Does anyone know what makes the scum and what is the scum that developes in the fuel tank upper section ? I think it has something to do with condensation. I would like to know if there is a solvent that will cut it.I notice it most in the old farm tractors that I run B100 in. I do not fill the tanks full but I am now thinking of doing that and seeing if the bio can disolve it.

Don't know for sure, but sounds like it may be tank fungus.

I'm sure an old hand will chime in.

Dave: I haven't seen scum on top of a tank of b100, but then I don't have a diesel tractor. Can you describe it a bit more? Have you taken a sample of it?

Unwashed biodiesel can develop a layer of floating soap as the methanol evaporates. The soap is, er, "biodiesel-phobic". Even though it is more dense than the BD, it won't sink until is it submerged, then it settles to the bottom. I hope this isn't what you're experiencing, I think, because it will clog fuel pickups and sedimenter screens and the like. Tell us a bit more about your biodiesel processing, as well.

I'm sure your fuel system returns some fuel to the tank while the engine is running. There may be something happening to the fuel as it passes through the Injector pump and Injectors that creates the scum you mention. It will have been heated, sheared, exposed to various hot metals, and maybe picked up some air bubbles or generated gas bubbles along the way. This wouldn't be much different from most automotive applications, other than the fact that you can look into the tank, and an auto owner can't. You may have discovered something quite common, but un-observed before. Describe you tractor engine, the injector pump, and how the route taken by the fuel.

Cheers,
JohnO

If your tractors are old (more than 3-4 years), maybe the bio cleaned out, or knocked loose a decade of accumulated petro diesel crud and that's where the scum comes from. If that's true, you might have to change the fuel filter (s) a couple times and clean the fuel tank to get that to go away.

HTH,

troy

I have been making bio for 9 years now and it goes into alot of tractors ( 5 farms)probably 15 tractors. I spray wash then bubble and have been doing this ever since I started , I never put out any bio until it passes my specs. After bubble washing the water is dead clear.
I always tell the farmers to exspect to change their filters often until the tanks are cleaned.
This scum is dark red and hardens a little , it will eventualy start to flake off and drop into the bottom , this has caused some plugging problems recently. The tank looks nice and clean under the fuel surface and the scum seems to get thicker towards the breather in the tank or the filler caps with vents. We usely run B100 all summer then switch to 100% dino in the winter, most use high sulfur dino but not all , some use low sulfur road fuel and the scum developes in both. I scaped off some scum and put it into test tubes. I tryed gasoline and paint thinner with no response to gasoline and slight disolving with paint thinner.

Red? Are you using an additive? Is your dino-diesel dyed? Do you have a storage tank or fuel cans that are coated with red sealant on the inside? Any other thoughts on where red stuff exists in your process?
Do you have the means to test the melting temperature of the scum? Proteins won't "melt" at reasonable temperatures, but high melting point esters, fats, waxes and soaps will. I've had filter goo analyzed at school that had a melting temperature close to 100C. It turned out to be fully converted esters (Linoleic? Linolenic? Something like that), with an unusual double-bond that we believed happened in the fryer. Wierd things can happen, as you've observed.
It may be a mundane substance (soap, for example) that has such a high melting point that room temperature solvents don't do much. We used heptane to dissolve our sample for the GC-MS. It wouldn't dissolve in methanol or 90%IPA or Acetone at the plant.
Oxidized/polymerized oil and biodiesel is often dark red. I've had one occurance of oxidized/polymerized oil (not biodiesel) in a tote that sat nearly empty for over a year. The substance was rubbery, but had hardly any tensile strength. It could be picked up and handled gently, but tore and fell apart even when I tried to preserve it. I got pictures, I'll try posting, if I can find them now. It was not like you describe, other than floating on top. Mine was thick - feeling like it was between 1/8 and 1/4 inch thick, but appearing thin as a heavy coat of paint when laid on a board. It had formed on a thin layer of old oil that sat undisturbed for a very long time, but covered nearly the entire surface. To get to it I filled the tote with water, floating the oil and its covering film into reach. I find it hard to imagine you substance forming the same substance so quickly, especially in a vibrating tractor that gets refilled occasionally. The only exception that comes to mind is if there is some form of polymerization taking place due to conditions inside the fuel system - heat, catalytic metals, and evil spirits. They're always a bad combination. Do NOT use Eye of Newt Esters, especially near Haloween.
I've never seen microbial growth on the top of diesel, always on the bottom, but I'm no expert. Then again I've been working around old farm equipment all my life, and have spent a fair amount of time working at a garage near a marina that cleaned out a lot of contaminated boat tanks. I don't recall anything like you've described, and am anxious to add to the knowledge base.

Cheers,
JohnO

Thanks for the reply JohnO.
I call it red or maybe brown/red. It developes in tanks that have never had red fuel added, it starts out like a flat rubbery paint and slowly gets thicker.I will try to melt some and use different solvents. I like your oxidized/polymerized bio that turned red idea because the fuel has gone through heating proceses and through engine fuel pumps and back to the tank and it seems to develope around open to air vents. I also see maybe the same scum developing around my mist washing hood that sets over my 55 gal. drum when I mist wash all the bio.

See Graydon's biodieselpictures website link for pictures of the polymerized layer found on top of old oil. If this is what is happening to Dave's fuel in his tractors, then something is making it happen much faster than what happened to mine.

An interesting mystery.

Cheers,
JohnO

OK Johno the aliens created a mess for you but what I have doesn't look like that at all.
What I have is much darker and more dark brown then red. It scrapes off like varnish . I tryed to disolve it with methanol but no go. It does however melt, I took a test tube of the stuff and added an inch of bio to it then with a flame I took it up to 120 C and I now have a very dark almost black liquid.
I think the key to this stuff is knowing its a mixture of bio, air, and water. I say that because I see the same thing developing on the inside of my water misting hood that has never seen diesel fuel only bio.

There are some fatty acids with very high melting points. Stearic acid for example melts at 70degC. There are others that are even higher, but I don't have them all on a chart. Hydrogenation increases the melting point even more.

Biodiesel melting point is typically close to the melting point of the fatty acids it was made from. Since all veg oils are blends of just a hand full of fatty acids, each with its own melting point, the resulting esters will also be a blend of different melting points. I mention this because of a hypothesis that what you are seeing is the preferential collection of one very high melting point component. Since it melts with relatively modest temperature and readily dissolves into biodiesel, some things can be ruled out. It's not a protein. It's probably not truly polymerized or oxidized fat or ester. It would seem to be some fraction that dissolves into hot biodiesel. Let's explore that statement a bit - if it's "something" that is a dissolved liquid in biodiesel at 120degC, then might it be "something" that comes out of solution below 120degC? I'm thinking of a saturated solution of "something" in biodiesel. When it cools it becomes super-saturated, and anxious to begin growing on some convenient seed point.
This still doesn't tell me what it is, only how it might behave.

My oil dryer skillet accumulates lots of oxidized and polymerized oil residue around the edges. It does NOT dissolve in hot oil or biodiesel at 120degC. It IS dark red to black, and sort of plastic-flakes that are a bit flexible. Your stuff doesn't sound like my stuff, but then you conditions are also quite different.
We need more ideas, I'm fresh out.
Cheers,
JohnO

OK Johno you think eye of newt will help now?
I like the way you discribed your (stuff) sounds alot like my (stuff) but I agree that they are formed in different ways. I picture in both tractor fuel tanks and my misting hood there are simular things happening. 1 there is biodiesel spashing around. 2 there is cool condensed water on the steel just above the bio. 3 there is oxygen present (may mean nothing). 4 there is alot of time these conditions are like this. I picture the bio starting to form on the condensation and by some chemical or physical action the condensation evaporates and I end up with some form of varnish.
I'm not sure as to how low of a temperature I could go and have this (stuff) melt, I just took it up to 120C and noticed it had melted.