UBC Theses and Dissertations
CFD simulation of aerosol flow and hydrocarbon fouling on a circular disk Lakghomi, Babak
Coking is one of the key technologies used in upgrading of oil sands bitumen. In coking units, the bitumen is thermally cracked in the presence of steam to produce valuable lighter species and by-product solid coke. Hot vapours which contain these valuable species from the fluid coker pass through cyclones before entering the scrubber section of the coker, so that coke and heavy droplets are removed. However, some micron-sized heavy hydrocarbon droplets are not removed in the cyclones and enter the scrubber grid packing. These droplets can deposit on the scrubber grid and react over time to form coke as a result of high temperatures. A model is developed for calculation of deposition from a droplet-gas mixture at similar conditions. A simple geometry of a circular disk was used to be able to evaluate the validity of model at different conditions. The model combined Computational Fluid Dynamics (CFD) for calculating the flow hydrodynamics and droplet transport to the surface, and HYSYS simulation for prediction of mixture phase equilibrium at different temperatures. Effects of parameters such as droplet size and gas velocity were studied. Based on modeling results, Stokes number seemed to be a very important parameter on deposition of droplets. At low Stokes number, the main mechanism for deposition was molecular and eddy diffusion, and deposition did not change very much with change in droplet size and velocity. At higher Stokes number impaction was the main mechanism, and the deposition rates increased with increases in droplet size and gas velocity. The effect of surface properties on deposition was also studied. For the applied conditions and surfaces the perfect sticking assumption was considered satisfactory. Calculations suggested that application of hydrophobic material might help to decrease deposition by increasing the possibility of rebounding for droplets. Model was tested against room-temperature data for air-droplet systems and hot-unit experiments with heavy hydrocarbons carried out in parallel with the modeling work. For the latter, increases in the temperature decreased the deposition rates both by decreasing the droplet concentration and by the evaporation of volatiles formed in coking reaction. Finally, the model showed good ability in the prediction of deposition rates at different conditions.
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