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Experimental Analysis and Computational-Fluid-Dynamics Modeling of
Pilot-Scale Three-Phase Separators
Article in SPE Production & Operations · August 2019
DOI: 10.2118/197047-PA
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Experimental Analysis and
Computational-Fluid-Dynamics Modeling
of Pilot-Scale Three-Phase Separators
Tariq Ahmed, Paul A. Russell, Faik Hamad, and Samantha Gooneratne, Teesside University
Summary
In this work, the performance of two pilot-scale separators was investigated using computational-fluid-dynamics (CFD) simulation with
one operating at low gas volumetric quality comprising a bucket-and-weir configuration, and the other operated at high gas volumetric
quality with a weir configuration. The pilot-scale separators were selected for this work because of their availability and the lack of data
on industrial separators. The effects of the liquid (oil and water) flow rate and weir height on separation performance have been investigated for the separator operating at low gas volumetric quality. For this separator, the design of experiments (DOE) and a preliminary
run of the separator were used to select the number of experiments and simulations to conduct and the levels (values) of the three variables investigated. For the second separator, the effects of the inlet flow rate on separation performance have been investigated.
R Fluent (Fluent 2019), combined with a k-e turbulence
Eulerian and volume-of-fluid (VOF) multiphase-flow models in ANSYSV
model, were used to simulate the fluid-flow pattern and phase behavior inside each of the separators. The numerical solutions were initialized with a water level set at 50% of the weir height using a patching tool. A mesh-independence test was carried out to ensure that
the results are not dependent on the mesh quality.
The separation efficiencies from both models were compared with that from the experimental data. The results indicated that the
two multiphase models, namely, Eulerian and VOF, predict the experimental results within 30% error. However, different separation
performances were obtained for the same flow conditions. For the separator operating at low gas volumetric quality, the results from the
Eulerian multiphase model produced a maximum deviation of 8%, while results from the VOF multiphase model produced a maximum
deviation of 23% of the experimental data. For this separator, the oil flow rate was found to have the greatest effect on the separation
efficiency. This is followed by the water flow rate and weir height.
For the separator operating at high gas volumetric quality, a maximum percentage error of 30% for the Eulerian model and 21% for
the VOF was obtained.
Introduction
Multiphase separators are the first surface equipment used for the separation of production fluid into gas, oil, and water. Horizontal
gravity three-phase separators are the most commonly used multiphase separators because of their ease of operation and lower capital
and operating costs when compared with other multiphase separators. The proper design and operation of these separators are crucial to
avoid downstream-equipment malfunction and failure.
The design of three-phase separators is quite challenging. It requires in-depth knowledge related to complex separation physics and
fluid-flow behavior, as well as feed properties and operating conditions. The design of separators has been studied by a number of
researchers using experimental data collected from the existing industrial systems. For example, Monnery and Svrcek (1994) and Arnold
and Stewart (2008a, 2008b) reported step-by-step procedures on how to size three-phase separators. Although useful in providing exploratory designs, these models tended to omit essential factors that affect the separation performance (Ghaffarkhah et al. 2018). The major
limitation of these conventional separator-sizing models is that they cannot predict the separation efficiency of the designed separator.
These models have also been reported to predict different separator dimensions for the same flow conditions (Ahmed et al. 2017).
These limitations highlight the need for an in-depth method for designing three-phase separators. The advances in CFD modeling in
recent years have led to the development of commercially available software that can be used to study the separation process. CFD is
an economical and more flexible option when compared to experimental techniques. It has been used for solving initial separator design
and sizing problems or the modification of the existing equipment to improve its performance (Lee et al. 2009; Laleh et al. 2012;
Kharoua et al. 2013).
CFD consists of a number of multiphase models, turbulence models, and droplet-interaction submodels available in commercial software,
giving it wide applicability. Focusing on the multiphase models, previous CFD studies on three-phase separators mainly used Eulerian
models (Vilagines and Akhras 2010; Kharoua et al. 2012; McCleney et al. 2017; Scapin et al. 2017; McCleney et al. 2018) and VOF models
(Frankiewicz et al. 2001; Laleh et al. 2012; Triwibowo et al. 2017; Ghaffarkhah et al. 2017, 2018). In a few cases, the population-balance
model has been used to model droplet breakup and coalescence in three-phase separators (Kharoua et al. 2013; Oshinowo et al. 2015).
However, no work has been reported in the literature in which both Eulerian and VOF models were used and compared. The objective of this work is to try to identify which among these two multiphase models can be used to accurately predict the separation efficiency in horizontal gravity three-phase separators; if no universal solution is found, then try to identify the optimal model for a given
operating range. Choosing the appropriate multiphase model for a given operating condition will result in more accurate CFD predictions that will minimize the gap between experimental and CFD results.
In this study we attempt to verify the accuracy of the ANSYS Fluent (Fluent 2019) software in experimental-data replication for
modeling three-phase separators. To perform this verification, real experimental data are required; however, there is a general paucity
of industrial-scale separator data available. The limited extant data can be summarized into two groups: the first group comprises data
related to separator internals (Lee et al. 2004; Lu et al. 2007; Kharoua et al. 2012, 2013), and the second group involves omission of
some relevant information regarding either the fluid properties (Hansen et al. 1993) or the separator specifications (Vilagines and
Akhras 2010).
C 2019 Society of Petroleum Engineers
Copyright V
Original SPE manuscript received for review 8 August 2018. Revised manuscript received for review 26 March 2019. Paper (SPE 197047) peer approved 8 May 2019.
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Two pilot-scale experimental units were available in the Teesside area. The first separator is a small laboratory demonstrator that
works at predominantly low gas volumetric quality, and the other is a proprietary system based on a 3-ft diameter for high gas volumetric quality. These permit a limited amount of measurements to be carried out at different flow conditions. While not truly reflecting
industrial length/diameter (L/D) ratios, these pilot-scale separators accurately represent the physics of operation of industrial separators.
Taken together, these provide reasonable test data for the models. The work presented in this paper is divided into three sections: experimental work, CFD analysis, and results and discussion.
Scope of the Study. This study focuses on using Eulerian and VOF multiphase models to model pilot-scale three-phase separators
used for demonstration purposes. Reynolds-averaged Navier-Stokes-based Eulerian and VOF models were used to simulate the fluidflow behavior inside the separators. For this work, droplet breakup and coalescence were not considered and a mean droplet diameter of
0.3 mm was used for the secondary phases. The factorial DOE was used to determine the number of experiments to be conducted for
the first separator. This method can be applied to separator design to identify the effect of independent variables and the interaction
between two or more variables. However, for the analysis of more than three variables with multiple levels, the experimental setup and
statistical analysis become quite complicated. In such cases, the Taguchi DOE should be used instead.
Experimental Work
The first stage of this work involved obtaining experimental data that could be used as inputs and reference values for the CFD models.
Therefore, we have decided to look at experimental apparatus available locally to obtain experimental data to verify the models. Most
CFD studies on three-phase separators are not compared with experimental data (Laleh et al. 2012, 2013; Ghaffarkhah et al. 2017). This
is largely because of the high cost associated with experimentation. However, in this work, appropriate experimental test data were
obtained from the pilot-scale separators in order to validate CFD predictions.
Pilot-Scale Separator Operating at Low Gas Volumetric Quality (Pilot-Sep-A). This is a commercially available three-phase separator simulator used for laboratory demonstrations, as shown in Figs. 1a and 1b. Pilot-Sep-A has a diameter of 300 mm and a length
of 600 mm (L/D of 2) and operates at atmospheric conditions with air, lubricating oil, and tap water. Table 1 presents the density and
viscosity of the three phases used for running Pilot-Sep-A. The rig is equipped with a bucket-and-weir configuration, an inlet diverter
and a wire-mesh mist extractor, a control monitor, oil and water pumps, oil and water tanks, an air compressor, and flowmeters for
measuring the inlet and outlet flow rates. The inflow into and outflow from the separator are controlled and adjusted manually using
globe valves until the desired flow rate and level are achieved. The separator was initially set up with gas, oil, and water flow rates of
0.6, 0.227, and 0.227 m3/h and a weir height of 120 mm.
A preliminary run of this separator indicated that it takes approximately 8 minutes for the flows inside the separator to reach a
steady state. This was established by monitoring the liquid level inside the separator. Therefore, a time interval of 10 minutes was
allowed after setting the flow rates. Four samples each for oil and water outlets were collected and centrifuged in a commercial bench
centrifuge for 10 minutes at 1500 rev/min. The quantity of oil in the water outlet and that of water in the oil outlet were then used
to determine the separation efficiency using Eqs. 1 and 2. It is important to mention here that 100% efficiency was obtained for
Pilot-Sep-A using Eq. 1, meaning that no water left this separator through the oil outlet. A simple white-rag test similar to the one carried out at rigsites (Smith 2017) to test for liquid in gas outlets was conducted to test if there is significant carryover of liquid in the gas
phase. This involves placing a white rag at the gas outlet. For Pilot-Sep-A, a white rag was placed at the gas outlet for 10 minutes. No
discoloration or moisture was observed on the rag, which indicates clean gas streams with no liquid carryover. Hence, it was concluded
that for Pilot-Sep-A, there is no liquid carryover through the gas outlet. Therefore, only liquid/liquid separation using Eq. 2 will be considered further.
water in oil outlet
100%; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð1Þ
gwo ¼ 1 water in inlet
oil in water outlet
gow ¼ 1 100%: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð2Þ
oil in inlet
Design of Experiments for Pilot-Sep-A. The DOE is a method of minimizing the number of experiments to determine the effects of
R DOE in order to deterthe input variables on the output (Anderson and Whitcomb 2016). Experiments were designed using MinitabV
mine the effects of the oil flow rate, water flow rate, and weir height on the separation performance of the pilot-scale separator. A major
advantage of DOE is that it allows for multiple input factors to be manipulated at the same time in order to determine their effect on the
output. With this, the multifactor interaction can be identified that might be missed when changing one factor at a time. The threefactors, namely, water flow rate, oil flow rate, and weir height, were set to have a low and a high level each. Using a full-factorial DOE
in Minitab, a total of eight experiments at different oil and water flow rates and weir heights were obtained (Table 2).
The experiments were conducted in a random order to minimize the effect of uncontrolled and unmeasured variables that are not
included as a response variable in the experiment but can affect the relationship between variables. The first experiment at Standard
Order (SO) 3 (Run Order 1) was carried out at a low oil flow rate (0.227 m3/h), a high water flow rate (0.454 m3/h), and a low weir
height (120 mm). The remaining seven experiments were carried out on the basis of the run order and appropriate levels as in Table 2.
Pilot-Scale Separator Operating at High Gas Volumetric Quality (Pilot-Sep-B). The pilot-scale separator operating at high gas volumetric quality, shown in Figs. 2a and 2b, has a weir configuration. The system consists of a transparent horizontal separator with a
diameter of 856 mm and a length of 2286 mm (L/D of 2.6), a gas-injection system, a coalescer, and oil and water pumping and collection systems. The properties of the fluids used for running this separator are presented in Table 1. The liquid levels in the separator are
controlled automatically using liquid-level controllers that operate the outlet dump valves. For safety reasons, this separator is operated
at high gas (nitrogen) flows to suppress any likelihood of combustion. Therefore, only three experiments were conducted at flow rates
shown in Table 3. It is important to point out that some uncertainties are expected because of the limited number of experiments conducted for Pilot-Sep-B.
At each run, four oil and water samples were collected and centrifuged at 1,500 rev/min for 10 minutes. The separation in terms of
water in the oil outlet and oil in the water outlet was then computed. Unlike Pilot-Sep-A, the results from Pilot-Sep-B indicated that no
oil droplets exit the separator through the water outlet. However, there is a significant amount of water in the oil outlet.
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(a)
160 mm
300 mm
100 mm
140 mm
370 mm
470 mm
550 mm
600 mm
(b)
Fig. 1—(a) Photograph of Pilot-Sep-A. (b) Schematic of Pilot-Sep-A.
Pilot-Sep-A
3
Pilot-Sep-B
3
Density (kg/m )
Viscosity (kg/m·s)
Density (kg/m )
Viscosity (kg/m·s)
1.225
0.000017894
1.165
0.0000176
Oil
856
0.046
810
0.00567
Water
1000
0.001
1000
0.001
Gas
Table 1—Physical properties of phases for Pilot-Sep-A and Pilot-Sep-B.
Standard
Order
Run Order
Oil Flow Rate
3
(m /h)
Water Flow Rate
3
(m /h)
Weir Height
(m)
1
3
0.227
0.227
0.12
2
6
0.454
0.227
0.12
3
1
0.227
0.454
0.12
4
5
0.454
0.454
0.12
5
2
0.227
0.227
0.14
6
8
0.454
0.227
0.14
7
4
0.227
0.454
0.14
8
7
0.454
0.454
0.14
Table 2—Three-factor, two-level DOE table.
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(a)
211 mm
PSH
PIT
201
6-in.NB, 150#
PI
206
575 mm
2-in.NB, 150#
TIT
201
400 mm
2-in.NB, 150#
250 mm
2-in.NB, 150#
250 mm
2-in.NB, 150#
250 mm
4-in.NB, 150#
350 mm
LIT
204
PSL
Gas outlet
325 mm
2-in.NB, 150#
Inlet deflector plate
10 mm
150 mm
Three-phase separator
960 mm
198
mm
297 mm
70
mm
125 mm
LIT
203
1250 mm
2-in.NB, 150#
2-in.NB, 150#
Water out
2-in.NB, 150#
792 mm
515 mm
50 type
150 mm
150 mm
Oil out
275 mm
6-in.NB, 150#
300 mm
Oil/water
mix inlet
205 mm
111
mm
4 number off 18-diameter
holes for M16 × 50LG
GR 8.8 bolts
70
mm
125 mm
1650 mm
636 mm
2286 mm
(b)
Fig. 2—(a) Photograph of Pilot-Sep-B. (b) Schematic of Pilot-Sep-B. NB 5 nominal bore; TIT 5 temperature indicating transmitter;
PI 5 pressure indicator; PIT 5 pressure indicating transmitter; PSH 5 pressure safety high; PSL 5 pressure safety low; LIT 5 level
indicating transmitter; LG 5 grip length (distance from head to full threads) on screws; GR 5 grade.
3
3
3
Cases
Gas (m /h)
Oil (m /h)
Water (m /h)
1
660
1.1
1.2
2
660
1.2
0.7
3
660
0.7
1.3
Table 3—Flow rates for pilot-scale separator operating at high gas
volumetric quality.
As for the gas phase, it is obvious that some carryover will be expected from this separator because of the high gas flows. Ideally, a
separator operating with such a gas/liquid ratio will be equipped with a mist extractor to minimize any liquid carryover. However, for
Pilot-Sep-B, the coalescer was located after the gas exits the separator. A total of 10 mL of liquid was collected in the coalescer over a
period of 4 hours of running the separator. From this, it was concluded that the gas/liquid separation is not an issue. The results will,
therefore, only consider liquid/liquid separation.
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CFD Analysis
The second stage of this work was to develop CFD models for the two pilot-scale separators. The plan was to develop CFD models for
each of the two separators described in the experimental section (base models) and also to look at the modification of Pilot-Sep-B by
adding a perforated baffle plate.
Development of the Base Models. Several steps must be followed to carry out any CFD simulation. These steps include the design
and discretization of the flow domain, setting of flow conditions, definition of materials, specification of boundary conditions, solution
initialization, and interpretation of results.
Design and Meshing of the Flow Domain. The computational domains were designed using an ANSYS design modeler to reflect
the real geometry of the two pilot-scale separators. The dimensions of the two pilot-scale separators are presented in Table 4. The
bodies of the separators were developed using the cylinder option from “primitive” in the create tab. The buckets and weirs were
extruded using cut-material operation and both symmetric directions from the x–y-plane at z ¼ 0. The inlets and outlets were also
extruded from newly created planes. The configuration of the inlet into the separators is different, with the top entry for Pilot-Sep-A
and the side entry for Pilot-Sep-B. Figs. 3 and 4 present the mesh schematic of the separators.
Pilot-Sep-A
Pilot-Sep-B
Inlet gas/liquid ratio
Low
High
Diameter (mm)
300
856
Seam-to-seam length (mm)
600
2286
Length/diameter ratio
2
2.6
Inlet size (mm)
25.4
152.4
Gas-outlet size (mm)
25.4
152.4
Water-outlet size (mm)
25.4
50.4
Oil-outlet size (mm)
25.4
50.4
Additional information
Top inlet entry equipped with
inlet diverter, bucket and weir,
and mist extractor.
Side inlet entry equipped
with flat plate inlet diverter
and oil weir.
Table 4—Vessel dimensions of pilot-scale separators.
0.000
0.200
0.100
0.400 (m)
0.300
Fig. 3—Mesh schematic of Pilot-Sep-A.
0.000
0.500
0.250
1.000 (m)
0.750
Fig. 4—Mesh schematic of Pilot-Sep-B.
The domain discretization of the designed geometries was carried out using the ANSYS meshing tool. Fig. 5 presents the overall
mesh quality in terms of skewness. A mesh independence test was carried out to ensure that results are independent of the mesh quality.
This was achieved by increasing the number of elements until the same results were obtained for each pilot-scale separator. The
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separation efficiencies obtained using Eq. 2 for Pilot-Sep-A and the percentage of water in the oil outlet for Pilot-Sep-B were determined for each of the grids. It is important to point out that the amount of oil in the water outlet was significant for Pilot-Sep-A, while
that of water in the oil outlet was significant for Pilot-Sep-B and hence the reason for reporting different parameters. Figs. 6 and 7
show an example of the grid independence test for Pilot-Sep-A (SO5 operating conditions in Table 2) and Pilot-Sep-B (Case 1 operating
conditions in Table 3). Selecting the optimal mesh size is usually a compromise between computational accuracy and simulation time.
It can be seen from these results that the mesh quality has more effects on the VOF model. For both Eulerian and VOF models, the simulation time was found to increase with the increasing number of elements. In all the cases studied, the Eulerian model was found to
take more time than the VOF model to converge. This is because the Eulerian model solves more equations than the VOF model. On
the basis of the grid independence test, optimal mesh sizes of 199,795 and 722,340 cells was chosen for Pilot-Sep-A and Pilot-Sep-B,
respectively. These mesh sizes were chosen because a further increase in the grid size to 240,780 for Pilot-Sep-A did not have any
effect on the result but increased the simulation time. Similarly, for Pilot-Sep-B, increasing the element size to 842,768 indicated a
deviation of less than 0.1% in the results, but the simulation time increased significantly. The next stage involves setting up the
flow conditions.
Mesh Volume (%)
80
Pilot-Sep-A
Pilot-Sep-B
60
40
20
0
.25
0–0
0
–0.5
0.25
0
–0.8
0.50
4
7
–0.9
–0.9
0.80
0.94
0
–1.0
0.97
Skewness Range
Fig. 5—Mesh skewness for Pilot-Sep-A and Pilot-Sep-B.
100
Efficiency (%)
90
80
70
60
Eulerian
VOF
50
0
50,000
100,000
150,000
200,000
250,000
300,000
Number of Elements
Fig. 6—Mesh independence test for Pilot-Sep-A.
14
Water in Oil Outlet (%)
12
10
8
6
4
2
Eulerian
VOF
0
0
200,000
400,000
600,000
800,000
1,000,000
Number of Elements
Fig. 7—Mesh independence test for Pilot-Sep-B.
Setting of Flow Conditions. In setting up the flow conditions, two aspects must be considered. The first deals with how the phases
interact with one another, and the second deals with the flow regime. To set up the phase interactions, two multiphase models, namely,
Eulerian and VOF, were implemented in modeling both separators in this work. The Eulerian model is the most complex multiphase
model in Fluent and has the ability to model multiple separate yet interacting phases. Continuity and momentum equations are solved
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for each phase, and a single pressure is shared by all the phases. The VOF model, on the other hand, simulates immiscible fluids and
tracks the volume fraction of each fluid throughout the domain. This is achieved by solving a single set of momentum equations. Both
Eulerian and VOF models have been reported to produce acceptable results when modeling gravity separators (Kharoua et al. 2012;
Laleh et al. 2012).
The nature of the flow regime depends on the Reynolds number. Almost all flows in three-phase separators can be categorized as turbulent flows. Because of their chaotic nature and infinite number of scales, solving the Navier-Stokes equations for these flows is impossible. Turbulence models are, therefore, required in order to model new unknowns obtained by using Reynolds decomposition.
Turbulence Models. Fluid variables such as velocity, density, and pressure fluctuate in time and space. To account for these fluctuations, closure models are used. ANSYS Fluent software (Fluent 2019) provides three methods for modeling turbulence in multiphase
flows within the k-e models and two options for Reynolds stress models. Previous studies on three-phase separators have used k-e turbulence models (Frankiewicz et al. 2001; Lu et al. 2007; Kharoua et al. 2012). Other turbulence models such as RNG k-e (Liang et al.
2013) and Reynolds stress models (Efendioglu et al. 2014) have been reported. A comparison performed by McCleney et al. (2017)
showed that a maximum deviation of 3% for the separation efficiency was obtained between standard k-e, realizable k-e, and k-omega
turbulence models. Given that the standard k-e turbulence model is most widely used for separator modeling (Laleh et al. 2012;
Triwibowo et al. 2017) because of its simplicity, robustness, and capability of providing optimal performance in terms of accuracy and
computational cost, it was adopted for this study.
Definition of Materials. Selecting the appropriate materials and their properties is crucial for obtaining accurate results, given that
different fluids exhibit different properties. The density, viscosity, and interfacial tension [obtained using ASTM D971-12 (2012)] of the
three phases are presented in Tables 1 and 5.
Interfacial Tension
Pilot-Sep-A (N/m)
Pilot-Sep-B (N/m)
0.024
Gas/oil
0.0304
Gas/water
0.072
0.072
Oil/water
0.048
0.053
Table 5—Interfacial tension for phases used in running pilotscale separators.
Specification of Boundary Conditions. The next stage involved the selection of the appropriate boundary conditions. The velocity
and volume fractions were specified at the inlet, while the pressure and backflow volume fractions were set for the outlets for all simulations. The oil and water outlet pressures were set to be higher than that of the gas outlet to account for the gauge pressure at the outlet
and maintain the interface levels at the desired levels. A no-slip boundary condition with standard wall functions was imposed at the
walls. This eliminates the need for a very-fine mesh near the wall. It is assumed that numerical errors that arise as a result of the choice
of standard wall function have a negligible effect on the overall flow behavior because of the scale of the flow.
Solution Methods and Initialization. The partial differential equations representing the multiphase flow were discretized using
the pressure-based finite-volume approach. For the Eulerian model, the phase-coupled simple scheme was used for pressure/velocity
coupling. First-order upwind was used for momentum, volume fraction turbulent kinetic energy, and turbulent dissipationrate discretization.
For the VOF model, the pressure implicit with splitting of operators algorithm was used for pressure/velocity coupling. The discretization method for momentum, turbulent kinetic energy, and the turbulent dissipation rate was carried out using the first-order upwind.
The body-force weighted scheme was chosen as the discretization method for pressure, while the geo-reconstruct scheme was used for
the discretization of the volume fraction. The geo-reconstruct scheme has been reported to be the appropriate scheme when the timeaccurate transient behavior of the interface between phases is important (Ghaffarkhah et al. 2018).
After setting up the model parameters, an iterative approach is required to resolve the complex nonlinear phenomena taking place
inside the separator. The proper setting of initial conditions will result in a quicker convergence, saving more computational time.
Therefore, the positions of the gas/liquid interface and liquid/liquid interface were specified using the patching tool in Fluent. This was
achieved by specifying the volume fraction of the phases above and below the interfaces. Similarly, relaxation factors for pressure, density, body forces, momentum, volume fraction, turbulent group, and discrete phase were set at 0.2, 0.9, 0.9, 0.005, 0.005, 0.7, and
0.5, respectively.
The solution was initialized with a fixed timestepping method of 104 seconds for 200 iterations. This was set to minimize any convergence problems related to high timestep sizes. The timestepping method was later modified to adaptive (variable for VOF), which
uses the Courant-Friedrichs-Lewy criteria that uses the Courant number. A truncation error tolerance of 0.01 (Eulerian) and a global
Courant number of 2 (VOF) with minimum and maximum timestep sizes of 109 and 10 seconds (for both models) were set for the
variable/adaptive timestep settings. As with most transient simulations, convergence was achieved on the basis of the decrease of residuals to 0.001. The iterative process was allowed to run until the flow field developed and a steady state was achieved. The iterative proR Core i7-4790 CPU at 3.6 GHz.
cedure was allowed to run continuously in order to simulate 5 minutes on an IntelV
Development of Model for Pilot-Sep-B With the Perforated Baffles. After obtaining the results for Pilot-Sep-B, a modification to
include a perforated baffle plate inside the separator just after the inlet diverter was investigated. Perforated baffles have been reported
to improve the gas-velocity profile, making it closer to ideal plug flow by straightening out the velocity profile of the liquid, enhancing
interface level control, and reducing sloshing effects (Bothamley 2013). The porous-media model in Fluent software (Fluent 2019) was
used to model the proposed perforated plates for this separator. A momentum source comprising a viscous term and an inertial
term was added as an extra term to the Navier-Stokes equations when using the porous-media model. The Navier-Stokes equation and
Si term are presented in Eqs. 3 through 6. For this model, the porosity and inertial resistance factor of the perforated baffle were
assumed to be 98% and 1920 m1.
dðquÞ
dP drxx dsyx dszx
þ r ðquV Þ ¼ þ
þ
þ
þ qbx þ F; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð3Þ
dt
dx
dx
dy
dz
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dðqvÞ
dP dsxy dryy dszy
þ r ðqvV Þ ¼ þ
þ
þ
þ qby þ F; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð4Þ
dt
dy
dx
dy
dz
dðqwÞ
dP dsxz dsyz drzz
þ r ðqwV Þ ¼ þ
þ
þ
þ qbz þ F; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ð5Þ
dt
dz
dx
dy
dz
where F represents the model-dependent source terms, such as porous media Si,
!
3
3
X
X
Si ¼ Dij lvj þ
Cij qjvjvi : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ð6Þ
j¼1
j¼1
Results and Discussion
In this section we compare the experimental data with the CFD predicted results. The CFD results presented in this work were obtained
at 300 6 60 seconds of computational time. In Table 6 we present the computational time usage for the two models for Pilot-Sep-A
(SO 1). The results for the two pilot-scale separators are presented in different subsections.
Eulerian
VOF
Computational time (seconds)
300
300
Average wall-clock time per iteration (seconds)
6.354
4.444
Timestep wall-clock time per iteration (seconds)
3.019
0.071
Total wall-clock time (seconds)
1,906.2
1,333.2
Table 6—Computational time usage for Pilot-Sep-A (SO1).
Results for Pilot-Sep-A. The results in terms of separation of phases, fluid-flow-pattern prediction, and effects of the oil flow rate,
water flow rate, weir height, and velocity magnitude are presented for Pilot-Sep-A.
Separation of Phases. The gas/liquid separation in both the CFD simulation and experimental work is very good, with no detectable
oil or water in the gas phase.
For water in the oil phase, for all experiments and CFD results, no water was again detected in the oil outlet and, therefore, the separation can be considered 100%.
The results for the oil lost in the water phase in terms of separation efficiency obtained from Eq. 2 are shown in Fig. 8. The analysis
of the samples obtained from the water outlet indicated a significant loss of oil in the water phase with a separation efficiency as low as
22%. The standard deviation for all eight experimental runs was less than 2%. It is clear that for the given fluid properties (Table 1), the
highest separation efficiency (95%) was obtained when Pilot-Sep-A was run at low oil and water flow rates and high weir-height settings (SO5), and the lowest separation efficiency (22%) was obtained at high oil and water flow rates and low weir-height settings
(SO4). Fig. 8 also shows a comparison between the experimental data and the two CFD predicted results.
100
Efficiency (%)
80
Experimental results
Eulerian
VOF
60
40
20
0
1
2
3
4
5
6
7
8
Standard Order
Fig. 8—Outcome of CFD and experimental analysis at different operating conditions for Pilot-Sep-A.
For ease of comparison, the percentage error between the experimental results and the two CFD predicted results is presented in
Fig. 9. The CFD predicted results overestimated the amount of oil in the water outlet. This led to the underprediction of the separation
efficiency. In all cases, the Eulerian model predictions were better than those the VOF model with a maximum error of 18% for the
Eulerian model and 23% for the VOF model. The percentage error between the experimental and CFD results was found to be less
when the weir height was set at a high level.
Comparison Between the Observed Flow Pattern and the CFD Predicted Flow Pattern. It is often important to understand the
flow pattern in multiphase separators, especially because industrial separators are made of opaque materials. Pilot-Sep-A is a transparent separator and, hence, it was easy to observe the flow pattern as the experiments were carried out. The results from the two CFD
models in terms of contours of oil-volume fraction are presented in Figs. 10 and 11. The red in these figures indicates the oil phase,
blue indicates no oil (i.e., gas and water phases), whereas any other color indicates an emulsion or foamy zone. Comparing the snapshots of Pilot-Sep-A in Figs. 10 and 11, it can be concluded that the Eulerian multiphase model is a better physical model for predicting
the flow pattern for this separator.
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Standard Order
Percentage Error (%)
5
0
1
2
3
4
5
6
7
8
0
–5
–10
–15
Eulerian
VOF
–20
–25
Fig. 9—Percentage error between Eulerian and VOF models, and experimental results for Pilot-Sep-A.
9.796×10–1
9.184×10–1
8.571×10–1
7.959×10–1
7.347×10–1
6.735×10–1
6.122×10–1
5.510×10–1
4.898×10–1
4.286×10–1
(h) Oil = 0.454 m3/h, water = 0.454 m3/h
3.673×10–1
(d) Oil = 0.454 m3/h, water = 0.454 m3/h
3.061×10–1
(g) Oil = 0.227 m3/h, water = 0.454 m3/h
2.449×10–1
(c) Oil = 0.227 m3/h, water = 0.454 m3/h
1.837×10–1
(f) Oil = 0.454 m3/h, water = 0.227 m3/h
1.224×10–1
(b) Oil = 0.454 m3/h, water = 0.227 m3/h
6.122×10–2
(e) 0.227 m3/h, water = 0.227 m3/h
0.000×100
(a) Oil = 0.227 m3/h, water = 0.227 m3/h
Contours of oil-volume fraction
Fig. 10—Contours of oil-volume fraction at different flow conditions for Pilot-Sep-A (Eulerian multiphase model).
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9.796×10–1
9.184×10–1
8.571×10–1
7.959×10–1
7.347×10–1
6.735×10–1
6.122×10–1
5.510×10–1
4.898×10–1
4.286×10–1
(h) Oil = 0.454 m3/h, water = 0.454 m3/h
3.673×10–1
(d) Oil = 0.454 m3/h, water = 0.454 m3/h
3.061×10–1
(g) Oil = 0.227 m3/h, water = 0.454 m3/h
2.449×10–1
(c) Oil = 0.227 m3/h, water = 0.454 m3/h
1.837×10–1
(f) Oil = 0.454 m3/h, water = 0.227 m3/h
1.224×10–1
(b) Oil = 0.454 m3/h, water = 0.227 m3/h
6.122×10–2
(e) Oil = 0.227m3/h, water = 0.227 m3/h
0.000×100
(a) Oil = 0.227 m3/h, water = 0.227 m3/h
Contours of oil-volume fraction
Fig. 11—Contours of oil-volume fraction at different flow conditions for Pilot-Sep-A (VOF model).
Effects of the Oil Flow Rate, Water Flow Rate, and Weir Height. To determine the effects of the oil flow rate, water flow rate, and
weir height and to further investigate any interaction effect between two or more of these factors, a regression analysis was carried out
using Minitab DOE.
Experimental and CFD results in the form of a Pareto chart for the efficiency of Pilot-Sep-A are presented in Fig. 12. The same pattern was observed between the three results with the oil flow rate (Term A) having the greatest effect on the efficiency of the separator,
followed by the water flow rate (Term B). The reason for this is that the criterion for separation are for the bulk flow velocity to be
lower than the droplet-settling or -rising velocity so that droplets/bubbles can reach their continuous phase before the phases exit the
separator. At a high-flow-rate setting (0.454 m3/h in this case), the bulk liquid velocity is higher and eventually exceeds the dropletsettling velocity, leading to improper separation.
As expected, the flow rate will also have an effect on the flow pattern and the separation efficiency of the separator. From Figs. 10
and 11, it can be seen that by changing the oil flow rate from a low to a high setting, the oil content in the water compartment of the separator increased. The effect of the higher water flow rate was observed more at the inlet zone of the liquid-collection section where yellowish green color was observed, which indicates more mixing of the phases.
The weir height (Term C) has the least effect of the three factors, as shown in Fig. 12. The weir height is an important parameter
that has to be optimized in separators equipped with a bucket and a weir because it determines the oil pad thickness and ultimately the
separation performance. If the weir height is too low, the oil/water interface falls. This can cause the oil to flow under the oil bucket and
over the water weir, and to leave the separator through the water outlet. However, if the weir height is too high, the oil/water interface
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level rises and can result in a significant loss of water through the oil outlet. For this work, better separation was obtained when the weir
height was at a higher level, as shown in Figs. 10 and 11. It is important to point out that the weir height is the reason for no water droplets being observed in the oil outlet.
Term
Term
26.82
A
Term
14.12
B
B
B
C
C
C
AB
BC
BC
BC
AC
ABC
ABC
ABC
AB
AC
AB
AC
0
10
20
30
40
18.35
A
A
0
Effect
(a) Experimental results
5
10
15
20
25
30
35
0
10
Effect
(b) Eulerian multiphase model
20
30
40
Effect
(c) VOF multiphase model
Fig. 12—Pareto chart for separator operating at low gas volumetric quality.
Two- and three-factor interactions that dictate the separator efficiency can also be seen in Fig. 12. However, these interaction effects
are not as significant as the single factor effects. This trend is seen in the experimental results and the VOF and Eulerian models. However, differences were noted for the interaction effects between the three results.
Velocity Magnitude. The velocity magnitudes for SO4 and 5 (with efficiencies of 22 and 95%) plotted at the symmetry plane (i.e.,
Z ¼ 0) using the Eulerian multiphase model are presented in Figs. 13a and 13b. A velocity magnitude of more than 1 m/s was observed
for both operating conditions at the separator inlet, oil bucket, and water weir. However, for SO4, the velocity magnitude at the separation section is four times greater than that of SO5. This high velocity will result in mixing of the phases and reduction in the phase residence time, thereby preventing the separation process.
0.4
0.2
0
–0.1
0
0.1
0.2
0.3
0.4
Position (m)
(a)
0.5
0.6
0.7
1.2
1.0
0.8
0.6
0.4
Separation section
0.2
0
–0.1
Water weir
0.6
Separation section
1.4
Oil bucket
0.8
Water weir
1.0
Oil bucket
1.2
Inlet section
Velocity Magnitude (m/s)
1.6
1.4
Inlet section
Velocity Magnitude (m/s)
1.6
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Position (m)
(b)
Fig. 13—(a) Velocity magnitude for Pilot-Sep-A (SO4). (b) Velocity magnitude for Pilot-Sep-A (SO5).
Results for Pilot-Sep-B. The results in terms of separation of phases, fluid-flow pattern prediction, velocity magnitude, and effects of
perforated baffle plates are presented for Pilot-Sep-B.
Separation of Phases. Similar to Pilot-Sep-A, gas/liquid separation for Pilot-Sep-B can also be considered good, with 10 mL of
liquid collected in the coalescer after 4 hours of running the separator. No liquid was obtained at the gas outlet for all three runs using
the Eulerian model, indicating 100% gas/liquid separation efficiency. However, the VOF model predicted a recovery of 99.6, 99.8, and
99.7% of the gas in the gas phase for the three cases.
The analysis of oil samples collected indicated a significant amount of water lost through the oil outlet of Pilot-Sep-B. Sampling
steps for the water in oil samples were performed according to ASTM D4007-11(2016)e1 (2016) to avoid sampling error. Similarly, the
standard deviation for all three experimental runs was less than 2%. The percentage of water obtained in the oil outlet is presented in
Fig. 14, where 8, 7, and 13% (with an uncertainty of 10%) water leaves the separator through the oil outlet for Cases 1, 2, and 3, respectively. Fig. 15 also shows a comparison between the experimental data and the two CFD predicted results. From these results, the
VOF-model predictions were closest to the experimental data.
The percentage error between the experimental and the two CFD predicted results is presented in Fig. 15. The results from the
Eulerian multiphase model overestimated the experimental data with a maximum percentage error of 29% for Case 2. A maximum percentage error of 21% was obtained for the VOF model, with overestimations in Cases 1 and 2 and underestimation in Case 3. It should
be noted that the deviation between the experimental data and CFD models is greater than the expected uncertainty in the measurements
and, therefore, the results are significant.
Finally, no oil droplets were obtained from the water outlet of Pilot-Sep-B from the experimental and CFD results. This is expected
because it is easier for the oil droplets to rise out of the water phase because of its lower viscosity. This is the reason most separators
are designed for the separation of water from the oil continuous phase.
Comparison Between the Observed Flow Pattern and the CFD Predicted Flow Pattern. Pilot-Sep-B is also a transparent separator. The contours of the oil-volume fraction for the VOF model for the three cases are presented in Fig. 16. The VOF model better represents the flow patterns observed experimentally. The Eulerian model gives a uniform color for high gas volumetric quality. This is
because the VOF model solves for the interface.
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Water in Oil Outlet (%)
20
Experimental
VOF
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Eulerian
15
10
5
0
1
2
3
Operating Conditions
Fig. 14—Outcome of CFD and experimental analysis at different operating conditions for Pilot-Sep-B.
Percentage Error (%)
30
20
10
0
–10
VOF
Eulerian
–20
–30
1
2
3
Standard Order
Fig. 15—Percentage error between Eulerian, VOF models, and experimental results for Pilot-Sep-B.
Oil-volume fraction
Contour 1
9.796×10–1
9.184×10–1
8.571×10–1
7.959×10–1
7.347×10–1
6.735×10–1
6.122×10–1
5.510×10–1
4.898×10–1
4.286×10–1
3.673×10–1
3.061×10–1
2.449×10–1
1.837×10–1
1.224×10–1
6.122×10–2
0
Case 1: Gas = 660 m3/h, oil = 1.1 m3/h, water = 1.2 m3/h
Case 2: Gas = 660 m3/h, oil = 1.2 m3/h, water = 0.7 m3/h
Case 3: Gas = 660 m3/h, oil = 0.7 m3/h, water = 1.3 m3/h
Fig. 16—Contours of oil-volume fraction at different flow conditions for Pilot-Sep-B.
Velocity Magnitude. To investigate the velocity distribution in the separator operating at high gas volumetric quality, the velocity
magnitude was plotted at plane z ¼ 0 (midway of the separator). Fig. 17 shows the velocity magnitude for Case 1 operating conditions
at this plane. A high velocity magnitude can be seen, which results in poor separation of the gas/liquid phases.
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Velocity Magnitude (m/s)
2.5
2.0
1.5
1.0
0.5
0
–0.5
0
0.5
1.0
1.5
2.0
2.5
Position (m)
Fig. 17—Velocity magnitude at z 5 0 for modified design (Case 1 operating conditions) for Pilot-Sep-B.
Pilot-Sep-B With a Perforated Baffle Plate. A modification to include perforated baffles was proposed to improve the velocity profile in the separator. Fig. 18 presents a plot of the velocity magnitude at plane z ¼ 0 after the perforated baffle plate was installed. By
comparing Figs. 17 and 18, it is evident that the inclusion of the perforated baffles significantly reduces the velocity inside the separator.
This reduction in velocity results in the increase in the gas/liquid-separation efficiency to 100% for both Eulerian and VOF models.
Velocity Magnitude (m/s)
2.5
2.0
1.5
1.0
0.5
0
–0.5
0
0.5
1.0
1.5
2.0
2.5
Position (m)
Fig. 18—Velocity magnitude at z 5 0 for original design (Case 1 operating conditions) for Pilot-Sep-B.
Similarly, the contours of the oil volume fraction for Pilot-Sep-B with a perforated baffle are shown in Fig. 19. By comparing
Figs. 16 and 19, it is clear that the carryover of water into the oil outlet is also significantly reduced.
Oil-volume fraction
Contour 1
9.796×10–1
9.184×10–1
8.571×10–1
7.959×10–1
7.347×10–1
6.735×10–1
6.122×10–1
5.510×10–1
4.898×10–1
4.286×10–1
3.673×10–1
3.061×10–1
2.449×10–1
1.837×10–1
1.224×10–1
6.122×10–2
0
Case 1: Gas = 660 m3/h, oil = 1.1 m3/h, water = 1.2 m3/h
Case 2: Gas = 660 m3/h, oil = 1.2 m3/h, water = 0.7 m3/h
Case 3: Gas = 660 m3/h, oil = 0.7 m3/h, water = 1.3 m3/h
Fig. 19—Contours of oil-volume fraction at different flow conditions for the separator operating at high gas volumetric quality
(baffle plates included).
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Conclusions
In this work we compared two commonly used multiphase models, namely, Eulerian and VOF, in predicting the separation performance
and fluid flow pattern of horizontal three-phase gravity separators. Pilot-scale separators operating at different flow conditions were
used to provide the test data for the models.
The first separator contains a bucket-and-weir configuration and operates at high gas volumetric quality (Pilot-Sep-A). For this separator, Minitab DOE was used to determine the effects of the liquid (oil and water) flow rate and weir height on the separation performance. A summary of the findings for this separator is given as follows:
1. The amount of oil leaving the separator through the water outlet was significant in comparison to the liquid leaving through the gas
outlet and water leaving through the oil outlet.
2. The highest separation efficiency of 95% was obtained when the separator was run at low oil and water flow rate and high weirheight operating conditions. The lowest efficiency of 22% was obtained at high oil and water flow rates and low weirheight conditions.
3. The results from Minitab DOE showed that the oil flow rate has the greatest effect on the separation performance, followed by the
water flow rate and weir height.
4. The Eulerian-model predictions for separation were better than those of the VOF model in all cases with a maximum percentage
error of 18% for the Eulerian model and 23% for the VOF model.
5. The percentage error between the experimental and CFD results was found to be less when the weir height was set at a high level.
6. The Eulerian model predicted the fluid-flow pattern better than the VOF model.
The second separator contains a weir configuration and operates at high gas volumetric quality (Pilot-Sep-B). Safety and operating
constraints limited the range of experimental work and precluded using designed experiments. A summary of the findings for this separator is given as follows:
1. The amount of water leaving through the oil outlet was significant in comparison to the liquid leaving through the gas outlet and oil
leaving through the water outlet.
2. CFD overestimated the quantity of water in the oil outlet when compared with the experimental results, with a maximum percentage
error of 30% for the Eulerian model and 21% for the VOF model.
3. The VOF model predicted the fluid-flow pattern better than the Eulerian model.
The maximum deviation between the experimental and CFD predicted results in this work is 30%. This might be caused by the
single droplet diameter used for the secondary phases. Other causes might be any of the following errors: physical approximation, computer round off, iterative convergence, and discretization errors. The overall conclusion is that for the two pilot-scale separators investigated in this work, the Eulerian model is a better model for predicting the separation performance and fluid-flow pattern inside the
separator operating at low gas volumetric quality. However, the VOF model is a better model for predicting the separation performance
and fluid-flow pattern inside the separator operating at high gas volumetric quality. These predictions are consistent with the basis upon
which the models were built. It is important to highlight that there might be some scaling distortions because of the differences in the
scaling factor between pilot scale and industrial scale models. However, the effect of these distortions is outside the scope of this work.
Finally, it is highly recommended to analyze the proposed dimensions from conventional separator-sizing models using the appropriate CFD model before the final selection of the separator diameter and length for given set of operation conditions.
Nomenclature
A ¼ oil flow rate, m3/h
b ¼ external body force, N/m3
B ¼ water flow rate, m3/h
C ¼ weir height, mm
D, C ¼ prescribed matrices
k-e ¼ kinetic energy epsilon
P ¼ pressure, kPa
Si ¼ source term for the ith momentum equation
t ¼ time, seconds
u ¼ velocity in x-direction, m/s
v ¼ velocity in y-direction, m/s
jvj ¼ magnitude of the velocity
V ¼ velocity, m/s
w ¼ velocity in z-direction, m/s
q ¼ density, kg/m3
s ¼ viscous stress tensor, N/m3
Acknowledgments
The authors thank the Petroleum Technology Development Fund (PTDF) for sponsoring this research. The authors also thank Darby
Tech and Middlesbrough College for their relevant contributions.
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Tariq Ahmed holds an MSc degree in petroleum engineering from Teesside University and is currently a PhD degree candidate
working on optimization of three-phase separators using CFD at the same institution. He is a lecturer at the Department of Chemical and Petroleum Engineering at Bayero University, Kano.
Paul A. Russell is a senior lecturer in chemical engineering at the School of Science Engineering and Design at Teesside University.
His research interests include the optimization of three-phase separators, the measurement of small flows using flux-response
technology, gas adsorption, distillation, and experimental studies of heterogeneous catalyzed reactions using microreactors.
Faik Hamad has been a senior lecturer in mechanical engineering at the School of Science Engineering and Design at Teesside
University since 2011. He worked for 4 years as a researcher/teaching fellow at Aberdeen University before he joined Teesside
University. Hamad has coauthored more than 40 articles in highly reputed journals and has been serving as a reviewer for several
journals. His research interest is in multiphase flows, energy storage, fluid mechanics, aerodynamics, and turbomachinery.
Hamad recently won two grants to extend his research to the new areas of microbubble generation and flow in porous media
and microchannels.
Samantha Gooneratne is a senior lecturer in chemical engineering at the School of Science Engineering and Design at Teesside
University. She holds a master’s in engineering and PhD degrees in chemical engineering from Cambridge University, and she is
currently the course leader in the bachelor’s in engineering/master’s in engineering program in chemical engineering at
Teesside University.
November 2019 SPE Production & Operations
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