By J.C. García, F.Z. Sierra, J. Kubiak and D. Juárez, CIICAP-UAEM, Cuernavaca, México

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Many industries have a variety of processes in which multiphase mixtures need to be separated. This is especially true in the oil industry. Among the separation devices available today, the Compact Cyclonic Separator (CCS) offers several advantages over other separator designs. It is easy to operate, requires little space for installation, and because of its simple geometry, has low maintenance costs. It is highly efficient, and has a small characteristic time scale for separating two- or three-phase systems.

Contours of density for a gas/oil-propane-coal (liquid-gas-particle) mixture in a separator

Using FLUENT, the separation performance of a CCS was recently analyzed. The two-phase flow studied was a water-gas/oil liquid-liquid) mixture, while the three-phase flow studied was a gas/oil-propane-coal (liquid- gas-particle) mixture.

CCS units were simulated with main cylindrical body height/diameter aspect ratios ranging from 2.5 to 3.8. The tangential inlet is a thin rectangular section located at the upper part of the main body. In the case of twophase flow separation, the CCS has two outlets: an axial one centered at the top, and a tangential one located at the bottom of the cylindrical body. For three-phase flow, an additional axial outlet is located above the bottom.

Tangential velocity vectors for a gas/oilpropane-coal (liquidgas-particle) mixture in a separator

The multiphase mixture enters tangentially through the upper part of the CCS, and forms two concentric vortices in the cylindrical body of the separator. An internal vortex hugs the vertical axis of the separator, while an external one develops in the region close the wall. The two vortices rotate in the same direction, but the internal vortex flows upward, carrying the lighter (gas) phase, while the external one flows downward, carrying the heavier (particulate) phase. In the case of three-phase flow, the intermediate (liquid) phase also migrates toward the bottom of the tank, and exits through the lower axial outlet.

A 3D solution of the complex flow in the CCS was obtained in FLUENT using a combination of the mixture model for the gasliquid mixture and the discrete phase model DPM) for the particulate phase. The RNG model was used for turbulence and the 3D mesh contained 67,000 elements.

Particle tracks colored by particle ID show the flow of particles through the cyclone

One of the first tasks in the project was the validation of CFD results against experimental data1,2 in a series of parametric studies. Among the parameters varied were the viscosity, inlet velocity, particle size, and exit pressures. The simulations conveyed detailed information about the behavior of the CCS under varying conditions, and how such conditions affect the separation process. In one case the tangential velocity was found to be in perfect agreement with experimental values3 near the wall, but over-predicted by 14% near the core. To demonstrate that threephase flows were in fact separating, contours of density on axial slice planes were used in conjunction with particle track displays. These results showed good separation performance for the three-phase mixture. FLUENT is now being used to further investigate other operating conditions of a CCS.

references:

1 F. Sierra, D. Juárez, J. García, J. Kubiak and R.
Nicolás, A Computational Analysis of Multiphase
Flow Cyclonic Separator for Clean Combustion in
Power Plants, 2003 International Joint Power
Generation Conference, Atlanta, GA, June 2003.

2 F. Erdal, S. Shirazi, O. Soham and G. Kouba,
CFD Simulation of Single Phase and Two-Phase in
Gas-Liquid Cylindrical Cyclone Separators, SPE
36645, SPIE 71st Annual Meeting, Denver,
October 6-9, 1996.

3 D. Farchi, A Study of Mixers and Separators for
Two-Phase Flow In M.H.D. Energy Conversion
System, MS Thesis, Ben-Gurion University,
Israel, 1990.

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