Breakage and coalescence mechanisms in oil/water gravity separators

Abstract

The equipment used for bulk separation operations in offshore activities to produce oil and gas is an important part of the oil and gas production chain. The part of the separation where you separate water from oil is often called liquid-liquid separation. The fluid dynamics inside the separators are of complex nature, and there is need for studying and understanding of processes such as breakup and coalescence which are present in the separators. The main focuses in this project are: 1. Review of liquid-liquid separation technology. 2. Investigation of breakage and coalescence mechanisms related to oil and water separation. 3. Presentation of the idea behind population balance modeling and CFD analysis for liquid-liquid separation. 4. The main part will be experimental with a droplet-surface coalescence experiment for an oil-water mixture. 5. Also droplet-wire impact and breakup of water droplets are experimentally investigated. The most important part of this project has been the experimental part. The measured parameters were droplet diameter, droplet impact velocity and coalescence time. The coalescence process was filmed with a high-speed Phantom camera. A mixture of Exxsol D80 and water made it possible to observe and get videos and images of falling water droplets that flows through an oil phase and coalesce and settles at the water phase in the bottom. The measurement of the droplet diameter and the droplet impact velocity gave consistent results and the impact velocity seems to increase linearly with increasing droplet diameter. The impact velocity was ranging from about 0,035 m/s to 0,15 m/s and the droplet diameter was ranging from about 1,4 mm to 5 mm. The numbers were compared with the analytical “Intermediate law”, and this equation gave results close to the experimental results. The results show that the coalescence time is a complex parameter to measure, and that the results are spread over a big interval. It is difficult to get a tendency and to see how the coalescence time is dependent on the droplet diameter and the impact velocity. We can from this say that the random nature of the coalescence time would need further investigation. Droplet wire impact was also investigated. Big droplets with diameter about 7,25 mm were breaking up into two daughter droplets of about 5-6 mm in diameter. Even this experimental part was not extensive it was enough to show that internal equipment in separators may change behavior, and by changing parameters like droplet diameter, impact velocity we can obtain different separation efficiency.