(199b) Modeling On Coupled Hydrodynamics and Catalytic Reaction in Core Region of Gas-Solids Riser Reactors

The conversion rates of Fluid catalytic cracking (FCC) riser reactors which convert heavy petroleum oils into light hydrocarbon products are dependent on both the hydrodynamics and reaction kinetics which are complexly coupled during the gas-solids transport process. The flow structure in gas-solids riser reactors is heterogeneous in both axial and radial directions, which is represented by an upward solids flow in the core region and a back-mixing downward solids flow in the wall region. Most models on FCC in riser reactors ignored the significant change of hydrodynamics of gas-solids flow along the riser and the potential influence of this change on the catalytic reaction kinetics along the riser by using reaction models which are based on a lumped-CTO of the entire riser and treating the hydrodynamics of riser flow as plug flow. Our recent research, which coupled a one-dimensional hydrodynamic model of gas-solids flow with a four-lump reaction model, showed that the traditional approaches with plug-flow simplification underestimated the reaction rates at the lower part of the riser where a dense-phase and highly heterogeneous gas-solids flow presents. In this paper, a better hydrodynamic model, which not only describes the hydrodynamic characteristics of gas-solids flow in axial direction but also considers the heterogeneous structure in radial direction by dividing the gas-solids flow into upward core flow and downward wall flow, is adopted to couple with a four-lump reaction model to predict the catalytic reaction process along the riser. The catalytic cracking reaction is assumed to be only taken place in the core region as the back-mixing downward solids flow is mainly composed of spent solids particles whose catalytic activity has already been lost. The effect of wall region, which is only on the hydrodynamics of flow, is taken into account in terms of wall-boundary and back flow mixing. Modeling predictions are satisfactorily compared with the yield pattern and exit temperature of industrial scale plant data. Some parametric effects on axial distributions of hydrodynamic properties of gas-solids with and without reaction are studied to demonstrate the coupling effect of reaction kinetics and hydrodynamics of gas-solids flow.