Chemical Engineering Science, Vol.157, 209-220, 2017
Effects of asymmetric feeding on gas-solid-liquid transport and catalytic cracking reaction in the feed zone of riser
Asymmetric feeding of liquid reactants and catalytic solids are common in the operation of fluid catalytic cracking (FCC) riser reactors. The asymmetric feeding of solids can be caused by the designed-geometry of J-bend right before the riser inlet or unbalanced operation of wall-steam feeding. In order to ensure uniform distributions in local catalytic to oil ratio (CTO) and temperature, a possible measure of matching the asymmetric feed of catalysts is to introduce liquid jets in an accordingly asymmetric feeding. The asymmetric feeding will cause complicated three-phase interactions, in terms of the transport and phase change characteristics of each interacting phase, in the feed zone of a riser reactor. This study has developed a theoretical framework with an inertia-based multi-spray interaction, which considers not only a complex geometric intervene of mutual-penetration sprays but also the individual spray properties and feeding operations. Based on the fact that the traverse gradient of pressure is insignificant compared to that of axial gradient in a riser flow, a sub-model is also established to redistribute the gas and solid phases within the cross-section of concern. Due to the lack of experiment data or available information on the asymmetric feeding of catalysts from J-bend, we have also conducted a CFD study of a J-bend dense gas solid flow to obtain the asymmetric distribution catalysts feeding to the feed-zone model. Our modeling examples suggest that the reaction and flow characteristics can be altered noticeably by adjusting asymmetric feedings of liquid reactants, such as changes in droplet penetration, spray coverage and local CTO. In addition, the result of over-penetration of droplets onto the feed zone wall shows the need for a various droplet size/flowrate model rather than mono-sized/constant-flowrate model of liquid phase. This can also assist in better design or selection of the characteristics for the spray nozzle. (C) 2016 Elsevier Ltd. All rights reserved.