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How to determine proper tank cleaning by acid wash onboard chemical tankers ?

The range of chemicals shipped in bulk now has been increased enormously and to maintain product quality proper tank cleaning is essential prior loading cargo.
The acid wash method is used if there is any suspicion that a cargo of aromatics may have been contaminated by a previous oil cargo.

The method is also used as a check that a tank is sufficiently cleaned before loading aromatics. A surface of about one m2 is carefully cleaned with clean cotton and an aromatic, e g toluene, and the liquid wrung out and tested as the cargo sample mentioned above. One can also take rust sediments (uncoated tanks) and dissolve any oil contaminants by means of toluene, which is then tested by this method.

This method is used to determine if petroleum hydrocarbons are present in aromatic compounds. Aromatics (e.g toluene, xylene, benzene) are unaffected by sulphuric acid, but oils and most other contaminants are affected, causing a discolouration. The method is suited for application on board and can be used as a check that proper tank cleaning has been accomplished.

A test tube is filled with 7 cc conc (96 %) sulphuric acid, on top of which is filled 21 cc of the product sample. The tube is then shaken 40 - 50 seconds (150 times) and is then left to settle for a period of time. The colour of the acid in the bottom of the tube is then compared with the standard shades where 0 corresponds to water and 14 to a dark yellow colour.

Finally a difficult question arises: how much of cargo contaminants can be tolerated on the tank wall? Make tests and train your own judgement!


Hydrocarbons in methanol - "water miscibility test"

Methanol is very sensitive to oil contaminants. The following test method uses methanol as a testing medium. The methanol must therefore be guaranteed free from any hydrocarbon contaminants.

About one m2 of the tank wall is carefully washed with cotton and about 40 cc methanol. The methanol is wrung out and put into a test tube. Then 60 cc distilled water is added. The mixture is well shaken and left to settle for 20 min. If the solution remains a clear liquid the tank wall can be considered to be clean, but if the solution has turned milky or only slightly milky it is an indication that the tank should be cleaned again before loading methanol or any other oil-sensitive cargo.

This method should be a standard practice to be carried out by the ship's personnel. The method can, of course, also be used as a check on the cargo upon loading and discharging.



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