| dc.description.abstract | Gravity separation is a critical operation in oil and gas processing, both in terms of attaining specifications for finished products and to protect equipment downstream of the gravity separator. Liquid-liquid gravity separation, and the slow migration velocities of the dispersed phase, is the main reason for the large size of gravity separators today. The principal effort in achieving a more efficient and compact gravity separator design is knowledge of the fluid dynamic behaviour of the dispersed phase. In this thesis, the settling velocity of a cluster of water droplets falling in oil was studied experimentally for three different oil/water systems. Focus has been directed on comparing the behaviour of droplets settling in the presence of other droplets, as opposed to the settling of an isolated droplet. Two different samples of produced water and crude oil were used, and clusters of droplets ranging from approximately 20 μm to 180 μm in diameter were generated using high voltage pulses. Visualisation of the droplets was achieved by the use of a near-infrared camera. The settling velocity of isolated droplets and of droplet clusters was measured and compared, and the effect of temperature on the settling velocity was studied. Similarly, the drag coefficient of single droplets and droplet clusters was calculated from the experimental measurements and compared. The viscosities of the crude oil and produced water samples were experimentally measured using a rheometer, and the densities of the samples were determined through high-precision weighing. The density and viscosity measurements were applied in theoretical equations describing the droplet settling velocity and compared to the experimentally measured velocities. The results of the crude oil/produced water experiments were compared to the results of similar experiments performed with Exxsol D80, a transparent lamp oil, and distilled water. Qualitative observations of droplet-droplet coalescence were made for each of the systems during the course of the experiments.Both for isolated droplets and for droplet clusters in all three oil/water systems, the settling velocity increased with increasing droplet diameter, or with increasing average droplet diameter in the cluster. The shape of the velocity curve for single droplets and for droplet clusters was found to correlate well with Stokes and Hadamard-Rybczynski theory, and the velocity of single, isolated droplets was ultimately found to correspond with Stokes' theory. For all systems investigated, most droplet clusters were observed to attain a higher velocity than isolated droplets of the same diameter as the average diameter of the droplets in the clusters, resulting from a reduction in the drag force on the droplets due to their mutual interaction. Regardless of the average diameter of the droplets in the cluster, the majority of droplet clusters exhibited settling velocities around 40% higher than the velocity of a corresponding single droplet. The drag coefficient of droplet clusters was approximately 20% to 40% lower than the drag coefficient of a single droplet with the same Reynolds number as the Reynolds number of the droplet cluster. The settling velocity of isolated droplets and of droplet clusters was observed to increase with increasing temperature, due to a decrease in the viscosity of the oil phase with increasing temperature. However, ageing of the samples was observed to result in an increase in the viscosity, most likely due to the evaporation of lighter components in the oil samples, counteracting the temperature-induced viscosity decrease. This phenomenon was also observed in the results of the viscosity testing of aged crude oil samples, even at ambient conditions. For droplet clusters, the influence of the volume fraction of droplets in the cluster on the cluster settling velocity was investigated, but no definite correlation was found. No cases of droplet-droplet coalescence was observed during the settling of droplet clusters in either of the two crude oil systems. The crude oil samples most likely contain a high amount of surfactant, inhibiting film drainage between colliding droplets in the clusters. The slow film drainage could possibly have lead to coalescence times that were longer than the amount of time the trajectories of the droplets could be followed with the current experimental set-up. Droplets in contact were observed to roll off of each other, or to remain in relative fixed positions while travelling downwards through the oil as a single droplet. Due to the lack of control over the shape, size and number of droplets in the clusters and the limited reproducibility of results with the current method of droplet generation, further experimental research on multiple droplet interaction and development of the experimental set-up is recommended. | nb_NO |
