Investigation of gravity based separation of immiscible liquids

E., Weiwei (2020) Investigation of gravity based separation of immiscible liquids. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Separation of oil and water plays an important role in oil production and other industries. Separation efficiency is a key factor for oil and gas production equipment, such as compressors and water treatment equipment. Thus, the improvement of oil / water separation efficiency is a task of increasing importance for the industry, especially manufacturers of separation equipment. This thesis analyzes the separation of oil from an oil / water mixture and, in particular, the coalescence of oil droplets formed during separation by using a combination of experimental and numerical modeling methods. A gravity-based separator is designed and built to conduct optimization experiments on a continuous oil/water separation process. Another laboratory-scale experimental setup is developed to investigate and optimize a batch separation process. Two-dimensional Computational Fluid Dynamics (CFD) models are developed using the Fluent Software package with the same geometric profiles of the two experimental models to investigate further the effects of a broader range of operating conditions on the separation process systematically. In addition, two other available software packages, OpenFoam and Flow-3D, are explored to model the oil / water separation process for the Base Case. The results show that they were not as accurate as Fluent but much faster. A new semi - analytical model is developed to predict liquid / liquid separation dynamics with a focus on water / oil mixtures. The model employs a force balance on the droplets to predict the rising velocity of the oil phase. The effect of droplet coalescence on the droplet’s rising velocity is investigated, and a new correlation is developed that predicts the coalescence rate based on the oil / water fraction, and the initial droplet diameter. Numerical simulations of a batch oil / water separation process are conducted to develop the droplet coalescence. An equivalent experiment is conducted, and the experimental results are found to agree well with the numerical predictions (relative error of 13.39 %). The proposed semi-analytical model can predict the rate of separation with a relative error of 6.35 % compared to full numerical predictions. The analytical model provides an alternative technique to predict the separation of liquid / liquid mixtures at a much lower computational cost, useful for initial design or analysis of separation scenarios. Finally, a new geometric design correlation is developed using a non-dimensional analysis method. A parametric study of numerical predictions conducted with CFD Fluent is employed to investigate several critical variables that affect the separator design. Additionally, a series of simulations are conducted to validate the correlation model by changing the value of dimensionless groups. The correlation results show that increasing the Reynolds and Euler numbers require a separator with a longer length to height ratio to achieve the same separation efficiency. However, an increase in the Weber number requires a separator with a smaller length to height ratio.

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