MODELLING OF OIL-WATER SEPARATOR USING COMPUTATIONAL FLUID DYNAMICS (CFD)

Abstract

The API design code is the existing design method for separators. It is based on rules of thumb; it does not look at complex phenomena that happen inside the separator. Building prototypes is both time consuming and expensive. Even if the design task is accomplished, the prototypes provide limited information as to why a particular design did or did not work. As a consequence of this, results may be obtained that are not exact and often lead to overdesign of the separator. Also, separation which is one component of a production phase poses a distinctive challenge on a floating platform because of the unavoidable wave motion to be expected at sea. These wave motions, i.e. pitch, heave, yaw, sway, surge and roll are present even in calm weather conditions. They tend to have a natural mixing effect on the oil, water and gas, thereby resulting in an increase in the time it takes to separate the mixture. The API design code has no answer to such a challenge. Computational fluid dynamics (CFD) can provide solutions to the aforementioned problems. The characteristics of fluid flow and phase separation were numerically analysed as part of the work presented herein. The effects of parameters like velocity and droplet diameter on the separator geometry were solved using the software package Fluent 6.2, which is designed for numerical simulation of fluid flow and mass transfer. Simulations were performed for different velocities and bubble diameters. The results showed that there is a strong dependency of phase separation on mixture velocity and droplet diameter. An increase in mixture velocity will bring about a slow down in phase separation and as a consequence will require a weir of greater height. An increase in droplet diameter will produce a better phase separation. The simulations are in agreement with results reported in literature and show that CFD can be a useful tool in studying a horizontal oil-water separator.