Two phase cluster model in riser reactors: impact of radial density distribution on yields

Abstract

Recent cold flow data (Miller et al., 1994) have shown that the manner in which the oil and catalyst are initially contacted in fluid catalytic cracker risers has a major impact on the radial density profile at the riser bottom. These results show that circumferential oil injection has a much more uniform radial density distribution than the traditional axial oil injection. In order to assess the impact of the improvements in radial density distribution on conversion and product selectivity, a two-phase heterogeneous model for riser reactors operating in pneumatic transport has been developed. The model consists of a dispersed cluster phase which contains all the catalyst and is where the bulk of the catalytic reactions occur, and a continuous oil phase with mass transfer between the phases. A simple three- component kinetic model is used to describe the rates of reaction which occur in the cluster phase. The average concentration of any component at any axial or radial position in the riser is then determined by the density-weighted average of the cluster phase and the oil phase at that position using experimental values for the radial density distribution. The radial density distributions were obtained in a cold flow riser unit simulating FCC riser reactor hydrodynamic performance. The resulting model matches commercial radial and axial conversion and gasoline yield profiles. This model provides a basis to determine the incentives for improving the mass transfer between the two phases in the riser by making hardware modifications. For example, the model predicts that the circumferential feed nozzle configuration would increase conversion by about 3 wt% over the traditional axial configuration.