Separator: Gas-Liquid

Gas Liquid Separator CFD Simulation, ANSYS Fluent Tutorial

Description

A gas-liquid separator is used across a wide range of industrial applications to separate a vapor-liquid mixture, and is variously known as a flash drum, break-pot, knock-out drum, knock-out pot, or — when specifically removing suspended water droplets from air streams — a demister. Separation itself is the fluid process by which mixtures of gas, liquid, and solid materials are divided into their component phases. Crude gas entering a refinery typically contains a mixture of gas, oil, and solids that must be separated before undergoing further processing, with oil and gas separators serving as a primary tool for dividing gas, crude oil, and water streams directly at the wellhead.

While gas-oil separators can be built in vertical, horizontal, or spherical configurations, horizontal designs are the most common. This project simulates liquid-gas separation using a horizontal cylindrical separator, with an oil-gas mixture entering at mass flow rates of 3 kg/s (oil) and 0.15 kg/s (gas). Since oil is heavier than gas, it settles toward the bottom of the separator while the gas phase exits through the top outlet.

The geometry was designed in Design Modeler as a horizontal cylinder, featuring a single horizontal inlet for the incoming mixture and two vertical outlets for the separated gas and liquid streams. The domain was meshed in ANSYS Meshing using an unstructured grid totaling 356,230 cells.

Methodology

This simulation models two phases — oil and hydrogen sulfide — using a multiphase approach. Since the phase boundary between them is sharply defined, the Volume of Fluid (VOF) model was applied, with hydrogen sulfide as the primary phase and oil as the secondary phase. The dispersed interface modeling option was used to capture the behavior at the boundary between the two phases.

Conclusion

Results include 2D and 3D contours of velocity, pressure, and the mass fractions of oil and gas, along with velocity vectors and streamlines throughout the separator. The results confirm that oil settles toward the lower portion of the separator due to its greater density, while hydrogen sulfide rises toward the top of the chamber due to its lighter weight — demonstrating that the separator system functions correctly, effectively dividing the oil and gas phases as intended.