Subsea separation is increasingly becoming a critical subject in the context of deepwater field development. Gas-liquid cyclonic separators are the most promising option for subsea separation due to their simplicity, cost effectiveness and ease of intervention. The performance of the existing cyclonic separators is undermined by liquid carryover phenomenon. An understanding of the behaviours of liquid holdup in upper section of cyclonic separator is required for proper design and prediction of operating envelope and pressure drop. This paper presents results of ERT and WMS measurement of liquid holdup in a 76.2 mm ID gas-liquid pipe cyclonic separator. The results showed that both inlet gas velocity and inlet liquid holdup affect liquid distribution and holdup in the upper section of the separator.


Liquid holdup in gas-liquid cyclonic separator

During normal operating condition of a gas-liquid cyclonic separator, liquid can exist in the upper section above the tangential inlet of the separator in form of upward swirling liquid film as schematically shown in figure 1. Ideally, the upward swirling liquid film could reverse and flow downward under the influence of gravity and not even a fraction of it would escape with the gas stream. When liquid exists in the upper part of the separator without liquid droplets entrained in it, the phenomena is termed zero-net liquid flow (ZNLF). The in-situ liquid volume fraction in the section above the inlet of the separator during ZNLF is referred to as zero net liquid holdup [1]. In gas-liquid separators, it is important to mention that ZNLF does not imply that there is no liquid flow into the separator. The occurrence of ZNLF is not limited to separators alone; it's also a common phenomenon in tubing of gas wells, gas lift oil wells; and multiphase reactors [2; 3]. Previous research shows that, liquid holdup during ZNLF condition is practically the limit above which liquid carryover in gas-liquid cyclonic separator would take place [6]. Liquid carryover (LCO) is one of the critical performance parameter of any separator whether gravity or cyclonic. In case of vertical gas-liquid cyclonic separators, LCO of the separator defines the operating range above which the separator cannot achieve required gas-liquid separation. The knowledge of ZNLF liquid holdup is critical to estimating the amount of LCO and also predicting separator inlet conditions at which LCO would occur. ZNLF liquid holdup is also required to calculate mixture density which is a key parameter in pressure drop prediction of the separator.

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