By Steve Turner, Zeta-pdm Ltd., Isle of Wight, UK

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Zeta-pdm Ltd. is a world leader in the oil & gas industry, focusing predominantly on separation processes. In the recent design of a separation vessel, Zeta-pdm engineers wanted to investigate the effect of different inlet designs and baffle arrangements on separation efficiency. The separator will be used to process a mixture of water, hydrocarbon gas, sand, and oil. During the process, water will separate from the gas and oil while sand particles settle to the bottom of the container. CFD was used to simulate the multiphase flow field for a number of prospective designs, so that engineers could improve their understanding of the separation process, and assess the designs for the improvement each would offer.

The interface between the water and gas close to the vessel inlet

The separation vessel consists of an inlet region, a series of specially designed baffles, and separate outlets for the water and gas. The flow pattern is a function of the inlet design, baffle design and position, vessel dimensions, inlet velocity, and mixture composition. Material enters the vessel in a highly turbulent state, consisting of liquid, bubbles, and particles. Since the quantity of oil in the mixture is very small, it is neglected for the purpose of the simulation. The material passes through the baffles, which work to calm the flow and enhance the separation process. The flow conditions need to be controlled so that the sand particles entering the vessel can settle, the gas bubbles can rise, and each constituent can be removed from the vessel through the desired outlet.

The multiphase mixture was simulated in FLUENT using the Eulerian multiphase model in combination with the discrete phase model (DPM). The Eulerian model, in which separate sets of fluid equations are used for each fluid, was used to track the bulk separation of the gas and water phases, the two primary ingredients in the incoming mixture. The sand particles were simulated using the DPM, since it is the most efficient way to track the motion of particles with a range of sizes. Gas bubbles were also tracked using this method, to assess the separation efficiency of the vessel as a function of bubble size. A hybrid mesh of approximately 600,000 cells was used. The solution was performed on a network of computers, using FLUENT's parallel processing capability.

Settling efficiency for sand for a range of particle sizes and two inlet designs

Examination of iso-surfaces of the gas-water interface close to the inlet is one way that engineers analyzed different inlet designs. The fluids are highly turbulent and well mixed in this region, but the large volume of buoyant hydrocarbon causes the gas to rise rapidly and separate from the water. Different designs were found to make this process more or less efficient. Examination of planar contours of volume fraction near the inlet and first few baffles were used to assess the inlet design as well as the calming effects of the baffles. Using the DPM results, different inlet designs were evaluated for their ability to separate bubbles and sand particles of a range of sizes. Results showed that while two designs may perform comparably for small bubbles and particles, one worked better for larger sizes. The results were used to finalize the design for the new separator, which is now in operation.

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