Electrocoalescence a multi-disiplinary arena

Transcription

1 Electrocoalescence a multi-disiplinary arena Electrical Engineering SINTEF Chemistry ELECTRO- COALESCENCE Physics NTNU CNRS Fluid Dynamics 1

2 Electrocoalescence project Need for smaller working equipment Objective: Fundamantal understanding of the electrocoalescence process under ac and turbulent conditions Clients: ABB, Statoil, Norsk Hydro, Petrobras Budget About 4MNOK/year in 4 years (3 researchers) + 3 PhD students + 1 postdoc. Today: Sedimentation: 5 meter diameter and 20 meter long Electrocoalsecers do not always work Project group: Electrical engineering, physics, fluid mechanics, chemistry. Our advantage: Internationally leading on liquid dielectrics. Good interdisciplinary environment. 2

3 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 3

4 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 surface layer (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 4

5 Our perspective: Electric ac fields induce charges that create forces between drops thereby increasing the coalsecencne efficiency when droplets meet barriers Turbulence creates shear movement in liquid. This results in more frequent drop meetings Turbulent energy profile 0.02 m R C V = 0 FLOW Turbulence and coalescence close to walls 5

6 Research on different scales Macroscale Industrial prototypes Microscale Drop drop interaction Coalsecence efficiency Mesoscale Systems with multiple droplets Turbulence Electrostatic forces Nanoscale Surface/interface characteristics Chemistry Electrochemistry 6

7 Microscale and mesoscale experiments 7

8 Experimental setup 8

9 Water drop instability A water drop will elongate due to the electric stress on its surface Above a critical field strength the drop becomes unstable and breaks up γ E crit = rε γ: surface tension ε: permittivity Defines the maximum applicable field in an electrocoalescer 9

10 Forces on the droplet Capillary pressure due to the surface tension 1 1 ( ) P = γ + c r 1 r 2 ε 2 ε 1 y (0,b) E v (a,0) x Electrostatic pressure P e = 1 E 2 2 ε Shape close to a rotational ellipsoid 10

11 Experimental results Critical field increases with decreasing drop size Excellent fit to theory Breakup modes depends on voltage waveform and frequency: Theory, IFT=40.04 No surfactant % surf. 0.1 % surf. Electric field [kv/cm] Theory, IFT=20 50 Hz square wave voltage Drop radius [mm] 2000 Hz sine wave voltage 11

12 Oscillating drop experiment Uncovered uniform field electrodes E Water drop rests on a teflon coated polypropylene rod Theory: A water drop will elongate in the direction of the electric field due to the electrostatic pressure Objectives: Automatic contour tracing of the drop circumference Calculate the interfacial tension γ from the drop deformation Measure time constant of relaxation of drop deformation (surface elasticity) Determine development of time constant over time to determine absorption of surface agents 12

13 Transient drop elongation Exxsol D80 Drop axis ratio a/b Ø=1.774mm, 0 ppm asph. Ø=1.753mm, 250 ppm asph. Ø=0.992mm, 0 ppm asph. Ø=0.995mm, 250 ppm asph Electric field [kv/cm] 10 ms Short excitation pulses enables observation of the relaxation time of the deformation Video shows deformation of Ø1.77 mm drop at 4.7 kv/cm 13

14 Surface elasticity 1.44 Width (2b) Width (2a) Heigth (2a) Electric field Heigth (2b) Electric field 150 Drop dimensions [mm] Electric field E 0 [V/cm] Drop dimensions [mm] Electric field [V/cm] t [ms] t [ms] Clean water/oil interface Asphaltene saturated water drop 14

15 Falling drop, high resolution Ø90 µm drop falling on a large, stationary drop Vertical electric field of 3 kv/cm, 50 Hz sine Instability, coalescence and formation of several satellite drops Video recording 2100 frames per second, 55 µs exposure time 15

16 Details from the video 1. 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) 16

17 Fast event the instability formation 2500 V/cm, 10 Hz, BSV. 10 µs camera shutter t 0 t µs t µs 0.26 mm 0.26 mm 0.26 mm <10 µs> <10 µs> <10 µs> Surface instability forming on the lower drop. A jet moves up towards the falling drop and initiates coalescence. 17

18 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. 18

19 Problem with particle stabilization 20 min. saturated falling droplet >24 h. saturated stationary drop E Electric Field: Frequency: Waveform: Capture rate: Playback: Frame Size: 670 V/cm 10 Hz Bipolar Square fps 250 ms/s 0.9 x 0.9 mm Nytro 10 X 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. 19

20 Electric forces on drop pairs 20

21 Dielectrophoresis E E 21

22 Drop-drop collision, clean oil E Electric Field: Frequency: Waveform: Capture rate: Playback: Frame Size: 230 V/cm 10 Hz Bipolar Square fps 250 ms/s 0.26 x 0.53 mm 0,6 0,5 0,4 0,3 0,2 0,1 0,0 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 Velocity (mm/s) Nytro 10 X, Distilled Water + 3.5w% NaCl Distance (mm) 22

23 Drop-drop collision, clean oil E Electric Field: Frequency: Waveform: Capture rate: Playback: Frame Size: 4000 V/cm 10 Hz Bipolar Square fps 250 ms/s 0.26 x 0.53 mm 12,0 10,0 8,0 6,0 4,0 Velocity (mm/s) 2,0 0,0 0,20 0,15 0,10 0,05 0,00 Nytro 10 X, Distilled Water + 3.5w% NaCl Distance (mm) 23

24 Experiments with suspended drops Drops resting on a Teflon surface 10 khz bipolar square voltage Clean water/oil interface Formation of instability leading to coalescence Longer distance between drops Formation of instability and jet Drops experience an adhesion force to the solid surface, resulting in immovable mass centers No coalescence 24

25 Effect of frequency AC vs. DC fields Insulating barriers are used to prevent breakdown due to water bridges (conductive water drops) limit charge injection from electrodes Local electric field determined by conductivity of oil and barrier permittivity of oil and barrier frequency of applied voltage DC voltage: Resisitive voltage distribution, E oil 0 (red line) AC voltage: Capacitive voltage distribution (blue line) barriers R C V = 0 25

26 The electric field is high when the drops are close Analytic expression exist The maximum electric field on the smallest drop (R 2 ): E3 100 E A = E cos E 0 ψ s/r 2 R1/R2 = 1 R1/R2 = 2 Field enhancement as for a single drop when the displacement s is more than one drop radius R 1 (largest drop) R1/R2 = 5 R1/R2 = 10 26

27 Electrostatic forces comparison of different models 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 Atten (asympt.) Dipole-dipole DID Davis (analytic) θ F1 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E s/r 2 R 1 /R 2 = 2, θ = 0 No net charge 27

28 Forces on multiple drops 28

29 Ph D work, Atle Pedersen (I) Forces on drop pair E r 0 Forces between multiple drops in an emulsion x, y R Ψ R 2 A B 1 z µ = µ 2 s µ =µ 1 Analytic expression based on forces between dipole - Two drops only 2 r ε Φ r F eˆ = n e ds n 2 S ( ˆ ) BEM (POLOPT) simulation is used to give charges, field and forces between droplets 29

30 Ph D work, Atle Pedersen (II) Forces between droplets in an emulsion 8 drops around one big drop one is closer than the 5 others Charges E-field Emulsion with E-field Numerical 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 30

31 Stagnant emulsions, case 1Hz E Electric Field: Frequency: Waveform: Capture rate: Playback: Frame Size: 5.0 kv/cm 1 Hz BSV 1000 fps 30 ms/s 2.5 x 2.5 mm Observations: Pronounced expansion of the emulsion column. Low coalescence efficiency Nytro 10X + 5 % water w. 3.5% NaCl % Span

32 Stagnant emulsion, case 100Hz E Electric Field: Frequency: Waveform: Capture rate: Playback: Frame Size: 2.5 kv/cm 100 Hz BSV 1000 fps 400 ms/s 2.5 x 2.5 mm Nytro 10X + 5 % water w. 3.5% NaCl % Span 80. Observations: Expansion of the emulsion column during several voltage periods. Formation of drop chains. Coalescence within and btw. chains. Charge movements. Good coalescence efficiency 32

33 Case: Hz E Electric Field: Frequency: Waveform: Capture rate: Playback: Frame Size: 2.5 kv/cm Hz BSV 1000 fps 400 ms/s 2.5 x 2.5 mm Observations: Isotrop coalescence. Rapidly increasing drop-size. High coalescence efficiency Nytro 10X + 5 % water w. 3.5% NaCl % Span

34 Simulation of hydrodynamic and electrostatic forces 34

35 Turbulence experiments impinging jets The problem observed The problem calculated E 35

36 Numerical simulation of the kinematics of water droplets emulsified in oil under the effect of a turbulent and electrical field m 0.02 m H2O volume fraction 2% U1 velocity profile Turbulent energy profile E0 We can observe that collisions are more frequent in the vicinity of the wall. The droplets move towards the middle of the geometry Flow A water-oil emulsion is injected at a velocity U2 along the inlet. 36

37 Direct element method (DEM) simulations Experimental Simulation 37