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Abstract

The goal of this work is to study computationally the flow induced by the collision between a single gasoil droplet and a spherical catalytic particle under realistic Fluid Catalytic Cracking (FCC) conditions. FCC reactors are found in the fossil fuel refineries and are used to upgrade heavy fuel (gas oil) to lighter products (gasoline or LPG), which are industrially more important. Gasoil is injected in the reactor and atomizes; the produced droplets vaporize intensely and come in contact with the hot fluidized solid catalysts. The “cracking” reactions accommodated at the particle porous surface (ex. zeolite) result in the decomposition of gasoil to lighter products.

The two-phase flow model developed solves the incompressible Navier-Stokes equations for mass and momentum, along with the energy conservation equation. The VOF methodology is used to track the liquid-gas interface, while a dynamic local grid refinement technique is adopted, so that high accuracy is achieved with a relative low computational cost. A local evaporation model coupled with the additional solution of the species transport equation is utilized to consider phase change. Cracking surface reactions are taken into account via a simplified 2-lump scheme.

The model is successfully validated in fundamental droplet dynamics flow conditions, such as droplet acceleration, droplet impingement onto flat and solid surfaces under isothermal conditions and droplet evaporation. Insights into these phenomena provide important information that are missing from experimental measurements. The numerical novelties of the current work include the implementation of a new Wetting Force Model to simulate drop-solid interaction, as well as the proposition of a sharpening scheme for the volume fraction field, to suppress diffusion.

Concerning FCC collisions, the numerical model is able to reproduce both the hydrodynamics (drop deformation, spreading, breakup), as well as the chemical products (gasoil converted to gasoline). It is found that droplets of similar size to the catalytic particles tend to be levitated more easily by hot catalysts, thus resulting in higher cracking reaction rates/cracking product yield, and limited possibility for liquid pore blocking. For larger sized droplets, solid-liquid contact increases.

The main ambition of the current Thesis, which is to combine the droplet hydrodynamics with the chemical reactions acts as a novel step towards the understanding of such micro-scale physical phenomena that are difficult to capture/measure in experimental apparatus. This fundamental numerical tool can provide insight to the spray system strategy of an FCC reactor for a wide range of operating conditions.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses

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