Phenomena in a Continuous Biodrying Process for Pulp and Paper Mixed Sludge

Effective sludge management is increasingly critical for pulp and paper mills due to high landfill costs and complex regulatory frameworks for disposal options such as sludge landspreading and composting. A novel continuous biodrying process has been developed to dry mixed sludge so that it can be combusted efficiently in a biomass boiler for energy recovery. Modeling this process is important in order to better understand the transport phenomena in the biodrying reactor and for design and scale-up of the process. A one-dimensional (1D) distributed model for heat transfer coupled with mass and biological transfer phenomena is introduced in this article that shows that the temperature of the sludge matrix is a critical parameter. The model assumes lumped parameters in the gas flow direction and distributed parameters in the (vertical) solids flow direction. Bioheat as a source term and evaporative heat as a sink term are critical issues. In order to evaluate the parameters and assess the model accuracy, a series of experiments was performed. The matrix temperatures predicted by the model were found to be in reasonable agreement with the experimental results, showing that the main transport phenomena were reflected in the model. Larger discrepancies between the water removal rates predicted by the model and the experimental values were indentified at higher aerobic exothermicity, which can be attributed to the complex mechanisms governing the growth cycle of mesophilic and thermophilic bacteria. A dimensionless analysis was performed to identify key dimensionless groups as well as the most dominant transport phenomena in the biodrying process. The results confirmed that convection processes dominated heat transfer at the top of the reactor, and the exothermic aerobic bioenergy dominated at its bottom.