Evaluation of Mixing Energy in Laboratory Flasks
Used for Dispersant Effectiveness Testing
Vikram J. Kaku
1
; Michel C. Boufadel, M.ASCE
2
; and Albert D. Venosa
3
Abstract: The evaluation of dispersant effectiveness used for oil spills is commonly done using tests conducted in laboratory flasks. The
success of a test relies on replication of the conditions at sea. We used a hot wire anemometer to characterize the turbulence characteristics
in the swirling flask SF and the baffled flask BF, the latter is being considered by the Environmental Protection Agency to replace the
prior. We used the measurements to compute the velocity gradient, G and the energy dissipation rate per unit mass, . The study shows
that the mixing in the BF is more uniformly distributed than that in the SF. Flask average energy dissipation rates in the SF were about
2 orders of magnitude smaller than those in the BF. The sizes of the microscales in the BF were found to be much smaller than that in the
SF. Also, in the BF, the sizes of the microscales approached the size of oil droplets observed at sea 50–400 m, which means that the
turbulence in the BF closely resembles the turbulence occurring at sea during breaking waves. Hence, the BF is preferable for dispersant
testing in the laboratory.
DOI: 10.1061/ASCE0733-93722006132:193
CE Database subject headings: Oil spills; Turbulence; Anemometers; Energy dissipation; Velocity; Time series analysis; Data
collection.
Introduction
The adverse economic and environmental effects of offshore oil
spills are greatest when the oil slick reaches the shoreline. For this
reason, much effort is put on preventing offshore oil spills from
reaching the shoreline. In calm seas, use of skimmers and booms
to collect the oil at sea is the conventional method of cleanup and
recovery. In situ burning is also used in such a situation, but it has
its limitations. In rough seas, skimming or burning the oil is not
effective, and the use of chemical dispersants appears to be the
promising approach for cleanup Naess 1979; Delvigne et al.
1987; NRC 1989; Fingas 2000.
A dispersant is a mixture of surfactants and solvents that
causes the oil slick to break into small droplets in a process
known as dispersion. The term “dispersion” used here is from the
oil literature and is different from the spreading of chemicals due
to the spatial variation of velocity. The generated small oil drop-
lets get transported or transferred into the water column due to
wave action and sea turbulence. They subsequently move away
from the contaminated area due to prevailing currents. They
could eventually adhere to suspended particulate matter and/or
biodegrade.
Dispersion of oil droplets is enhanced by turbulence due to the
mixing energy imposed by waves, especially breaking waves
Delvigne 1993. This means that the artificial dispersion of oil is
a chemico-physical process that depends both on the type of
dispersant/oil pair and on the sea state. Typically, light and heavy
oils are not easily dispersible. In the case of light oils, the formed
droplets have to be very small to overcome buoyancy. Hence, a
high dosage of dispersant is required to cause the formation of
such small droplets. Heavy oils are much more resistant to dis-
persion because their high viscosity prevents the dispersant from
penetrating them, which is a necessary condition to produce dis-
persed oil droplets. The use of dispersants in very calm or very
rough sea is not effective. In very calm seas, the applied dispers-
ant tends to run off the oil and gathers in small pools within the
slick. The use of dispersants in very rough seas might not be
needed because a high degree of dispersion occurs naturally due
to the high energy at sea.
Various field studies and laboratory experiments have been
conducted to evaluate the effectiveness of dispersants under vari-
ous sea conditions. Field studies are accompanied by large experi-
mental uncertainties in the sea; replicates are usually difficult to
achieve due to constantly changing climatic conditions and for
economic reasons. Hence, smaller scale testing is extensively
used to study dispersant effectiveness. Fingas 1991 reports that
there are about 50 different laboratory test methods available for
determining the effectiveness of dispersants on oil. Examples of
commonly used tests include the Swirling flask SF test method
Fingas et al. 1987b, 1991; Clayton et al. 1993; Fingas 2000, the
Warren Spring Laboratory test method Byford and Green 1984;
Martinelli 1984; Lunel 1993; Lunel and Davies 1996; Fingas
2000, and the Exxon dispersant effectiveness test method Nor-
dvik et al. 1993; Fiocco et al. 1999; Canevari et al. 2001.
The SF Fig. 1a test consists of placing a mixture of oil,
1
Research Assistant, Dept. of Mechanical Engineering, Temple Univ.,
1947 N. 12th St., Philadelphia, PA 19122.
2
Assistant Professor, Dept. of Civil & Environmental Engineering,
Temple Univ., 1947 N. 12th St., Philadelphia, PA 19122 corresponding
author. E-mail: boufadel@temple.edu
3
Program Manager, Oil Spill Research Program, U.S. Environmental
Protection Agency, National Risk Management Research Laboratory, 26
W. Martin Luther King Dr., Cincinnati, OH 45268.
Note. Discussion open until June 1, 2006. Separate discussions must
be submitted for individual papers. To extend the closing date by one
month, a written request must be filed with the ASCE Managing Editor.
The manuscript for this paper was submitted for review and possible
publication on November 18, 2003; approved on April 11, 2005. This
paper is part of the Journal of Environmental Engineering, Vol. 132,
No. 1, January 1, 2006. ©ASCE, ISSN 0733-9372/2006/1-93–101/
$25.00.
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