Experimental Study And Reacting Flow Simulations Of Coupled High-Density Downer-to-Riser Reactor

With the growing demand on the production of ethylene and propylene as well as the environmental concerns on the cleaner fuels for automobile, conventional fluid catalytic cracking (FCC) riser reactors can hardly facilitate the multifunctional purpose for the integration of refinery and petrochemical processes. A multi-zone reactor design, i.e., a coupled downer-to-riser reactor (DTRR), was proposed in this work to carry out the main primary and secondary reactions in two different reaction zones with different hydrodynamics and process conditions. The feed oil first enters into a downer reactor, undergoing a high severity operation but a short contact time between phases, for production of light olefins. After a certain conversion, the reacting flow transfers into a riser for the further cracking reactions of the heavy components adsorbed on the catalysts. Here a lower temperature and a longer residence time are favorable, especially to promote hydrogen shift reactions which reduce the olefins content in the product of gasoline.A pilot-scale experimental setup (cold model) was built to demonstrate the feasibility of running gas-solid flows smoothly in the DTRR system. High solids flux conditions, i.e., above 400 kg/m2/s, were achieved in the experimental apparatus, where the solids volume fraction in the fully developed region of the downer reaches up to 5%. Hydrodynamics in such a reactor coupling system was studied using a compressive model that considered the pressure balances around all the sub-units in the prototype. The continuity closure condition was used to determine the material balance of the solid particles flowing in the circulating fluidized bed system. The model predictions had good agreement with the experimental data in rather wide operating conditions.A two-dimensional convection-diffusion-reaction model incorporated with hydrodynamics and a 14-lump kinetic model was established to predict the conversion and yields of different products in a single riser, a single downer and different coupling schemes of the reactor columns. The results showed that downer reactor has better control on the desired intermediate products (e.g., gasoline) than riser reactor due to the plug-flow performance, especially under the conditions of higher catalyst to oil ratios. The DDTR design combines the advantages of both riser and downer, in which more yields of gasoline and light olefins can be achieved. Furthermore, it was predicted that the coupled reactor design has more potential to obtain cleaner gasoline with less olefin content.For a better description of the gas-solid reacting flows, a computational fluid dynamics (CFD) model for gas-phase flow combined with a discrete element method (DEM) for particle movement, i.e., CFD-DEM, was extended to incorporate the heat transfer behaviors between particles and between gas and particles. This is a so-called cross-scale modeling of gas-solid reacting flows due to the particle movement, heat transfer and catalysis at the particle-scale together with gas-phase reactions at the grid-scale. This Eulerian-Lagrangian model has clearer physical meanings by considering the particle behavior in a discrete manner than a Eulerian-Eulerian (two-fluid) model by assuming the particle phase as a continuous medium. The transient simulations for riser/downer based FCC process showed that downer reactor benefits from the plug-flow performance in comparison with riser for the FCC process. The predicted reactor performances had good agreement either qualitatively or quantitatively with the corresponding hot-model data available in the literature.In order to measure the multiphase flow behavior in a non-intrusive way, two types of X-ray computed tomography techniques were proposed in theoretical schemes: one is for one-dimensional measurement of concentration field with axisymmetric character, another is a novel method for fast measurement of dynamic concentration variations. The proposed new methodologies were demonstrated by both different experiments and numerical simulations. For the reduced request on the X-ray hardware, the two approaches are expected to serve the experimental studies in lab-scale even in some industry applications.