Simulation Of Two-phase Flow Characteristics In Feedstock Injection Zone For FCC Risers With Double-layer Nozzles

In the feedstock injection zone of a FCC riser reactor,there were some problems decreasing the contacting efficiency between catalysts and oil and then deceasing the yield of desired products(such as the gasoline,the diesel,the propylene).It is mainly covers the long time catalyst-oil contact,the in-uniform mixing,and the coking due to severe back-mixing of oil and gas near the riser wall.In order to improve the product distribution of FCC process,a new structure of injection zone with double-layer nozzles was proposed.This type of riser was achieved by symmetrically introducing two supplementary nozzles below the primary nozzles.However,the investigation on gas-solid flow characteristics in the injection zone of this novel riser is rarely reported until now.In this study,the combination of Euler-Euler method and energy minimization multi-scale(EMMS)-based drag model were employed to simulate the mixing and flow of gas-solid characteristics in the injection zone of novel riser.The grid size,the time step and the particle-wall specularity coefficient were determined by comparing simulated and experimental data.Their perfect agreement showed that the method could be used to simulate the gas-solid flow characteristics in the injection zone of this novel riserFor the riser with double-layer nozzles,four combination types of risers were constructed by fixing the distance of 0.5 m between primary and supplementary nozzles and changing their feeding directions.They were the down-down type,the down-up type,the up-down type and the up-up type.The hydrodynamic behaviors of gas and particles in these four types of risers were investigated and further compared with the results in the risers with single-layer nozzles.The results showed that the introduction of the supplementary nozzles caused an uneven distribution of solids volume fraction in the region above the secondary nozzles.For the double-layer nozzles risers with upward main-nozzles,the jet from supplementary nozzles could accelerate gas and particles to reach the uniform and stable state at the region above the primary nozzles.For the double-layer nozzles risers with downward main-nozzles,the downward jet from supplementary nozzles could decrease the back-mixing of the particles and feed at the riser wall with axially positioned between the primary and supplementary nozzles.However,the upward jet from supplementary nozzles could intensify the back-mixing of particles and feed in the same position.For all risers with downward main-nozzles,small mean residence time and heterogeneity occurred in the injection zone of riser with the single-layer nozzles.For all risers with upward main-nozzles,smallest values were in the up-up type of riser.These results indicated that the two types of risers could achieve the strong mixing within a smaller contact time and inhibit the over-cracking of desired products,thus increasing the yield of desired products and reducing the coking.It was discussed to the influence of nozzle intervals between primary and supplementary nozzles on gas-solid flow characteristics in the double-layer nozzles risers with downward and upward main-nozzles.And the optimal nozzle intervals were determined in each type of riser by analyzing solids volume fraction,the matching ratio of catalysts to feed and the radial nonuniformity indexes of solids.In the riser with the optimal nozzle intervals,the gas-solids flow and the mixing characteristics were compared in the single-and double-layer nozzles of risers with downward and upward main-nozzles to determine the optimal feeding schemes.The optimal schemes were the down-up type of risers with the nozzle intervals of 0.25 m and the up-down type of risers with the nozzle intervals of-0.25 m.