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Electrocoalescence of Drops in a
Water-in-Oil Emulsion
Jorunn Holto, Gunnar Berg and Lars E. Lundgaard
SINTEF Energy Research AS,
Sem Saelands vei 11, 7465 Trondheim, Norway
Abstract- Electrocoalescence of water drops in a stagnant
water-in-oil emulsion has been studied in a newly developed
test cell. An homogeneous electric field was applied across the
emulsion volume. It was used to observe drop behaviour in a
multi-drop system. The emulsion was made of napthenic oil
containing chemically stabilized water drops with size in the
range of 5-100 µm. Insulated electrodes were used to prevent
direct charge injection and electrophoresis. Drop dynamics in
the stagnant emulsion was observed by use of a high speed
optical camera and high resolution optics. Three parameters
were varied systematically: water cut, frequency and
amplitude of the bipolar square voltage. Different events
during electrocoalescence are described and discussed,
including dynamics of both collapse and break-up of drops,
chain formation and charge transfer in the emulsion.
Destabilization of emulsions is a costly and difficult
process, and an important commercial problem. Crude oil
usually contains some vol% of water distributed as water
drops, which should be removed before further processing
takes place. It can be achieved in large separation tanks, as
water will sink to the bottom because of having higher
density than oil. However, this is a slow process, especially
for small droplets. To make the process more efficient
merging of drops into larger ones is beneficial, and for this
purpose electrocoalescence is a viable alternative. However,
to make the technique more efficient a more basic
understanding of the processes and their voltage- and
frequency dependence is desired.
I. BACKGROUND
Application of an homogeneous electric field E across a
water-in-oil emulsion results in two main types of
electrostatic forces [1]: Drops with a net charge q will
experience an electrophoretic force F=qE. For drops with
no net charge dielectrophoretic forces dependent on material
properties will occur in an inhomogeneous electric field,
here formed around drops. As water has a much higher
dielectric permittivity than oil positive dielectrophoretic
forces will draw the water drops together. The relative effect
of both forces depends on frequency, voltage magnitude and
distance between drops. In addition mechanical forces
caused by gravity, inertia and viscous effects both inside the
drop itself and in the surrounding media play a role. An
homogenous background field will polarise the drops, with
978-1-4244-4559-2/09/$25.00 ©2009 IEEE
direction of the dipole moment opposing the field direction.
The drops are stretched to ellipsoids, and later at high
electric field disruption of the drops will occur. For a clean
system under dc stress the instability occur when E is higher
than the critical field strength Ec for maximum stable
elongation of a drop [2]
0,648
2
where γ is surface tension, r is the drop radius, ε0 and εoil is
the permittivity of vacuum and oil respectively.
No analytical expressions are readily available for
calculation on the effect of electrostatic forces in a system
consisting of more than two drops. Numerical solutions are
possible, but demand a lot of data force.
In a multi-drop system drops tend to form chains which may
result in breakdown when bridging the electrode gap, unless
the electrodes are covered by a solid insulation. The
trajectory of drops relative to each other and the forces
exerted on neighbouring drops has been studied
experimentally and modelled [2]. Coalescence between
drop pairs depends on their inter-distance and alignment
relative to the background field [1]. Normally it is assumed
that the drops in an emulsion are charge neutral, as the
charge relaxation time constant of a crude oil is short [1].
The destabilisation of drops in an emulsion can be described
by three processes [3]: First, flocculation of drops assisted
by applied field and shear forces. Second, fusing or
coalescence of drops thus increasing the drop size. Finally,
phase separation, either by creaming or sedimentation
depending on the density difference between the two phases
in the emulsion. Coalescence itself can be described by three
different steps [4]: Collision of two drops followed by
drainage of the oil between the drops and finally
destabilization of the interfacial film leading to rupture.
The objective of this work is to present an initial study on
the behaviour of a multi-drop system in a new test cell
designed for studies of both stagnant and flowing emulsions
when applying homogenous electric field. Here only results
in stagnant emulsions are presented.
II. EXPERIMENTAL SETUP
To allow observation of an emulsion during application of
an electric field a cell with three axes was designed, as
outlined in Fig. 1. The emulsion was inserted along the yaxis, while electric field was applied along the z-axis and
the events was than observed horizontally along the x-axis.
Emulsion was made of a naphtenic model oil (Nynäs Nytro
10XN) mixed with saline water (3,5wt% NaCl). The
emulsion was chemically stabilised using 500 ppm of
Sorbitan Monooleate (Span®80). A shaking table was used
to make the emulsion. The test matrix is presented in Table
1, showing the combinations of water cut, frequency and
magnitude of the applied voltage which were used.
The emulsion was observed using shadow-graphic
techniques with collimated background light from a LEDsource. Events were recorded in 8 second sequences using a
Phantom V4 high speed CMOS camera, with a resolution of
512×512 pixels which for this magnification corresponded
to a 1×1 mm section of the gap. The electrode separation
was 3 mm and the width of the emulsion volume along the
x-axis was 2 mm. The electrodes were insulated using
transparent paint.
Bipolar square ac voltage was used to keep the electric
forces on the drops constant (F
E02) and to avoid a
voltage drop across the insulating film which would have
occurred for dc conditions. During earlier experiments the
critical electric field for disintegration of singular drops was
found to be lower by use of such a voltage shape compared
to a sinus or triangular voltage [1].
Fig. 2. Picture of the experimental cell used for application of voltage on
water-in-oil emulsion.
TABLE 1
TEST MATRIX USED DURING EXPERIMENTS
Variable
Values
Water content (vol%)
1,5
3
Frequency (Hz)
1
100
Voltage (kV)
0,4
0,9
4
1000
1,5
III. RESULTS AND DISCUSSION
A variety of phenomena was observed in the experiments.
We will here comment on some of the main processes that
could be revealed from the videos.
Our main aim is to study the coalescence process between
two free drops, as shown in Fig. 3. Here two spherical drops
of diameter about 100 µm slowly approached each other and
coalesced fully without showing any elongation in the field
direction. This example occurred in oil with 1,5 vol% water
cut with a distribution of drop sizes and low drop density.
For an emulsion with low density of drops the rate limiting
step of coalescence seems to be the number of drop
collisions. In this case no flow is applied and only gravity
and effects of the applied voltage influence the emulsion.
The intention is in later studies to investigate such two-drop
coalescence in real crude oil emulsions to find the critical
voltage.
At low frequencies a strong agitation of the emulsion was
seen. This is explained by charged drops having enough
time during each half-cycle to move a long distance in the
gap. This seemed not to increase the collision frequency and
few cases of coalescence were observed. Turbulence also
disrupted any chain formation, and at higher voltages or
high water cuts observation was made near impossible due
to rapid movement. A complicating factor is that at low
frequencies the voltage drop over the insulation is unknown,
as a capacitive voltage distribution is not established.
Fig. 1. Experimental cell for observation a water-in-oil emulsion during
application of voltage.
0 ms
8 ms
18 ms
28 ms
Fig. 3. Coalescence of two drops with approximate diameter 100 µm without much interaction with the surrounding drops, in a 1.5 vol% water-in-oil-emulsion
and an applied voltage 0.4 kV and frequency 100 Hz.
0 ms
20 ms
28 ms
40 ms
Fig. 4. During coalescence of two drops a cone is formed and a jet of small droplets is ejected, in a 3 vol% water-in-oil emulsion and an applied voltage 1.5 kV
and frequency 1 kHz.
0 ms
10 ms
14 ms
28 ms
Fig. 5. Interactions in a chain formed parallel to the electric field, with coalescence normal to the field direction. 4 vol% water, 0.9 kV and 1 kHz.
Fig. 4 shows a merger of two drops into a larger one, which
becomes asymmetrically unstable. According to Eq. 1 the
critical voltage for drop instability decreases with increasing
drop radius. The merged drop has a size above the critical
limit for this voltage. However, for singular drops the
instability is expected to be symmetric at both poles [5]. These
instabilities will produce small, very stable droplets. In this
case the instability is asymmetric, which can be explained by
an inhomogeneous field distribution in the emulsion. Such
single sided instability will leave the drops charged, giving
rise to dielectrophoretic forces and movement. This behaviour
was mainly seen at higher voltages.
Frequently it was seen that when drops touched without
coalescing they would exchange charge and start being
repelled from each other. This could occur both for drop pairs
and for drops within a chain. For the free charged drops and at
lower frequencies one could see that they started oscillating
with the field polarity.
When a chain of drops formed, charge exchange along the
length would take place, and more drops would easily be
attracted, to be included either by coalescence or reorganising
of the chain. This could both take place at the end and in the
middle of the chain. Often larger drops were interspersed by
smaller droplets, stabilizing the chain, of which an example is
shown in Fig. 5. However, in this special case drop attraction
and coalescence seemed to occur between a large drop in the
chain and another drop alligned perpendicular to the electric
field direction. Two polarized drops are normally repelled
when aligned normal to the field direction, at angles exceeding
54.7° [1]. However, for very small separation distances drops
nearly always attract each other [1], which this is an example
of. In addition interaction between the two left-most drops
cause a chain of small droplets being left behind. An
uncertainty here is the interpretation of which drops that exists
in the same plane.
At high voltage (1.5 kV) and high frequencies (1 kHz) little
agitation was seen and the coalescence rate seemed high, but
drop impacts also frequently resulted in charge exchange
without any merger. Other phenomena were also observed:
Charged drops creating local agitation, partial coalescence and
drop break-up due to elongation, which resulted in smaller
drops than originally. Also drop chains were readily formed,
which reduced the probability of coalescence when two drops
collided. When reducing the voltage to 0.9 kV the coalescence
rate remained high, while many of the other phenomena
became less frequent.
Tests were made at 0.5 vol% water cut, but no activity was
observed within this time frame. With 3 vol% and 4 vol%
water cuts chains of drops formed across the gap, at
combinations of the two highest values of voltage and
frequency. However, electric breakdown of the gap never
occurred, proving that the method for insulation of the
electrodes was effective. This was also confirmed by
observations of the emulsion close to the high voltage
electrode; no significant charge injection was observed.
During these experiments high water cut or an emulsion with
drops in the range below 50 µm gave very limited visibility.
We therefore here focus on singular events in an emulsion
with some larger drops. The experiments are a continuation of
the work presented by Ingebrigtsen et al. [6], in which case the
emulsion volume was able to expand into surrounding model
oil. Especially at high voltages and low frequencies the
emulsion volume was shown to expand significantly. In
important aspect of the present work was to use a confined
volume to avoid uncertainty from drops movement being
influenced by this expansion. In both cases it was observed
that electrophoretic stirring at low frequencies may hinder
coalescence [6]. Based on our limited number of experiments
we can qualitatively conclude that high voltages promote
chain formation.
Our aim has been to look at the frequency and voltage
dependency of coalescence and chain formation in model oil
in a confined volume, as a first step toward the goal of
observation of crude oil emulsions with continuous flow. The
experimental cell seems well suited for this purpose and
further work will be performed.
CONCLUSION
For the intended purpose the experimental cell works well at
low water cuts.
Earlier assumptions, that drops in an emulsion will be charge
neutral seems to be a gross simplification; we have seen –
primarily at higher voltages - that drops in the emulsion may
be charged both by contact between drops and break-up of
single large drops.
While coalescence between drops will increase at higher
voltages there is a risk that the large ones may disintegrate.
ACKNOWLEDGMENT
This work is funded by the project “Electrocoalescence – Criteria
for an efficient process in real crude oil systems”; co-ordinated by
SINTEF Energy Research. Contact person is L.E. Lundgaard. The
project is supported by The Research Council of Norway, under
the contract no: 169466/S30, and by the following industrial
partners: Aibel AS, Aker Solutions AS, StatoilHydro ASA, BP
Exploration Operating Company Ltd, Shell Technology Norway
AS, Petrobras, Saudi Aramco.
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