Experimental and modeling study of the droplet dynamics related to liquid-liquid separators
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Separators are an integral part of oil and gas processing equipment. A high enough level of separation is needed both due to the sales specifications of the finished products, but also due to the effect poor separation may have on processing equipment. Today?s separators are of considerable size and weight and it is desirable to reduce both properties. Important factors affecting the sizing of separators are the settling velocity of droplets and the time needed for coalescence. For this thesis the dynamics of droplets, and their interaction with a liquid-liquid interface has been studied. The work has consisted of an experimental part and a modeling part using commercial CFD software. For the experimental part, the work has involved the generation of single water droplets inside a continuous oil phase. The trajectory of the droplets was recorded using a IR high speed camera, and the images processed with an in-house code filtering the images, and determining the terminal velocity of the droplets. The droplet-interface interaction was also studied, determining the coalescence time for the droplets at the interface. All experiments were performed with oil samples available in the laboratory, and the effect of the temperature on the dynamics of the system was evaluated. For the experimental part, the main tasks were the generation of droplets of varying sizes, temperature control of the system and image recording. For the modeling part of the thesis the simulations have consisted of modeling single sedimenting fluid particles in a stagnant fluid. The program used for the simulations has been Ansys Fluent. Experiments were performed to determine the viscosity of the oil and water samples available in the laboratory, enabling more accurate simulations. The use of high voltage provided control of the droplet size to some extent. Droplets in the range 200 to 2500 µm were generated and further studied. The effect of droplet size, temperature and aging of the sample on both settling velocity and coalescence time was studied. It was found that for the chosen size range of droplets, the settling velocity increased with increasing droplet size, however a clear temperature dependency could not be seen. The coalescence time showed a trend corresponding well with existing literature. However, aging of the sample showed to have an effect on which trend the coalescence followed. A clearer temperature dependency than that of the terminal velocity was found, which showed a decrease in coalescence time with increase in temperature for higher temperatures. The experimental results obtained during the work for this thesis show compliance with both theoretical predictions as well experimental predictions, however further work is suggested. The experiments were limited to the use of atmospheric conditions, and only one sample of crud oil was studied. Further experimenting on crude oils and water samples with varying temperatures to study the effect on droplet dynamics and reproducibility of results, along with aging effects, is recommended.The simulation results for the terminal velocity showed oscillating tendencies and lower values than what was experimentally obtained, expected to be caused by the simplicity of the simulation model and differences in properties of the materials. Further work with simulations and more advanced models is recommended, including coupling with population balance modeling.