Flow-Reaction Coupled Simulation Of The Riser Reactor

Riser reactor has become the main device of heavy oil processing in refinery,which is generally used in petro-chemical industry. The axial and radial distribution of flow, heat and mass transfer of gas-solid two phase is very uneven in the reactor. Traditional research largely focused on the cold model, without considering heat transfer and reaction, which can not accurately simulates the riser reactor. In this paper, numerical simulations based on computational fluid-dynamic technique have been performed to investigate the hydrodynamic of gas-solid two phase flow in a bench scale experiment of Heavy Oil Processing State Key Laboratory and a suitable model was developed. Based on the flow model and detailed lumping kinetics of heavy oil catalytic pyrolysis, a two dimensional gas-solid two-phase flow reaction model of the FCC riser reactor was established, taking into account comprehensively the complex details of flow, heat transfer, mass transfer and reactions in the riser reactor. And the simulation results are consistent with the experimental data.The results show that the particle size has a core-annular flow pattern in radial position and solid volume fraction is much denser at the bottom of the riser. With the decreasing of superficial gas velocity or increasing of solid mass flux, solid volume fraction at different axial positions increases simultaneously, while the particle velocity decreases. As it is very difficult for the conventional riser reactor to achieve high catalyst-oil ratio and appropriate residence time simultaneously, riser reactor with changing-diameter could be a probable solution, which can better achieve both objectives. Solid volume fraction can significantly increase in the diameter expanded section of the diameter-changing riser, while it doesn't vary greatly in the upper part. Particle velocity is relatively low in the diameter expanded section, thus the residence time of gas and solid increases. And high velocity could be gained in following small diameter riser section, so the reaction depth and undesired by-pass products could be controlled reasonably.The uneven distribution of heat and mass transfer is caused by the uniform distribution of the flow. The temperature of oil increases at the entrance of the riser because of gasification and the heat required for gasification is provided by catalyst. Therefore, the temperature of particles drops in the area around the entrance and down the most significantly. Although the reaction is endothermic, the main reaction has been almost completed at this time, so the temperature will decrease slowly in the upper part. With the riser height increases, the mass fraction of heavy oil reduces, while the weight of gasoline, LPG, gas and coke has been increasing. The concentration of each component changes rapidly near the entrance, and becomes gently in the upper part. The concentration of diesel increases first and then reduces slowly. As the riser reactor becomes longer, the depth of the reaction increases and the conversion of heavy oil improves correspondingly, the concentration of LPG, gas and coke increases. The mass fraction of diesel and gasoline has emerged firstly increases and then decreases trend.