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Chemical Dispersants
Dispersants are a group of chemicals designed to be sprayed onto oil slicks, to accelerate the process of natural dispersion. Spraying dispersants may be the only means of removing oil from the sea surface, particularly when mechanical recovery is not possible. Their use is intended to minimize the damage caused by floating oil, for example to birds or sensitive shorelines. However, in common with all spill response options, the use of dispersants has its limitations and should be carefully controlled. Dispersant use will be dependent upon national regulations governing the use of these products.
How chemical dispersion works?
Natural dispersion of an oil slick occurs when waves and other turbulence at the sea surface cause all or part of the slick to break up into droplets and enter into the water column. The addition of dis
persants is intended to accelerate this process.
Dispersants have two main components, a surfactant and a solvent. Surfactants are molecules which have an affinity for two distinct liquids which do not mix, acting as an interface between them. A part of the surfactant molecule used in dispersants has an attraction to oil (i.e. it is oleophilic) while another part has an attraction for water (i.e. it is hydrophilic). Common washing-up liquid is another example of a product that contains surfactants.
When a dispersant is sprayed onto an oil slick, the interfacial tension between the oil and water is reduced, promoting the formation of finely dispersed oil droplets. These droplets will be of varying sizes and although the larger ones may rise back to the surface some will remain in suspension. If dispersion is successful, a characteristic plume will spread slowly down from the water surface a few minutes after treatment. However, the effective distribution of surfactant throughout the oil is crucial to the success of the process. To achieve the required distribution, most dispersants contain a suitable solvent which allows the dispersant to penetrate into the slick and acts as a carrier for the surfactant.
Limitations
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 10-20ºC. 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.
Types of dispersant
Type 1 dispersants are based on hydrocarbon solvents with between 15 and 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.
Methods of application at sea
Dispersants can be applied to open water by a variety of methods. In general workboats are more suitable for treating minor spills in harbours or confined waters. Large multi-engine planes are best equipped for handling large off-shore spills. Small, single-engine aircraft and helicopters are suitable for treating smaller spills and near shore areas. Regardless of the method used, it must be able to apply the dispersant effectively. In order to minimise losses due to wind drift, a uniform spray pattern of larger droplets, "rain drops", are required rather than a fog or a mist.
Dispersants are a group of chemicals designed to be sprayed onto oil slicks, to accelerate the process of natural dispersion. Spraying dispersants may be the only means of removing oil from the sea surface, particularly when mechanical recovery is not possible. Their use is intended to minimize the damage caused by floating oil, for example to birds or sensitive shorelines. However, in common with all spill response options, the use of dispersants has its limitations and should be carefully controlled. Dispersant use will be dependent upon national regulations governing the use of these products.
How chemical dispersion works?
Natural dispersion of an oil slick occurs when waves and other turbulence at the sea surface cause all or part of the slick to break up into droplets and enter into the water column. The addition of dis
persants is intended to accelerate this process.Dispersants have two main components, a surfactant and a solvent. Surfactants are molecules which have an affinity for two distinct liquids which do not mix, acting as an interface between them. A part of the surfactant molecule used in dispersants has an attraction to oil (i.e. it is oleophilic) while another part has an attraction for water (i.e. it is hydrophilic). Common washing-up liquid is another example of a product that contains surfactants.
When a dispersant is sprayed onto an oil slick, the interfacial tension between the oil and water is reduced, promoting the formation of finely dispersed oil droplets. These droplets will be of varying sizes and although the larger ones may rise back to the surface some will remain in suspension. If dispersion is successful, a characteristic plume will spread slowly down from the water surface a few minutes after treatment. However, the effective distribution of surfactant throughout the oil is crucial to the success of the process. To achieve the required distribution, most dispersants contain a suitable solvent which allows the dispersant to penetrate into the slick and acts as a carrier for the surfactant.
Limitations
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 10-20ºC. 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.
Types of dispersant
Type 1 dispersants are based on hydrocarbon solvents with between 15 and 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.
Methods of application at sea
Dispersants can be applied to open water by a variety of methods. In general workboats are more suitable for treating minor spills in harbours or confined waters. Large multi-engine planes are best equipped for handling large off-shore spills. Small, single-engine aircraft and helicopters are suitable for treating smaller spills and near shore areas. Regardless of the method used, it must be able to apply the dispersant effectively. In order to minimise losses due to wind drift, a uniform spray pattern of larger droplets, "rain drops", are required rather than a fog or a mist.
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