Predicting Liquid Re-Entrainment in Horizontal Separators
- J.C. Viles (Paragon Engineering Services)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- May 1993
- Document Type
- Journal Paper
- 405 - 409
- 1993. Society of Petroleum Engineers
- 4.6.3 Gas to liquids, 4.1.1 Process Simulation, 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment
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The design procedure for horizontal-separator sizing results in a range ofconfigurations of vessel diameter and length that will perform adequate-liquidseparation. The actual diameter chosen depends on a trade-off between smaller,more economic diameters and the larger diameters needed to preventre-entrainment of previously separated liquid droplets that can break away fromthe gas/liquid interface. The lower diameter limit has been determinedpreviously by design guidelines based on the slenderness ratio of the vessel.This article presents a procedure for determining the lower diameter limit andcalculating the maximum gas capacity of a horizontal separator on the basis ofliquid re-entrainment. The method is based on correlations for predicting theonset of liquid re-entrainment developed by Ishii and Grolmes. The procedureuses known and predicted liquid and gas properties and may be used inconjunction with normal design procedures for more economichorizontal-separator designs.
Entrainment refers to liquid droplets breaking away from a gas/liquidinterface to become suspended in the gas phase. The term re-entrainment is usedin horizontal-separator design because it generally is assumed that dropletshave settled to the liquid phase and then are returned to the gas phase.
Liquid re-entrainment is caused by high gas velocities. Momentum transferfrom the gas to the liquid and associated pressure variations on the gas/liquidinterface cause disturbances in the two-phase boundary. These disturbancesmanifest themselves as waves and ripples. Gas-to-liquid momentum transfer to adisturbed interface is more efficient than to a smooth surface, which allowsdroplets to break away from the liquid phase.
Re-entrainment must be avoided in horizontal separators because it is thereverse of the gas/liquid separation desired. This necessity imposes an upperlimit on the gas velocity allowed across the liquid surface in the gravitysettling section of the separator, which places a lower limit in the vessel onthe cross-sectional area for gas flow. Vessel design therefore is limited by acombination of minimum vessel diameter and maximum liquid level because thesedetermine the cross-sectional area. Previously, such rules of thumb as amaximum slenderness ratio of 4:5 have been used to avoidre-entrainment.1
This article presents a procedure for predicting when re-entrainment ispossible on the basis of previously developed correlations and discussesmodifications to design procedures to produce more economicalhorizontal-separator designs.
Re-entrainment is a physical phenomenon of two-phase stratified fluid flow.The onset of re-entrainment occurs at the boundary of the stratified wavy andannular mist two-phase flow regimes at relatively high gas/liquid velocities,as Fig. 12 shows. Re-entrainment is caused by rapid momentumtransfer from gas to liquid. For the purposes of this article, only the onsetof re-entrainment must be predicted because no amount of re-entrainment can beallowed in a horizontal separator.
Ishii and Grolmes3 and Ishii and Kaichiro4 proposedcorrelations for predicting the minimum velocity required for re-entrainment ofliquid into gas for concurrent flow. The equations (see Appendix) are based oninterpretation of experimental data taken from several gas/liquid systems,including water or oil and nitrogen or helium. The correlations use theReynolds film number and an interfacial viscosity number to characterize thetwo-phase flow. These are defined as
and Equation 2
NRef is a measure of the turbulence of the liquidphase and indicates which mechanism of re-entrainment is most likely for theflow conditions considered.
Ishii and Grolmes3 proposed three distinct mechanisms forre-entrainment. For NRef<160, a wave undercutmechanism was proposed where gas impinges on the gas/liquid interface,undercutting it and breaking displaced liquid away from the interface. Athigher NRef, roll wave shear, where the tops of wavesare sheared off by high relative velocities between gas and liquid, becomes thedominant mechanism. NRef>1,635 indicates a highlyturbulent condition dominated by interfacial properties. AsNRef increases, the liquid surface becomes rougher,and the importance of NRef diminishes asymptotically.These mechanisms occur in three flow regimes, called low Reynolds number(<160), transition (160 to 1,635), and rough turbulent (>1,635).Re-entrainment is more likely at high NRef.
Nµ is a measure of the resiliency of the liquid surfaceunder turbulent conditions. In physical terms, it is the ratio of viscousforces induced in the liquid by flow to the surface tension maintaining thegas/liquid interface. Re-entrainment becomes more likely with higherNµ. The tendency of liquids to re-entrain increasesappreciably as Nµ exceeds 1/15.
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