Transformer Oil Purification: Dewaxing

Paraffin can be removed from transformer oil in several ways.  The most common methods are removal of solid crystals from the solution at low temperature and removal of n-paraffin hydrocarbons in combination with carbamide.

Dewaxing combines the treatment of oil with a special solvent (a mix of methyl ethyl cetone, benzene and toluene), heating the oil to 25-30 degrees (Celsius) higher than the oil’s cloud point (Wax Appearance Temperature or WAT), cooling to the required temperature and separation of solid hydrocarbons.

Acetone and methyl ethyl cetone solve some paraffin, but almost no oil.  Benzene is added to increase solvent capacity.

A mix of acetone (methyl ethyl cetone) with benzene can solve some paraffin and all liquid oil components at low temperatures.

An alternative way of dewaxing is the use of carbamide (urea) which forms a solid complex with paraffin.  The latter may be filtered and, after treatment with hot water, regenerate the carbamide. Filtered oil must also be treated with hot water to remove traces of urea.

The main advantage of carbamide dewaxing is that the whole process occurs at normal temperature and there is no need to purchase and install cooling systems.

Just like with any with other method however, this method also has its disadvantages.  The first disadvantage is the high melting temperature of isoparaffinic hydrocarbons that contain some methyl and ethyl groups.  Some naphthenic and aromatic hydrocarbons can melt at relatively low temperatures.

At the same time, carbamide treatment removes n-paraffin and isoparaffin hydrocarbons, but only if they contain one ethyl or methyl side chain.  Today the process is mostly used for the treatment of transformer oils made from acid-base oil refining.

The “Cold Dewaxing” process dominates the processing of transformer oils made by phenol purification of some crude oils.

Activated charcoal is different from silica gels, alumina gels and bleaching clays in the way that it can adsorb hydrocarbons with long chains.  It can adsorb just like solid paraffins of normal structure.  It has been suggested to utilize  activated charcoal for oil dewaxing, but this method has not been adopted by the industry or placed into widespread practice.

The main method of transformer oil distillate purification is adsorption.  It can also be the finishing operation in the final purification of oils produced by other methods.  Contact purification is also a possible method.


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