electrocoalescence – a multi-disiplinary arena · 2014. 11. 17. · electrocoalescence project...
TRANSCRIPT
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
2SINTEF Energy Research
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
3SINTEF Energy Research
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
4SINTEF Energy Research
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
11SINTEF Energy Research
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
24SINTEF Energy Research
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
25SINTEF Energy Research
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
30SINTEF Energy Research
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
32SINTEF Energy Research
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
33SINTEF Energy Research
Simulation of hydrodynamic and electrostatic forces
34SINTEF Energy Research
Turbulence experiments – impinging jets
The problem observed The problem calculated
E
35SINTEF Energy Research
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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