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Le Floch - What are Dispersants?
Photo-oxydation
Air
Water
Evaporation Oil slick
Volatilization
Dissolution Dispersion
Sedimentation
Emulsification
Figure 4.3: Oil weathering processes.
The weathering of oil at sea results in an increasing viscosity, and as a consequence it will
reduce the efficacy of chemical dispersion. However, the speed of these phenomena de
pends directly on the environmental conditions in the zone, i.e. the same oil may remain dis
persible depending on the sea state and on the temperature, as temperature greatly affects
the oil viscosity. Therefore, oil, which is fluid in tropical climates and thus potentially dispersi
ble, may become highly viscous in arctic regions and thus be non-dispersible.
Therefore, after a certain time, floating oil will become non-dispersible due to weathering
processes. In operational terms, this is reflected by a “window of opportunity” for dispersant
use, i.e. strategically speaking the decision whether or not to disperse an oil slick has to be
taken quickly.
Chemical composition of oil
Even though information on the viscosity of the oil is an important parameter for the assess
ment of its dispersibility, this information alone is not sufficient. On two oil types with similar
viscosity, dispersants may show very different efficacy depending on the chemical composi
tion of the oil (Mukherjee et al., 2011). It is commonly accepted that the higher the ratio of
polar compounds in the oil, the more the oil is dispersible, as long as the viscosity does not
exceed 500 cSt. By contrast, paraffin type oils are regarded as having a low dispersion po
tential.
Experimental research
There is no simple rule on how to evaluate the efficacy of dispersants with regard to a given
oil type and given environmental conditions. Therefore, Cedre has developed an experi
mental test chamber in order to conduct efficacy tests in controlled conditions. This tool
(Flume Tank, Figure 4.4) can be used to implement controlled weathering of an oil under
specific conditions (Table 4.3) and to subsequently assess its dispersibility over time in order
to derive the “window of opportunity” (Figure 4.5).
