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Dropout in tank may be due to petrodiesel

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March 12, 2015, 08:30 AM
Johann Cape Town
Dropout in tank may be due to petrodiesel
Diesel generator running on petrodiesel. Diesel may be up to two years old. Generator starts and runs well.

As the fuel level dropped it was topped up with bio but the ratio would have peaked at 50/50. This is on one tank of fuel.

Dipping the lank which was getting low I picked up a heavy dark layer on the bottom of the tank. I siphoned the fuel out and then the dropout which I thought was glycerol. I tried to mix a sample with water and it had no effect. I then used toluene to disolve the dregs in the tank and siphoned it out.

Does anyone have any idea what that layer may be?. I think it may be a reaction between bio and stale diesel but I'm basing that on the fact that it wasn't soluble in water. If it is caused by petrodiesel it may explain some of the horror stories one reads about when problems arise when changing over to bio where layers of gunk appears in tanks and filters.

This is the same bio that I use in all of my vehicles (PD, VE, IDI) without problems. I have no dropout in any fuel tanks and might get a thimble of soap/glycerol in 20,000 km.
March 12, 2015, 11:07 AM
Hi Johann, My immediate thought, as was yours, was that all the junk from dinodiesel was dissolved by the bio and dropped to the bottom. Another thought, the diesel might have a bug seeing as its been standing for so long. Jim.
March 12, 2015, 04:14 PM
Jon Heron
Sounds like bugs to me. Not fun.
See my horror story here;
Good luck!


Simple schematic for a pump and heater control with a high limit
Sensor for the biodiesel/glycerin layer
March 13, 2015, 12:30 AM
Johann Cape Town
Before adding bio, the tank appeared clean and the fuel clear. The filter too. The dropout appears to have precipitated from the fuel bulk. The tank is only 10 liters or so and I dip to check the level. I may have missed the dropout before but it was very obvious on my dipstick after the bio. I'll set some petrodiesel aside in an open container and see what happens in a year from now when bio is added. Moisture may also have played a part as it was outside for some time with a loose cap before I acquired it.
It is possible that the gunk was there but never adhered to the dipstick until it became sticky when bio was added.
Jon thanks for the link. My dropout looked more refined and appeared to be plain glycerol with no other crud. Was any bio used in that tank prior to the failure? Toluene (because I have plenty) easily dissolved my mess.
The dropout seems to be an excellent rust proof coating!
March 13, 2015, 02:26 AM
john galt
It could easily be 'all of the above', including microbes, crud from the tank, residual glycerol and soap, and any products from interacting with whatever diesel fuel was in the tank.

March 16, 2015, 12:47 AM
Johann Cape Town
Not sure about glycerol and soap. The dropout wasn't soluble in water and needed a petro solvent to loosen it.
March 16, 2015, 10:20 AM
Johann; I believe you might have oxidized, polymerized, biodiesel. It is not soluble in water, but loosens with petrodiesel liquids. I don't know your level of chemistry education.
Molecules of biodiesel might be imagined as short pieces of rope. Or as a snake with a head. Many biodiesel molecules are about 19 carbons long (incuding the one carbon from methanol). Along the chain of atoms linked together by bonds that are shared electron pairs are also double bonds. In a biodiesel molecule may be none, one, two or three double bonds.
By a complicated mechanism, oxidation occurs, where two lengths of rope, two snakes with heads, two chains of carbon atoms, link to each other, say halfway down the chain, the two molecules bond to each other through one oxygen molecule.
It might be pictured as two lengths of rope 30 centimeters long, parallel to each other with a golf ball between the two lengths of rope. The golf ball is the oxygen atom. The lengths of rope are the carbon chains in the free fatty acids.
One way to imagine polymerization is molecules attach to each other, First 2, then ,3, then another bonds to the growing molecule 4, and so on. Without looking it up, polymers exist that are hundreds of units long, like a freight train.
I expect that the precipitate, solid you've described is a polymer of biodiesel molecules. You can find more and better information by searching the internet using the terms, Biodiesel, Oxidation, Polymerization or Polymerisation, and/or possibly other related words. Thanks