Using Oil Spill Dispersants on the Sea (1989) / Chapter Skim
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2 Chemistry and Physics of Dispersants and Dispersed Oil
2 Chemistry and Physics of Dispersants and Dispersed Oil
Pages 28-80
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From page 28... ...
Applying chemical dispersants to an of} slick greatly increases the amount of surfactant available and can reduce oil-water interfacial tension to very low values it therefore tales only a small amount of mixing energy to increase the surface area and break the slick into droplets (Figure 2-1~. Dispersants also tend to prevent coalescence of of!
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From page 29... ...
(1985) , API Task Force (1986)
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From page 30... ...
HLB is important in determining the effect of salinity on dispersant performance, since hydrophobic portions of the surfactant molecule tend to be salted out. Laboratory measurements on weathered crude of} with a dispersant sensitive to salinity showed that, at a comparable treatment rate and mixing energy, the amount of oil dispersed is approximately 58 percent in seawater compared to 1 percent in fresh water.
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From page 31... ...
Above the CMC level, there is little change in interfacial tension, and additional surfactant molecules form new micelles. Below the CMC, additional surfactant molecules accumulate at the water-air or oil-water interfaces.
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From page 32... ...
. Light Arabian crude with three dispersant formulations at 28°C aIld 38 percent salinity; interracial tension is measured by the dro~weight method.
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From page 33... ...
33 m ~ m ~ Z ~ to\ ~ ~ '.
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From page 34... ...
waters (Chapter 6~. Matching Dispersant Formulations With Oil Type for Increased Effectiveness Because oils vary widely in composition, it is reasonable to hypothesize that a particular dispersant formulation could be more effective with one of} than another; indeed that a dispersant could
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From page 35... ...
For example, at similar dispersantoil ratios, water salinity, and wave energy, dispersant OSD-1 was found to be 100 percent effective (aD of! was removed from the water surface)
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From page 36... ...
Some work has been done on the rate of breakdown and environmental concentrations of surfactants in the water column (Kozarac et al., 1983; I'acaze, 1973, 1974; Penrose et al., 1976; Una and Garcia, 1983~. Surfactants are also transferred from water to air via sea spray, and increase the production of marine aerosol.
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From page 37... ...
, 1982~. Some investigators believe that using average thickness, although formally consistent, is misleading and obscures one of the most important aspects of an oil slick from the cleanup team's point of view its nonuniformity.
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From page 38... ...
becomes emulsified to form mousse, which occurred during the Amoco Cadiz disaster and many other incidents. Slick Spreading Oil slicks are usually nonuniform in thickness because of the interaction of interfacial tension, gravity, and viscosity in spreading processes, the accumulation of oil at downwehing convergence zones procluced by water movement, and the formation of high-viscosity water-in-oi} emulsions (mousse)
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From page 39... ...
Subsurface release from a well blowout produces a thinner slick than a release directly onto the water surface because of the entrainment of water (and oil) in rising gas bubbles (Fannelop and Sjoen, 1980~.
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From page 40... ...
3. The sea surface was 20 to 50 percent covered by light-brown emulsion oriented in streaks parallel to the wind direction, apparently in the convergence zones of the Langmuir surface circulation.*
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From page 41... ...
is distributed and transported by motion of the water mass in which it resides. Such motion includes drift of the slick caused by wind, tides, and other forces, motion of dispersed oil with the water mass, and redistribution of of} with respect to the water as a result of turbulent diffusion and vertical shear.
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From page 42... ...
Mechanical dispersion occurs primarily when waves break, which requires wind speeds greater than 10 kn. When wave action provides sufficient energy to overcome interfacial tension and create new oil-water interfacial area, an of} slick breaks into small droplets, usually less than 10 Am in diameter depending on slick thickness, that become suspended in the water column.
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From page 43... ...
I\ . 41 _ · tA 1` 1 1 1 1 1 1111 1 1 `.1 1 1111 10° FIGURE 2-5 Droplet size distributions of No.
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From page 44... ...
Concentration = 100 p9/1 FIGURE 2-6 Oil concentrations (pa/liter) in the water column following the Ixtoc I blowout.
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From page 45... ...
19GMT 1 Okm JUNE 30. 1 2GMT FIGURE 2-7 Time variation of an oil slick observed by remote sensing during the Halten Bank experiment.
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From page 46... ...
References Horizontal Diffusion New York Bight 5,500 Bering Sea 2,800 Harrison Bay 780 Bering Sea Beaufort Sea Okubo, 1971 Coachman and Charnell, 1979 Wilson et al., 1981 Vertical Diffusion 185 Cline et al., 1982 25 Liu and Leendertee, private communication SOURCE: Pelto et al., 1983. currents.
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From page 47... ...
components in the water column depends on many physical factors, chiefly sea state, which breaks a stick into droplets and during storms can mix the upper layers of water to a depth of 10 m or more. (This vertical distribution has been studied in a number of field experiments described in Chapter 4.)
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From page 48... ...
Biological toxicity of the remaining surface of! or droplets dispersed into the water column should thereby be greatly reduced (Anderson et al., 1974; McAuliffe, 1971, 1974; WeDs and Sprague, 1976~.
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From page 49... ...
Although some oxygenated products have been isolated from samples taken at large of} spins, most predictions are based on smaDscale laboratory experiments, and little or no fieldwork has been done on this process (Overtop et al., 1979, 1980; Payne and McNabb, 1984; Payne and Phillips, 1985~. Mousse Formation The formation of stable water-in-oi} emulsions appears to depend on the simultaneous presence of asphaltenes and paraffins (Bridle et al., 1980; Payne and Phillips, 1985~.
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From page 50... ...
Some of the processes discussed earlier, particularly advection and turbulent diffusion, apply to chemically dispersed oil as wed as untreated oil. However, chemical dispersants can cause major changes in slick-spreading characteristics; droplet formation, stabilization, coalescence, and resurfacing; and adherence of of!
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From page 51... ...
Indeed, under optimum conditions (very low interfacial concentration for some dispersants) , oil-water interfacial tension can be reduced to less than ~ dye/cm, and almost any minor agitation wiD suffice.
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From page 52... ...
Differences in performance of different dispersants with the same of} also occur, despite similarities in some dispersant compositions. An example from laboratory experiments with four dispersants on various crude oils is shown in Table 2-2 (Labofina test, discussed later in this chapter)
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From page 53... ...
Laboratory studies (NRC, 1985) have shown that metahoporphyrins, which are naturally occurring components of crude oils, with some surface-active properties, favor formation of water-inoil emulsions (mousse)
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From page 54... ...
points out that the upper viscosity limit for chemical treatment of oils and water-in-oi} emulsions is specific for different oils. It is therefore not possible to use a general viscosity limit, particularly on water-in-oi} emulsions, where the dispersant has to break the emulsion into oil and free water at the surface before dispersion of the of} into the water column can take place.
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From page 55... ...
at sea for 12 to 24 hr must be considered to determine whether and how long after a spill dispersants can be usefully applied (van Oudenhoven et al., 1983~. According to Cormack, all crude oils are initially amenable to dispersion except those crude oils with high initial viscosities, those that would be solid at seawater temperatures, and petroleum products normally carried in heated cargo tanks.
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From page 56... ...
For of} or mousse with a viscosity of 10,000 cSt, the standard WSL Labofina test gives almost zero effectiveness, but with a contact time of several minutes the effectiveness could
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From page 57... ...
The surfactant lowers the surface tension of the water thereby causing the oil slick to contract in a few seconds. This herding soon subsides and is not important after a few minutes.
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From page 58... ...
Oil droplets can be removed from the water column by combining with sediment and other abiotic particles, or becoming bound to or ingested by biota, such as plankton. Size distribution of dispersed of} droplets in the water is an important measure of dispersant effectiveness (Mackay et al., 1986~.
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From page 59... ...
(1984) summarized the situation by proposing that dispersed oil droplet size be regarded as a major factor in judging dispersant effectiveness.
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From page 60... ...
The concentrations of dispersed oil measured in these tests should be compared with toxic thresholds estimated for the organisms and crude oils reviewed in Chapter 3. For example, during tests off southern California, La Rosa crude was dispersed by aerial application of Corex~t 9527.
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From page 61... ...
Cormack and Nichols (1977) measured concentrations of Ekofisk crude oil chemically dispersed within the first 2 men after spraying from a boat: 16 to 48 ppm at 1 m.
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From page 62... ...
was removed from the water column, and the settling velocity of oiled particulate matter was estimated to be as rapid as 1 m/hr. From 5 to 30 percent of the oil was incorporated in the settled sediment.
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From page 63... ...
State-of-the-art models include some, but not all, of these processes at varying degrees of sophistication; but field or laboratory experiments designed to calibrate or test models usually focus on only one process. Comprehensive models tend to be created in response to a need, such as the following (Mackay, 1986 and private communication)
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From page 64... ...
Weathering processes included in the model are evaporation, dispersion into the water column, dissolution, waterin-oi} emulsification, and slick spreading. Good agreement has been obtained between predicted and observed weathering behavior.
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From page 65... ...
Most of the of} particles rise again to the slick and coalesce there, but smaller droplets diffuse downward and are retained by sedimentation or biologically mediated transport (Mackay et al., 1980b; Sleeter and Butler, 1982~. Breaking Waves Dispersion rate is likely to be a function of slick thickness, oilwater interfacial tension, sea state, and fraction of the sea covered by breaking waves.
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From page 66... ...
Thinner slicks damp turbulence less effectively on the water surface, and fewer breaking waves are affected.
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From page 67... ...
on the water surface into thick and thin slicks, the proportion of oil in each and the amount of dispersant sprayed on each are part of the input data. The influence of chemical dispersants is included in the form of an effectiveness factor, X; that is, the amount of of} dispersed is X times the amount of dispersant applied.
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From page 68... ...
The advantage of chemically dispersed oils, from the physical viewpoint, is that the of] is dispersed into the water column rather than remaining as a surface slick.
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From page 69... ...
enters the water column even in calm weather, is more influenced by vertical diffusion and vertical shear, and is less affected by horizontal advection. Resurfacing of of} in the water column is an important process that greatly complicates attempts to model, conceptually or numerically, the distribution of dispersed oil.
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From page 70... ...
Tank tests with water volumes of 6 to 150 liters, including test vessels agitated using circulated seawater, and tests that employ breaking or nonbreaking waves to generate more realistic turbulent mixing energy. Examples are the EPA test (U.S.
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From page 71... ...
This is another important criterion since larger droplets resurface some time after dispersal in the water column. The volume mean diameter in the MNS test was 14 to 226 ,um depending on the dispersant-to-oi} ratio (DOR)
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From page 72... ...
The nearly infinite capacity of the open ocean for diluting hydrophilic dispersant is not normally accounted for in laboratory tests. Typical oil-t~water ratios are 0.02:1 for the Labofina test, 0.0017:!
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From page 73... ...
is frequently a cause for major errors. Need for Standard Testing Oils A number of investigators (Canevari, 1985; Mackay et al., 1986; Fingas, private communication)
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From page 74... ...
Two primary differences are continuous washout of soluble materials in the IFP test, and greater mixing energy of the Labofina test (500 W/m3 versus 1.5 W/m3) , although the longer run time of the IFP test (1 to 5 hr)
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From page 75... ...
The samples are collected under static conditions, and the results depend on precisely when the samples are collected after mixing stops. Mackay-Nadeau-Steelman Test The MNS test uses a 20-liter closed glass, temperature-controlled tank, with a stream of air blowing tangentially on the water surface to generate reproducible waves and turbulence (Mackay and Szeto, 1981; Mackay et al., 197S, 1984; U.S.
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From page 76... ...
, and six tests can be completed per day by one operator. The disadvantages of the MNS test are that its mixing energy, while reproducible, is difficult to quantify, and wave dampening by the materials under study can affect the results.
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From page 77... ...
A flume, with its large volume, permits low oil-water ratios and greatly reduces wall effects. In a flume, the resurfacing of dispersed of} droplets can be studied, and droplet size distributions in the water column can be measured in the course of the test.
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From page 78... ...
It appeared, however, that of} droplet size may have decreased slightly with some evaporation. Photochem~cal oxidation increased naturally dispersed oil concentrations, with no change in the chemically dispersed concentration.
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From page 79... ...
There is no strong correlation between laboratory and field tests (see Chapter 4~. A simple strategy for screening dispersants is to apply a reliable test such as the rotating flask test or the MNS test (see CONCAWE, 1986~.
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From page 80... ...
For example, it appears that oil composition (as distinct from physical properties) affects dispersibility, but the reason for this is not known.
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