Most owners / operators of chemical tankers that are able to carry such products will already have procedures in place for the handling of these chemicals.

The most important item is to ensure that the tanks, lines and pumps are completely free of any moisture, purging with nitrogen to a dewpoint of -40C being the most common method of ensuring this requirement.

However, there are cases where moisture has entered the tank, causing polymerisation with dried residues remaining on the tank surfaces.

A common cause is due to water in the fixed tank cleaning system, which when used for the recirculation of PCE, reacted with the MDI with somewhat inevitable results. It would seem that these lines are frequently forgotten and have not been purged before use.

Another, more common cause, is that of an inadequate quantity of solvent being used, resulting in a the failure of the deepwell cargo pump to maintain suction causing a loss of pressure at the tank cleaning machines and the recirculation wash to reach the tank surfaces. This results in significant quantities of MDI to remain on the tank surfaces with the subsequent seawater wash causing these residues to polymerise and form hard residues.

Removal of Polymerised Residues

To remove residues that have not had time to solidify, hand-cleaning with white spirit, or paint thinners, will often give good results.

For large quantities of hardened residues, washing with sulphuric acid would seem to be the only solution, however this would seem to either produce good results, or fail to remove the residues.

Tanks washed with sulphuric acid may require re-passivating, so ensure check are conducted.

Some articles on MDI / PMDI are downloadable via the links below: