Understanding of electro-coalescence for enhancing oil … velocity is used to design a separtion...

Optimal algorithm

for nearest drop

detection

Effects of electric fields on

multiple drops with

simultaneous tracking of all

drops

Experimental

observations

Computed

forces Compact separation

technology for oil,

water, and gas

Understanding of electro-coalescence

for enhancing oil-in-water separation

../SeminarmedStatoil/Emulsion.mpg



General Introduction

The oil extracted from offshore

reservoirs will normally contain a large

and during the reservoir lifetime,

increasing percentage of water in oil.

When the water-oil mixture is passed

through the pressure relief valve an

emulsion with high percentage of small

water droplets is formed.

Before the oil is pumped onshore or into

tankers it is desirable to extract the

water from this emulsion.

http://intranet.statoil.no/ktj/kom/svg01766.nsf/Attachments/magnor+1+2002.pdf/$FILE/magnor 1 2002.pdf

The separation tanks are mainly built or

operated as gravity separator with low

flow rates and long residence times.

The residence time mainly depends on

the sedimentation velocity of the

smallest droplets (d<100µm)

The electric fields are to some extent

used to help smaller droplets to coalesce

in to larger droplets that sediment

quicker.

The sedimentation velocity increases

proportionally to the square of the

diameter and therefore one wishes to get

the smallest water droplets to coalesce

General Introduction

A multidisciplinary investigation

Surface chemistry

Surface/interface characteristics

Chemistry

Electrochemistry

Continuum mechanics and Multiphase flow

Drop-drop interaction

Electrostatic forces

Systems with multiple droplets

Turbulence

Electrical Engineering

Critical field strength

Effect of frequency AC vs. DC fields

Coalescence efficiency

Industrial application

Industrial prototypes

Novel separation philosophy

Oil

Water

The Stability of Water in Oil emulsion is due to:

Production history

Oil characteristics

Water characteristics

Crude oil composition and characteristics play a role in stabilizing water-in-oil emulsion:

Asphaltenes and Resins natural emulsifying agents

Brief overview on how does chemical and electrostatic emulsion resolution methods work

My contribution:

Hypothesis on how external field applied to emulsion influences the aggregation

properties of natural emulsifying agents and thereby the emulsion stability

Apparatus for measuring the effect of an electrostatic field on crude oil, inline at an

offshore installation. (Patent owned by Aibel)

The importance of surface chemistry

Short overviews

Emulsion stability due to production history

A blend of oil and water is forced to the surface through vertical

casing. The resulting shear energy and pressure decline produces a

“tight” oil/water emulsion once at the surface.

To enroute the blend into the production equipment, where primary

separation and dehydration takes place may require that many

emulsions are heated, pressurized, pumped, and pushed through

pipelines.

Once at the production facility, the emulsion is generally a

homogenized blend of oil, water, gas and contaminants. An

understanding of the production history can provide an insight into

the nature of the emulsion.

Consider:

Different lift techniques

Pressure in the reservoir

Pressure cycles

The mixture is often heated

Injection of the optimum de-emulsifier

Varying water content

1

10

100

1000

10000

100000

0 20 40 60 80 100 120 140 160

Temperature (oC)

Vis

co

sity (

mP

as)

0,915

0,925

0,935

0,945

0,955

0,965

0,975

De

nsity (

g/m

l)

viscosity

shear rheology (100/s)


density

T [C]

Vis

cosi

ty [

mP

a]

Den

sity

[g/m

l]

Emulsion stability due to oil characteristic

Oil specific gravity and viscosity are traditionally the fundamental

physical properties used to evaluate emulsion stability. They are

principally responsible for the separation rates of a water-in-oil

emulsion.

Interfacial tension and conductivity are equally important.

Crude oils, in addition to a mixture of hydrocarbon fractions, also

contain a non-homogeneous blend of a variety of compounds, such

as surfactants, anions, cations, clay, sand, silt, and bacteria.

The degree of emulsion stability depends on most of the following

factors:

The size of the dispersed water droplets

The age of the emulsion

The viscosity of the oil

The difference in the density of the two liquids

The volume percentage of the water cut

The interfacial tension

The asphaltenes, paraffin and suspended solids content.

Wavelength (nm)

Ap

pa

ren

t ab

so

rban

ce

2200 12001700

Wavelength (nm)

Ap

pa

ren

t ab

so

rban

ce

2200 12001700

Near-Infrared spectroscopy

Wavelength (nm)

Appar

ent A

bso

rban

ce

Production history and oil characteristics

In order to increase the sedimentation velocity and

enhance oil waterseparation one can:

A water droplet falling in oil can be in first approximation

considered as a sphere. In a very conservative way the

sedimentation velocity is used to design a separtion vessel.

Maximise the density difference

Decrease viscosity

Increase the droplet size

The viscosity varies exponentially.

The greatest effect is experienced for

temperatures below 80º C

One can observe that the curves

become more flat at higher

temperature

1

10

100

1000

10000

100000

0 20 40 60 80 100 120 140 160

Temperature (oC)

Vis

co

sity (

mP

as)

0,915

0,925

0,935

0,945

0,955

0,965

0,975

De

nsity (

g/m

l)

viscosity



density

Emulsion stability due to water characteristic

Low water pH neutralizes the naturally occurring

basic surfactants so that better water quality can be

achieved

Low brine pH may contribute to the stability of the

emulsion by chemically altering the water droplet

interface.

Generally, produced water salinity varies directly

with oil gravity and inversely with oil viscosity:

low salinity brines accompany heavy, high

viscosity oils

high salinity brines accompany light, low

viscosity oils

Generally, water wet solids should not interfere with

the coalescence and separation processes.

Oil-wetted solids tend to hinder separation by

accumulating at the droplet interface.

Asphaltenes and Resins:

natural emulsifying agents

Emulsion stability is governed primarily by the state

of solubility of the asphaltenes in the crude oil.

Asphaltenes at or near the point of precipitation are

more surface-active than those which are sufficiently

solvated or molecularly dispersed.

The characteristics of the crude oil which should play

a role in determining the solubility state of the

asphaltenes include:

The resin-to-asphaltene ratio,

The aromaticity ratios of the crude medium and

resins with respect to the asphaltenes

The concentration of polar functional groups in

the

Cartoon from Kilpatrick

Asphaltenes and Resins

natural emulsifying agents

Resin-asphaltene association to form a colloidal aggregate.

Asphaltenes interact through hydrogen bonding and -bond

overlap.

Resins solvate asphaltene aggregates through polar functional

group interactions.

Resin-asphaltene colloidal aggregate association to form an

interfacial film.

Primary asphaltene aggregates cross-link to form a rigid,

viscoelastic structure at the oil-water interface

Cartoon from Kilpatrick

Chemical resolution methods

Asphaltene

”molecuke

”

Resin

molecuke

Oil

Water

Oil –Water interface

Resin-solavated

asphaltene aggregate

Resin-solavated


+

Properly designed chemicals react with the interfacial

film in an orderly way to promote complete

coalescence.

These chemical blends are designed to neutralize the

effect of natural emulsifying agents that stabilize the

emulsion.

Demulsifiers are surfice-active compounds, and when

added to the emulsion, they migrate to the oil/water

interface, rupture or weaken the rigid film, and enhace

coalescence of water droplets.

A properly selected chemical for the given emulsion

Adequate quantity of this chemical

Adequate mixing of the chemical in the emulsion

Sufficient retention time

Additional heat

Optimum emulsion breaking with a demulsifier requires:

Electrostatic resolution methods

A variety of electrostatic treatment techniques are available to

the designer and operator. They include AC, DC, AC/DC,

modulated and pulsed fields.

AC field Water Dipole

The driving force for droplet coalescence is based on the

dipole of the water drop.

The water molecules are aligned creating a chain of water

droplets with positive and negative poles.

Droplets that are close together will migrate towards each

other and coalesce.

At 50 Hz AC field alters polarity up to 100 times per second

and the droplet interface is most probably not charged.

DC field Charging

The DC field transfers a charge to dispersed water droplet

proportional to the voltage gradient and the oil conductivity.

The charge at the interface promotes coalescence.

The DC field promotes droplet stretching that ruptures the

outer film and enhances the droplet coalescence rate.

Oil

Oil –Water interface

Resin-solavated

asphaltene aggregate Resin-solavated


Water

Asphaltene

”molecuke

”

Resin

molecuke

+

Electrostatic resolution methods

E

E

Asphaltene

”molecuke

”

Resin

molecuke

+

Preliminary results show that resin-asphaltene association

form a colloidal aggregate somewhat influenced by the

presence of the electric field.

The viscoelastic properties of the film formed by cross link of

primary aggregates are dependent on the presence of the

electric field.

This suggests that electric field can be favorable for enhancing

separation and should be applied as far upstream as possible

Significance of my contribution :

Literature: Chiesa M., Ingebrigtsen S, .Melheim J.A., Hemmingsen P.V. and Hansen B.E.:“The role of viscosity on electrically induced coalescence

of water drops in oil”. Separation and Purification Technology, Volume 50, Issue 2, 2006, pp 267-277

Oil

Oil –Water

interface

Resin-solavated

asphaltene aggregate Resin-solavated


Water

Combined Chemical and Electrostatic

resolution methods

The chemical selection and evaluations are typically based

on widely accepted “bottle test” methods

This technique may fail to select the proper chemical

demulsifier for electrostatic coalescence and separation.

Just as the chemical demulsifier acts at the water droplet

interface, so does electrostatic coalescence.

If not properly selected, the applied electrostatic field may

interfere with the demulsifier action.

However, when properly selected, the electrostatic field can

significantly augment the chemical activity and ultimately

separation

E

E


Apparatus for inline measuring the effect of

electrostatic fields on crude oil, offshore.

The apparatus is to be connected to a sample point

upstream a separator. The stream is ported through a

pipe section with electrodes on each side.

After electrodes, a three-way valve is installed so that

one can choose either to port the treated oil back to the

separator inlet or to a measuring device.

The batch test device allows for variation in:

Electric field

Temperature

Chemicals

Flow conditions

Laminar

Turbulent

The data obtain using the apparatus take into account of

production history issues and physiochemical properties

of the crude.

Patent application: Erik Hansen, Matteo Chiesa and Pål Jahre Nilsen “An apparatus for measuring the effect of an electrostatic field on crude

oil, inline at an offshore installation.”. Patent application number at Oslo Patentkontor AS: 2007 3231


VIEC batch test for Ofon

10% WC, 80degC, no chemicals

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50

Time (min)

Re

ma

inin

g w

ate

r in

oil W

iO (

%)

WiO(%) OFF

WiO(%) ON

Settling curves for real crude oil, 80°C (176 ° F)


Surface chemistry


Chemistry

Electrochemistry





Turbulence








Oil

Water

The water drops motion is due to different

forces. A framework that takes into

account such forces is proposed.

The framework is tested on two drops

system and semi-stationary multi drops

system

The development of efficient data

structure and numerical algorithms is

necessary to describe dynamic system.

Turbulent electro-coalescence is

described and the results resemble

qualitatively the results obtained

experimentally

Continuum mechanics & Multiphase flow

Modelling and experimental efford to describe

Turbulent electro-coalescence

Experiments are performed to observe the behavior of a droplet falling

towards a stationary one.

Mathematical models for these forces are presented and discussed with

respect to the implementation in a multi-droplet Lagrangian framework.

The droplet motion is mainly due to buoyancy, drag, film-drainage, and

dipole-dipole forces. Attention is paid to internal circulations, non-ideal

dipoles, and the effects of surface tension gradients.

The optical observations are compared with the results from numerical

simulations.


Literature: Chiesa M., Melheim J., Pedersen A., Ingebrigtsen S. and Berg G: “ Forces acting on two water droplets in oil under the influence of an electric

field: numerical predictions versus experimental observations”. European Journal of Mechanics B/Fluids, Vol. 24 717-732 2005.

h

2r

Experimental setup

Experimental set-up designed for visual

observation of the behavior of water

droplets in oil, exposed to an electric field.

A movable 15 mm electrode-gap

arrangement is placed vertically inside a

cubic test cell.

The cell is placed in an optical bench to

obtain a shadow-graphic representation

of the water droplets as shown.

FIRST STEP: A TWO DROPLET SYSTEM

The forces acting on each droplet

determine its motion:

Fext (external forces on the

droplet)

Ffluid (forces from the fluid on

the droplet)

Fd-d (inter-droplets forces)

ii

dt

du

x

dd-extfluidi

idt

dm FFF

u

E=0 V/mm E=400 V/mm

MODELLING FRAMEWORK

In the Lagrangian formulation of the

droplet motion, the best results is

obtained with:

Drag force of LeVan [1]

Film-thinning force of Vinogradova [2]

Analytical expression of the electric force of Davis [3]



THE EFFECT OF INTERNAL

RECIRCULATION ON THE DRAG FORCE

LeVan formula:

1

1

1

1

d)(233

|)|(3/2)(223

Re

24

r

rC

c

cc

vu N/m 10 5

1

0

vuvuF ACdcd 2

1

E = 300 V/mm, r = 110 μm

THE EFFECT OF FILM THINNING

rrr

f fh

ae

ev *2

c )(6

F

The model of Vinogradova:

1

61ln

61

6

2*

h

b

b

h

b

hf

E = 300 V/mm, r = 110 μm

where are complicated series depending on and

DIPOLE-DIPOLE FORCE

The analytical expression of Davis:

Dipole induced dipole model (DID) of Siu et al

1cos312 2

1

43

1

3

2

22

KrrF or dE

2sin12 2

43

1

3

2

22 KrrF ot

dE

2

2

2

1

2

2

2sinFcosF4 rF or E

2sinF4 3

2

2

2rF ot E

31 - FF / 2rd / 21 rr

42

2

2

1

2

2

2

2

1

23

2

3

1

2

42

1

2

53

2

42

2

2

53

1

1

331

rr

rrrr

r

r

r

rK

d

d

d

d

d

d

32

2

2

1

2

3

2

3

1

2

32

1

2

33

2

32

2

2

33

1

2

3

221

rr

rr

r

r

r

rK

dd

d

d

d

E = 300 V/mm, r = 110 μm

WATER DROPS FALLING UNDER THE

INFLUENCE OF AN ELECTRIC FIELD

t = 0s

t = 0.56s E

t = 0s

t = 0.56s E

t = 0.8s t = 0.808s

Experimental observation Numerical prediction



In a stagnant water-in-oil emulsion subjected to an

external AC electrical field adjacent drops will

attract each other.

The discrete drop model of the emulsion presented

earlier is used to calculate the two-dimensional

motion of the individual, spherical water drops

directly from the forces acting on them.

The hydrodynamic interaction between the drops

and the interstitial oil phase is taken into account,

together with the effect of the electrical field.

Coalescence is assumed to occur when two drops

collide.

DIRECT ELEMENT METHOD SIMULATION OF

A STATIONARY MULTI DROPLET SYSTEM

DIRECT ELEMENT METHOD SIMULATION OF

A STATIONARY MULTI DROPLET SYSTEM

Literature: Chiesa M., Norheim S., Pedersen A.:“Predicted and measured droplet growth in an electrostatic field”. To be submitted to “European Journal of

Mechanical Engineering B/Fluids” 2006.

Predicted and measured droplet growth due to the electrical field

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 5 9 13 17 21 25 29 33 37

Droplet diameter (px=6.67e-6m)

Acc.

vo

lum

etr

ic d

rop

let

dis

trib

utio

n

Fig 9c

Fig 9d

Fig 9b

Fig 9a

Fig 11a

Fig 11b

Fig 11c

Fig 11d

CELL STRUCTURE FOR THE DETECTION OF

NEIGHBOURING PARTICLES

A separate particle cell structure

A priority list for events to handle

Events are for instance collisions and transfer

between particle cells

Adaptive cell structure

Literature: Melheim J. and Chiesa M.: “Formulation and numerical performance of an adaptive algorithm for efficient

collision detection.” ASME Fluids Engineering Summer Conference USA 2005

THE CLUSTER INTEGRATION METHOD FOR

THE EFFICIENT CALCULATION

Literature: Melheim J. and Chiesa M.: “Formulation and numerical performance of an adaptive algorithm for efficient

collision detection.” ASME Fluids Engineering Summer Conference USA 2005

Virtual radius for each drop.

A test move (hard-sphere algorithm) is

employed to search for droplets with

overlapping virtual radii during the next global

time step.

The clusters are integrated separately using an

embedded Runge-Kutta scheme.

After a local time step is successfully

performed, the fluctuating velocities and the

turbulence frequency of those droplets are

updated using a Runge-Kutta scheme for

stochastic differential equations.

CORRELATED DROP MOTION

The instantaneous velocity ``seen'' by the droplets is needed to calculate the forces.

A model for the fluctuating fluid velocity, that correlates the fluid velocities seen by close droplets is used.

The model is based on a stochastic differential equations, Langevin Eq.

We obtain correlated fluctuating velocity for each drop

Un-correlated

Correlated

Literature: Melheim J. A..:“Correlated motion of inertial particles in turbulent flows”. Submitted to “Physics of Fluids” 2006.

Simulation of turbulent electro-coalescene

E=250 V/mm

1000

2.2

cP4

kg/m1000

kg/m800

wE,

oilE,

oil

3

3

oil

w

m20

sm216.0

sm05.0

m/s3.0

%2

3-2

2-2

d

k

U

In

In

w

3cm

Literature: Melheim J. and Chiesa M.: “Simulation of

turbulent electrocoalescence ” Chemical Engineering

Science, Volume 61, Issue 14, 2006, pp 4540 - 4549

SIGNIFICANCE OF MY CONTRIBUTION

A numerical framework able to describe

turbulence electro-coalescence

Big step forward to provide process

engineers a design tool for separation

equipment

Possibility to take into account for the

presence of electrostatic internals.

Modelling and experimental efford to describe

Turbulent electro-coalescence


Surface chemistry


Chemistry

Electrochemistry





Turbulence








Oil

Water

WATER DROPLET 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

– : surface tension

– : permittivity

• Defines the maximum applicable field in an

electrocoalescer

rEcrit

2648.0

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

1 2

E

21

11rrcP

2

21 EPe

SOME EXPERIMENTAL RESULTS

• Critical field increases with

decreasing drop size

• Excellent 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

ctri

c fi

eld

[kV

/cm

]

Theory, IFT=40.04

No surfactant

0.025 % 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

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, Eoil 0 (red line)

• AC voltage: Capacitive voltage

distribution (blue line)


Surface chemistry


Chemistry

Electrochemistry





Turbulence








Oil

Water

Electro-coalescence

Compact coalescers -> Turbulent flow and electrostatic fields

HV

AC •The turbulence causes collisions between droplets.

•The electric field enhances the chance of

coalescence when drops collide.

The evolution equation

• The continuous equation (PBE);

• The discrete equation;

• The proportionality coefficient;

00

),(),(),(),(),(),(2

1

d

),(dvdtvnvvQtvnvdtvntvvnvvvQ

t

tvnv

max

3 33

00

)(

0

),(),(d

d im

m mmii

il

l

liIntm

m

mmlli nddQnnddQnt

n

),(),(),( mieffmiturbmi ddQddQddQ

MODELLING COMPACT COALESCERS

The collision frequency

32

37

31

1

)(2

)(8

),(

ji

ji

jiturb

rrC

rrC

ddQ

RL

RL

MODELLING COMPACT COALESCERS–1D

The collision efficiency

The collision efficiency is a measure of the probability of

coalescence when two droplets collide.

The probability of coalescence is determined by the time

required for the film to rupture and the interaction time of

the droplets.

Predicted and measured droplet growth in coalescer

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 50 100 150 200 250 300 350 400

Droplet diameter (m)

Acc. vo

lum

etr

ic d

rop

let

dis

trib

uti

on

inlet

6kV experiments

6kV simulation

12kV experiments

12kV simulation

MODELLING COMPACT COALESCERS–1D


Surface chemistry


Chemistry

Electrochemistry

Inter-droplet interaction




Multiphase flow


Turbulence




Oil

Water

Electrocoalescence - VIEC

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Understanding of electro-coalescence for enhancing oil … velocity is used to design a separtion...

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