Hydraulic modeling and axial dispersion analysis of UASB reactor

In this work, the axial distribution of a conservative tracer in the upflow anaerobic sludge bed (UASB) reactor was modelled by a two-compartment model describing sludge bed and liquid zones. The orthogonal collocation algorithm was used for numerical solution of the model. The model parameters were estimated using on-line fluorescence-based measurements of an inert tracer rhodamine. Rhodamine concentrations were simultaneously measured at the bottom, middle and top of the sludge bed, as well as in the reactor effluent. The model used either a constant or variable axial dispersion coefficient in the sludge bed zone. A significantly better agreement between model outputs and observed tracer distribution was obtained when the dispersion coefficient was proportional to a constant raised to the power of the reactor height. In addition, a near linear dependence of the dispersion coefficient on the liquid upflow velocity was found. These findings, along with high efficiency of the orthogonal collocation algorithm used for model solution, enable future development of a comprehensive model, which accounts for both hydraulics and biokinetics of the anaerobic digestion process.