SPE 124205

CFD Characterization of Liquid Carryover in Gas/Liquid Separator With Droplet Coalescence due to Vessel Internals Yaojun Lu, SPE, and John Greene, FMC Technologies Inc., and Madhusuden Agrawal, Ansys Inc.

Copyright 2009, Society of Petroleum Engineers This paper was prepared for presentation at the 2009 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, USA, 4–7 October 2009. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

ABSTRACT

Liquid carryover in gas/liquid separators has been numerically characterized by using the Euler-Lagrange multiphase formulation together with correlations specifically developed to account for droplets coalescence due to the vessel internals. With this approach, contribution of vessel internals to flow distribution is addressed with porous medium treatment, and contribution to separation performance is considered using the local changes of equivalent mean-droplet size across each vessel internal. Numerical verifications on two vessel configurations operated at high and low pressures have shown that the vessel with proposed internals outperform the vessel with original internals, hence minimized liquid carryover rate is achieved. This CFD prediction agrees well with the existing separation theory and practical observation since only a partial perforated baffle and a vane pack are available in the original vessel. The proposed vessel however integrates the perforated baffles as flow distributor, mesh pad as droplet coalescer, and spiral flow cyclone as mist demister. Of course, combination of these vessel internals makes the proposed vessel a great upgrade to the original vessel. It is therefore demonstrated that not only CFD can be applied to characterize the overall flow distribution, but also can be extended to quantify liquid carryover, hence the separation performance of gas/liquid separators, no matter they are operated in onshore, offshore, or subsea upstream processing industries. Key words: gas/liquid separator, vessel internal, CFD simulation, droplet coalescence, liquid carryover

INTRODUCTION

Gas/liquid separators are commonly used to remove oil, condensate, and water from wet gas stream during oil/gas production. Due to the complex mechanisms involved, the gas/liquid separators are largely designed based on empirical principles. As a consequence, the designated vessel performance can hardly be achieved in practical operation. In most cases either the designed vessels cannot function as expected or the output stream cannot fulfill quality standards due to high liquid carryover rate. It is, therefore, essential to quantitatively characterize and effectively control the liquid carryover in a gas/liquid separator.

Liquid carryover in a gas/liquid separator depends not only on the vessel configuration and operating condition, but also on the droplet break up and coalescence processes due to vessel internals. Because of the sensitivity of droplet size to the gravity settling process, a variety of vessel internals (inlet cyclones, perforated baffles, vane packs, mesh pads, and spiral flow demister, etc) have been developed to enhance droplet coalescence and reduce liquid carryover involved in a gas/liquid separator. Based on the Souder–Brown equation (Sounders and Brown 1934), API SPEC 12J (API specification 12J 2008) sizes a gas/liquid separator using the maximum allowable gas velocity, at which the minimum droplet can settle out of a moving gas stream. To prevent re-entrainment of liquid droplets from gas/liquid interface, a simplified Kelvin-Helmholtz criterion is practically used to estimate a critical interface velocity. If the actual gas velocity under a given operating condition exceeds the critical interface velocity, it is then assumed that the liquid droplets will be entrained into the gas stream from the gas/liquid interface, and eventually leads to a higher liquid carryover at gas outlet. As a work-around approach, both the K-factor and K-H criterion simplify the separation process based on empirical correlations, and thus large uncertainty may exist in characterization of liquid carryover in practical applications. Overall, there is at present no theoretical approach available to account for the contribution of vessel internals, while experimental quantification of each vessel internal is time consuming and costly

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Although Computational Fluid Mechanics (CFD) has demonstrated effectiveness in modeling flow distribution in various separation vessels, and holds potential to quantify the liquid carryover by tracking trajectories of droplets in a gas/liquid separator, the standard Lagrangian transport model does not account for the local change of droplet size and number due to the vessel internals. Therefore, the objective of this paper is to extend the existing CFD capability by developing an applicable strategy to practically characterize the droplet breakup and coalescence effect due to the enhancement internals. To achieve this, specific correlations have been proposed based on experimental observation and theoretical analysis and implemented into the commercial CFD code by user defined subroutine to address the local changes of droplet size caused by each vessel internal. PROCESS CONSIDERATION

Gas/Liquid Separator

Gas/liquid separators can be categorized into vertical, horizontal and spherical types. Only horizontal vessel is discussed in the present work, and a similar analysis can be applied to the vertical and spherical vessels. As illustrated in Figure 1, four major functional zones can be generally identified in a gas/liquid separator (Ken and Maurice 2008). The section between the inlet nozzle and first baffle can be considered as the primary separation zone, which is designed to separate the bulk liquid from the gas stream. Downstream from the primary separation zone is the gravity settling zone, which is used for the entrained droplets to settle from the wet gas stream. This section normally occupies a large portion of the vessel volume through which the gas moves at a relatively low velocity. Following the gravity settling zone is a droplet coalescing zone, which could be parallel plates, vane packs, mesh pads, and spiral flow demisters. This zone helps to remove very small droplets based on impingement and inertial separation principles. In some designs, the droplet coalescing zone and the gravity settling zone work sequentially. In other designs, however, they are integrated together. The last section is the liquid collection zones, which is located below the interfaces, and behaves like a sump/receiver for liquid removed from the gas stream in all the primary separation, gravity settling, and droplets coalescing zones. Depending on process requirements, the liquid collection zone may have a certain amount of surge volume over a minimum liquid level necessary for control system to function properly.

Generally, gas/liquid separators without any enhancement internal can only remove liquid entrainment with sizes above 100 micron. By adding efficient internals, the corresponding droplet size can be reduced to 5-10 micron. This indicates that the gas/liquid separation efficiency can be enhanced considerably by properly designed vessel internals. It is for this reason that varieties of vessel internals have been developed, which include inlet devices, perforated baffles, mesh pads, vane packs, spiral flow demisters, etc. Vessel Internals

Inlet devices The basic function of the inlet device is to re-distribute the incoming fluids and create an even mass flux along the flow direction. The simplest inlet devices can be a baffle plate or an inverted dished head, which behave like an obstructer mounted at the end of inlet nozzle, and direct the incoming flow within the separator vessel. With these simple devices, the strong jet flow and poor flow distribution are normally created downstream from the inlet nozzle. To enhance separation performance and increase utilization of the vessel volume, it is essential to improve the flow distribution near the inlet nozzle. Therefore, more sophisticated inlet devices have been developed to absorb momentum of the incoming stream and introduce pre-separation effect. On this aspect, the even flow inlet and inlet cyclones shown in Figure 2 and 3 are increasingly gaining importance in gas/liquid separators. The even flow inlet is more specific to guide the incoming stream over vessel across section, while the inlet-cyclone seeks to separate the gas and liquid phases prior to entering the separator vessel. Perforated baffles The perforated baffle can be just a single plate with uniformly distributed holes as shown Figure 4. Some advanced baffles are constructed as double plates with varied hole-size and pitch pattern or combination of the full and partial baffles properly spaced. As the gas stream approaches the baffle surface, flow is forced to change direction and spread along the baffle surface. Due to the presence of perforated baffle, additional pressure drop is created, kinetic energy of the gas stream is dissipated, and flow across the baffle is re-distributed accordingly. Since the gravity settling process is closely related to the flow distribution, the perforated baffles are commonly used to manage flow condition and further to control re-entrainment of droplets from the gas/liquid interface. In conjunction with the inlet devices, the perforated baffles are frequently utilized to establish a primary separation section, where the momentum is reduced prior to entering the gravity settling zone where the conditions are optimized for settling separation. Mesh pad The most common type of mesh pad is knitted-wire-mesh pad. As illustrated in Figure 5 the mesh pads are usually constructed from wires of diameter ranging from 0.1 to 0.28 mm, typical void fraction from 0.95 to 0.99, and thickness from

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100 to 300 mm. During operation, gas stream carrying entrained droplet passes through the mesh pad. The gas moves freely through the mesh pad while the entrained droplets are forced to impinge on the wire surfaces and coalesced due to the inertial effect. Droplets formed in the mesh pad ultimately drain and drop out of the mesh pad. It is evident that a well designed and operated mesh pad can effectively remove droplets larger than 3-5 micron from the gas stream, and the corresponding pressure drop is less than 0.25 kPa. In addition, mesh pads can be operated between 30-110% of the design capacity, thus exhibits excellent turndown behavior. The collection efficiency and pressure drop in a mesh-pad can be estimated by equations of the following forms (Paul etc 1993):

⎟⎠⎞

⎜⎝⎛ ⋅⋅⋅−−= Tvane HA ηπη

32exp1 (1)

2

2

981ερ GG

vaneVAHfP ⋅⋅⋅⋅=Δ

(2)

Vane pack As shown in Figure 6, a conventional vane pack consists of a series of spaced blades to provide passage for gas flow and profiled with angles to provide sufficient change of direction for liquid droplets to impact, coalesce and drain from the blades (. Space between two adjacent blades ranges from 5 to 75 mm with a total depth in the flow direction of 150 to 300 mm. By passing the wet gas through the vane pack, the mist droplets undergo changes in momentum, causing impingement and coalescence on the vane blades. The coalesced droplets then drain down along the vane surfaces. The demisted gas stream flows forward, and eventually escapes from the gas out. Figure 7 illustrates the vane packs with strategically designed slots or pocket hooks, which allows the coalesced liquid on the blade surfaces to be collected and directed into the internal channels shielded from the gas flow. Once droplets get into these channels, the collected liquid is directed to drains and lead to a liquid sump in the separator vessel. Since the liquid is isolated from the gas stream and less subject to re-entrainment, the gas velocities can go significantly high both in horizontal and vertical applications.

It is reported that the conventional vane pack can separate droplets larger than 40 micron, while the vane pack with pocket design can remove droplets down to less than 15 micron. In most cases, the pressure drop through the vane pack ranges from 0.1 to 0.15 kPa, and following correlations can be use to estimate the collection efficiency and pressure drop across a vane pack (Paul etc 1993):

⎥⎦

⎤⎢⎣

⎡⋅⋅⋅⋅⋅⋅−−=

θθη

tan3.57exp1

bVWmV

G

Tmesh (3)

∑ ⋅⋅⋅×=Δ−

C

PAGDmesh A

AVCP2

1002.1 23 ρ (4)

Spiral flow demister Spiral flow demisters consist of multiple cyclone tubes mounted into housing. For illustration purpose, Figure 8 shows only one of the cyclone tube. As can be seen, the gas stream enters the cyclonic inlet and flows through the spiral flow element that imparts a high centrifugal force. The droplets are then flung outward and are coalesced into a liquid film on the inner wall of the cyclone tube. The liquid film is purged out of the cyclonic unit through slits in the wall, along with a small portion of gas flow, into an outer chamber where most of the gas and liquid separate. The gas, along with some remaining mist, is educed back into a low pressure zone of the cyclone unit and the remaining entrainment is removed. The demisted gas is then discharged from the top and the separated liquid is drained from the bottom. Depending on nominal diameter of the spiral elements, droplets of 25 micron and above can be effectively separated, and the corresponding pressure drop ranges from 2.5 to 7.5 kPa. An important advantage of the spiral flow demister is its high gas handling capacity combined with excellent droplets removal efficiency even at elevated pressures. Its downside, however, is sensitivity to flow change. Therefore, the spiral flow demisters are more suitable for applications where the flow fluctuations are not very significant. Vessel sizing equation

Several techniques are available to size a gas/liquid separator. The most commonly used sizing equation was developed by Sounders and Brown (1934). The basis of the Sounders and Brown equation is force balance on a droplet in a moving gas stream. By assuming all droplets as sphere, the corresponding terminal settling velocity can be determined by

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G

GLSBT KV

ρρρ −= (5)

Here KSB is known as the Sounders and Brown value or K-factor, which is defined by.

D

PSB C

gdK34= (6)

Based on the Sounders and Brown equations and API specification 12J (2008), Table1 provides API recommended K-factor for sizing the gas capacity in a gas/liquid separator, while Table 2 gives API design criteria for determining the liquid capacity in term of retention time.

Table 1 API recommended K-factor

Separator type Height or length Typical K-factor range

Vertical 1.524 m 0.12 to 0.24

3.048 m 0.18 to 0.35

Horizontal 3.048 m 0.40 to 0.5

Other 0.4 to 0.5x(0.1L)0.56

Spherical All 0.2 to 0.35

Table 2 API recommended liquid retention time

Oil gravity Liquid retention time

Above 35°API 1 min

20-30°API 1-2 min

10-20°API 2-4 min

In practical applications, high gas velocity at gas-liquid interface may cause momentum transfer from the gas stream to the bulk liquid, creation of waves and ripples in the liquid phase, and eventually separation of broken-away droplets from the liquid phase. Regarding the liquid re-entrainment, neither API recommendation nor theoretical solution is currently available. To prevent re-entrainment of liquid droplets from gas/liquid interface, a simplified Kelvin-Helmholtz criterion is frequently used to estimate a critical interface velocity. If the actual gas velocity under operating condition exceeds the critical interface velocity, it is then assumed that the liquid droplets will be entrained into the gas stream, and eventually leads to a higher liquid carryover rate at gas outlet.

It can be seen that both the Sounders and Brown theory and the K-H criterion, as a work-around approach, focus on the droplet settling and re-entrainment processes in the gravity settling zone, and little attention has been paid to the flow and separation behaviors in the primary separation, droplet coalesce, and liquid collection zones. According to the Sounders and Brown equation, the separation process is simplified as settling process of a single droplet in the gas stream, and no considerations have been given to the coalescence effect due to the vessel internals. Apparent limitation exists in the K-H criterion. Consequently, large uncertainty may be introduced, and the gas/liquid separator thus designed may potentially be confronted with a higher liquid load than that expected, and eventually lead to higher liquid carryover rate from the gas outlet. NUMERICAL SIMULATION

Vessel configurations

Figure 9 schematically shows the original design of a test separator. It is basically a horizontal oil-water-gas separator with two elliptical heads. The corresponding vessel internals include a revolution tube, an inclined perforated baffle, and a vane-pack demister. The revolution tube is attached to the inlet nozzle to separates the bulk gas from the liquids before the incoming fluids get into the separator vessel. The inclined baffle is used to regulate the flow distribution in the liquid phase. The vane pack is applied to enhance separation of entrained droplets from the gas stream. In practical applications, it is,

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however, found that this vessel experiences significant liquid carry over, especially, when new wells are put on stream and the well flow exceeds the capacity of the test separator.

To overcome the liquid carryover issue in the original design, a new vessel configuration has been proposed as shown in Figure 10. The proposed vessel internals include two perforated baffles, a mesh pad, and a spiral flow demister. The inlet stream is first directed through the inlet cyclone for bulk separation. The perforated baffles are used to improve flow distribution by reducing short circuit and channel flow involved in the gas, oil and water phases. The gas stream then flows through the mesh pad. With continuous impact on the knitted and crimped wire surfaces, the liquid droplets are coalesced into a liquid film, which is then dragged down as large droplets, and the rest of uncoalesced droplets are carried down by the gas stream to the spiral flow demister for further treatment. By forcing the mist stream into the multi-cyclone or cyclonic bundle, the gas stream spins and the entrained droplets migrate to the cyclone wall. The liquid then drains out of the cyclone body from recycle opening, whilst the gas stream, free of liquid, flows out the bottom of the cyclone tubes.

To check if the proposed design could deliver an improved working performance, it is therefore necessary to perform CFD evaluation. With bulk separation of the liquid phases from the gas stream inside the inlet cyclone, only small liquid droplets are entrained in the gas stream, and contributed to the liquid carryover phenomenon observed. By focusing on the liquid carryover issue, only gas space above the gas/liquid interface need to be considered. To minimize the scale-up effects, full scale and three-dimensional geometric models are built, which include the inlet cyclone, major process nozzles, and associated internals. In case it is impossible to mathematically describe internal of complex geometry, a simplified treatment is applied based on a sound physical basis. For instance, the perforated baffle, vane pack, mesh pad, and spiral flow demister are approximated by porous mediums. Figure 11 illustrates the geometric models for the original vessel, while Figure 12 shows the proposed new vessel. Operation parameters

To generate representative prediction, actual operating conditions and fluid properties are implemented. In case precise data for a specific parameter is not available, approximations based on best practices and experience are substituted. In fact, two operating conditions are of major concern in this study, which are the high pressure case of 7688 kPa and the low pressure case of 3482 kPa, respectively. Considering that density difference between the gas and liquid phases is less at the high pressure, while gas flow rate is higher at the low pressure, it is not straight forward to tell which case is worse. Therefore, both cases need to be investigated. The corresponding operating parameters and fluid properties are summarized in Table 3.

Table 3 — Operating parameters

Parameters LP HP

Temperature (°C) 44 44

Pressure (kPa) 3428 7688

Flow rate (m3/h) 1408 592.7

Density (kg/m3) 24.8 58.9

Viscosity (cP) 0.016 0.016

Liquid Level (mm) 1016 1016

CFD approach

Advances in Computational Fluid Dynamics (CFD) have provided the basis for insight into multiphase flow dynamics. Currently there are two simulation approaches available for numerical analysis of multiphase flows: the Euler-Lagrange approach and the Euler-Euler approach (FLUENT 2008). Considering that bulk liquid in the two separator vessels is knocked out within the inlet cyclones, Lagrangian formulation can be applied to simulate the mist flow downstream the inlet cyclone. With the Euler-Lagrange approach, the fluid phase is treated as a continuum by solving the time-averaged Navies-Stokes equations, while the dispersed phase is solved by tracking a large number of droplets through a given flow field based on formulations of the following form

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( ) ( )

⎪⎪⎪⎪

⎩

⎪⎪⎪⎪

⎨

⎧

−⋅=

⋅=

+−+−=

G

GPPGe

eD

PP

GD

P

GPPGD

P

VVdR

RCd

F

FgVVFdt

dV

μρρ

μρ

ρρ

2418

2

(7)

Although the Euler-Lagrange approach has been extensively used to capture dynamic behavior of particles/droplets in a processing vessel, the following challenges exist for quantitatively characterizing the liquid carryover in the gas/liquid separator considered.

(1) A fundamental assumption made in the Euler-Lagrange approach is that the dispersed phase is sufficiently dilute so that particle-particle interactions and the effect of the particle volume fraction on the gas phase are negligible. This implies that no droplets interaction/collision can be considered.

(2) The particle trajectories are computed individually at specified intervals. This indicates that the dispersed phase can exchange momentum, mass, and energy with the fluid phase, but the droplet size remains constant from the inlet all the way down to the outlet. Consequently, no droplet breakup and coalescence are accoutered for.

(3) Although it is possible to simulate the detailed configuration of the vessel and associate internals, a more effective way is to simplify the associated vessel internals as porous mediums to facilitate the simulation process. This, however, will remove the physical boundaries available in the vessel internals, and makes it difficult to reflect the contribution of the vessel internals.

With these limitations, the Euler-Lagrange approach actually can’t feel the presence of vessel internals, thus can't correctly account for the coalescence contribution of the vessel internals. In order to capture the liquid carryover effect in the gas/liquid separator, it is, therefore, essential to develop specific models to describe the lcoal change of droplet size due to breakup and coalescence across each vessel internal. By implementing the model into the Euler-Lagrange approach with a user defined subroutine, contribution of the vessel internals can then be quantitatively characterized. This way, CFD can not only predict multiphase flow distribution in the separator vessels, but also able to characterize the liquid carryover associated with separation performance. Droplets coalescence

Liquid carryover in a gas/liquid separator depends not only on the vessel hydraulics, but also droplets break up and coalescence due to the vessel internals. By introducing an equivalent mean droplet diameter, the droplets break up and coalesce across each vessel internal can be addressed based on its contribution to gas/liquid separation. As illustrated in Figure 14, the equivalent droplet size increases from d1 to d2 due to the presence of vane pack and mesh pad ( d2 > d1 ). Enhancement of the spiral flow demister is accoutered for by a critical droplet dcritical. As illustrated in Figure 15, all droplets with size greater than critical are removed from the gas stream, which are actually trapped at outer surface of the spiral flow unit. The three diameters of d1, d2, and dcritical are all determined based on experimental observation and empirical correlations. With this treatment, liquid carryover can then be qualitatively characterized. RESULTS AND DISCUSSION

Vessel with original internals at HP

Trajectories of liquid droplets can visually show how these droplets behave in the test separator under a given operating condition, thus providing valuable information about liquid carryover in the separation vessel. In view of this importance, Figures 16 illustrates the trajectories of 20 micron, 40 micron, and 60 micron oil droplets predicted in the vessel with original internals operated at the high pressure. By releasing 426 droplets from top of the inlet cyclone, it is noticed that most of the 20 micron oil droplet are carried over to the gas outlet, none of 40 micron oil droplets reaches the gas outlet, and the 60 micron oil droplets are even separated prior to the vane pack.

To quantify the liquid carryover in the test separator with original internals, Figure 17 shows the oil and water droplet numbers escaped from the gas outlet in an extended size range. As can be seen, the liquid carryover is mainly caused by droplets with diameter less than 40 micron, which can be considered as a critical size for the given vessel configuration and operating condition. A closer examination on Figure 17 reveals that the liquid carryover rate is gradually reduced as the droplet size increases from 10 micron to 40 micron; and the oil droplets always shows a higher possibility to be carried over than the water droplets due to the less density difference between the oil and gas phases.

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Vessel with original internals at LP

Figure 18 presents the trajectories of oil droplets predicted in the vessel with original internal operated at the low pressure. Similar to the observation in Figure 16, majority of 20 micron oil droplets escape from the gas outlet, only a few of 40 micron oil droplet come out from the gas outlet, and none of 60 micron oil droplets reaches the gas outlet. Compared to the corresponding prediction at the high pressure, it is noticed that more oil droplets escape from the gas outlet due to the increased gas flow rate at the low pressure.

Figure 19 summarized the oil and water droplet numbers captured at the gas outlet under the low operating pressure. Similar to the observations at the high pressure, the liquid carryover rate is reduced as the droplet size increases from 10 micron to 60 micron; the oil droplets exhibited higher tendency to be carried over in the whole size range. With reference to Figure 17, the critical droplet size was increased from 40 micron at the high pressure to 50 micron at the low pressure. All these are attributed to the increased gas rate at the low operating pressure.

Vessel with proposed internals at HP

Figures 20 show the trajectories of oil droplets predicted in the vessel with proposed internals operated at the high pressure. It can be seen that most of 20 micron oil droplets released from the inlet cyclone are carried to the gas outlet, no 40 micron oil droplets escapes from the gas outlet, and none of 60 micron oil droplets reaches the gas outlet.

Figure 21 further compares the oil and water droplets received at the gas outlet. Apparently, no liquid carryover is observed for droplets larger than 30 micron. This indicates that the critical size for this vessel configuration is 30 micron at the high operating pressure. Compared to Figure 16, the critical droplet size decreases 10 micron due to the introduction of the proposed internals. Except for this, all the other observations remains the same, which includes that the water droplets exhibits less tendency to be carried over than the oil droplets of the same size; and the liquid carryover rate is reduced as the droplet size increased from 10 micron to 60 micron, etc. Vessel with proposed internals at LP

Figures 22 show the trajectories of oil droplets predicted in vessel of proposed internals at the low operating pressure. Compared to the predictions in Figures 20, more oil droplets are carried over to the gas outlet due to the reduced operating pressure hence increased gas flow rate.

Similarly, Figure 23 compares the liquid carryover rate for the oil and water droplets with diameter from 10 micron to 40 micron. Apparently, a descending trend is observed over the whole size range, and a corresponding critical size of 40 micron is observed at the low operating pressure. With reference to the 30 micron observed in Figure 21, the critical droplet size increases 10 micron due to change in the operating pressure. Compared to the critical droplet size of 50 micron observed in Figure 17, introduction of the proposed internals leads to a 10 micron reduction in the critical droplet size. Comparison between two vessels

Figures 24 and 25 further compare the oil and water carryover rates predicted in the two vessel configurations operated at the high pressure, while Figures 26 and 27 illustrate the corresponding results at the low operating pressure. As can be seen, the vessel with proposed internals experience less liquid carryover, hence exhibit better separation performance under the both operating pressures. To better understand the underlying physics, it is necessary to look at the difference in configurations between the two vessels. With reference to Figures 9 and 10, there is only the vane pack available in the original vessel to enhance the gas and liquid separation downstream the inlet device, while the proposed vessel integrates the perforated baffle, mesh pad, and spiral flow demister in the same vessel, thus provide significant enhancement on top of the gravity separation. CONCLUSIONS

1. An empirical correlation has been proposed to account for the droplets break up and coalesce effect introduced by the vessel internals in a gas/liquid separator. By implementing the proposed correlation into the Euler-Lagrange model using user defined subroutine, CFD capability has been extended to quantify both the multiphase flow dsitribution and separation behavior, thus quantitatively evaluate the overall performance of the gas/liquid separators, no matter that they are operated in onshore, offshore, or subsea upstream processing industries

2. Based on the trajectories of liquid droplets predicted in the two vessel configurations operated at high and low pressures, liquid carryover rate decreases as the droplet size increases due to the greater driving force available for separation; oil droplets exhibit more potential to be carried over than the water droplets due to the less density difference; vessel operated at lower pressure is prone to carry more droplets to the gas outlet due to the higher gas flow rate. All these observations agree well with existing separation theory and practical observations.

3. The vessel with proposed internals well outperform the vessel with the original internals, and eventually minimizes the liquid carryover rate. This is because only the vane pack is available to enhance gas/liquid separation in the

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original vessel, while the proposed vessel integrates the perforated baffles as flow distributor, mesh pad as droplet coalesce, and spiral flow demister as cyclonic separator. Combination of these enhancement internals makes the proposed vessel a great upgrade of the original vessel.

ACKNOWLEDGEMENTS

The Authors thank FMC management for supporting this research and permitting to publish this work. The Authors are grateful to Adrian S Villarreal for preparing some of the Figure files, and Egidio Marotta for providing proof reading of the manuscript. NOMENCLATURE

A = Surface area of mesh pad (m2)

CA =Cross section area of vane pack (m2)

PA =Projected area of a vane blade (m2) b =Space between adjacent vane blade (m)

DC =Drag coefficient

Pd =Droplet diameter (m) f =Friction factor

F = Additional forces (virtual mass, lift force, pressure induced force, etc) (N)

DF = Drag force (N) g = Gravity acceleration ( ms-2 ) H = Thickness of mesh pad (m)

SBK = Sounders and Brown value or K-factor (dimensionless) m = Number of bends

eR = Droplet Reynolds number (dimensionless) W = Width of a vane baffle ( m )

AV = Actual gas velocity (ms-1)

GV = Gas velocity (ms-1)

PV = Droplet velocity (ms-1)

TV = Terminal velocity (ms-1)

Gρ = Gas density (kgm-3)

Lρ = Liquid density (kgm-3) ε = Void fraction of a mesh pad (dimensionless)

vanePΔ = Pressure drop across vane pack (Pa)

meshPΔ = Pressure drop across mesh pad (Pa)

Tη = Target collection efficiency of a single wire (dimensionless)

vaneη = Separation efficiency of a vane pack (dimensionless)

meshη = Separation efficiency of a mesh pad (dimensionless)

Gμ = Gas viscosity (Pa.s) REFERENCE

[1] API specification 12J, Specification for oil and gas separators, 8th edition, October 2008. [2] Fluent 6.3 user guide, 2008, Fluent Inc. [3] Ken Arnold and Maurice Stewart, 2008, Surface production operations, 3rd edition, Gulf Professional Publishing

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[4] Putnam, A., 1961, Integral form of droplet drag coefficient. ARS Journal:1467-1468 [5] Paul Fabian, Roger Cusack, Paul Hennessey, and Mark Neumann, Demystifying the selection of mist eliminators, part I: the basics, Chemical Engineering, November, 1993 [6] Paul Fabian, Roger Cusack, Paul Hennessey, and Mark Neumann, Demystifying the selection of mist eliminators, part II: applications, Chemical Engineering, December, 1993 [7] Sounders, M. and Brown, G.G., 1934, Design of fractionating columns, Industrial and Engineering Chemistry: 2691-2698

Figure 1 Major function zones in a separator vessel

Figure 2 Revolution-tube inlet Figure 3 Even-flow inlet

Figure 4 Typical configuration of perforated baffle

Primary separation zone Gravity settling zone

Droplet coalescence zone

Liquid collection zone

Gas outlet

Oil outlet Water outlet

Inlet nozzle

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1 layer 4 layer

Figure 5 Typical configuration of mesh pad

Flow

Flow

Figure 6 Conventional vane pack Figure 7 Vane pack with pocket

1829

HHLL 965

4877 Figure 8 Spiral-flow demister Figure 9 Configuration of vessel with original internals

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Figure 10 Configuration of vessel with proposed internals

. Figure 11 Geometric model of vessel with original internals Figure 12 Geometric model of vessel with proposed internals

Figure 14 Droplet coalescence in associated vessel internals Figure 15 Droplet size change in spiral-flow demister

Inlet cyclone

Gas outlet

Gas/liquid interface

Vane pack

Inlet cyclone

Perforated Baffler

Gas outlet

Gas/liquid interface

Mesh pad Spiral flow

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Figure 16 Trajectory oil droplets in vessel with original internals at HP

Figure 17 Oil and water carryover rate in vessel with original internals at HP

Figure 18 Trajectory of oil droplets in vessel with original internals at LP

Figure 19 Oil and water carryover rate in vessel with original internals at LP

20 micron droplet 40 micron droplet 60 micron droplet

20 micron droplet 40 micron droplet 60 micron droplet

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Figure 20 Trajectory of oil droplets in vessel with proposed internals at HP

Figure 21 Oil and water carryover rate in vessel with proposed internals at HP

Figure 22 Trajectory of oil droplets in vessel with proposed internals at LP

Figure 23 Oil and water carryover in vessel with proposed internals at LP

20 micron droplet 40 micron droplet 60 micron droplet

20 micron droplet 40 micron droplet 60 micron droplet

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Figure 24 Compared oil carryover rate between original and proposed vessels at HP

Figure 25 Compared water carryover rate between original and proposed vessels at HP

Figure 26 Compared oil carryover rate between original and proposed vessels at LP

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Figure 27 Compared water carryover rate between original and proposed vessels at LP