An oil dispersant is a mixture of emulsifiers and solvents that help break down oil into small droplets after an oil spill. Small droplets are more easily spread throughout the volume of water, and small droplets may be more easily decomposed by microbes. The use of dispersants involves a trade-off between exposing coastal life to the surface of oil and exposing aquatic life to dispersed oil. While drowning oil with dispersant can reduce exposure to marine life on the surface, it increases exposure to animals living underwater, which may be harmed by the toxicity of both oil and dispersant dispersant. Although dispersants reduce the amount of oil landed on land, this allows faster and deeper oil penetration to coastal areas, where it is not readily biodegradable.
Video Oil dispersants
History
Torrey Canyon
In 1967, the Torrey Canyon supertanker leaked oil to the coast of England. The alkylphenol surfactants are primarily used to break down oils, but are shown to be highly toxic in the marine environment; all kinds of marine life are killed. This causes the dispersant reformulation to be more environmentally sensitive. After Torrey Canyon spilled, a new boat spraying system was developed. Reforms then allow more dispersant to be conceived (at higher concentrations) to become aerosols.
Exxon Valdez
Alaska has less than 4,000 gallons of dispersant available at the time of the Exxon Valdez oil spill , and no aircraft are used to dispose of it. The dispersion introduced is relatively ineffective because the wave action is insufficient to mix oil and water, and its use is soon abandoned.
A report by David Kirby for TakePart found that the main component of the Corexit 9527 formulation used during the Exxon Valdez cleansing, 2-butoxyethanol, was identified as "one of the agents that causes the liver, kidneys, lungs, nervous system, and blood disorders between they." a cleaning crew in Alaska after the spill of 1989 Exxon Valdez . "
Initial use (by volume)
Dispersants were applied to a number of oil spills between 1967 and 1989.
Deepwater Horizon
During the Deepwater Horizon oil spill, an estimated 1.84 million gallons of Corexit were used in an effort to reduce the amount of surface oil and reduce the destruction of coastal habitats. BP buys one-third of Corexit's supply in the world as soon as the spill begins. Nearly half (771,000 gallons) of dispersants were applied directly to the wellhead. The main dispersants used are Corexit 9527 and 9500, which are controversial due to toxicity.
In 2012, a study found that Corexit makes oil up to 52 times more toxic than oil alone, and that the dispersant emulsifying effect makes oil droplets more bioavailable to plankton. The Georgia Institute of Technology found that "Mixing oil with dispersant increases the toxicity of the ecosystem" and makes the oil spill in the bay worse.
In 2013, in response to increased laboratory-derived toxicity data, some investigators discussed the checks that should be used when evaluating the extrapolated laboratory test results using procedures that are not fully reliable for environmental assessment. Since then, guidelines have been published that improve the comparability and relevance of oil toxicity tests.
Rena oil spill
Maritime New Zealand uses Corexit 9500 oil dispersant to assist in cleaning process. Dispersers were applied only for a week, after the results proved unconvincing.
Maps Oil dispersants
Theory
Overview
The surfactant reduces the oil-water interface tension, which helps the waves break down the oil into small droplets. The mixture of oil and water is usually unstable, but can be stabilized by the addition of surfactant; This surfactant may prevent the dispersion of dispersed oil droplets. The effectiveness of dispersants depends on oil weathering, ocean energy (waves), water salinity, temperature and oil type. Dispersion is not possible if the oil spreads to a thin layer, because dispersants require a certain thickness to work; otherwise, the dispersing agent will interact with water and oil. More dispersion may be needed if sea energy is low. Water salinity is more important for ionic-surfactant dispersants, because salt filters out electrostatic interactions between molecules. Oil viscosity is another important factor; viscosity can inhibit the migration of dispersant to the oil-water interface and also increase the energy required to cut a drop of slick. Viscosity below 2,000 centipoise is optimal for dispersant. If the viscosity is above 10,000 centipoise, no dispersion is possible.
Requirements
There are five requirements for surfactants successfully spreading oil:
- Dispersant must be on the surface of the oil in the right concentration
- Dispersant must penetrate (mix with) oil
- The surfactant molecule must be oriented to the oil-water interface (hydrophobic in oil and hydrophilic in water)
- The water-oil interface voltage must be lowered (so oil can break).
- Energy must be applied to the mix (for example, by wave)
Effectiveness
The effectiveness of dispersants can be analyzed by the following equation. The area refers to the area under the absorbance/wavelength curve, which is determined using the trapezoidal rule. Absorbance is measured at 340, 370, and 400 nm.
Area = 30 (Abs 340 Abs 370 )/2 30 (Abs 340 Abs 400 )/2 (1)
The effectiveness of dispersant can then be calculated using the equation below.
Effectiveness (%) = Total oil dispersed x 100/(< oil V oil )
- ? oil = oil test density (g/L)
- V oil = oil volume added to test tube (L)
- Total dispersed oil = oil mass x 120mL/30mL
- Mass oil = oil concentration x V DCM
- V DCM = final volume of DCM sample extract water (0.020 L)
- The oil concentration = area is determined by Equation (1)/slope of the calibration curve
The dispersion model
Developing well-constructed models (accounting for variables such as oil types, salinity and surfactants) is needed to select the appropriate dispersant in certain situations. There are two models that integrate the use of dispersants: the Mackay model and the Johansen model. There are several parameters to consider when creating dispersion models, including oil-slippery thickness, advection, resurfacing and wave action. A common problem in dispersant modeling is that they change some of these parameters; surfactant decreases film thickness, increases the amount of diffusion into the water column and increases the number of breaks caused by wave action. This causes more slick oil behavior to be dominated by vertical diffusion rather than horizontal advection.
One equation for oil spill modeling is:
dimana
- h adalah ketebalan minyak-licin
- adalah kecepatan arus lautan di lapisan pencampuran kolom air (dimana campuran minyak dan air bersatu)
- adalah tegangan geser yang didorong angin
- f adalah koefisien gesekan minyak-air
- E adalah perbedaan relatif dalam kepadatan antara minyak dan air
- R adalah laju penyebaran tumpahan
The Mackay model predicts an increased rate of deployment, as the slick becomes thinner in one dimension. Models predict that thin slicks will spread faster than thick slick for several reasons. Thin layers are less effective in reducing waves and other sources of turbidity. In addition, the droplets formed on the dispersion are expected to be smaller in a thin slick and thus easier to dissolve in water. This model also includes:
- Expression for oil droplet diameter
- Temperature dependence of oil movement
- Expression for oil coating
- Calibration based on data from experiment spills
This model is lacking in some areas: does not take into account evaporation, seafloor topography or spill zone geography.
Johansen's model is more complex than the Mackay model. It considers the particles to be in one of three conditions: on the surface, trapped in a water column or vaporized. The empirical model employs a probabilistic variable to determine where the dispersant will move and where it will go after breaking the oil slick. The deviation of each particle is determined by the state of the particle; this means that the particles in the steam state will move farther than the particles on the surface (or below the surface) of the oceans. This model improves on the Mackay model in several key areas, including provisions for:
- Possibility of entrainment - depends on the wind
- Possibility of resurfacing - depending on density, droplet size, submerged time and wind
- Possible evaporation - matched to empirical data
The oil dispersant modeled by Johansen uses a different set of entrainment and resurfacing parameters to be treated than unrefined oil. This allows slippery oil areas to be modeled differently, to better understand how oil spreads along the surface of the water.
Surfactant
Surfactants are classified into four main types, each with different properties and applications: anionic, cationic, nonionic and zwitterionic (or amphoteric). Anionic surfactant is a compound containing anionic polar group. Examples of anionic surfactants include sodium dodecyl sulphate and dioctyl sodium sulfosuccinate. Included in this surfactant class are sodium alkylcarboxylic (soap). Cationic surfactants have properties similar to those of anionic surfactants, except that the surfactant molecule carries a positive charge in the hydrophilic part. Many of these compounds are quaternary ammonium salts, as well as cetrimonium bromide (CTAB). Non-ionic surfactants are uncharged and together with anionic surfactants form the majority of oil-dispersant formulations. The hydrophilic portion of the surfactant contains a polar functional group, such as -OH or -NH. Zwitterionic surfactants are the most expensive, and are used for certain applications. These compounds have positively and negatively charged components. An example of a zwitterionic compound is phosphatidylcholine, which as a lipid is largely insoluble in water.
HLB value
The behavior of the surfactant depends heavily on the hydrophilic-lipophilic balance (HLB) value. HLB is a coding scale from 0 to 20 for non-ionic surfactants, and considers the chemical structure of the surfactant molecule. The zero value corresponds to the most lipophilic and the value 20 is the most hydrophilic to the nonionic surfactant. In general, compounds with HLB between one and four will not mix with water. Compounds with HLB values ​​above 13 will form a clear solution in water. Usually the oil dispersant has a HLB value of 8-18.
Comparative industry formulation
Two different dispersing agent formulations for oil spills, Dispersit and Omni-Clean, are shown below. The main difference between the two is that Omni-Clean uses ionic surfactants and Dispersit uses entirely non-ionic surfactants. Omni-Clean is formulated for little to no environmental toxicity. Dispersit, however, is designed as a competitor with Corexit. Dispersite contains non ionic surfactants, which allow both especially the soluble in oils and especially water-soluble surfactants. The inter-phase surfactant partition allows effective dispersion.
Degradation and toxicity
Both the degradation and the toxicity of the dispersant depend on the chemical selected in the formulation. Compounds that interact too hard with oil dispersants should be tested to ensure that they meet three criteria:
- Must be biodegradable.
- In the presence of oil, they do not have to be exclusively used as a carbon source.
- They should be non-toxic to native bacteria.
Usage methods
Dispersant can be delivered in aerosol form by airplane or boat. Sufficient dispersion with droplets of the right size is required; this can be achieved with an appropriate pumping rate. Droplets larger than 1,000 Âμm are preferred, to ensure they are not blown off. The dispersion ratio to oil is usually 1:20.
References
Source of the article : Wikipedia
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