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

Presentation topics

• Company overview;

• Problem description;

• Goals;

• Methodology;

• Results;

• Conclusion and next steps.

Company overview

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.

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.

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;

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

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;

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;

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.

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.

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.

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

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)

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

Results – Level control + droplet increase

Case 2:

• Without level control;

• With droplet size increase.

Results – Level control + droplet increase

Case 3:

• With level control;

• Without droplet size increase.

Results – Level control + droplet increase

Case 4:

• With level control;

• With droplet size increase.

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

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%.

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

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%.

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;

Thank you!