Vegetable and animal oils

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.

Sticky residues could be created

A negative side effect during the saponification is 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.

Saponification reaction

The chemical reaction that produces soap is called saponification. In this reaction, fat or oil reacts with a strong base (sodium hydroxide or potassium hydroxide) to produce glycerin (glycerol) and soap.

Reaction of fat with Sodium Hydroxide

Fat + NaOH = Glycerol + Hard Soap

This hard soap is more difficult to remove because it will dissolve slowly in water.

Reaction of fat with Potassium Hydroxide

Fat + KOH = Glycerol + Soft Soap

This soft soap (green soap) is less difficult to remove because it will dissolve quickly in water.