Mechanistic model of an in-line liquid-liquid swirl separator

Abstract

The world’s ever-increasing demand for energy and the inevitable depletion of the available fossil resources in the foreseeable future lead to an increasing necessity to optimize the exploitation of oil fields. The in-line liquid-liquid swirl separator is a separation method that is interesting for two reasons: It separates oil and water faster than the conventional settling tank and the fact that it is placed in-line is a practical advantage. In this report a study has been conducted on the modelling of an in-line liquid-liquid swirl separator using a mechanistic model. This model can be used as a thinking tool to better understand the separation performance of an in-line swirl separator: which parameters play an important role, what are the key processes? This model could also serve as an engineering design tool to optimize the swirl separator for industrial use. The goal of this research is to build a solid foundation for a relatively uncomplicated mechanistic model to describe the underlying physical processes of an in-line swirl separator.
To model the process of separating an oil-water mixture using an in-line liquid-liquid swirl separator different research steps have been taken. The first step was building a base model. The idea of this model was to provide a solid foundation for a more sophisticated model. Based on the findings from this base model and by comparing the base model results to results from CFD simulations and experimental results, a more sophisticated model has been built. In this report this model is referred to as the swirl decay model. These models calculate the separation efficiency based on certain key parameters. To find the efficiency, the trajectory of the particle in the axial and radial directions are modelled based on a balance of forces constructed under the assumption of a quasi-steady state. From these trajectories it is possible to determine whether an oil droplet starting at a certain radial position at the beginning of the swirl tube ends up in the collection tube at the end of the pipe.
The mechanistic model that was built in this research manages to capture the general processes involved in an in-line swirl separator, although the calculated efficiency values are generally larger than the experimental values. However, in order to capture the whole process, several features should be added to the model. These improvements include: more accurately determining the swirl decay coefficient, finding a way to model the droplet size distribution, model droplet coalescence and breakup, implementing a way to alter the flow split.