United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-88/023 June 1988
oEPA Project Summary
An Improved Laboratory
Dispersant Effectiveness Test
J. S. Shum
The U.S. Environmental Protection
Agency (EPA) initiated this program
to evaluate an Improved Laboratory
Olspersant Effectiveness Test
(ILDET) which was developed to
replace EPA's Revised Standard
Oispersant Effectiveness Test
(RSDET). The ILDET has an improved
scientific basis and uses more up-
to- date laboratory techniques to
evaluate dispersants. It is also more
precise, easier to carry out, and less
expensive than the existing EPA test
The full report summarizes the
development of the ILDET. The
improved test provides a method to
evaluate dispersant effectiveness in
a physically realistic condition. The
test energy level is dynamically
similar to the small-scale ocean
turbulence responsible for droplet
formation. A preliminary evaluation of
the test was conducted to assess the
various factors that may affect the
precision of the test. The preliminary
evaluation shows a- possible
improvement in precision over the
existing EPA method.
This Project Summary was
developed by the EPA's Hazardous
Waste Engineering Research
Laboratory, Cincinnati, OH, to
announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
This program was initiated to evaluate
the U.S. Environmental Protection
Agency's (EPA) Improved Laboratory
Dispersant Effectiveness Test (ILDET),
which was developed to overcome
certain inadequacies in the previously
described (Federal Register, 1984)
Revised Standard Dispersant
Effectiveness Test (RSDET) The
fundamental physical principles
governing the dispersion process are
generally missing from the RSDET which
also lacks the desired degree of
repeatability and reproducibility.
Laboratory-scale dispersant effective-
ness tests vary in test apparatus design,
method of applying mixing energy,
oil/water volume, dispersant volume,
dispersant application method, and
method of sampling and analysis. Many
of these laboratory-scale tests were
developed to reproduce the physical
appearance of seawave mixing action at
laboratory-scale. The ILDET program
has developed an improved dispersant
effectiveness test method that
incorporates the fundamental principles
governing the process of dispersing oil
from the water surface and more up-
to-date and efficient laboratory
procedures. The full report describes an
Improved Laboratory Disperant
Effectiveness Test (ILDET) and the
results of the initial evaluation tests
The dispersion process and the role of
the dispersant are well known A
chemical dispersant is used to cause
floating oil to disperse permanently from
the water surface. Dispersants reduce the
interfacial tension, which, with fluid
dynamic forces, results in enhanced
droplet formation and dispersion The
increased surface area from droplet
formation accelerates the natural
purification process through
biodegradation, evaporation, and
dissolution. Once the oil droplets are
dispersed into the water column, the
resurfacing process is governed by
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Dispersant Effectiveness
amount of oil permanently removed
amount of oil originally present at the surface
-xlOO
(1)
Stokes' law, which establishes that
smaller droplets stay submerged longer.
It is therefore desirable to create droplets
small enough to stay down in the water
column long enough. We can then
consider the droplets to be effectively or
permanently removed. According to time
and size criteria established through
Stokes' law, a droplet with a diameter of
10 iim has a typical rise time of 51 hr/m.
For all practical purposes, droplets of this
size are permanently dispersed.
In this study, we define dispersant
effectiveness in terms of the amount of
oil permanently removed from the
surface. Specifically, dispersant
effectiveness can be expressed as
shown in Equation 1.
This, in turn, can be related to the ability
of the dispersant to promote the
formation of droplets that are small
enough to stay down long enough to be
considered permanently removed.
Test Design
Method
The dominant force creating fine
droplets in the ocean is turbulence.
Because laboratory simulation of the
entire ocean turbulence spectrum is
unrealistic, the ILDET design addresses
the small-scale turbulence structure that
controls the dynamics of small-droplet
formation.
Turbulent flow theory provides a guide
to identify the controlling parameters
prescribing the energy level associated
with the small-scale turbulence
structure. These parameters are the
macroscale Reynolds number and the
turbulent microscale. The theory,
together with the empirical data for ocean
waves, constitutes the basic design
criteria for the ILDET.
Test Apparatus
Apparatus design follows the test
design criteria. Specifications for the
apparatus and the operating parameters
attempt to provide a test energy level
that is characteristic of small-scale
ocean turbulence. The test apparatus
(Figure 1) consists of a 356-mm (14-
in.) square by 610-mm (24-in.) high
clear plastic tank. The test uses 38.19
liters (10.09 gal) of water and 40 ml of
Prudhoe Bay crude oil. Mixing energy is
provided by a 305-mm (12-in.)
diameter propeller mixer at 210
revolutions per minute. The dispersant
and the oil are premixed and applied with
a syringe onto the turbulent water
surface. Samples are withdrawn from
sampling ports in the side of the tank at a
specified height. Table 1 shows the
essential test parameters and the test
conditions.
Procedures
The test apparatus was set up in the
laboratory, where the ambient air
temperature was maintained at 23 ±
4°C (73 ± 7°F). Water for the test was
prepared in accordance with the EPA
RSDET specification for "synthetic
seawater.'The test tank was filled to a
depth of 305-mm (12-in.) with the test
water at 23 ± 1°C (73 ± 2°F). A line
marked at the outside of the clear plastic
tank indicated the correct water level. A
sample of the test water was then taken
for salinity and pH measurements. The
salinity of the water was adjusted to 25
± 1.5 parts-per-thousand (ppt), with
additional sodium chloride, as needed.
The pH of the water was adjusted to 8.0
± 0.1 with concentrated NaOH of HCI.
Dispersant-oil mixture was prepared
by mixing 50 ml of the test oil with the
dispersant in a 100-ml beaker. The
dispersant volume was determined
according to the desired D/0 ratio for the
test. The volume measurements were
performed using syringes of appropriate
sizes. A magnetic stirrer continuously
mixed the oil and dispersant until the
mixture was ready for addition into the
test tank. When the disperant-oil
mixture was ready, the propeller mixer
was started, the mixer speed was
adjusted by regulating the supply air
pressure, and the dispersant-oil mixture
was added onto the water surface with a
syringe or syringes as appropriate. The
mixture volume was 40 x (1 + D/O) ml.
At the end of the predetermined mixing
time, the mixer was turned off, which
marked the start of the rising time.
Samples were taken at various
sampling times for analysis of the
concentration of the dispersed oil. The
samples were taken with a 50-ml glass
syringe through the septa-sealed
sampling ports The sample volume was
100-ml. Each sample, therefore
consisted of two 50-ml portions.
Two extraction procedures were use
during the test program. At first
modified version of the extractio
procedure for the EPA RSDET was usei
The RSDET uses 25 ml of chloroforn
and the extract is dried by sodiui
sulfate. This method did not recover <
the oil from the samples and resulted
errors in the analyses
A revised extraction procedure we
later developed The revised methc
uses 50 ml of chloroform to increase tr
extraction efficiency and does not us
any salt to dry the extract. The extractic
is performed in a 250-ml separator
funnel in four 12-ml chloroform additic
steps.
The chloroform extract was analyze
for oil concentration by a Baush & Lorr
Spectronic 20 visible-light spectre
photometer. The concentration <
dispersed oil in the sample we
computed as
C x (volume of chloroform used)
(volume of sample}
(2
where C is the concentration of oil in tr
chloroform extract, and Cdo is th
concentration of dispersed oil in tr
sample. The percent of oil dispersed w<
then computed as
ioo
(3
where D is percent of oil dispersed, ar
C100 is the concentration of oil equivale
to 100 percent dispersion.
Results and Discussion
A preliminary evaluation w<
conducted to assess the ILDET desic
and the various factors affecting the te:
Major results and findings from tt
preliminary evaluation tests a
summarized as follows.
Sample extraction procedure w,
found to be an important factor
achieving a more precise measureme
of dispersant effectiveness Most of t!
test data obtained using the fir
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/./ kW Motor •
Clear Plastic Tank •
305-mm Diam. Propeller
Mixer @ 210 rpm ,
Sampling Ports @ 51. 102,
152 & 203 mm Elev. with
22 mm-Diam. Teflon Coated
Silicone Septa ^—^—
610 mm
305 mm
356 mm Square
75 mm
Figure 1. Schematic of the test apparatus.
extraction procedure show excessive
scattering and lack the desired
repeatability. Detailed evaluation of the
extraction procedure shows 10 to 40%
oil loss from the samples due to sorption
by both the sodium sulfate salt and the
filter paper. The revised extraction
procedure improves the test
repeatability. Statistical evaluation of the
replicate test data shows that a precision
(or standard deviation) of ±5 in
dispersant effectiveness measurements
can be expected Figure 2 shows typical
values of percent oil dispersed as a
function of sampling t,ime (or settling
time) for three replicate tests.
The need for Oil Blank Correction
(OBC) and Dispersant Blank Correction
(DBC) were evaluated following the
methods specificed for the RSDET. Test
results using Corexit 9527 show that
there is no contribution from the
dispersant blank test The contribution
from the oil blank test can be quantified
However, OBC is not necessary in
dispersant evaluation tests The field
objective for using a dispersant is to
disperse the floating oil. The
measurement of interest is the total
amount of oil dispersed, not the increase
over natural dispersion
Table 2 summarizes the tests that
were conducted with Corexit 9527. The
results shown are based on sample
analysis using the revised extraction
method without sodium sulfate salt
drying. The samples were taken at
various times after mixing was stopped.
Stokes' law provides the relationship
between settling time and height for oil
droplets of various sizes. Using Stokes'
law, various sampling times and heights
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Table 1. Design Specifications for Improved Laboratory
Dispersant Effectiveness Test
Tank: 356 mm x 356 mm x 610 mm
Water Depth: 305 mm
Water Volume. 38.19 liters
Freeboard: 305 mm
Test Water: Synthetic Sea water at 23 ± 1°C
Test Oil: Prudhoe Bay Crude at 23 t 1°C
Oil/Water Ratio: 1:955
Oil Volume: 40 ml
Slick Thickness: 0.3 mm
Dispersant Application Method. Premixed
Dispersant/Oil Ratio. Variable*
Mixing Method: Propeller Mixer
Mixer Type: INDCO Model AM1 -A
Propeller Size: 305 mm
Speed 210rpm"
Mixing Duration: Variable"
Sampling Height: Variable"
Settling Time' Variable*
Sample Volume. 100 ml
"To be investigated during the evaluation tests.
can be related to a cutoff diameter, DC
This diameter represents the largest oil
droplet size that should be present in the
sample Droplets larger than DC should
float up above the sampling height. At a
sampling height of 51 mm, the
corresponding DC for settling times of 10,
30, 60, and 120 minutes are 35, 20, 15
and 10 nm, respectively Thus, Stokes'
law can be used in dispersant
effectiveness tests to collect samples
that contain oil droplets less than a
specified size. This approach allows
dispersant effectiveness to be expressed
in terms of the amount of oil dispersed
into droplets less than the specified size
The results for tests at D/0 ratio of 0.1
show oil dispersion greater than 100%.
This is physically not true. Later
laboratory experiments with control
samples containing known dispersant
and oil concentrations show that the
presence of dispersant in oil was biasing
the analyses. The samples containing
dispersant and oil average 20% higher
oil analysis than the samples containing
oil only. Although the EPA RSDET's DBC
approach shows no dispersant blank
contribution, some kind of dispersant
correction factor is necessary
The results in Table 2 also show the
effect of varying the test mixing speed
and the test mixing duration. An increase
in either the mixing speed or the mixing
duration increases the distribution of the
smaller droplets and results in an
increase in oil dispersion. Other factors
of interest to the evaluation of the ILDET
that were investigated included:
horizontal homogeneity of the test fluid,
sampling duration, measurement
precisions, and oil loss by adherence to
test apparatus. These data provide a
basis to evaluate sensitivity of the test to
these factors and form a basis for
establishing the various test operating
parameters
Conclusions
Preliminary evaluation of the ILDET
confirms the general usefulness of the
test in evaluating dispersant
effectiveness The ILDET presents
several improvements over other existing
laboratory tests:
• The test has a better scientific basis.
Test design is formulated from a fluid
mechanical consideration of the
dispersion process and empirical data
on ocean turbulence.
•The test uses simpler test procedures
and more up-to-date laboratory
techniques. There is noted
improvement in the precision of
dispersant effectiveness mea-
surement.
•The test apparatus is simpler.
•The test requires less laboratory
space, is portable,and is easy to
perform.
Tests using the ILDET method wer
conducted to evaluate the test apparatus
the procedures, and other factor
affecting the test. Conclusions from th
preliminary evaluation of the ILDET are
•The test can distinguish an effectiv
dispersant from an meffectiv
dispersant.
•The test is reproducible. A precision c
±5% in the overall dispersar
effectiveness measurement can b
achieved with the ILDET procedures.
•Sample extraction is the single mo:
important factor affecting the precisio
of the test method Sodium sulfat
drying causes a negative bias in th
analysis. Extraction without sodiur
sulfate improves the analytical and th
overall measurements
• Mixing duration affects the fin;
dispersion value. The longer the flui
content is mixed, the better
dispersion is produced Thus th
effectiveness measurement vanes wit
the mixing duration.
• Mixing speed also affect
effectiveness measurements A bettf
dispersion is produced at high*
speed.
•The effect of time taken to withdraw
sample varies with the settling tim
After the dispersion has been settle
for 30 minutes, the condition of th
dispersion is relatively stable Errors
the sampling time become le:
critical Because of the rapid chanc
in the dispersion condition during tr
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J£
.
0
130 -
120-
110-
100^
90-
80-
70-
60-
50-
40-
30-
20-
10-
a Test No. 38
+ Test No. 39
« Test No. 40
•k
* ^Ss-s^>°
1 1 i 1 1 1 1 1 1 1 1 1
0 20 40 60 80 100 120
Settling Time (min)
Figure 2. Percent oil dispersed with time (Tests No. 38, 39, & 40). Tests were conducted using Corexit 9527 at D/0 at 0.02.
Mixing during tests was at 210 rpm for 10 minutes.
initial settling period, the samples
taken at 10 minutes are more
susceptible to error.
• Dispersant blank and oil blank
corrections are not necessary in
evaluating dispersant effectiveness.
However, our study noted that the
presence of dispersant-in-oil may
have contributed to the positive biases
in the spectrophotometric analyses.
•The dispersion in the test tank is
horizontally homogeneous. The test
results are, therefore, independent of
the horizontal sampling position.
•The test apparatus design meets the
objectives. The compact design is
easy to set up The amount of oil loss
due to adherence to the propeller is
minimal The construction of the test
tank needs improvement to facilitate
cleaning between tests
The evaluation tests were not able to
establish any correlation between
sampling height and time, and the
droplet size through measurements. This
is partly due to errors caused by the
extraction procedure used initially. No
further measurements were taken to
correlate these parameters after the
extraction procedure was revised due to
project budget constraints.
This study was conducted at the
EPA's Oil and Hazardous Materials
Simulated Environmental Test Tank
(OHMSETT) facility at Leonardo, New
Jersey The full report was submitted in
fulfillment of Contract No. 68-03-3203,
Work Assig'nment No. 117 by Mason &
Hanger- Silas Mason Co., Inc under the
sponsorship of the U.S Environmental
Protection Agency.
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Table 2. Test Results at 51-mm Sampling Height
Test
No.
38
39
40
41
42
43
44
45
46
47
O/O
0.02
0.02
0.02
0.1
0.1
0.1
0.02
0.02
0.02
0.02
Nominal
Mixing
Speed
(rpm)
210
210
210
210
210
210
210
210
210
175
Mixing
Duration
(minute)
10
10
10
10
10
10
30
120
2
10
Percent Oil Dispersed at Time (minute)3
0
102
102
102
109
104
108
104
100
NA
104
10
52
52
46
109
109
108
74
96
67
52
30
43
35
39
109
113
128b
61
87
33
41
60
39
35
35
109
117
115
61
91
37
26
720
35
30
30
113
113
113
56
87
28
26
aTests were performed with Corexit 9527. Percent oil dispersed are gross values without
oil blank and dispersant blank corrections. Calculations based on oil concentration at
100% dispersion equals 937.4 mg/l. Analyses performed with 100-ml samples extracted
with 50 ml of chloroform without sodium sulfate drying.
bSample contamination suspected.
J.S. Shum is with Mason & Hanger-Silas Mason Co., Inc., Leonardo, NJ 07737.
Richard Griffiths is the EPA Project Officer (see below).
The complete report, entitled "An Improved Laboratory Dispersant Effectiveness
Test," (Order No. PB 88-184 8821 AS; Cost: $19.95, 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:
Releases Control Branch
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Edison, NJ 08837
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