Gas-solid transport and reaction via intervened evaporating sprays

Fluid catalytic cracking (FCC) is a major process used for converting heavy oils to transportation fuels and light olefins. The gas-solid transport with reaction via intervened evaporating sprays in the FCC riser is specially important but complicated, with coupled mechanisms of chemical reaction and heat, momentum and mass transfer among multiple phases (liquid, solid and gas) in the restriction of wall boundary. Recent developments in FCC process models have progressed along two lines. One aims to develop composition-based kinetic models derived from molecular characterization of petroleum fractions while overlooking the hydrodynamic effect on local catalyst to oil ratio (CTO). The other aims to develop computational fluid dynamics (CFD) based models which cost too much on emphasizing flow dynamics yet not suitable for real time on-site monitor/control/optimization in industry. This work shows the efforts in developing an FCC model that strikes a right balance between the kinetics- and CFD-dominated approaches. Specifically, the feed injection zone, with multiple evaporating sprays penetrating throughout the hot gas-solid flow ambient and overlapping among each other, is integrally modeled (with coupling of FCC kinetic reactions) by geometrically cascading sub-models of single across spray and gas-solid transport. An innovative experimental method is proposed to obtain the statistical characteristics of solids wetting and solid-droplet collision probability distributions from spray impingement onto free-fall particles. This feed-zone modeling, quantifying liquid feed trajectory, droplet vaporization, gas-solid transport and vapor cracking, is capable to provide hydrodynamic and pre-cracking inlet conditions for downstream gas-solid transport in the remaining part of riser. A two-zone analytic model for FCC riser, consisting of an entrance zone and a fully developed riser zone, is thus developed. Using a four-lump cracking kinetic model, this work shows that for the first time the commercial data of Derouin et al. (1997, [14]) can be explained and predicted. The success of the prediction reflects an inherent two-zone character of the FCC riser. Inside the entrance zone, cracking intensity is high and changes rapidly, resulting in a sharp rise in VGO conversion. Outside the entrance zone, cracking intensity is low and becomes slowly varying, giving rise to a sluggish increase in conversion. The results show that the two-zone theory is a simple, practical way of capturing the essence of physicochemical phenomena underpinning the FCC process. Further exploitation of this approach is to quantify solid back-flow in gas-solid transportation due to wall restriction. The continuous modeling, which takes into account mechanistic of radial heterogeneity by considering radial mass and momentum balances between the collision-induced diffusion and the turbulent convection of solids, is proposed. Results are partially validated against published experiment data for radial and axial distributions of both solids and gas characteristic properties. Back-flow ratio can be thus predicted quantitatively for further optimization of riser reactor