Modeling on hydrodynamic coupled FCC reaction in gas-solid riser reactor

The fluid catalytic cracking (FCC) riser reactor consists of a bottom section of liquid feed injection and vaporization and an upward straight riser of vapor-catalysts transport and reaction. The product yield, obtained at the top of riser, is an accumulative result of liquid feed injection, vaporization by liquid contacting with hot catalysts, and subsequent catalytic cracking of feed vapor while being transported concurrently with catalysts through the riser. The FCC process involves not only these sequential sub-processes but also complicated coupling among multiphase fluid hydrodynamics, heat and mass transfer between phases, and catalytic kinetic reactions of vapor components in each sub-process. It is essential to build up a model covering all sub-processes/mechanisms mentioned above through riser reactor and giving prompt results, especially for real-time online optimization of industrial operation. This paper aims to develop a parametric model, integrated from bottom feed nozzle to top exit of riser, that can quickly predict both hydrodynamic and kinetic characteristics throughout the riser as well as various parametric effects on production yield and selectivity. Highlights of modeling contributions in this integrated model include a mechanistic and spatial-structural model of multiple-nozzle feeding with strong interactions not only among sprays themselves but also with cross-flowing steam and catalysts, a heat transfer model between gaseous and catalyst phases, and a more-rigorously derived model of reactant conservation in the multiphase flow transport. The convective nature dominating the nozzle feeding, riser transport and kinetic reactions allows us to simplify the governing equations in this integrated model to a set of coupled first-order ordinary differential equations whose solutions can be obtained quickly via Runge-Kutta algorithm. Compared to the published plant data, the predicted VGO conversion and gasoline yield from the proposed model shows a much better agreement to those from previous parametric models, which suggests the newly-added sub-models of previously overlooked mechanisms can be quite important. Some parametric effects, such as the effect of catalyst-to-oil ratio and catalyst inlet temperature, on production yield and selectivity are further predicted. The results show that a higher CTO or catalyst temperature normally leads to higher cracking conversion, higher gasoline production and lower coke content. However, a very high inlet temperature of catalysts does cause overcracking and lower the gasoline selectivity.