flow field analysis to improve a liquid/liquid separator … field analysis to improve a...
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Flow field analysis to improve a liquid/liquid
separator vessel design using CFD
Rodrigo Peralta, ESSS
Lucilla Almeida, ESSS
Karolline Ropelato, ESSS
Thiago Anzai, Petrobras
Robson Pereira Alves, Petrobras
João Cláudio Bastos, Petrobras
Erick Quintella, Petrobras
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Presentation topics
• Company overview;
• Problem description;
• Goals;
• Methodology;
• Results;
• Conclusion and next steps.
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Company overview
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Problem description
• Crude oil contains water and other contaminants which need to be removed for economical
transport and before further processing of the crude oil.
• Water in oil emulsion is a particularly problematic issue, since its formation makes it difficult
to separate with only the gravitational field, demanding, for instance, electrostatic devices.
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Goals
• Simulate the multiphase fluid dynamic behavior of an oil/water
separator in different geometries configurations aiming further
construction;
– Several approaches were performed to improve separation process:
– inclusion of inlet device;
– The goal is to allow a smooth flow of the mixture inside the
equipment;
• Implementation of level control strategy with UDF;
• Implementation of simplified eletrocoalescence with UDF.
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Methodology – Geometry
mixture inlet
oil outlet
brine outlet
• Initial geometry:
• Further configurations:
– Included an inlet device:
Simplifications: Electrodes and
perforated plates were not considered;
• Simulation result will predict their
location;
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Methodology – Mesh
Mesh generated in ANSYS Meshing® software;
• Hybrid mesh: Tetra + Prism;
– Prism layer near walls to capture boundary layer effects and also in the expected oil/water interface
region;
• 1,46 millions of elements.
Prismatic elements
Finer mesh in regions of higher gradients
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Software : ANSYS FLUENT 14.0®;
• Multiphase flow simulation
– Euler-Euler approach;
• Each phase has his own velocity flow field;
• Dispersed phase diameter:
– Study 1: 100 microns;
– Study 2: UDF to control this parameter inside equipment.
– Incompressible fluids;
– Continuous phase and dispersed phase interaction;
• Surface tension: 0,0377 [N/m];
• Drag: symmetric;
• Isothermal
• Turbulence: k-ε model;
• Transient.
Methodology – Mathematical model
Physical setup:
• Prescribed inlet velocity;
• Prescribed outlet pressure;
• Prescribed outlet velocity (controlled by an User Defined
Function);
• No slip walls;
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Methodology – Additional variables for CFD analysis
• Flow uniformity coefficients analysis;
– Objective: Set the most appropriate position for internal devices, such as the
electrodes;
𝐶𝑉 =1
𝐴𝑡 𝐴𝑖
𝑢𝑥 − 𝑢𝑥𝑢 𝑥
2𝑁
𝑖=1
12
𝛼 = 𝑎𝑟𝑐𝑡𝑎𝑛𝑢𝜃
𝑢𝑥 𝑆 =
𝑢𝑥 𝛼 𝑑𝐴
𝑢𝑥 𝑑𝐴
0 m 4 m
Region of flow uniformity analysis:
Swirl number (S):
Coefficient of variation (Cv):
Measure the flow uniformity in tangential
direction.
• 100% uniform flow: S = 0;
Measure the flow uniformity in the flow
direction.
• 100% uniform flow: Cv = 0;
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Results – without level control
Without a modified inlet device,
separation efficiency was lower;
• Small fraction of brine in the bottom of
vessel;
• Small fraction of oil in the top of vessel;
Including an inlet device:
• Improved oil/brine separation efficiency;
• Brine level still less than expected;
– Loss of oil by bottom outlet.
Low separation efficiency due to low
droplet size (without eletrocoalescence).
Without inlet device:
With inlet device:
*Simulations without level control and a simplified eletrocoalescence phenomenon.
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Results – without level control
Without inlet device:
With inlet device:
More uniform flow for case with inlet device.
*Simulations without level control and a simplified eletrocoalescence phenomenon.
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Results – without level control
Oil volume fraction in outlet plane:
Without gutter Horizontal gutter without vents Horizontal gutter Vertical gutter
• Different configurations of inlet device:
*Simulations without level control and a simplified eletrocoalescence phenomenon.
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Definitions: Level controller
• UDF to calculate brine level on measurement
plane (Execute At End);
– Calculate volumetric flow rate at outlet (Define
Profile).
Plane of level measurement
• Brine level obtained from the averaged volume fraction by trigonometric
relations.
LAL
LAH Normal level
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Definitions: Eletrocoalescence phenomenon
• Modeling the brine droplets size increase due to the electrocoalescence;
– Phenomenon treated in a simplified way, without solving population balance;
– The droplet size change inside vessel by a step function defined by droplet
position in equipment;
• Two points for droplet size change chosen as they had greater flow uniformity.
Length (m)
Dro
ple
t siz
e (μ
m)
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Cases definitions
• 4 cases defined to study droplet size increase and level
control effects;
– Geometry: best result obtained from study 1 Horizontal gutter without vents
Cases Droplet size increase Level control
Case 1
Case 2 X
Case 3 X
Case 4 X X
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Results – Level control + droplet increase
Case 2:
• Without level control;
• With droplet size increase.
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Results – Level control + droplet increase
Case 3:
• With level control;
• Without droplet size increase.
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Results – Level control + droplet increase
Case 4:
• With level control;
• With droplet size increase.
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Results – Level control + droplet increase
• Higher separation for cases with the consideration of increase in droplet
size diameter (Cases 2 and 4);
– Fundamental consideration for simulation of an electrostatic separator;
• Increase of volume with high brine VF for cases with brine level control;
– Significant improvements compared to case without controller (Cases 1 and 2).
Cases Droplet
size increase
Level control
Case 1
Case 2 X
Case 3 X
Case 4 X X
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Results – Level control + droplet increase
Cases Water volume fraction
(%) – top outlet Water volume fraction
(%) – bottom outlet
Case 1 18,79 25,73
Case 2 6,18 75,39
Case 3 19,10 100,0
Case 4 8,52 100,0
Cases Droplet size
increase Level control
Case 1
Case 2 X
Case 3 X
Case 4 X X
• Use of level controller allows volume with brine
VF of 100% in bottom outlet;
– All of oil collected by the top outlet.
• Low BS&W for cases with increase of droplet
size;
• Case 4: Separation efficiency: 60,5%.
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Results – Level control + droplet increase
Cases Droplet size
increase Level control Brine level [m] Flow rate [m³/h]
Case 1 0,079 5,00
Case 2 X 0,127 5,00
Case 3 X 0,195 0,25
Case 4 X X 0,246 3,01
• For all cases brine final level remained lower than set-point;
– Set-point: 0,294 m.
• Brine level higher for Case 4 (with level control and increase of droplet size diameter);
Oil interface
position
Brine interface
position
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Conclusion
According to the assumptions and considerations made, it was observed that:
• Inclusion of the inlet device reduced mixture inside vessel and increase the separation
efficiency;
– Lower uniformity coefficients;
• Interface brine/oil below expectations: Loss of oil by lower output;
– Motivation of the simulation with controller level;
• The implementation of the level controller allowed variation of the bottom outlet flow rate
over time, according to the measured brine level in a plane near the exit;
– Controller allowed 100% of the oil to be recovered by the output exceeding;
• However, there were loss of the brine at top outlet due to the increase of mixing region inside the vessel;
• The inclusion of oil droplets size increase (due to eletrocoalescence effect) increased the
separation efficiency ~0 to 60%.
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Next steps
• Modeling brine droplets size increase due to the electrocoalescence;
– Use of more combinations of droplet size diameters;
• Test different droplet sizes and calculate separation efficiency;
• Simulate different positions of inlet device;
• Simulate different inlet devices to improve separation efficiency;
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Thank you!