Abstract
The purpose of the study is to develop Design of a Crude Oil
Dehydration Unit and compact electrocoalescer. To improve oil/water
separation, definition of optimized conditions for coalescence of water
droplets under an electrical field in combination with optimal
hydrodynamic performances was performed. Numerical simulations
carried out with the CFD code fluent permit to design a new dehydrator
integrating the three functions of electrocoalescence, centrifugation and
phase separation. A first part consists of two concentric cylindrical
vessels used as electrodes with a tangential flow introduction. Then the
speed of water droplets is accelerated in a second step by centrifugal
effect. Finally, the water flow is separated from the oil phase in a third
part. A laboratory prototype was built and tested in various conditions of
flow rate and with different types of crude oil/water emulsions.
Encouraging results led to define and patent a new concept of compact
centrifugal electrocoalescer .
Introduction
Crude oil dehydration is the removal of water or water vapor from crude.
oil, by separating the oil from the water, often in a rotating centrifuge
Crude oil contains water that must be removed to levels that meet
pipeline specifications before refining into upgraded petroleum products.
This process is known as crude oil dehydration . The aim of crude oil
dehydration is to remove free water from the oil and then to break the oil
emulsions to reduce the remaining emulsified water in the oil.
High levels of water in crude oil make fractional distillation
difficult. Water has a hard time vaporizing from the boiling flask and
getting through the distillation column packing. Water drops in the
distillation column head causes fluctuation in the vapor temperature
causing inaccurate vapor temperature readings. D2892 requires that
crude oil samples have 0.3% water or less. If the crude oil sample has
more than 0.3% water then it must be dehydrated before the D2892 test
can be performed. Conversely, heavy, sour crude oils are usually “wet”.
As the nature of crude oil changes continuously as the producing field is
depleting, the production process must be adapted to this evolution. In
offshore conditions, the challenge of primary and secondary separation as
well as dehydration becomes critical due to the lack of place on the
platform. A compact electrocoalescer will have the advantage of being
smaller and lighter than traditional ones and thus easier to install on a
platform and reduce the CAPEX. Furthermore, the possibility of
revamping is restricted. Moreover, it is noteworthy that the efficiency of
desalting and dehydration of produced oil decreases in the presence of
heavy components in oil that induce the formation of stable emulsions.
Traditional application of electrostatic methods implies that the
separation unit has to be sized in order to keep a satisfying efficiency
during the life-time of the producing field, while emulsion breakers
additives are used.
Water in crude oil can be in various forms
 free water
 emulsified water
 chemically bound water
Problems caused be water in crude oil:
1. Because water is heavier than crude oil it sits on the bottom of the
boiling flask with all the crude oil above it. When the water
vaporizes a “bump” may be heard and there may be violent shaking
of the distillation apparatus as the water tries to break through to
the surface of the crude oil. Many times the water does not escape
and returns to the bottom of the boiling flask.
2. When water does escape from the crude oil into the distillation
column packing. It may not having enough energy to stay in the
vapor phase. Water beads on the stainless steel column packing
and so it doesn’t freely flow back down the column packing to the
boiling flask. The water has a difficult time going up the
distillation column or down. It is trapped in the packing.
3. When water finally makes it to the condenser it can get “stuck”
there. Water beads on glass and so doesn’t freely flow back down
to the reflux area to be collected. When water does flow back
down there are 6 to 1 odds that it will be returned to the distillation
column.
4. Water forms a droplet on the vapor temperature probe and the
vapor temperature decreases. When the water droplet falls from
the end of the temperature probe, the vapor temperature goes up.
Numerical Part
Design of a new dehydrator system was intensively supported by
numerical simulations carried out with the CFD code FLUENT.
Previous design
A first step was to evaluate the electrostatic coalescer previously
patented by IFP. In this system, elctrocoalescence of water droplets and
their separation from oil by centrifugation are performed simultaneously.
Water is assumed to be collected at the bottom of the system and
dehydrated oil recovered at the top. There is no great difficulty to
simulate this straightforward configuration and, at first glance, its main
deficiency can be detected: the centrifugal flow induced by the tangential
input cannot be sustained all along the coalescer because of oil high
viscosity (Figure 2). It resulted in a very poor separation between oil and
water as it is shown by simulation post calculus: for an oil recovery ratio
of 80 % (the quantity of oil recovered) the mean separation rate was 30 %
(the percentage of water separated from oil). This rate depends very
weakly on oil flow, confirming that water separation is not at all achieved
and that fluids have probably been re-mixed when they have reached the
bottom of the system.
Conclusions clearly came out :

it is certainly more convenient to operate coalescence and
centrifugation sequentially, because separation is more efficient
when water droplet size is maximum. Thus, centrifugation must
start after coalescence.
 due to oil high viscosity, fluid rotation cannot be maintained
without a guiding device, for instance an helical wall.
 turbulence has to be carefully managed because it may break-up
water droplets and minimize coalescer performance. Flow must be
laminar all along coalescer and centrifugal part.
New design
A new system has been designed keeping in mind last conclusions. It
consisted of three parts which are from upstream to downstream :
1. a coalescer, made of two concentric tubes of 1 m length with an
annular space of 1 cm. Electrocoalescence is made possible by applying
an electric field in this annular space.
2. a centrifugal part, made also of two concentric tubes of 500 mm length,
with between them an helical wall which creates an helical duct.
Diameters and annular space are reduced in order to increase flow speed
and maximize centrifugal acceleration.
3. a separator, which has the hard task to pick-up water droplets that, after
centrifugation, are assumed to stay close to the wall of the external tube.
There are two ways to collect water, one tangential and the other axial,
which correspond to two outlets. Dehydrated oil is directed upward,
inside the inner tube. Turbulence is intense in this part and the challenge
is to collect water droplets before they have broken and been dispersed.
Design of this part is very critical and needs imperatively the help of a
CFD code to be achieved in a reasonable time.
Simulation results
All results presented, corresponded to simulations carried out with the
same design and the same computation procedure. Volume meshing has
been carefully sized with almost 100,000 cells, mainly hexahedral.
General CFD is of a classical type, with turbulence taken into account by
a K-epsilon model and near wall flow handled by standard wallfunctions. Interactions between water droplets and oil are treated by a
special model named “Algebraic Slip Mixture Model” and proposed by
FLUENT among multiphase models. It differs from classical Volume of
Fluid (VOF) models in enabling phases to be interpenetrated and to move
at different velocities. Thus, a special algebraic equation must be solved
for the relative velocity and acceleration of particles takes into account all
forces in presence, such as gravity and centrifugal force, and proximity of
other particles. The model only needs data specifying physical properties
of fluids in presence and droplet diameter of secondary phase (water in
our case). Boundary conditions must also specify volume fraction of
secondary phase at inlets. Main flow patterns are completely determined
when pressure values at inlet and outlets are fixed. For all calculus,
balance between inlet and outlets are adjusted in order to achieve an
outflow of 20 % at bottom of dehydrator (“water” outlets) and 80 % at the
top (“oil” outlet). It is important to point out these data, because separator
performances are closely linked to them. Simulations led to general flow
patterns that corresponded to the expected flow design. Figure 4 and 5
showed that the fluid streams had normal characteristics and the velocity
magnitude levels were sufficiently low to maintain Reynolds number well
beyond critical values. Thus, flow remained laminar all along coalescer
and centrifugal parts.
presents profiles of rotating velocity component induced by helical duct
in centrifugal section. Centrifugal acceleration can be evaluated from
spinning magnitude of this component and, in this case for 1 m3 /h (1000
l/h), it reaches nearly 270 m/s2 , that is 27 G. Values for other flow rates
can be roughly estimated in proportion to the square of flow rate.
Simulations enabled to good analysis and understanding of the
process.For instance, examining Figure 7 it is easy to track and study
water droplet concentration all along the centrifugal part. It clearly
appears on Figure 8 that concentration is completed very gradually and
that at end water is confined like a skin close outside centrifugal wall.
The separator task is then to trap the water layer without breaking it and
mix again the oil/water phases.
Spinning velocity component in centrifugal
part (1000 l/h)
Performances
1- Water droplet size : presents separation performances versus droplet
size, this for different oil flow rates. These results are computed with oil
data recovery fixed at 80 % and water volumetric ratio at 5 % at the inlet.
Curve shapes indicated that centrifugal forces segregates water droplets
according to their weight. The abrupt and quasi-linear variation at
midpoint resulted from a competition between two forces: the centrifugal
force (proportional to the cube of diameter) and the drag force
(proportional to the square of diameter).
System performances versus water droplet size for different oil flow rates.
2- Total flow rate : confirms the great influence of flow rate on
performances. Linear variation of fraction of water separated versus flow
rate could be related to the facts that if centrifugal force is proportional to
the square of flow velocity, residence time in centrifugal device is
inversely proportional to flow rate. Slope decreasing observed for flow
rate above 3 m3 /h is a representative of insufficient resisdence time in
the centrifugal part. This underlines that centrifugal length is a significant
parameter, just like helical pitch.
Influence of oil flow rate on water separation
Conclusion
By removing the saline solution and free aqueous formation from the
crude oil, the mineral salts represent calcium chloride, sodium chloride,
and other inorganic solids.
The removal of these impurities is done with separation and separation
technology by using electrostatic separation technology where it exposes
the emulsion of water and oil to a controlled high-voltage electric field,
this causes the electric field of emulsion suspended in the oil to become
polarized and heal with each other to form larger drops As the
It is used in such units as high .gravitational pulls down, it expels out
voltage transformer (12kV, 16.5 kV, 20 kV, 23 kV, 25 kV), 100%
Impedance and impedance reactor.
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