(583s) Numerical Simulations of Mixing of Supercritical Water and Hydrocarbons in 3-D Reactor Geometries
Supercritical water desulfurization and upgrading (SCWDS) is a new concept in the oil refining industry wherein, crude oil is mixed with supercritical water in a reactor leading to chemical breakdown of the sulfur containing compounds (desulfurization) and cracking of long chain hydrocarbons to shorter chain compounds closer to commercial fuel components (upgrading). The focus of the present work is the development of a numerical tool to investigate the mixing of water and hydrocarbons under supercritical fully-miscible conditions (water and hydrocarbon forming a single phase) in a realistic 3-D reactor geometry so as to develop an understanding of the effects of reactor geometry, flow rates and fluid properties on the mixing dynamics which in turn would influence the conversion of sulfur compounds and the final product distributions. The work includes a consistent treatment of near-critical fluid mixture thermodynamics using the cubic Peng-Robinson equation of state. Variations in the transport properties of the real-fluid mixture with temperature and pressure are also accounted for. The impact of reactor geometry and flow Reynolds number on the mixing of supercritical water and n-decane in a cylindrical tee reactor under fully miscible conditions is investigated. At low Reynolds numbers, mixing is mainly brought about by the action of a counter-rotating vortex pair in the hydrocarbon jet entering from the top. The first onset of instability in the water-decane mixing layer is observed at a water inlet Reynolds number close to 1000. The instabilities in the shear layer trigger the stretching and breaking of the large counter-rotating vortices in the hydrocarbon jet leading to enhanced mixing action. This flow regime in which sustained vorticity brings about fast transport of both species and energy over large length scales would be conducive for the desulfurization and upgrading process since it would ensure simultaneous quick transfer of hear and water to the hydrocarbon stream. This would ultimately result in less coking due to the radical capping of coke precursors by water. In order to isolate the major driving force causing the early onset of instability, different test simulations are performed which capture the effects of different probable causes. It is found that the strong density gradients across the water-decane mixing layer in the presence of the adverse pressure gradient, lead to the baroclinic generation of vorticity in a counter-clockwise sense. This aids the rollup of the shear layer and eventually destabilizes it. This mechanism of baroclinic vorticity generation could be potentially exploited to enhance mixing rates by designing mixers and reactors with stronger adverse pressure gradients. Finally, the effect of gravity on the flow and mixing in a cylindrical tee reactor with a vertically oriented hydrocarbon arm is studied. Gravity forces strongly accelerate the heavier n-decane as it flows down the vertical arm of the tee leading to hydrocarbon jet impingement on the wall and a highly turbulent flow and consequently, enhanced mixing. Buoyancy forces also cause the lighter water to rise into the vertical arm of the tee leading to further enhancement in mixing. At low Reynolds numbers the gravity forces dominate the flow and mixing behavior. However, the impact of gravity will continue to decrease as we go to higher Reynolds numbers with a view to enhance mixing rates.
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