Disperser

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Dispersants are mixtures of substances which are used to accelerate or enable the dispersion of oil spills in water. This removes the oil from the surface of the water and, when used correctly, finely distributes it in a relatively low concentration in the water column and thus makes it available for oil-degrading microorganisms .

composition

Dispersants consist of

As a rule, dispersants nowadays contain at least two surfactants with different HLB values . The solvent either consists of hydrocarbons or is water-based and then usually also contains alcohols , glycols or glycol ethers . The solvent enables handling (further dilution, spraying) and has a major influence on the speed and completeness of the transport of the surfactants into the oil phase. It usually has a decisive share in the ecotoxicity of a dispersant. Stabilizers are necessary to keep the dispersants storable for a long time.

Use and mode of action

Dispersants such as Corexit are applied directly to the oil, a dosage according to the film thickness makes sense. Deployment resources for application are aircraft, ships or helicopters with spray devices or hand sprayers, depending on the extent of the contamination and the location of the deployment site. The spread and thickness of oil spills at sea are recorded with specially equipped surveillance aircraft. During the accident on the Deepwater Horizon drilling rig in 2010, a disperser was used for the first time on an underwater oil leak.

Mechanical energy is also required for the actual dispersion. At sea this is applied by the wave energy , so that a minimum sea swell is required for meaningful use. From ships, however, otter boards can also be used to a limited extent. It is also no longer useful in rough seas, as the natural dispersion takes place sufficiently quickly here. Further application limits are water depth and exchange. Shallow water initially lead to higher oil and dispersant concentrations, so that a higher toxic effect occurs. Where there is insufficient water exchange, all of the oxygen in the water can be used up due to the highly oxygen-consuming oil breakdown. In Europe, these factors are taken into account when planning oil spill measures, but in the event of a disaster, a further assessment of the circumstances of the individual case is always necessary, in which, as a rule, different consequences have to be weighed.

Due to dispersants, an increased concentration of PAHs can occur locally and temporarily , which are then increasingly taken up by some fish species and accumulate in them.

history

The first documented use of dispersers (approx. 70 tons), which were practically still identical to industrial cleaners, took place in 1966 when Anne Mildred Brøvig was damaged by the Cuxhaven Waterways and Shipping Authority. These so-called first-generation dispersants were used for the first and last time on a large scale during the Torrey Canyon disaster off Cornwall in 1967, which released 117,000 tons of crude oil . Out of 14,000 tons of cleaner, 10,000 tons were used against around 14,000 tons of stranded oil. This use of very toxic agents led to extensive poisoning of marine life. The damage from the use was greater than it would have been from the oil. In the 1970s, this led to the development of the second generation of dispersants with the actual specific purpose of combating oil, which was characterized by a greatly reduced toxicity. However, the ratio to the oil when used was still 1: 2 to 1: 3. The third generation dispersers, developed from the late 1970s to the early 1990s, with application ratios of 1:20 to 1:30, enable significantly longer periods of use for seagoing vessels and, finally, for the particularly effective use of aircraft due to the large area coverage. On seagoing ships, water-thinnable agents were initially mixed with seawater in a ratio of 1:10 and then applied in the usual ratio. In the undiluted state, highly viscous agents are applied from aircraft using specially developed spray devices. A successful example of the use of dispersants is the Sea Empress accident in 1996 on the Welsh coast, where, with 446 tons of dispersant, the amount of dispersed crude oil was roughly tripled compared to exclusively natural dispersion, so that of the 72,000 tons released, only about 4,000 were stranded (Approx. 40% of the total amount evaporated, 1500 t were taken up at sea).

Classification and approval

Second generation dispersants are still used as type 1. Type 2 denotes diluted and type 3 undiluted dispersants of the third generation. Most products can now be used either as type 2 or type 3. In most countries, dispersants require official approval for use. In Europe it is required that their toxicity must not exceed that of the pollution to be controlled. In addition, the toxicity of the contamination must not increase significantly through mixing with the dispersant. In addition, the effectiveness must be proven.

Web links

literature

  • Zhengkai Li, Kenneth Lee, Thomas King, Michel C. Boufadel, Albert D. Venosa: Assessment of chemical dispersant effectiveness in a wave tank under regular non-breaking and breaking wave conditions . In: Marine Pollution Bulletin . tape 56 , no. 5 , 2008, p. 903-912 , doi : 10.1016 / j.marpolbul.2008.01.031 .

Individual evidence

  1. ↑ Entry on dispersants. In: Römpp Online . Georg Thieme Verlag, accessed on July 6, 2017.