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Dispersibility

Cedre applies a standard test protocol to estimate the time window of opportunity for dispersant use and to identify the most efficient dispersants.

In addition to oil in its initial state, for each crude oil 3 samples are analysed representing weathering periods of 6, 36 and 96 hours.

The environmental conditions applied in this simulation are a wind speed of 5 m/s and a temperature of around 20°C but which can be adjusted according to the areas liable to be affected.

In order to estimate the degree of weathering for these conditions, the behaviour of different light crude oils is simulated using the modelling software ADIOS 2, using the parameters outlined above.

A relationship is therefore established between the evaporation rate after 6 and 36 hours and the corresponding distillation fractions. It appears that the fraction which evaporates in the first 6 hours is composed of the molecules distilled below 200°C, while the 200-250°C fraction evaporates during the 6-36 hour period (these observations concur with the conclusions of the DIWO study conducted by IKU/SINTEF (1997)).

Finally, to simulate the most advanced stage of weathering at sea, obtained after around 96 hours, the 250°C residue is photo-oxidised by exposing it to a light intensity equivalent to 4 days in a tropical environment.

Consequently, to simulate evaporation at sea for periods from 6 to 36 hours, the samples are distilled respectively at 200°C and 250°C (vapour temperature).

The sample representing 96 hours at sea is obtained by exposing the 250°C residue to a light intensity of 1,000 W for two days.

 

In terms of the emulsification process, a similar analysis gives respective water contents of 50%, 75% and the maximum water content for the 3 above-mentioned residues.

The efficiency of the dispersant selected for this study is assessed on the crude oil as well as on the weathered samples using the IFP test method. The viscosity of the emulsions is checked at each stage in this study in order to guarantee the reliability of comparisons. According to the results and samples considered, it is possible to define the viscosity limit for dispersant application, together with the time window of opportunity, for each crude oil.

Furthermore, characterisations of the emulsions formed at laboratory scale at a given test temperature can be used as input data for modelling software in order to extrapolate the results and operational conclusions to other environmental conditions.

The test matrix applied for this type of study is presented in Table 2. These tests are performed by applying a dispersant specified by the client, used as a reference for the dispersibility trials, with a Dispersant Oil Ratio (DOR) of 1:20.

The tests are carried out according to the IFP procedure, however the MNS protocol can also be used.

All the tests are performed in duplicate and the deviation between two repetitions of the same trial must not exceed 14% based on the relative difference (deviation between two values relative to their mean).

This criterion is applied for mean efficiency values of over 40%. When the mean value is less than 40%, the absolute difference between the two measurements must be less than 6%. When the deviation between the efficiencies measured exceeds these limits, a third test is carried out.

 

  Table 2: Test matrix for defining the window of opportunity for dispersant use

Sample

 

Water content

Crude

150°C+

≈ 2-3 hours

200°C+

≈ 6 hours

250°C+

≈ 36 hours

250°C+ Photo-ox.

≈ 96 hours

0%

X

 

 

 

 

50%

 

 

X

 

 

75%

 

 

 

X

 

% max.

 

 

 

 

X

 

 

Comparison of dispersant efficiency

 

The viscosity measurements taken for the emulsions, combined with the results obtained during the dispersibility study, can be used to select the most relevant laboratory samples for comparative dispersibility tests.

These samples should, wherever possible, correspond to IFP efficiencies in the ranges 40-50% and 10-20% in order to highlight the differences between dispersants.

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