Optimization Of The Flow-field Structure Of Naphtha Thermal-cracking Reactor Tube Based On CFD Simulation

The concept of structured flow field has been applied to optimize the heat transfer and reactive mixing behavior of the tube in cracking furnace, which is the core equipment of ethylene production. Specifically, the hollow cross disk (HCD) is adopted to enhance the heat transfer and turbulent reactive mixing process in the reactor tube and hence to intensify the cracking process.Firstly, the heat transfer and flow characteristics of reactor tube embedded with different static mixers are analyzed by SST k-w turbulent model. The simulation results indicate that Nusselt number of HCD and twisted tape are32.06%and13.68%higher than that of empty tube, respectively. Also, the performance evaluation criteria(PEC) of HCD are higher than that of twisted tape with73.07%. Secondly, CFD method coupled the Direct Quadrature Method of Moments and the Interaction by Exchange with the Mean(IEM) model is utilized to simulate the turbulent reactive mixing behavior of the tube reactor incorporated with the novel inserts HCD. The simulation result implies that the HCD can induce vortex flowing which strongly enhances mixing without much pressure lose. Such flow pattern is beneficial to increase main products yield and inhibit side reaction.Thirdly, the naphtha thermal-cracking experiments are carried out, and the experimental results show that incorporation of HCD in the reactor tube can effectively improve the yield of ethylene and propylene. Also this enhancement effect becomes more intense with the increasing temperature.For the purpose of uniform heat supplying for reactor tube in the furnace, a three-dimensional CFD model which coupled standard k-ε turbulence model with eddy dissipation concept model (EDC) model is established to simulate the industrial USC-style ethylene cracking furnace. The flow field, temperature field, gas concentration distribution in the furnace is analyzed. To optimize the flow field of furnace, a novel diffusion nozzle is developed, and the simulation results show that this nozzle could increase the mixing rate of fuel with the air, and hence effectively reduce low temperature zone and improve the overall temperature of furnace. In addition, the comparison of simulation results of different heat release modes (combined heating and hearth heating) indicates that combined feeding can generate more uniform heat flux distribution of reactor tube.