Numerical Simulation And Thermodynamic Analysis Of The Riser Reactor

The riser reactor is the most significant part of the Fluid Catalytic Cracking(FCC)unit in oil refining.Investigations on the reaction process occurring inside the riser reactor is of crucial importance,furthermore,quantitative thermodynamic analysis of the reaction scheme will bridge the gap between the industrial need and the theoretical research.In this investigation,based on the Computational Fluid Dynamics(CFD),a numerical hydrodynamics and reaction coupling model capable of predicting gas-solid flow behavior,catalytic cracking reaction performance and heat transfer was firstly established applying the Euler-Euler Two Fluid Method(TFM).A thermodynamic analysis procedure was put forward afterwards according to the entropy balance equation.Then the thermodynamic analytic process was combined with the previous two fluid model to quantitatively evaluate the irreversible catalytic reactions inside the riser.The established two fluid model was first applied to simulate a large scale riser with novel geometry and then validated through the comparison of simulation results and design data.The flow pattern,catalyst density,reacting performance and its endothermic characteristics were intensively investigated during the CFD numerical simulation.It was revealed that the high speed injected oil gas through atomizing nozzles would significantly affect the flow behavior in feed injection zone,moreover,change gas-solid contact and reaction performance.Velocity gradient,catalyst density gradient and temperature gradient were aroused attributing to the high injecting speed,however,this inhomogeneity decreased along the riser height as the fully developed flow was given rise to.Attention was drawn to the nozzle angle as a result of the radial inhomogeneous hydrodynamic and thermal distribution.To further justify the assumption of the two fluid flow inside the riser,a multiphase model incorporating the evaporation process of atomized droplets was additionally established to explore the existence and evolution of feed oil droplets.According to the simulation,the penetrating depth of the liquid phase was just 3~4m above the nozzles,which further nationalize the simplification of droplets’ evaporation in the gas-solid two phase model.In the establishment of thermodynamic analysis procedure,evaluation of thermodynamic properties was first conducted for all the lumps in the used 11 lump kinetic model based on the available experimental data and published reports.Procedure for thermodynamic analysis was then put forward and coded as a user defined function in C language.The thermodynamic analysis function was incorporated into the previous two fluid model and applied to a small scale riser in the laboratory along with another diameter enlarging riser reactor.It is indicated that,larger catalyst to oil ratio and desirable residence time make the diameter enlarging section favorable to the catalytic reaction path,the reaction scheme was intensified and consequently more irreversible compared to that occurring in the traditional riser.As a result,entropy generation and destroyed exergy in the diameter enlarging riser were demonstrated to be quantitatively larger than that in the traditional riser.