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

Abstract

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.

Keywords:

• 1-D modeling
• Biodrying
• Bioheat
• Dimensionless analysis
• Pulp and paper mixed sludge
• Transport phenomena
• Water removal rate

ACKNOWLEDGMENTS

This work was carried out with the financial support of NSERC and FQRNT for the research and CFI for the cost of the biodrying reactor. The dedicated assistance of Carl Tchoryk, Kheng Huynh, Omar Ben Ndiaye, and Antonin Paquet for the experimental part is gratefully acknowledged. Fruitful discussions concerning the project were held with Professors Jamal Chaouki and Mario Jolicoeur from the Department of Chemical Engineering at École Polytechnique of Montreal, Professor Tom Richard from the Department of Agricultural and Biological Engineering at Pennsylvania State University, and Professor Stephen Whitaker from the Chemical Engineering Department at California State University.

Notes

^a Numbers in parentheses represent the value of air flow rate for compartments 1, 2, 3, and 4, respectively.

^a Total water in is the summation of water in by air flow and feed and water generation terms.

^b Total water out is the summation of water out by air flow and discharge materials.

^a Superficial velocity is the gas velocity at the inlet (u _{g ,i} ) divided by the porosity of the matrix (ϵ _i ).

Cp _mix,i and ϵ _i vary as a function of sludge dry solids content.

^a Excluding inlet (T _in) and outlet (T _out) temperatures.