Advances in
Electrostatic Treatment
of Crude Oil
Pam Boschee
Oil and Gas Facilities Editor
A
s operators produce more
opportunity crude oils, oils that
provide a higher refining margin
relative to other crudes, dehydration of
the oil becomes more difficult. Physical
properties of the crude oil, such as API
gravity, oil viscosity, and water density,
determine the challenges and drive the
selection of technologies to be used for
effective dehydration.
Opportunity crude oils are
generally heavy, described as crude oils
with API gravities of approximately
20 or less. Very heavy oil is defined as
below 14°API, and extra heavy oil is
11°API and below. Known problematic
crude oils include Venezuelan Merey
(16°API), Mexican Maya (21°API),
and Doba (21°API), produced
in Chad.
Characteristics of the heavy crude
oils that contribute to difficulty in
dehydration include reduced oil/water
density differences that decrease the
forces needed for separation of water
droplets, higher crude oil viscosity, and
smaller water droplets resulting from
heavy crude oil production techniques.
To dehydrate crude oils with high
viscosities, the dehydration treaters
October 2012 • Oil and Gas Facilities
17
1800
Conductivity, nS/m
1600
1400
Typical
1200
High
1000
800
600
400
200
0
50
100
150
200
250
300
Fig. 1—Crude oil conductivity as a
function of the crude oil temperature.
Temperature, °F
must be larger than those used for
lower-viscosity oils or the temperature
must be increased to lower the viscosity
to achieve effective dehydration.
However, there is an upper limit
of approximately 300°F for most
dehydration processes.
Higher operating temperatures
require increased size of treater power
units. Larger supply systems and
substations are required to provide the
power to these units. A disadvantage
to this approach is that the increased
power results in electrical current
that heats the crude oil and causes
other issues.
conductivity can increase several times
from ambient temperature to process
temperature in the treater. The effect
of increased crude oil conductivity
in an AC treater is a decreased
electrostatic field, leading to less
effective dehydration because the lower
voltage fields cannot reach the small
water droplets.
Coalescence of water droplets
dispersed in crude oil occurs when the
droplets collide with enough energy
to overcome barriers to coalescence,
such as surface films adsorbed on
the droplet’s surface, dispersion
of fines around the droplets, and
interfacial tension.
Electrostatic fields generate
forces that can create conditions
for improved coalescence of water
droplets, resulting in improved
separation of water from the
crude oil.
Paper SPE 156299 described
three types of primary electrostatic
forces (Fig. 2).
Electrostatic Technologies
Used in Treaters
Typically, crude oil dehydration
treaters use retention times, heat,
and alternating current (AC)
electrostatic technology to treat the
oil. Paper SPE 156299 described
methods of enhancing the electrical
field in bulk oil treaters by the use of
combined AC and direct current (DC)
electrostatic technologies.
Crude oil conductivity affects the
electrostatic fields inside the treaters. If
the crude oil conductivity is very low,
the electrostatic charge has difficulty
reaching the dispersed water droplets.
If conductivity is very high, some of
the electrostatic charge may dissipate
into the crude oil and never reach the
water droplets.
The process temperature in the
crude oil treater affects the conductivity
of the crude oil (Fig. 1). The crude oil
18
Oil and Gas Facilities • October 2012
Retrofitting a dual polarity unit to a dual frequency unit requires the installation of a
transformer and an electronic control system.
Dipolar attraction is the electrostatic attraction force
between oppositely charged ends of water droplets.
Electrophoresis is the electrical attraction between the
charged electrode and oppositely charged water
droplets in a uniform electric field.
Dielectrophoresis is the movement of polarized water
droplets in a nonuniform electrostatic field with the
movement toward the direction of convergence
of the electrostatic field.
Fig. 2—Three primary electrostatic forces.
High-voltage transformer
-
+
-
+
AC power
supply
Oil flux
Electrodes
DC
field
DC
field
AC
field
DC
field
Bulk water
removal
Fig. 3—Typical dual polarity configuration.
Dipolar forces result in the
water molecules aligning themselves
in an electrostatic field. A water
droplet made up of such aligned
molecules is polarized with a positive
charge on one end and a negative
charge on the other end. Polarized
water droplets are attracted by other
water droplets.
Dipolar attraction is effective for
high-water-cut emulsions because the
water droplets are larger and more
closely spaced. When water droplets
are smaller, as in a low-water-cut
emulsion, dipolar attraction is less
effective because the water droplets are
smaller and farther apart.
Electrophoresis moves water
droplets horizontally between
electrode plates in a uniform DC
electrostatic field. With a force of up to
four orders of magnitude larger than
dipolar attraction, the water droplets
collide more frequently.
Dielectrophoresis, the weakest
of the three electrostatic forces, is
approximately half the strength
of the dipolar attraction force.
Polarized water droplets move in a
nonuniform electrostatic field toward
the direction of convergence of the
electrostatic field.
Electrical Field Enhancement
AC crude dehydration technology
applies an alternating electric field at
50 to 60 Hz to the emulsion, causing
the water droplets to deform because
of dipolar attraction force. The
attraction force between oppositely
charged ends of the water droplets
speeds up their coalescence.
Treaters using AC fields are
effective for bulk water removal, but
their performance suffers when lower
water cuts are processed. The lower
water cut means the water droplets
are farther apart, which weakens their
dipolar attraction.
DC fields use electrophoresis to
enhance water droplet collision rates
and promote coalescence. Because
the application of a DC field to an
emulsion with significant water
content results in electro-corrosion,
the method is limited to treaters used
in refined products.
Dual polarity technology (Fig. 3)
applies a weak AC field to remove bulk
water, followed by a stronger DC field
to remove remnant water droplets.
The potential for electro-corrosion is
eliminated because the DC field exists
only between the electrode plates
In the DC field, the water droplets
acquire a charge and are accelerated
to the electrode of opposite polarity.
The droplet then acquires the charge of
that electrode and is accelerated back
to the opposite electrode. The droplets’
movement in the DC field is mainly
because of electrophoresis. The AC
field’s dipolar forces aid in deforming
the droplets. As the droplets collide,
they become larger and separate out
of the DC field, settling down to the
separated brine phase in the bottom of
the treater.
The most recently developed
of the enhanced electrostatic field
technologies is dual frequency. The
October 2012 • Oil and Gas Facilities
19
600
Primary voltage, volts
dual frequency technology was
developed by Cameron; the first
patent was issued in 2004, Gary Sams,
managing director of research and
development at Cameron Process
Systems, said.
In AC/DC treatment of crude
oils with high conductivity, one set of
electrode plates experiences charge
decay while the alternate set of plates is
being charged. The decay can result in
the loss of the DC field. To minimize
this effect, the time between charges
must be decreased by increasing the
frequency of the power source.
The DC field in a treater can
be strengthened by replenishing the
voltage on the electrodes at a faster rate.
Dual frequency technology restores
voltages on the electrodes at frequencies
of 800 to 1,600 Hz compared to 50 to
60 Hz for a traditional AC system. As
a result, “the benefit of the DC field is
regained in high-conductivity crude
oils,” Sams said.
The low interfacial tension of many
crude oils can be used to promote
droplet coalescence by energizing the
water droplet surface. Modulating
the power at a frequency close to the
resonant frequency of the dispersed
water droplets can achieve this.
The frequency of the power supply
(base frequency) is set to a value high
enough to minimize field decay and is
modulated (pulse frequency) at a rate
sufficient to energize the water droplet
surfaces. The high sustained field
strength energizes the droplets so that
they readily coalesce.
Fig. 4 illustrates the amplitude
modulation of the base frequency
possible with this technology. The
bimodal frequency modulation in the
dual frequency system uses the same
400
Critical
voltage
20–40 kV
Sinusoidal wave
(or trapezoidal,
triangular, square,
etc.)
200
0
–200
–400
–600
Base
frequency
Threshold
voltage
12–25 kV
Droplet
charge follows the
modulation waveform
Pulse
frequency
Time
Fig. 4—Bimodal frequency modulation in the dual frequency system.
Dual
frequency
Dual
polarity
Conventional
AC
50,000
100,000
150,000
200,000
BOPD per treater
Fig. 5—Dehydration treatment capacities for 12×80 ft treater using different tecnologies
for 20.6° API crude oil.
electrode configuration as that used in
the dual polarity technology.
Sams said that, for retrofitting a
dual polarity unit to a dual frequency
unit, the only physical changes
required are the power unit and the
electronic control system for adjusting
the base frequency and modulating the
DC field.
He said that there are more than
50 installations of the dual frequency
technology worldwide, representing
more than 100 transformers. “Some
operators using the dual polarity
technology have pushed it beyond
its capacities or their oil proved to be
more difficult than they expected it
to be. Following replacement of the
transformers at the existing vessel to
convert to the dual frequency technology,
operators typically see a performance
benefit. They’re back to spec and can
potentially push more crude.”
Process tests performed at
the Cameron Technology Center
in Houston verified that crude oil
dehydration capacity (flux) can
be improved by using AC/DC
technologies instead of conventional
AC technology.
Paper SPE 156299 cited
improvements of flux rate of
25% volume/volume by using dual
polarity technology, and
125% volume/volume by using
dual frequency technology.
TABLE 1—TREATER SIZES REQUIRED TO TREAT 50,000 BOPD OF WET CRUDE TO 0.2% BASIC SEDIMENT AND WATER
Technology
Crude
API°
Brine salinity
(%)
Inlet BS&W
(%)
Temperature
°C
Vessel diameter
(ft)
Length
(ft)
Length Savings
(%)
Conventional AC
18
4
15
100
14
94
–
Dual polarity
18
4
15
100
14
78
17
Dual frequency
18
4
15
100
14
58
38
Conventional AC
30
12
15
40
12
56
–
Dual polarity
30
12
15
40
12
46
18
Dual frequency
30
12
15
40
12
36
36
20
Oil and Gas Facilities • October 2012
Fig. 5 shows the dehydration
capacities for three different
treaters with identical size but
using different electrostatic
technologies (conventional AC,
dual polarity technology, and dual
frequency technology).
Table 1 shows treater size
comparisons using the three electrostatic
technologies on 30°API crude oil
with 12% salinity formation water
and 18°API crude oil with 4% salinity
formation water at 50,000 BOPD, 0.2%
basic sediment and water.
Besides requiring a smaller
footprint, the use of shorter treaters
on floating production systems is that
trim issues with the floater will have less
of a negative effect on the treater level
control system. With longer treaters
containing level sensors in one end of
the treater, the effect of a 1° trim on the
floater can push the oil/water interface
outside the measuring range of the level
sensor, requiring the operator
to use manual level control of the
treater interface.
Smaller treater size is also
important for lower installation
cost and projects with shipment
size restrictions.
Implementation of Dual
Frequency Technology
Operators of the dual frequency
technology require initial training.
“Once the technology is tuned and
demonstrating that the parameters
are operating successfully on the
crude being processed, the parameters
really don’t need to be adjusted again,”
Sams said. “The operators need
to understand that they shouldn’t
be tweaking knobs for the sake of
tweaking knobs, because that will get
the process out of sync quickly.”
“Operators can also be reticent to
make adjustments to the technology
until they gain an understanding of
what it can do for them. Once they
ARE YOU READY TO
EXPLORE THE FRONTIERS
OF KNOWLEDGE?
have that understanding and realize
that they have another tool available to
them besides chemistries, temperatures,
and reduction of flow rates, they realize
they now have a means to tune their
operations at a fairly modest cost. They
are no longer incurring the same costs
for chemistries or heating, and they’re
not losing production,” he said. OGF
For Further Reading
SPE 156299 Benefits of Using
Advanced Electrostatic Fields in
Crude Oil Dehydrators and Desalters
by Erik Sellman, Gary W. Sams, and
S. Pavan Kumar B. Mandewalkar,
Cameron Process Systems.
OTC 23200 Field Implementation
of New Electrostatic Treating
Technology by John M. Walsh,
Shell International, Gary Sams and
Joseph Lee, Cameron.
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October 2012 • Oil and Gas Facilities
21