United States
Environmental Protection
Agency
Municipal Environmental Researctr
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-032 August 1982
Project Summary
Apparatus and Procedure for
Determining Oil Droplet Size
Distribution
Raymond A. Meyer, Milton Kirsch, Fred Howard, and Frank Freestone
This study was undertaken to char-
acterize the oily brine resulting from
the production of oil and to develop an
apparatus and a procedure for ac-
curately determining oil droplet size
distribution. Knowing that the size
range and distribution of oil droplets is
a major governing factor in the
successful treatment of oily brine, the
end result of this work may be to
reduce the hydrocarbon discharge to
the environment.
An oil-specific, automated photo-
micrographic system was developed
to fulfill the need for determining size
distribution on production platforms
where explosive concentrations of
hydrocarbons may exist. This system
can be used to measure the diameter
of particles in the 2- to 100-jum micro-
meter range under flowing conditions
without introducing significant shear
forces, which can affect the oil-drop
population.
The system uses a newly developed
technique that applies time-lapse
photography to the determination of
number, size, and density of spherical
entities. Specifically, a microscope
with a horizontal orientation of the
viewing axis is used to photograph the
movement of oil droplets through a
flow-through cell.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory, Cincinnati.
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction to the Problem
Production of oil-brine mixtures,
pumping, and pipeline flow all result in a
dispersal of oil in the associated brine.
In an effort to reduce the hydrocarbon
discharge to the environment, the
Offshore Operators Committee and the
U.S. Environmental Protection Agency's
Municipal Environmental Research
Laboratory cooperated in an oil produc-
tion platform study. The goals of the
study included characterizing the oily
brine at several points in the oil removal
treatment process, evaluating the
effectiveness of several treatment
techniques, and comparing analytical
methods.
Oil-brine separation methods ulti-
mately depend upon the oil drops rising
through the brine to a collection area.
This rise rate is proportional to the
density difference between the oil drops
and the brine, and is also proportional to
the square of the diameter of the drop.
Thus, a major governing factor in the
success of brine treatment for oil
removal is the size range and distribu-
tion of oil drops. Knowing the oil drop
size dispersion at several places in the
produced water treatment system
would aid in applying present separa-
tion techniques and developing future
systems. Therefore, a part of the
platform study wgs directed toward
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measurement of the drop size distribu-
tion in the primary oil-water separator
feed, the final treatment unit feed, and
the final discharge. Emphasis was on
offshore production, but the develop-
ment is equally applicable to onshore
production.
A number of nonspecific techniques
are used to characterize particle size
distribution. All are based on measuring
the particle's effect in the interruption of
the flow of some energy such as light,
radiation, or electricity. These methods
are nonspecific and cannot distinguish
between gas bubbles, solid particles,
and oil drops. Such methods are
inadequate because the produced brine
contains such particulate material as
sand, shells from microorganisms, gas
bubbles from depressurized dissolved
gas, and gas bubbles intentionally
introduced to assist the oil removal
flotation process. Therefore, any of the
nonspecific techniques would be ex-
pected to give erroneously high oil
contents and misleading oil drop size
distributions. Additionally, to obtain
meaningful size measurement, only one
particle may be in the measuring path at
a time. This is typically achieved by
severely limiting the path length. While
this is quite acceptable when dealing
with solids, the high shear introduced
by passing the sample through the small
orifice required would be expected to
alter the oil drop size distribution and
render the data meaningless. For these
reasons, the existing techniques were
considered unsuitable for determining
oil drop size dispersions.
The Solution
A micrographic technique may be
implemented that does not introduce
significant shear forces on the oil drop
population. The technique was devel-
oped into an automated photomicro-
graphic system (PMS) that met the
requirements of the National Electrical
Code Class 1, Division 1, Group D. This
permitted its use on production plat-
forms where explosive concentrations
of hydrocarbons were known to exist.
The system can be used to measure the
diameters (and densities) of particles in
the 2- to 100-Aim-diameter range under
flowing conditions. The battery-oper-
ated device is 63-cm long, 55-cm wide,
and 55-cm high, and weighs 16 kg.
Conventional micrography involves
capture of a sample, placing it on a slide,
perhaps in a shallow well, and counting
or measuring the entities of interest.
This typically is done at leisure since the
sample is stable over a relatively long
time. Such is not the case when
studying oil drop dispersion. As soon as
the turbulent mixing motion dissipates,
the sample starts to stratify because of
the density disparity between the oil
drops and the brine matrix. This
dispersion alteration proceeds at such a
rapid rate that the sample would be
useless within 20 seconds.
A flowthrough microscope cell was
constructed, and the microscope illumi-
nation changed from the conventional
continuous light source to electronic
flash Illumination. When the sample
was flowing fast enough to maintain
turbulent mixing, however, the linear
drop movement was too rapid to give
sharp photographs. Accordingly, an
interrupted flow system was designed
in which the flow was blocked by a
downstream valve and the photographs
taken a few seconds later.
The spherical entities were initially
thought to be nothing but oil drops.
However, the oil content of a Wemco*
flotation unit (final oil removal unit)
outlet sample, calculated from mea-
sured drop diameters and count, was
found to be much larger than that
determined by conventional solvent
extraction techniques. The rationale
was offered that not all the photo-
graphed, measured, and counted
spherical entities were oil. This seemed
logical since the function of the Wemco
treating unit was to mix gas bubbles into
the water in an effort to "parachute" the
oil drops to a surface skimmer. These
bubbles could well be covered with a
film of oil and be photographically
similar to actual oil drops. This compli-
cation led to the development of a
technique to apply time-lapse micro-
photography to determine number, size,
and, most importantly, the density of the
spherical entities.
In the normal, vertical orientation of
the microscope viewing axis, the oil
drops rise toward the top of the cell and
thus move in and out of focus (Figure 1).
If the axis of the microscope is turned
horizontal, however, the drop move-
ment would be across the field of view of
the microscope (Figure 2). Thus, the
drop would remain in focus and only
change position in the field of view as it
rises. The vertical movement of the oil
drops and air bubbles could be mea-
Figure 1. Drop movement with normal
vertical microscope axis
orientation.
'Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
Figure 2. Drop movement with non-
typical horizontal microscope
axis orientation.
sured by comparing their positions in
two photographs taken a known-time
interval apart. If the rise rates and
diameters were known, the densities of
the spherical entities could be deter-
mined by applying Stokes Law.
Figure 3 shows a line diagram of the
system as viewed from above while in
its operating position. The horizontal
orientation of the microscope viewing
axis and its relation to the flowthrough
cell and film plane are shown. Both the
camera focusing magnifier and the
microscope oculars are designed to be
used from the side when in the operat-
ing position. The camera was positioned
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^.
0
_^
o
o
o
35mm film
f- ,
2,5 X magnifier it. I
Binocular
eyepieces //
\
^ x
Discharge
Camera
Microscope
Cell
Condenser
lens
Strobe and reflector assembly
Standpipe
_^>_Lr-
Jg> «~
^_J Sample
Figure 3. Top diagrammatic view of photomicrographic system.
with the long, 34-mm axis of the film
vertical. An electronic control circuitry
was developed to sequence the sample
flow and the three time-lapse photo-
graphs. Figures 4, 5, and 6 are photo-
graphs of the apparatus at its present
stage of development.
Sampling
If the pipeline and sample flow are
maintained in the turbulent region
(Reynolds numbers over 3000), the
sample may be obtained and transferred
to the analytical apparatus without
altering the oil drop size dispersion.
Sample flowrate.is a function of liquid
pressure in the pipeline, which typically
ranged between 2.8 kPa (30 cm water)
and 28 kPa (300 cm water). The
additional requirement — that the
sample presented to the microscope cell
should have been in turbulent flow
since its removal from the pipeline —
dictated some form of bypass sampling.
The overflow standpipe system fulfilled
both the requirements of pressure
regulation and of bypass sampling
(Figure 7). This particular apparatus
used a 21.5-cm-tall inner pipe and has
been operated with pipeline pressures
ranging from 2.8 kPa (30 cm water) to
44.8 kPa (457 cm water). Minor size
modifications could extend this range.
Normal operating conditions resulted
in a flow of 2075 mL/min during the
microscope cell flushing period. The
lowest Reynolds number in the sample
flow system was 4000 in the cell body,
and over 7000 elsewhere in the sample
system. Attention must be directed
toward elimination of any shear-
inducing restrictions such as partly
opened valves. If these precautions are
taken, the microscope cell may be'
assumed to be filled with a completely
representative and uniformly dispersed
sample of the pipeline flow.
Stopped Flow Period
Previous applications of the photo-
micrographic principle for measuring oil
drop size dispersion relied upon the
ability to photograph moving drops in a
flowing stream. If one applies the
requirement that the stream must be in
continuous turbulent movement to
eliminate sample stratification, the
exposures must be extremely short. For
example, the linear transit rate of a drop
in the microscope cell at a Reynolds
number of 4000 is 4.3 x 105 jum/sec.
Common shutters of 1/100 of a second
capacity would result in an image of a 1-
//m-diameter drop that would be 430/um
long. Photography with an electronic
flash lamp having a 50-fjsec duration
would give an image 21 -/um long. Even if
the 5-fjsec "Strobotac" source were
used, the image would still be twice as
long as it was wide. We have estab-
lished 0.1 /um as the desired limit of
movement during photography. This
would impose an exposure duration of
0.2 /usec, which is beyond the range of
available portable illumination sources.
Accordingly, a stopped flow system was
designed that would not induce sample
degradation. The previous discussion of
sample flow defends the assumption
that the sample cell is filled with an
uncompromised and uniformly dispersed
sample at the time turbulent flow is
interrupted. Drop motion at this time
results from inertia and the random
movement of turbulence. The sample
must remain static until movement from
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Standpipe
pressure
reducer •
Overflow
line
Sample ,,
inlet
line
250-exposure
camera
Figure 4. Photomicrographic apparatus, left side view.
Control
switches
\Cell flow
outlet
Case
•essurization
connection
Figure 5. Photomicrographic apparatus, right side view.
4
both of these sources dissipates and
vertical movement from the density
difference is established between the
drop and the liquid matrix. It will be
proven that a 10-sec static period does
not compromise the sample.
Microscope Viewing Cell
The microscope viewing cell is the
liquid volume in focus by the micro-
scope optics. Its size is defined by the
length and width of the film, the
diameter of the drop (D), and the
magnification and depth of focus of the
objective. Under the conditions used in
this work, the cross-sectional cell
dimensions are 535 + D x 349 + D A*m.
The apparent depth of focus was found
to be dependent on the drop diameter.
As used in the field studies, a slide of
oil drops captured in gelatin was photo-
graphed with color film and electronic
flash. The microscope stage was moved
in 4-/vm steps over a wide range with
photographs taken at each step. Twelve
drops were selected ranging in size from
2 to115;umindiameterandtherangeof
stage positions resulting in sharp image
photographs wasdetermined by inspec-
tion of the photographs. The data were
fitted to various equations with a
Hewlett Packard statistics program.
The following equation shows the best
fit and may only be applied under the
exact conditions used in its determi-
nation.
Depth of focus (fjm) =
3.861 + (5.088 Ln D)
where D is the drop diarneter in //m
Black and white film, for example, gave
significantly different results. The
determination of "in focus" is very
subjective, and the data analyst must be
well trained by inspection of the
calibration photographs. Retraining by
viewing the calibration photographs
should be performed at periodic inter-
vals to eliminate "subjective drift."
There is a marked tendency to "find"
drops in sparsely populated exposures
that lead to high drop counts and oil
contents. The retraining minimized this
effect.
Once the concept of the viewing cell
as a boundaryless volume of liquid
sample located someplace within the
25,000 x 3175-Aim microscope cell is
established, the preferred location may
be selected. The objective side of the
viewing cell is typically located 600 jum
into the liquid to minimize the effect of
the cell wall on drop rise. The top of the
viewing cell is a nominal 1500 jum
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Microscope
13!
Salenoid valve
Figure 6. Photomicrographic apparatus, internal detail photograph.
below the top of the nicroscope cell to
avoid optical distortion from the cell
curvature. If one discounts a 1000-//m
section at the bottom of the cell, the
drops may be said to have a conserva-
tive rise path of 22,000/urn to the bottom
of the 535-/um-high viewing cell.
Representative Sample
The assumption has already been
proven that the sampling procedures
have placed an uncompromised and
uniformly dispersed sample within the
microscope cell when flow is inter-
rupted. It remains to be shown that
photographs taken 10, 10.3, and 12.0
sec later capture a representative
sample of the oil drops in the water.
Figure 8 shows calculated rise distances
in 10.3 sec for oil drops in various
matrices. Platform ST177 conditions
resulted in the most rapid rise and
therefore were used to prepare Table 1.
According to the 10-sec column in
Table 1, a 2-/urn-diameter oil drop rises 7
fjm, a 100-/um drop rises 10,488, and a
^2S-(Jm drop rises 24,762. As previ-
ously indicated, the free vertical rise
section of the cell to the bottom of the
microscope viewing cell is a conserva-
tive 22,000 /urn.
Consider a zero time photograph
where a 2-, a 10-, and a 100-jum drop
are just inside the lower edge of the film
image. The water sample had been
stationary for 10 sec before the photo-
graph was taken and all oil drops had an
opportunity to rise at their diameter and
density determined rate for the 10 sec.
Thus, the2-/um drop had risen 7yum; the
1 Q-fim drop, 165; a nd the 100-yum drop,
16,488. Another way to say the same
thing is that the 2-/urn drop originated in
a microvolume 7 //m below the viewing
cell; the 10-/um drop, 165; and the 100-
fjm drop, 16,488.
Over-
flow
Sample
Figure 7. Standpipe pressure reducer.
Considering the previously estab-
lished fact that all drops are uniformly
dispersed, all microvolumes have equal
chances of containing any drop size. It
therefore makes no difference if the 2-
jum drops originate in one microvolume
and the 100-/vm drops in another. All
microvolumes are equivalent and the
sample volume photographed will still
be representative of the entire sample.
This condition holds until the drop-rise
distance during the static period ex-
ceeds the available path within the cell.
This was defined as 22,000 fjm and a
115.5-/um-diameter oil drop will then be
the cutoff point, since it will rise 21,995
fjm in 10 sec. Any larger drops may
exceed the rise path, and even if they
originated in the lowest acceptable
microvolume, they may have escaped
the viewing cell when the photograph
was taken. System limitations, there-
fore, indicate a conservative cutoff point
of 100-/um diameter, and the above
discussion shows that the viewing cell
contains a representative sample at the
end of 10 sec. The rise rate is dependent
upon water density and viscosity and oil
density. Thus, the actual cutoff point
will change somewhat because of
sample conditions.
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/ 2 345
50,000-1
/ Platform ST177
2 Platform SP65B
3 Sea water and 0.8412 density oil
4 Platform WD45C
5 Fresh water and 0.8412-density oil
0
i I i i i i I i r i i i i i i i i i i i i i i i i i i i i i i i i i i i i
25 50 75 100 125 150 175 200
Drop diameter, micrometers
Figure 8. Drop position after 10.3 seconds.
Table 1. Platform S T177 Drop Movement*
Drop diam.,
micrometers
2
10
50
100
125
Rise rate.
micrometer/ sec
0.7
16
412
1,649
2,576
Micrometer rise, seconds
10
7
165
4.122
16.488
25,762
10.3
7
170
4.246
16.983
26.535
11.7
8
193
4,823
19.291
30. 142
*Brine density = 1.151; Oil density = 0.8418; Viscosity = 0.01021 Stokes
Oil drop movement over a 12-sec
period is diagrammed in Figure 9.
Column A diagrams three equal-sized
cells. Cell 1 is at the top of the
microscope cell; cell 2 is near the center
of the microscope cell; and cell 3 near
the bottom. Each cell contains the same
drop size distribution since the drops are
uniformly dispersed throughout the cell.
The drops carry the number of their eel I
of origin to permit following their
movements. Column B shows the
position of the drops 10 sec later at the
time of the first exposure of the time-
lapse series. The large drops from cells
1 and 2 have risen out of view, but the
large drop from cell 3 has replaced them
in the top viewing cell. The medium-
sized drop from the viewing cell has
escaped from view, but has been
replaced by an identical drop from cell 2,
the next lower one. The small drop's
movements are minimal and each
remains in its cell of origin.
Columns C and D diagram the drop
position after 10.3 and 12 sec. After
10.3 sec, the large drop from cell 3 is
still in view but is about to escape. This
drop has, however, been photographed,
and its diameter and rate of rise
measured and used to calculate density.
Twelve sec after flow interruption, the
largest drop from cell 3 has escaped, the
medium-sized drop originating in cell 2
and the small drop originating in cell 1
have moved measurable distances.
Thus, although a drop may move out of
the viewing cell, it is replaced by a drop
of the same size from a lower volume,
and the photographs capture a repre-
sentative sample of the oil drops in the
microscope cell. This will occur until
sufficient time has passed (12 sec here)
to allow the largest drop of interest to
escape the viewing cell, even though it
was originally at the bottom of the
microscope cell.
The preceding discussion of drop size
cutoff applies only to the capture of a
drop in a single photograph. Density
measurement requires capturing the
same drop in two photographs of the
photo-triad to provide for measurement
of rise rate. This requirement seriously
decreases the effective vertical height
of the liquid viewing cell because:
1. The camera motor drive limits
successive photographs to 0.3-sec
intervals.
2. During this time a 100-/um drop
rises 495 /urn.
3. Cell height is only 40 m greater
than the rise distance of the
largest drop during 0.3 sec.
Thus, a 100-^m drop has to be in the
bottom 40 /urn of photograph 1 to be at
the top of photograph 2, taken 0.3 sec
later. Therefore, the effective viewing
cell height for a 100-/ym drop is only 40
/urn. When a 50-yum drop is under
consideration, the effective cell height
is 411 //m. Thus, the dynamics of drop
movement as well as photographic
aperture combine to fix the volume of
the viewing cell during density deter-
mination. Understanding this factor is
vital to the calculation of oil content
based on the drop volume and the
viewing cell volume.
There are many possible combina-
tions of vertical position of the viewing
cell within the microscope cell, static
period before photography, and time
between photographs that wil) result in
capturing divergent size and density
entities. For example, if the viewing cell
were positioned near the bottom of the
microscope cell, the system would be
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No. 1
Near top of
microscope
viewing cell.
©
©
No. 2
Near center
of cell.
No 3.
Near bottom
of cell.
©
©
©
©
A
0 Time
B
10 sec.
C
10.3 sec.
D
12 sec.
Figure 9. Drop movement during 12 seconds.
optimized for entities heavier tnan
brine. The photographic timing sequence
has been optimized for oil drops. If the
density measurement feature is elimi-
nated, however, a photograph can be
taken within 0.1 sec of flow interruption
and capture almost all entities in a
single photograph. This is a switchable
option in the electronic circuitry. Such
adaptations must be at the well-con-
sidered discretion of the user.
The full report was submitted in
partial fulfillment of Contract No. 68-
03-2648 by Rockwell International
under the sponsorship of the U.S.
Environmental Protection Agency.
•US GPO:1M2-559-M2-457
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Raymond A. Meyer and Milton Kirsch are with Rockwell International, Environ-
mental Monitoring & Services Center, Newbury Park, CA91320; Fred Howard
is with Esoteric Systems, Inc., Thousand Oaks, CA 91360; and the EPA author
Frank Freestone is with the Municipal Environmental Research Laboratory-
Cincinnati, Edison, NJ 08837.
John S. Farlow is the EPA Project Officer (see below).
The complete report, entitled "Apparatus and Procedure for Determining Oil
Droplet Size Distribution, "(Order No. PB 82-231317; Cost: $ 12.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Oil and Hazardous Materials Spills Branch
Municipal Environmental Research Laboratory-Cincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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