x°/EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-199 Oct. 1981
Project Summary
Oil Slick
Dispersal Mechanics
C. A. Osamor and R. C. Ahlert
This study investigates the spread-
ing and dissolution behavior of small
oil slicks formed from spills of 12 oils.
The increases in area covered by the
oils during spreading experiments
were determined using photographic
techniques. Spreading equations
were derived and used to correlate
experimental data. Derivation of the
equations parallels Fay's development.
The rates of dissolution of the oils in
tap water at 25 C were investigated by
equilibrating oils with water in open
static tests. Limits of solubilities were
established for the oils from results of
long-term equilibration in closed
vessels. Six oils were also equilibrated
with salt water. A segmented mathe-
matical model was derived and used to
correlate experimental data. The
model describes two processes that
occur during equilibration: Soluble
and volatile components of oil leach
into solution initially and later evapo-
rate.
Finally, a detailed description was
made of the mass transfer processes
occurring during chemical dispersion
of oil spills. The primary mechanisms
were quantified by analogy to homo-
geneous and heterogeneous catalysis
and detergency. To evaluate the
effectiveness of five commercial
dispersants, a large-scale laboratory
system was designed. Parameters
investigated include oil and dispersant
types, oil-to-dispersant ratio, degree
of agitation, and the effect of salt
water. The results of these evaluations
indicate that the use of dispersants to
control oil slicks must be based on
knowledge of the action of specific
chemicals.
This Project Summary was devel-
oped by EPA's Municipal Environmen-
tal 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
Crude oils and petroleum-based
products are extremely complex systems
and behave differently when discharged
to marine environments. Spills of
chemicals that are less dense than
water are usually marked by slick
formation. Then several natural
processes such as spreading and
dissolution begin to act on the oil and
cause it to disperse. Some processes
operate on certain oil components more
rapidly than others, and there are
numerous interactions between the
processes. The interactions are complex
and poorly understood.
The rate of a specific dispersal
process depends on a combination of
factors such as oil type. To predict the
fates of oils and their effects on the
environment, it is necessary to quantify
the rates of different dispersal processes
for a variety of petroleum-based products
with potential for spillage.
Spreading is one of the most important
mechanisms causing dispersal of crude
oils and petroleum products. The extent
of the surface area of a spreading oil is a
function of time and influences the
rates of other dispersal processes. Thus
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knowledge of how fast oils spread on
water is important to management
decisions to control oil slicks. The rate of
oil dissolution in water is useful for
estimating hydrocarbon concentration
levels in water to which marine orga-
nisms will be exposed. Although some
research has been done on these
dispersal processes, information is
lacking on a variety of oils. An objective
of this study was to investigate the
spreading and dissolution behavior of
nine crude oils and three petroleum-
based products in laboratory experi-
ments. Mathematical models were
developed and used to correlate experi-
mental data.
Although containment and physical
removal are favored methods for con-
trolling oil slicks, they are not practical
in all spill situations. The use of
nontoxic, chemical, surface-active
agents is gaining wide acceptance.
The effectiveness of dispersants and
their toxicity to marine life forms have
been investigated many times, but
laboratory results are' not always
consistent with field data because of
poor test equipment and procedures.,
The effectiveness of five commercial
dispersants was evaluated in this study
during experiments conducted under
controlled conditions in the laboratory.
A major concern was the duplication of
the mixing forces that exist in nature
and field practices during chemical
treatment of oil slicks.
This report identifies the mechanisms
of oil/water/dispersant interactions,
which are poorly understood. A compre-
hensive picture of chemical dispersion
of oil slicks is presented, and mathe-
matical descriptions of several funda-
mental processes are provided by
analogy to partial detergency theory and
catalysis.
Conclusions
Oily discharges to aquatic systems
are usually marked by slick formation.
Surface oil layers that form after spills
have many undesirable impacts on the
environment, such as aesthetic damage
to beaches and shorelines, reduction of
oxygen exchange at the air-water inter-
face, fouling of wild fowl, etc. The
potential for damage by oil spills
depends on the rates of the dispersal
mechanisms and other factors. Spread-
ing and dissolution of oils in the
underlying water are important dispersal
mechanisms, but these rates have not
been quantified for many petroleum-
based systems. In this study, the rates of
dissolution and spreading of 12 oils
were investigated under laboratory
conditions.
Spreading Rates for Oil Spills
Rates of spreading were determined
experimentally by measuring the area
covered and the time of small oil spills
on calm water. Four different volumes
of oils were spilled, and the areas
covered by the slicks were determined
photographically. Because the oils have
different physical and chemical proper-
ties, the variables investigated included
density, viscosity, surface tension, and
interfacial tension between oil and
water. Observations of the configura-
tions of the spreading slicks indicated
that the oils did not spread preferentially
as rectangular, circular, or elliptical
slicks. The shape of a slick generally
varied according to the type of oil spilled,
the rate of discharge, and other factors
such as thermal convection currents
and molecular motions in the water
column.
Mathematical models were derived
for oils spreading on calm water. The
derivation of the spreading equations
follows Fay's work. The principal forces
influencing the spread of oils on calm
water are gravitational, viscous, inertia),
and net surface tension. Gravity ac-
celerates spreading, causing the oil
slick thickness to decrease and the oi> to
spread laterally. Viscous and inertia!
forces retard spreading, but the effect of
the latter appears to be small. The net
surface tension determines whether
spreading is accelerated or retarded.
These forces are related to the physical
properties of the oil and water phases.
By equating an accelerating force to a
retarding force, several spreading
models containing only one empirical
constant can be derived. The models fit
the experimental data with varying
accuracies. The order of goodness of fit,
from best to worst, was generally as
follows: gravity/viscous; surface-ten-
sion/viscous; gravity/inertia; and sur-
face-tension/inertia. The surface-ten-
sion/inertia spreading model is inde-
pendent of the volume spilled; this
equation is not valid for predicting the
area! extent of slicks. The effects of
physical properties of the oil and water
phases can be determined from the
spreading equations. The effect of
temperature on spreading rate was not
investigated, but it can be determined
from indirect influences on the physical
and chemical properties of the oil and
water phases.
Calm conditions do not persist in-
definitely in the field, and ultimately,
transport of gross oil by mechanical
forces is superimposed on natural
spreading. The interactions of wind,
waves, and tides in the presence of oil
slicks cannot be adequately simulated
in che laboratory. The mechanical
transport of oil as a result of these forces
is probably more important than natural
spreading in the overall dispersion of oil
slicks if the damage to a coastline is
considered. But the effects of these
forces on oil slicks are known in general
terms—oil slicks become elongated and
distorted. Usually, the slick breaks into
patches that drift in the direction of the
wind at a speed proportional to the sum
of the vector velocities created by
transport forces. The influence of tides
should be minimal because of the
periodic and oscillatory nature of tidal
movements.
Dissolution Rates
The dissolution rates of the oils were
determined by measuring their solubili-
ties in water during equilibration in
open static tests. The oils were equili-
brated with tap water at 25 C for 2 to 3 A
weeks. Experimental data show in- *
creases in oil concentrations initially
and decreases later during the period of
equilibration. Similar trends were
exhibited by the experimental data
generated by equilibrating six oils with
salt water solution; however, solubilities
were lower in tests with salt water, and
the oils attained maximum solubilities
at slower rates. Saturation data were
determined from long-term, closed-
system experiments. Solubilities vary
for different oils and depend on oil
composition. Organic species in solution
were not characterized, but they are
believed to be low-molecular-weight
hydrocarbon compounds, e.g., aliphatics,
aromatics, and substituted organics.
The experimental data suggest that as
oil slicks equilibrate with water, volatile
hydrocarbon species evaporate into the
atmosphere from the air/oil interface,
and soluble species dissolve into the
underlying column of water from the
oil/water interface. These processes
are not in equilibrium; hydrocarbons
continue to evaporate from solution
after the oil layer has been depleted of
volatile hydrocarbons. This process
occurs even when oil is present in
solution at less than saturation con-
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centrations. A segmented mathematical
model was proposed to quantify the
rates of dissolution for the duration of
the equilibration period. This model
consists of equations for the solution
and evaporation phases. The model was
used to fit experimental data. The
results of the numerical simulation
show that the model fits experimental
data for a majority of the oils fairly well.
The conditions during the dissolution
experiments correspond to an unlikely
worst case of an oil spill in which the oil
completely covers the water surface.
Under these conditions, experimental
data suggest that low concentrations of
oil will persist in the water phase after 2
weeks of equilibration. In the field, the
concentration of oil in the water column
below surface slicks will be influenced
by several factors, such as water
quality. Dissolved organic matter is also
present at varying concentrations in
aquatic systems. Dissolved organic
matter can solubilize organic compounds
and increase oil concentration. Water
movements will have a dilution effect on
concentration, but it may cause oil
droplets to be transported to the under-
lying column of water. Other processes
that disperse and degrade petroleum
operate simultaneously with dissolu-
ion. The concentration of oil in the
aqueous phase will be influenced by the
rates of these mechanisms.
Use of Chemical Dispersants
Management decisions to disperse oi I
spills with chemical dispersants must
be based on knowledge of the action of
commercial preparations and their
toxicity to marine organisms. This study
has identified the mass transfer
processes that lead to the formation and
dispersion of droplets in chemically
treated oil slicks. Mathematical equa-
tions are proposed to quantify the rates
of some of the principal mechanisms.
These equations were not verified with
experimental data, since necessary
input data includes information that is
considered proprietary or cannot be
determined experimentally.
The efficiencies of five commercial
products for dispersing three oils of
varying physical and chemical charac-
teristics were evaluated in a large-scale
laboratory system. The design of the
wave-tank was based on current dis-
persion practices, and the tank permits
spatially distributed sampling. Variables
investigated include oil-to-dispersant
ratio, oil and dispersant types, and the
effects of agitation and sea salts.
Experimental data show that efficiency
increases with the volume of dispersant
added. Oil concentrations decrease
gradually with time after dispersion:
The rate of decrease varied for each
dispersant and oil combination. When
the system is mixed continuously,
agitation causes unstable and stable
droplets to go initially into the aqueous
phase. When agitation ceases, unstable
droplets coalesce and migrate to the
water surface. Oil concentration in the
aqueous phase decreases with time and
finally stabilizes. The dispersants are
classified according to the efficiency of
their action under test conditions by
measuring the quantity of extractable
organic materials in water samples.
The dispersal of spilled oil by applica-
tion of chemical dispersants appears to
be a promising method for cleaning up
oil spills. Proper use of dispersants
could result in the efficient dispersal of
oil even in the absence of wave action. A
gap in knowledge exists: the mechanism
of dispersant action must be better
understood, and the rates of spreading
and dissolution must be established for
a larger variety of crude oils and
petroleum products.
The full report was submitted in
fulfillment of Grant No. R805901 by
Rutgers, The State University of New
Jersey, New Brunswick, New Jersey
08903, under the sponsorship of the
U.S. Environmental Protection Agency.
C. A. Osamor and R. C. Ahlert are with Rutgers, The State University of New
Jersey, New Brunswick, NJ 08903.
Leo McCarthy, Jr., is the EPA Project Officer (see below/.
The complete report, entitled "Oil Slick Dispersal Mechanics," (Order No.
PB 82-105 560; Cost: $24.50, 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
U.S. GOVERNMENT HUNTING OFFICE 1981-559-017/7398
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Environmental Protection
Agency
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Information
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