| Circulating fluidized beds, with the advantages of high gas-solids throughput, continuous operation as well as excellent heat and mass transfer characteristics, have been widely applied in petrochemical industry, especially for FCC process in oil refining. However, a nonuniform core-annulus flow structure is formed in the conventional CFB risers, consequently resulting in gas-solids segregation. Meanwhile, the low solids concentration further restricts CFB risers’ application in processes demanding for high solids-to-gas ratio and vigorous contact. Therefore, to increase solids concentration, improve flow structure and intensify gas-solids contact in CFB risers, it is essential to develop a novel riser reactor.On a CFB cold-model apparatus, gas-solids flow patterns inside a novel riser integrated with an enlarged bottom section were systematically studied. The results demonstrated that, a multi-regime flow was obtained in this novel riser, with turbulent fluidization established in diameter-enlarged section and pneumatic transportation achieved in upper conveying section. Compared with the bottom section of a conventional riser, higher cross-sectional averaged solids concentration with lower radial gradients of solids concentration, particle velocity and solids flux were observed in the diameter-enlarged section of this novel riser, i.e., the flow structure was improved. Further analysis of transient solids concentration signals indicated that, gas-solids turbulence in the diameter-enlarged section was enhanced and corresponding probability density distribution was uniformized.To further investigate gas distribution and gas-solids contacting behavior, catalytic ozone decomposition reaction was introduced. Compared with the conventional riser, where the reaction mainly occurred near the wall, notable ozone decomposition in the column center was representative for this novel riser, consequently leading to a more uniform radial ozone concentration profile along axial direction. Although the performance of both riser reactors deviated obviously from the ideal plug-flow reactor, due to improved flow structure and intensified gas-solids fluctuation over the conventional riser, higher ozone conversion and gas-solids contact efficiency were achieved in this novel riser.Based on experimental data from this work and the literature, an empirical correlation of solids concentration in the turbulent section of various multi-regime risers was developed. This correlation was further extended to predict axial solids concentration profile in the diameter-enlarged section of this novel riser. Afterwards, Boltzmann function was adopted to correlate local solids concentration with cross-sectional averaged solids concentration and radial position. In general, these correlations gave a satisfactory prediction of axial and radial solids distributions in the diameter-enlarged section. Besides, gas-solids contact efficiency was correlated with operation conditions. And combined with the definition of contact efficiency, axial ozone concentration profile in the diameter-enlarged section was predicted. Accordingly, empirical correlations of radial ozone concentration profiles were also developed.Ultimately, with the feasibility of applying this novel riser reactor in a residue catalytic cracking process for maximum ethylene and propylene production being verified, the reactor performance was evaluated in a pilot-scale FCC unit. The results indicated that, the novel riser reactor enhanced the conversion of light cycle gasoline and recycled butenes remarkably, especially for the light cycle gasoline. Meanwhile, the semispent catalyst still retained sufficient activity for residue cracking. When treating heavy oil, one-stage riser cracking could yield up to 13 wt.% ethylene and more than 27 wt.% propylene. In contrast, although the ethylene yield decreased, two-stage riser cracking produced equivalent propyelene and more gasoline, achieving higher economic benefit. |
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