What causes the colour of transformer oil to go bright yellow?

[Mebing]

Used transformer oil has high total acidity number (TAN) and is dark coppery in colour. Does lowering the TAN gives oil its bright yellow colour? If yes, what actually gives the oil its colour with respect to acidity? If not, what actually happens when the transformer oil is cleaned via media such as attapulgite? What causes used oil to go from dark coppery colour to bright yellow colour after going through the recycling media? What actually gives the colour of oil? Since it can be used to give a rough idea if the oil has high acidity or low?

Any help or suggestions or any books or journals is very much appreciated, thanks.

 

[pmureiko]

Here are my general observations. The acid is due to the oil oxidizing and forming organic acids.

As far as the dark color, this is due to the presence of oxidized oil compounds. Trace levels of color bodies have a big impact on the overall color. To repeat a phrase, a little color goes a long way.

 

[25362]

Colour is the eye perception of a tiny (400-700 nm) range of wavelenghts (the visible range) of the immensely wide electromagnetic radiation spectrum. White light is in fact a mix of wavelengths. Below and above this range human eyes cannot see and the radiation is invisible. When some of the wavelengths in white light are absorbed we see what is left over as coloured light.

When blue is absorbed we see yellow, when violet is absorbed one sees yellow-green, absorbed cyan lets us see orange. When all of the white light components are absorbed we see black. The opposite is also true.


Molecules containing configurations represented by C, O, S, N, and having alternating double and single bonds, such as benzenic (aromatic) groups -present in transformer oils- have delocalized electrons capable of absorbing light wavelengths, showing colour.

Usual (chromophoric) configurations are : -C=C-, -C=N-, -C=O-, -N=N-, -NO₂ and quinoid rings.

Upon oxidation the resulting R-COOH acidic groups enhance these effects. For this reason they are called auxochromes (colour intensifiers). Among the usual auxochromes we find:-NH₂, -COOH, -HSO₃, and -OH.

Fuller's earths, specially the type containing active attapulgite, a Mg,Al hydrated silicate, have a lot of internal surface, and adsorb these colour bodies -together with some oil- the removal of which results in a clearer nice-looking reclaimed transformer oil.

I suggest you visit the inernet for subjects such as: "colour", "active clays",
"adsorption", "re-refining of lube oils", "transformer oils" and items related to.

I hope you find the above note satisfactory .