Análise numérica do escoamento água-óleo em um separador gravitacional horizontal

Abstract

A typical oil reservoir contains oil, natural gas and water. This mixture is transferred to a primary processing plant, which consists of a set of equipment with the purpose of separating the phases. Generally, the gravity separator is the first and largest processing equipment in a primary processing plant. After the initial separation carried out by the gravity separator, the oily water needs to be treated to meet environmental demands, or operational if used as an input in another production activity. The treatment of oily water is usually carried out by hydrocyclones and a flotation system, which aims to remove the oil droplets dispersed in the water. Gravitational separators are large equipment, which often becomes a critical factor for their installation in the small spaces of oil production platforms. The general objective of the present work is to verify gains in efficiency with the injection of bubbles in a gravitational separator through a numerical study, aiming to improve the quality of the produced water. The simulations were performed using the Eulerian-Lagrangian multiphase approach, with the SST turbulence model available in ANSYS CFX R1 2021. The numerical test grid was based on the experimental data available for the validation of this study. The water flow at the inlet varied from 200 to 450 ml/s, and the oil flow at the inlet varied from 25 to 50 ml/s, forming a total of 6 numerical points. The droplet diameter distributions were implemented in two ways, by the Rosin-Rammler distribution model and by the discrete diameter distribution. The validation of this study was based on the estimate of the cut-off diameter of the oil droplets at the water outlet, where the minimum and maximum error were 0% and 58% in relation to the experimental data. The results found showed that the greater the water flow at the inlet, the greater the cut-off diameter of the oil drops at the water outlet, as expected. The injection of bubbles in the gravitational separator reduced by up to 22% the cut-off diameter of the oil droplets at the water outlet and increased the efficiency of phase separation by up to 1%. It was then concluded that bubble injection can be beneficial for phase separation, especially at higher flow rates.