Electrocoalescence – a multi-disiplinary arena · 2014. 11. 17. · Electrocoalescence project...

Physics

Fluid Dynamics

Electrical Engineering

SINTEF

CNRSNTNU

ELECTRO-COALESCENCE

Chemistry

Electrocoalescence –a multi-disiplinary arena

1SINTEF Energy Research

Electrocoalescence projectObjective:

Fundamantal understanding of the electrocoalescence process under ac and turbulent conditions

Clients:ABB, Statoil, Norsk Hydro, Petrobras

BudgetAbout 4MNOK/year in 4 years (3 researchers) + 3 PhD students + 1 postdoc.

Project group:Electrical engineering, physics, fluid mechanics, chemistry.

Our advantage:Internationally leading on liquid dielectrics.Good interdisciplinary environment.

Need for smaller working equipment

•Today:

•Sedimentation: 5 meter diameter and 20 meter long

•Electrocoalsecers do not always work

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Motivation for the work

Establish a basic understanding of the physical mechanisms active in the electrocoalescence process

Find restrictions for when the process can be used

Establish possibilities for optimizing equipment and process technologies

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Hypothesis for coalescence efficiency in AC fields

Large field and forces between drops due to induced charges from the ac field

1. Longer contact times and higher impact velocities between water drops gives more efficient film draining

2. Instability of surfaces of adjacent water drops from forces acting on induced charges. This also may give a thinning of the surfacelayer (Maragoni effect)

3. Thinning of surface layers from electrostrictive forces acting on electric dipoles in the surfaces

4. Shockwaves from electric discharges between water drops

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Our perspective:

Electric ac fields induce charges that create forces between drops thereby increasing the coalsecencneefficiency when droplets meet

Turbulence creates shear movement in liquid. This results in more frequent drop meetings

0.02 m

Turbulent energy profile

0.02 m

Turbulent energy profile

Turbulence and coalescence close to walls

FLO

W

barriers

R

C∆V = 0

barriers

R

C∆V = 0

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Research on different scales

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MicroscaleDrop drop interactionCoalsecence efficiency

NanoscaleSurface/interface characteristicsChemistryElectrochemistry

MacroscaleIndustrial prototypes

MesoscaleSystems with multiple dropletsTurbulenceElectrostatic forces

Microscale and mesoscale experiments

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Experimental setup

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Water drop instability

A water drop will elongate due to the electric stress on its surfaceAbove a critical field strength the drop becomes unstable and breaks up

γ: surface tensionε: permittivity

Defines the maximum applicable field in an electrocoalescer

εγr

Ecrit 2648.0=

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Forces on the droplet

Capillary pressure due to the surface tension

Electrostatic pressure

Shape close to a rotational ellipsoid

x

(0,b)(a,0)

y

ε1ε2

Ev

( )21

11rrcP +=∆ γ

221 EPe ε=

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Experimental resultsCritical field increases with decreasing drop sizeExcellent fit to theory

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0 100 200 300 400

Drop radius [mm]

Ele

ctric

fiel

d [k

V/c

m]

Theory, IFT=40.04No surfactant0.025 % surf.0.1 % surf.Theory, IFT=20

Breakup modes depends on voltage waveform and frequency:

50 Hz square wave voltage

2000 Hz sine wave voltage

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Oscillating drop experimentTheory:

A water drop will elongate in the direction of the electric field due to the electrostatic pressure

Objectives:Automatic contour tracing of the drop circumferenceCalculate the interfacial tension γ from the drop deformationMeasure time constant of relaxation of drop deformation (surface elasticity)Determine development of time constant over time to determine absorption of surface agents

Water drop rests on a teflon coated polypropylene rod

Uncovered uniform field electrodes

E

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Transient drop elongation

Short excitation pulses enables observation of the relaxation time of the deformationVideo shows deformation of Ø1.77 mm drop at 4.7 kV/cm

Exxsol D80

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

0 1 2 3 4 5

Electric field [kV/cm]

Dro

p ax

is ra

tio a

/b

Ø=1.774mm, 0ppm asph.Ø=1.753mm, 250ppm asph.

Ø=0.992mm, 0ppm asph.

Ø=0.995mm, 250ppm asph.

10 ms

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1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

0.00 25.00 50.00 75.00 100.00 125.00 150.00

t [ms]

Dro

p di

men

sion

s [m

m]

-200

-150

-100

-50

0

50

100

150

200

Ele

ctric

fiel

d [V

/cm

]

Width (2a)Heigth (2b)Electric field

Surface elasticity

1.24

1.26

1.28

1.30

1.32

1.34

1.36

1.38

1.40

1.42

1.44

0 25 50 75 100 125 150 175 200

t [ms]

Dro

p di

men

sion

s [m

m]

-200

-150

-100

-50

0

50

100

150

200

Ele

ctric

fiel

d E

0 [V

/cm

]

Width (2b)Heigth (2a)Electric field

Clean water/oil interface Asphaltene saturated water drop

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Falling drop, high resolution

Ø90 µm drop falling on a large, stationary dropVertical electric field of 3 kV/cm, 50 Hz sineInstability, coalescence and formation of several satellite dropsVideo recording

2100 frames per second, 55 µs exposure time

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Details from the video1. Formation of instability,

most noticeable on lower surface

2. Coalescence 35 µs after first contact between drops

3. Formation of first satellite drop (radius 22 µm). String of droplets observed

4. Formation of second satellite drop (radius 7 µm)

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Fast event – the instability formation

2500 V/cm, 10 Hz, BSV. 10 µs camera shutter

0.26 mm 0.26 mm 0.26 mm

t0 t0 + 167µs t0 + 334µs

<10 µs> <10 µs> <10 µs>

Surface instability forming on the lower drop. A jet moves up towards the falling drop and initiates coalescence.

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Collapse, with and without asphaltenes

E

Clean water/oil interface. Very fast draining of small drop with formation of satellite drop.

Saturated water drops (oil with 100 ppm Asphaltenes). Very slow draining of small drop.

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Problem with particle stabilization

E

• 20 min. saturated falling droplet

• >24 h. saturated stationary drop

• Electric Field: 670 V/cm

• Frequency: 10 Hz

• Waveform: Bipolar Square

• Capture rate: 2 000 fps

• Playback: 250 ms/s

• Frame Size: 0.9 x 0.9 mm

Nytro 10 X + 100 ppm Asphaltenes, Distilled Water + 3.5w% NaCl

Observations• Droplet starts to oscillate at contact.

• Much particles on stationary surface.

• Effective coalescence is hindered.

• No satellite drop.

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Electric forces on drop pairs

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Dielectrophoresis

E E

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Drop-drop collision, clean oil• Electric Field: 230 V/cm

• Frequency: 10 Hz

• Waveform: Bipolar Square

• Capture rate: 6 000 fps

• Playback: 250 ms/s

• Frame Size: 0.26 x 0.53 mmE

Nytro 10 X, Distilled Water + 3.5w% NaCl

0,00,10,20,30,40,50,60,70,80,91,0

0,00,10,20,30,40,50,6

Distance (mm)

Vel

ocity

(mm

/s)

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Drop-drop collision, clean oil• Electric Field: 4000 V/cm

• Frequency: 10 Hz

• Waveform: Bipolar Square

• Capture rate: 6 000 fps

• Playback: 250 ms/s

• Frame Size: 0.26 x 0.53 mmE

0,0

2,0

4,0

6,0

8,0

10,0

12,0

0,000,050,100,150,20

Distance (mm)

Velo

city

(mm

/s)

Nytro 10 X, Distilled Water + 3.5w% NaCl

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Experiments with suspended drops

Drops resting on a Teflon surface10 kHz bipolar square voltageClean water/oil interfaceFormation of instability leading to coalescence

Longer distance between dropsFormation of instability and jetDrops experience an adhesion force to the solid surface, resulting in immovable mass centersNo coalescence

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Effect of frequency – AC vs. DC fieldsInsulating barriers are used to

prevent breakdown due to water bridges (conductive water drops)limit charge injection from electrodes

Local electric field determined byconductivity of oil and barrierpermittivity of oil and barrierfrequency of applied voltage

DC voltage: Resisitive voltage distribution, Eoil → 0 (red line)AC voltage: Capacitive voltage distribution (blue line)

barriers

R

C∆V = 0

barriers

R

C∆V = 0

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The electric field is high when the drops are close

Analytic expression exist

The maximum electric field on the smallest drop (R2):

Field enhancement as for a single drop when the displacement s is more than one drop radius R1 (largest drop)

30 cos EEEA ⋅= ψ

1

10

100

1000

10000

0.0001 0.001 0.01 0.1 1 10 100

s/R2

E3

R1/R2 = 1 R1/R2 = 2

R1/R2 = 5 R1/R2 = 10

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Electrostatic forces –comparison of different models

R1/R2 = 2,θ = 0No net charge

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

0.001 0.01 0.1 1 10 100

s/R 2

F1

Atten (asympt.)

Dipole-dipoleDID

Davis (analytic)

θ

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Forces on multiple drops

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Ph D work, Atle Pedersen (I)

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Ψ R2

R1

s

A

x, y

µ =µ1

µ = –µ2

zB

0Er

Forces on drop pair

( )2

ˆ ˆ2 S

F e n e dSn

ε ∂Φ⎛ ⎞⋅ = ⋅ ⋅⎜ ⎟∂⎝ ⎠∫r r

Forces between multipledrops in an emulsion

Analytic expression based on forces between “dipole”- Two drops only

BEM (POLOPT) simulation is used to give charges, field and forces between droplets

Ph D work, Atle Pedersen (II)Forces between droplets in an emulsion

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8 drops around one big drop one is closer than the 5 others

Charges E-field Emulsion with E-fieldNumerical Simulation BEM (POLOPT) of distributed droplets

Forces between droplets

Measurements of drag forces on droplets in an emulsion when a field is applied

Forces between droplets

To organisation chart

Stagnant emulsions, case 1Hz• Electric Field: 5.0 kV/cm

• Frequency: 1 Hz

• Waveform: BSV

• Capture rate: 1000 fps

• Playback: 30 ms/s

• Frame Size: 2.5 x 2.5 mm

E

Observations:• Pronounced expansion of the emulsion column.

Low coalescence efficiency

Nytro 10X + 5 % water w. 3.5% NaCl + 0.05 % Span 80®.

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Stagnant emulsion, case 100Hz• Electric Field: 2.5 kV/cm

• Frequency: 100 Hz

• Waveform: BSV

• Capture rate: 1000 fps

• Playback: 400 ms/s

• Frame Size: 2.5 x 2.5 mm

Nytro 10X + 5 % water w. 3.5% NaCl + 0.05 % Span 80®.

E

Observations:• Expansion of the emulsion column during several voltage periods.

• Formation of drop chains.

• Coalescence within and btw. chains.

••• Charge movementsCharge movementsCharge movements.

Good coalescence efficiency

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Case: 10 000 Hz• Electric Field: 2.5 kV/cm

• Frequency: 10 000 Hz

• Waveform: BSV

• Capture rate: 1000 fps

• Playback: 400 ms/s

• Frame Size: 2.5 x 2.5 mm

Nytro 10X + 5 % water w. 3.5% NaCl + 0.05 % Span 80®.

E

Observations:• Isotrop coalescence.

• Rapidly increasing drop-size.

High coalescence efficiency

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Simulation of hydrodynamic and electrostatic forces

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Turbulence experiments – impinging jets

The problem observed The problem calculated

E

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Numerical simulation of the kinematics of water droplets emulsified in oil

under the effect of a turbulent and electrical field.H2O volume fraction 2%

0.02 m

U1 velocity profile

0.02 m

Turbulent energy

profile

We can observe that collisions are more frequent in the vicinity of the wall.

The droplets move towards the middle of the geometry

A water-oil emulsion is injected at a velocity U2

along the inlet.

E0

Flow

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Direct element method (DEM) simulations

Experimental Simulation

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