Feedstock injection zone is a key section for fluid catalytic cracking (FCC) riser reactor. The mixing between oils and catalysts in this zone largely determines the reaction yield. In the traditional commercial FCC riser, the injection angle of the feedstock nozzles is usually 30° upward with the riser axis, easily causing non-uniform oil-catalyst contacting and severe back-mixing in the feedstock injection zone. In this work, a 3-D computational fluid dynamics (CFD) simulation of the gas-solid two-phase flow and mixing is carried out by combining the two-fluid model (TFM) and the energy-minimization multi-scale (EMMS) drag. Comparison with experimental data indicates that the flow and mixing behavior in the feedstock injection zone can be well predicted by using the EMMS drag model. The occurrence of the intense back-mixing of catalysts and oil feed near the riser wall is closely related to the development of the feed sprays and the wall effect, while its feature can be well explained by the Kutta-Joukowski lift theory. Seven different feedstock injection angles are selected which include three inclined upward (i.e. 30°, 45° and 60° upward), one horizontal (i.e. perpendicular to the riser wall) and three inclined downward (i.e. 30°, 45° and 60° downward). Those profiles of the catalysts and feed concentration, the matching ratio of catalyst to feed and the back-mixing in the feedstock injection zone are compared to discuss the effects of changing the feedstock injection angles on the gas-solid two-phase flow and mixing. The results show that, the gas-solid flow and mixing under the downward injection angles (especially, 30° downward) condition are more uniform than those with upward or horizontal injections. When the feed sprays inclines upward at a larger injection angle than the traditional 30°, higher radial momentum component makes the feed sprays quickly reach the riser center to mix into the pre-lift particle flow, which can improve the uneven mixing of the catalysts and feed to some extent. However, larger upward injection angle such as 60° will cause seriously intense back-mixing of clusters above the feed nozzles exits, easily leading to coke deposition in the commercial FCC riser. If the feedstock injection angle is adjusted to incline downward, it normally takes less time to reach a sufficient mixing and then a uniform mixing will be easier to obtain. However, if the downward angle is too large, such as 60° downward, it will also cause severe accumulation of catalysts near the riser wall and severe back-mixing above the feed nozzles exits. On the other hand, when the high-speed feed spray inclines upward into the riser wall, the transverse Kutta-Joukowski lift causes the feed sprays bend toward the riser wall to slowly reach the riser center, the intense back-mixing of clusters occurs near riser wall above the nozzle exits in particular at the traditional 30° upward angle under the common effects of the feed sprays and the riser wall. However, the direction of transverse Kutta-Joukowski lift under the condition of the downward injection angle changes toward the riser center, which is conductive to the feed sprays quickly leaving the riser wall, thereby weakening the effect of the feed sprays on the back-mixing. In all, a suitable feedstock injection angle, in this case, 30° downward, can realize a complete and uniform mixing and at the same time, relieve the back-mixing in the feedstock injection zone.
The Gas-Solid Two-Phase Flow and Mixing in a FCC Riser with Different Feedstock Injection Schemes
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