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GULF OIL SPILL BP's Oil Dispersants - itopf

Dispersants have two main components: a surfactant and a solvent. Surfactants molecules are made up of two parts: an oleophilic part (with an attraction to oil) and a hydrophilic part (with an attraction to water). When dispersants are sprayed onto an oil slick, the solvent will transport and distribute the surfactants through the oil slick to the oil/water interface where they arrange themselves so that the oleophilic part of the molecule is in the oil and the hydrophilic part is in the water. This creates a reduction in the surface tension at the oil/water interface and small oil droplets will break away from the oil slick with the help of wave energy. These droplets will be of varying sizes and although the larger ones may rise back to the surface some will remain in suspension and will drift apart and become degraded by naturally occurring bacteria. If dispersion is successful, a characteristic brown plume will spread slowly down from the water surface a few minutes after treatment. Dispersants have little effect on very viscous, floating oils, as they tend to run off the oil into the water before the solvent can penetrate. As a general rule, dispersants are capable of dispersing most liquid oils and emulsions with viscosities of less than 2000 centistokes, equivalent to a medium fuel oil at 10-20?C. They are unsuitable for dealing with viscous emulsions (mousse) or oils which have a pour point near to or above that of the ambient temperature. Even those oils which can be dispersed initially become resistant after a period of time as the viscosity increases as a result of evaporation and emulsification. For a particular oil, the time available before dispersant stops being effective depends upon such factors as sea state and temperature but is unlikely to be longer than a day or two. Dispersants can, however, be more effective with viscous oils on shorelines because the contact time may be prolonged allowing better penetration of the dispersant into the oil. • Type 1 dispersants are based on hydrocarbon solvents with between 15% to 25% surfactant. They are sprayed neat onto the oil as pre-dilution with sea water renders them ineffective. Typical dose rates are between 1:1 and 1:3 (dispersant:oil). • Type 2 dispersants are dilutable concentrate dispersants which are alcohol or glycol (i.e. oxygenated) solvent based with a higher surfactant concentration. Dilution is normally 1:10 with sea water. • Type 3 dispersants are also concentrate dispersants with a similar formulation to type 2 products. However, they are designed to be used neat and typical dose rates are between 1:5 and 1:30 (neat dispersant:oil). Type 1 and 2 dispersants require thorough mixing with the oil after application to produce satisfactory dispersion. With type 3 products, the natural movement of the sea is usually sufficient to achieve this. The lower application rates required with concentrates mean that types 2 and 3 have largely superseded type 1 dispersants for application at sea. - itopf
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