Polymerisation

The initial wash of products that tend to polymerise should be carried out with cold (ambient) water. Washing with hot water may result in polymeric residues being left in tanks and lines, which are very difficult to remove. To avoid any polymerisation by loss of inhibitor-protection it is also recommended that tanks be washed immediately after discharge. More details about polymerisation

Evaporation of volatile substances

Cargoes consisting of mixtures with different vapour pressures should neither be cleaned by evaporation nor prewashed hot. The evaporation of the light substances from a mixture could result in low boiling residues or non-volatile residues, which are very difficult to remove.

Reaction with Water

Isocyanates must never come into contact with water, not even the residues because the reaction product and insoluble urethane (plus CO2) are very difficult to remove. Such products must be washed with a suitable solvent, that does not contain any water.

Reaction with Oxygen

(Drying Products) Drying and semi-drying vegetable and animal oils react with oxygen to form a varnish-like polymeric film. This is very difficult to remove from the bulkheads etc. Heat increases the reaction speed. Therefore the initial washing of these products must be done with water at ambient temperature without any delay after unloading the cargo.

Heated cargoes in adjacent tanks should be avoided. In case of doubt as to whether a vegetable or animal oil is non-drying or drying, always use water with ambient temperature first.

Reaction with water hardness compounds

Water hardness is formed by the calcium and magnesium content of the water. Seawater has a very high water hardness. Some products like fatty acids and vegetable oils with a high free fatty acid content will form white sticky residues (calcium and magnesium soaps) if they are cleaned with water of high water hardness (e.g. sea water). Heat accelerates these reactions. If sea water must be used the initial wash must be done at a temperature of 20º C above the melting point of the product. More details about water hardness

Saponification

Cargo residues which are saponifiable like vegetable and animal oils can be cleaned by using an alkaline cleaner based on sodium- or potassium hydroxide. The saponification product beside glycerin will be a hard soap if sodium hydroxide is used and a soft soap if potassium hydroxide is used. Soft soaps (green soap) have a quicker water solubility than hard soaps and can be easier washed out of the tank thus potassium hydroxide based cleaners are preferable.

A negative side effect during the saponification is the formation of white powdery or sticky residues.

To avoid this undesired reaction the use of fresh water with a low water hardness is mandatory.

Seawater contains a large amount of calcium and magnesium and therefore has a very high water hardness. Thus remove the seawater residues by flushing the tanks with fresh water before any re-circulation cleaning with an alkaline cleaner (saponification) is performed. More details about saponification

Smell

Minor residues of a smell-producing cargo left in lines, valves and pumps (including pump cofferdams) can contaminate a sensitive cargo.

To neutralise the smell of some chemicals (e.g. Acrylate, Nitrobenzene or Pygas) the use of a smell remover may be recommended. Special attention should be given to gaskets in the tank and cleaning hatches. If the next cargo is a sensitive cargo the gaskets should be cleaned or renewed.

A smell killer/remover should preferably be used in combination with an emulsifier. The tanks should be cleaned by recirculation to guarantee that every drop of the smell-producing cargo inside cargo lines, stripping lines, pumps and valve seats are removed.

Drying Oil

Any of several natural oils which, when exposed to the air, oxidise to form a tough, elastic film. The oil dries through oxidation, a chemical reaction between the oxygen in the air and the oil, and not through evaporation. This oxidation process creates a remarkably durable and flexible paint film.

Putrefaction

Most animal and vegetable oils undergo decomposition over time.

Putrefaction (rottening) is a biochemical process that decomposes these oils partly under generation of noxious vapours and depletion of oxygen in the tank. Attention: Putrefaction generates carbon monoxide.