Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 11.3.4 - Phase inversion studies using conductivity probes and electrical resistance tomography
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00283
 Material Information
Title: 11.3.4 - Phase inversion studies using conductivity probes and electrical resistance tomography Experimental Methods for Multiphase Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Ngan, K.H.
Ioannou, K.
Rhyne, L.D.
Angeli, P.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: phase inversion
flow pattern transition
horizontal oil-water flow
electrical resistance tomography
 Notes
Abstract: Phase inversion during the horizontal flow of a liquid-liquid mixture is investigated in an acrylic pipe with 38 mm ID. The investigation was carried out at two mixture velocities, 3m/s and 4m/s with oil (5.5mPas, 828 kg/m3) and water as test fluids. The changes in the continuity of the mixture and the distribution of the phases in a pipe cross section were monitored using electrical resistance tomography (ERT) and conductivity probes. The ERT system, consists of 16 sensors located on the pipe periphery, provided information on the phase distribution and the continuity of the mixture. The results were supported by a wire conductivity probe, with two point sensors that detect the continuity of the oil-water mixture at the centre of the pipe cross section and a ring probe located at the periphery of the pipe that detects the continuity of the mixture in contact with the pipe wall. Pressure gradient was also recorded. It was found that inversion of a water continuous dispersion with increasing oil fraction at a particular mixture velocity appears initially at the pipe centre, followed by the regions close to the top and finally the bottom of the pipe. The range of volume fractions where the transition from water to oil continuous fully dispersed flow occurred decreased with increasing mixture velocity. The pressure gradient of the water continuous mixture was found to decrease as the dispersed oil fraction increased and only started to increase again when the oil came in contact with the pipe wall after the mixture close to the top of the pipe inverted.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00283
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 1134-Ngan-ICMF2010.pdf

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Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Phase inversion studies using conductivity probes and electrical resistance tomography


Kwun Ho, Ngan1, Karolina loannou2, Lee D. Rhyne2, Panagiota Angeli'*


1Department of Chemical Engineering, University College London, Torrington Place, London, WC1E 7JE, U.K.
2 Chevron Energy Technology Company Limited, U.S.A.
Corresponding author: Dr P Angeli. Email: p.angeli@ucl.ac.uk


Keywords: Phase inversion; flow pattern transition; horizontal oil-water flow; electrical resistance tomography


Abstract

Phase inversion during the horizontal flow of a liquid-liquid mixture is investigated in an acrylic pipe with 38 mm ID. The investigation
was carried out at two mixture velocities, 3m/s and 4m/s with oil (5.5mPas, 828 kg/m3) and water as test fluids. The changes in the
continuity of the mixture and the distribution of the phases in a pipe cross section were monitored using electrical resistance tomography
(ERT) and conductivity probes. The ERT system, consists of 16 sensors located on the pipe periphery, provided information on the phase
distribution and the continuity of the mixture. The results were supported by a wire conductivity probe, with two point sensors that detect
the continuity of the oil-water mixture at the centre of the pipe cross section and a ring probe located at the periphery of the pipe that
detects the continuity of the mixture in contact with the pipe wall. Pressure gradient was also recorded.


It was found that inversion of a water continuous dispersion with increasing oil fraction at a particular mixture velocity appears initially at
the pipe centre, followed by the regions close to the top and finally the bottom of the pipe. The range of volume fractions where the
transition from water to oil continuous fully dispersed flow occurred decreased with increasing mixture velocity. The pressure gradient of
the water continuous mixture was found to decrease as the dispersed oil fraction increased and only started to increase again when the oil
came in contact with the pipe wall after the mixture close to the top of the pipe inverted.


1. Introduction


continuity.


Multiphase flow is commonly encountered in the
petroleum industry during oil production and
transportation. Water is often present in the oil pipe line
particularly at the later years of production. At high
velocities, oil-water dispersions can form with either the
oil (water-in-oil, W/O, dispersion) or the water
(oil-in-water, O/W, dispersion) as the continuous phase. At
a critical range of phase volume fractions phase inversion
is likely to occur where the initial continuous phase
becomes dispersed and vice versa. A number of
investigators have reported that inversion does not happen
simultaneously across the whole pipe cross section but,
rather, occurs firstly at certain locations before spreading to
the rest of the pipe cross sectional area (Chakrabarti et al.,
2006; Piela et al., 2008). The changes in phase continuity
as well as the distribution of phases in the pipe cross
section during inversion can have a significant impact on
pressure gradient.


In this paper, the changes in phase continuity and
distribution in a pipe cross section during the flow of an
oil-water dispersion are investigated for increasing
dispersed phase fraction until phase inversion occurs and
beyond. Two types of measuring techniques are used, (1)
conductivity ring and wire probes, and (2) electrical
resistance tomography (ERT). Pressure gradient is also
measured to relate any variations to the changes in phase


Nomenclature
D Pipe diameter
ERT Electrical resistance tomography
O/W Oil-in-water dispersion
W/O Water-in-Oil dispersion



2. Experimental Facility


The experimental study on phase inversion was conducted
in the pilot scale liquid-liquid flow facility available at
UCL. The test section is an acrylic pipe with 38mm I.D.
Exxsol D140 (density of 828 kgm-3, viscosity of 5.5mPa.s)
was used as the oil phase and tap water was used as the
aqueous phase. The two fluids were pumped from their
respective storage tanks via variable area flow meters into
the test section inlet and joined in a modified Y-junction. A
plate was fitted at the end of the junction to minimise
mixing between the two phases so that they enter as
separate layers with oil at the top. The phase inversion
experiments were conducted at mixture velocities 3m/s and
4m/s where previous work but with a different inlet
(Y-junction and no split plate; Ioannou, 2006) had
indicated that the flow is in the dispersed regime.
During an experiment, the water phase was first introduced






Paper No


into the test pipe at the same as the required mixture
velocity. Oil was then added to form an oil-in-water
dispersion. The oil flow rate was then gradually increased
while at the same time that of the water decreased to
increase the dispersed oil fraction while keeping the
mixture velocity constant. The experiment continued until
100% oil fraction. Pressure drop was measured via a
differential pressure transducer (Validyne DP103) between
two measuring ports 1.5m apart, with the first port located
at 3.8m (-100D) from the inlet where previous
measurements showed that pressure gradient was already
fully developed


An electrical resistance tomographic (ERT) system (by ITS
plc) with sampling frequency of 10Hz was used to obtain
phase continuity and distribution in a pipe cross section.
The tomographic sensor consists of 16 electrodes equally
distributed around the pipe periphery (Figure 1). Each
electrode is made of a 4mm diameter circular stainless
steel piece embedded on the acrylic pipe wall and in
contact with the mixture inside the pipe. Care was taken to
ensure that each electrode was flush with the inside pipe
wall. The sensor was located at approximately 5.8m
(-153D) from the inlet. The system used at UCL has a
resolution of 316 pixels over the 38 mm pipe cross section
with each pixel representing approximately 2mm x 2mm.
ERT is non-intrusive but only gives results when the
conducting water phase is in contact with at least 2 of the
electrodes. During the phase inversion experiments, at a
particular mixture velocity after a change in the phase flow
rates the mixture was allowed to run for about 5min before
any recordings were made. About 50 frames were then
recorded which represent approximately 30s of run time.
The raw data from ERT represent conductivity values on a
pipe cross section. From the raw conductivity data the
phase concentration is back calculated (using the ITS
toolsuite).


Figure 1: Photograph of the ERT sensor. 16 equal-sized
stainless steel electrodes are embedded on the pipe wall
periphery.


In addition to ERT, a local conductivity wire probe (Figure
2) was used to obtain phase continuity at specific locations
within a pipe cross section. The probe consists of two
copper wire sensors, 0.5mm in diameter, that are placed
10mm apart and have an L shape (see Figure 2b). The
probe gives a zero signal when an oil continuous phase
surrounds the sensors and a non-zero signal when a water
continuous phase surrounds the sensors. For the results


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

shown here the probe was located in the middle of the pipe.
A conductivity ring probe was also used (Figure 2a). The
probe consists of two 4mm wide stainless steel ring
sensors embedded on the pipe wall, 38mm apart. The ring
probe identified the continuity of the phase in contact with
the pipe wall. When oil was the continuous phase
throughout the pipe cross section, the probe signal was
zero. When there was a water continuous phase in contact
with the pipe wall (e.g. dispersed water continuous pattern;
dual continuous pattern with water continuous at the
bottom of the pipe and oil continuous at the top; oil
continuous patterns with a thin water layer at the bottom of
the pipe), the signal became non-zero. Both conductivity
probes are located at about the same downstream location
(see Figure 2 for setup) as the ERT sensor and their results
can be compared with those from ERT.


(b)
Figure 2: (a) Photograph of the conductivity ring and wire
probes. (b) Schematic diagram of the conductivity wire
probe and the L-shaped wire sensors.


3. Results and Discussion


3.1 Changes in phase continuity during phase inversion


The results showed that the conductivity data normalised






Paper No


against the values of pure water of the ring and wire probes
are similar to those of the ERT system. This can be seen in
Figure 3 where the data on phase conductivity in the pipe
centre, from the wire probe, and close to the pipe wall,
from the ring probe, are compared against those from ERT
at corresponding locations for a mixture velocity of 3m/s.
For the data close to the wall the conductivity values from
ERT over a concentric ring area of 2mm from the wall
were averaged. The small difference between the ERT and
ring probe data at medium phase fractions is perhaps
expected since at these conditions, as will be discussed
below, the distribution of the phases is very
inhomogeneous and can vary significantly from one
location to the other in a pipe cross section. The results
from the ring probe represent the conductivity of the
mixture in contact with the pipe wall while those from ERT
represent conductivity close to the pipe wall but further
inside the pipe.


.0.8 -

100.6

0.4A

;10.2

Zo.0
0 20 40 60
Input water fraction [%]
APipe Centre (WireProbe)
APipe Centre (ERT)
EPipe Periphery (Ring Probe)
E Pipe Periphery (ERT)


80 100


Figure 3: Comparison of local conductivity data between
the ERT system and the conductivity ring and wire probes
at a mixture velocity of 3m/s. The arrow denotes the
direction of experiment.


The changes in phase continuity at 3m/s mixture velocity
and increasing oil fraction are shown in Figure 4 for 3
locations in a pipe cross section, near the top wall (4mm
from the top), at the centre of the pipe (19mm from the
top) and on the pipe periphery. For the pipe periphery, the
data from the ring probe were used because they can
indicate precisely when no more water continuous mixture
is in contact with the wall. The results shown are averaged
values over several experimental runs. Phase distribution
in a pipe cross section diagrams (tomograms) are shown in
Figure 5 for representative phase fractions. The water
phase is shown as blue while the oil phase as green.


As can be seen in Figure 4 at high water fractions, the
conductivity is high at all locations indicating a water
continuous mixture throughout the pipe cross section. The
tomograms indicate that at these conditions the dispersion


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

is homogeneous (Figure 5, 80% input water fraction), also
suggested by the similar conductivity values of the various
probes in Fig. 4. As the oil fraction increases, the
dispersion becomes less homogeneous but remains water
continuous (Figure 4) and the oil drops tend to accumulate
at the pipe centre (Figure 5, 60% input water fraction).
This concentration of the oil phase leads to a local phase
inversion in the pipe centre at 54% water fraction, where
as can be seen from Figure 4 the conductivity reduces to
values close to zero. However, the conductivity close to the
top of the pipe, although low is non-zero (about 5% of the
normalized water conductivity) indicating that at the top
water is still the continuous phase. The flow pattern
resembles annular flow with an oil continuous core and a
water continuous annulus. The conductivity at the top
reduces to zero at 40% water fraction and the pattern
becomes dual continuous


1.0 Q
1.0 APipePeriphery -
O Pipe Centre Ex
0.8 -XPipeTop

0.6 Transition A

to.4

.2 A20% x 40%

o.o
0 20 40 60 80 100
Iuput water fraction [%1

Figure 4: Normalized conductivity data of the oil/water
system at a mixture velocity of 3m/s from the ring and
wire probes and the ERT system. The direction of the
experiment from water to oil continuous is shown by the
arrow. The vertical lines denote the boundaries of the
phase inversion transition region and are drawn at the first
and last near zero conductivity values recorded using the
various probes. The percentages represent the water
fraction where conductivity approaches zero at the various
locations in the pipe cross section. The arrow denotes the
direction of experiment from water to oil continuous
dispersion.


with oil as the continuous phase at the top and water
continuous at the bottom of the pipe (Figure 5, 40% water
fraction). This flow pattern persists until 20% water
fraction. At 20% and beyond, conductivity becomes
dispersion zero at all three locations. Phase inversion is
completed and a fully dispersed water-in-oil flow is
established. Because there is no conductive phase in
contact with the ERT electrodes, phase distribution data
cannot be obtained at these conditions. Under these
conditions, phase inversion does not happen at one critical
phase fraction but over a range of volume fractions (phase
inversion transition region) which is bound by vertical
lines, drawn at the first and last near zero conductivity
values recorded using the various probes. The percentages
represent the water fraction at which the different probes
show near-zero conductivity values (Figure 4).






Paper No


Figure 5: Phase distribution in a pipe cross section during
the transition from a water continuous to an oil continuous
mixture at a mixture velocity of 3m/s. The percentages in
brackets represent the input water fraction.


3.2 Changes in pressure gradient during phase inversion


Pressure drop was recorded during the conductivity
measurements and at each condition the data was averaged
over a period of 30s. Pressure gradient data for mixture
velocity 3m/s can be seen in Figure 6, where the
experiments started from a water continuous mixture and
the oil fraction was gradually increased.


As can be seen in Figure 6 when the dispersed oil phase
fraction is increased in the water continuous phase a
decrease in pressure gradient is observed. This drag
reduction effect during liquid-liquid dispersed flow has
been reported in the literature previously (e.g.
Arirachakaran et al., 1989, Pal, 1993, Ntidler and Mewes,
1997). The pressure gradient continues to reduce even after
the first boundary of the phase inversion transition region
(where the mixture in the core of the pipe has inverted)
while water is still in contact with the pipe wall. When the
water fraction reaches about 40%, pressure gradient starts
to fluctuate significantly. The fluctuations are believed to
be caused by the alternating passage through the pipe of
large water and oil continuous structures as the mixture is
inverting. Piela et al. (2008) reported similar fluctuations
and found that they could be related to the passage of water
or oil continuous regions, detected by a conductivity probe.
With the increase in the oil fraction the top part of the
mixture also inverts (see Figure 4) and oil finally comes in
contact with the pipe wall. This causes the pressure
gradient to start increasing. The increase continues after
the second phase inversion transition boundary as more
and more oil contacts the pipe wall and the pressure
gradient approaches the single phase oil value.


Drag reduction appears to be stronger in the oil continuous
than the water continuous dispersion. Similar trends have


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

also been observed by other investigators (Pal, 1993;
loannou et al., 2005; Hu and Angeli, 2006).


-3.0

2.5

2.0

1.5

,1.0

Z0.5
- P


0 20 40 60 80
Input water fraction [%1


Figure 6: Experimental pressure gradient against input
water fraction at a mixture velocity of 3m/s. The error bars
represent the standard deviation of the fluctuations at each
phase fraction. The arrow denotes the direction of
experiment from water to oil continuous dispersion and the
vertical lines represent the boundaries of the phase
inversion transition region.


3.3 Effect of mixture velocity on phase inversion


In order to investigate the effect of increasing mixture
velocity on phase inversion and the transition region,
experiments were also conducted at 4m/s mixture velocity
following the same methodology as before. The
conductivity data from 3 locations in the pipe cross section
can be seen in Figure 7 and tomograms for selected input
phase fractions can be seen in Figure 8.


. 0.8

0.6
8
0.4

| 0.2

Z 0.0


0 20 40 60
Input water fraction [%1


80 100


Figure 7: Normalized conductivity data of the oil/water
system at a mixture velocity of 4m/s from the ring and
wire probes as well as the ERT system. The vertical lines
denote the boundaries of the phase inversion transition
region and are drawn at the first and last near zero
conductivity values recorded using the various probes. The
percentages represent the water fraction where
conductivity approaches zero at the various locations in the
pipe cross section.


Expt





W/ T-itio

S ra uu Transition


APipePeriphery
OPipeCentre Expt
XPipeTop

Transition A
o

W/O /
A 22 x 24 0o 0
0 40%


v.u






Paper No


I 1 [ (22%) I
Figure 8: Phase distribution in a pipe cross section during
the transition from a water continuous to an oil continuous
mixture at a mixture velocity of 3m/s. The percentages in
brackets represent the input water fraction.


Similarly to 3m/s, the dispersion at high water fractions is
water continuous with the dispersed oil phase
homogeneously distributed through it (Figure 7 and Figure
8, 80% input water fraction). As the oil fraction increases
the oil starts to accumulate in the middle of the pipe
(Figure 8, 70% and 60% water fraction) until the mixture
in the core inverts at 40% water fraction (Figure 7). The
flow pattern resembles annular flow with an oil continuous
core and a water continuous annulus as was also found for
3m/s. At 4m/s mixture velocity, however, this annular flow
is more symmetric. At 24% input water fraction the
mixture at the top of the pipe also inverts (Figure 7; Figure
8, 24% input water fraction) and the pattern becomes dual
continuous. Only a thin water continuous layer remains
now at the bottom of the pipe until 22% input water
fraction when the mixture becomes oil continuous
throughout the pipe cross section and the inversion is
completed (Figure 7).


The phase inversion transition region appears to be
narrower at 4m/s compared to 3m/s. This is because the
higher mixture velocity promotes better mixing of the two
phases and limits their segregation induced by gravity. As a
result, a higher amount of input oil fraction is required to
reach the critical concentration for inversion in the pipe
centre which appears at 40% at 4m/s compared to 54% at
3m/s. Because of the reduced gravity effect, water remains
continuous at the top of the pipe resulting in annular flow
for lower water fractions at 4m/s compared to 3m/s. It is
interesting that despite the better mixing at 4m/s, the thin
water layer at the bottom of the pipe is finally consumed
by the oil continuous region at a similar input water
fraction as in 3m/s.
Pressure gradient at 4m/s is plotted in Figure 9. Similarly
to 3m/s, addition of dispersed oil in the water continuous
flow results in drag reduction. The pressure gradient starts
increasing again when the top part of the mixture inverts


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

and oil comes in contact with the pipe wall (at about 24%
input water fraction). This increase continues as the oil
fraction increases towards the single phase oil value. As
before, drag reduction appears to be larger in the oil
continuous dispersion than the water continuous one.


4.0
3.5

3.0
S2.5
I 2.0
c 1.5
-1.0
o 0.5
0.0


0 20 40 60
Input water fraction [%1


80 100


Figure 9: Experimental pressure gradient against input
water fraction at a mixture velocity of 4m/s. The error bars
represent the standard deviation of the fluctuations at each
phase fraction. The arrow denotes the direction of
experiment from water to oil continuous dispersion and the
vertical lines represent the boundaries of the phase
inversion transition region.


4. Conclusions


Phase inversion during the horizontal flow of an oil-water
mixture was investigated using conductivity probes and an
Electrical Resistance Tomography (ERT) system. These
sensors provided information on the continuity of the
mixture as well as the distribution of the phases in a pipe
cross section.


It was found that starting from a water continuous
dispersion an increase in the input oil fraction leads
initially to phase inversion in the middle of the pipe. With
a further increase in the input oil fraction water remains
continuous in an annulus around the oil continuous core
until the mixture close to the top of the pipe inverts and the
pattern becomes dual continuous. At even higher input oil
fraction the water continuous layer at the bottom of the
pipe finally inverts and the pattern becomes fully dispersed
oil continuous flow. When the mixture velocity is
increased the transition region between a water continuous
and an oil continuous fully dispersed flow becomes more
narrow.


Pressure gradient was found to depend on the continuity of
the phase in contact with the pipe wall. It decreased in a
water continuous flow as the dispersed oil fraction
increased and started to increase again when oil, after
inversion of the mixture on the top of the pipe, came in
contact with the pipe wall.






Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

This detailed information on the phase inversion process is
particularly important for understanding and controlling
multiphase pipeline flows.


Acknowledgements


Kwun Ho Ngan would like to thank Chevron Energy
Technology Limited for the financial support for his
research at UCL as well as the technical support on the
research work. The authors would also like to thank the
Mechanical Workshop in the Department of Chemical
Engineering, UCL, for the construction of the conductivity
probes and the ERT sensor.


References


Arirachakaran, S., Oglesby, K.D., Shoulam, O., and Brill,
J.P (1989). An investigation of oil water flow phenomena
in horizontal pipes. SPE Proc. Prod. Operation Symp., SPE
18836, 155-167.


Chakrabarti, D. P, Das, G., Das, P K. (2006). The
transition from water continuous to continuous flow
pattern. AICHE Journal, 52(11): 3668-3678.


Hu, B. and Angeli, P (2006). Phase inversion and
associated phenomena in oil-water vertical pipeline flow.
Can. J. Chem. Eng., 84: 94-107.


Ioannou, K. (2006). Phase inversion phenomenon in
horizontal dispersed oil/water pipeline flows. Ph.D. thesis,
University College London, U.K.


Ioannou, K. Nydal, O. J., Angeli, P. (2005). Phase
inversion in dispersed liquid-liquid flows. Expt. Thermal
and Fluid Sci., 29: 331-339.


Naidler, M., Mewes, D. (1997). Flow induced
emulsification in the flow of two immiscible liquids in
horizontal pipes. Int. J. Multiphase Flow, 23 (1): 55-68.


Pal, R. (1993). Pipeline flow of unstable and
surfactant-stabilized emulsions. AIChE J., 39 (11):
1754-1764.


Piela, K., Delfos, R., Ooms, G., Westerweel, J., Oliemans,
R.V.A. (2008). On the phase inversion process in an
oil-water pipe flow. Int. J. Multiphase Flow, 34(7):
665-677.




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