Sciencemadness Discussion Board

Biodiesel problems

Nixie - 12-6-2008 at 22:54

I tried using the acid/base two step method at and ended up with what seems a lot of soap. Which is weird, because the point of this method is that it avoids soap...

I need help figuring out what went wrong so I don't waste another batch.

I used 250 mL fresh vegetable oil for a test. Two thirds virgin olive oil and one third canola/sunflower oil--because that's what I had on hand.

Due to the small batch I used an excess of methanol in both stages, as I noticed it evaporating (covering the beaker with a watch glass caused methanol condensation).

I also used an excess of sulfuric acid in the first stage cause 0.25 mL was hard to measure and I erred on the extra side. Maybe as much as 50% more acid. In the second stage I used an excess of (sodium) lye since I had used an excess of acid, and also my lye was old and slightly carbonated. It's possible I had used much too much, but I'd have expected that the first acid stage would have prevented any soapification...

Mixing was with a magnetic stirrer since I had no other way to do it, and thus I couldn't do the heating simultaneously. What I'd do is microwave the oil together with its water bath until it got 5 degrees C warmer than specified, put it back on the stirrer, and when it cooled to 5 degrees C cooler than specified (about 10-15 minutes), repeat the warming. So there was a bunch of temperature swing.

I got about 30 mL of glycerine settling on the bottom, so I thought things were fine.

However, putting a test sample 1:9 in methanol, not all dissolved and some would settle to the bottom of the test tube.

Putting a test sample 1:1 in water, thick white layer forms in between which I read is indicative of either soap or incomplete reaction. Shaking the two together turned into an emulsion that won't separate--meaning it would be impossible to wash the batch. Trying to break the emulsion with vinegar as suggested elsewhere just turned the white stuff to scum, so I'm guessing it's soap and not incomplete reaction... help!

[Edited on 12-6-2008 by Nixie]

YT2095 - 12-6-2008 at 23:18

soap is often down to water in the reaction, either in the Alc or the hydroxide catalyst. not letting the reaction go to 100% completion can also do this when it comes to washing it after.
Mono and Di glycerides present also have a similar effect.

Sometimes you can get your diesel back by adding the glycerine you removed back to the pot, mix well and then let it stand again, but it`s only really effective if it`s not Too badly "wet".

Nixie - 13-6-2008 at 00:30

But how can I know the reaction has reached "100% completion"?

not_important - 13-6-2008 at 00:35

First off, fresh vegetable oils usually have litte FFA content, it's likely that the acid stage is a waste of time. You could titrate the oils to get the FFA level to tell.

An excess of methanol is needed, 2X theoretical is common.

Some of the following has already been mention:

where the reagents dry?

did you let the reaction go on long enough? Ester formation is fairly slow, and you've got little forcing the reaction to the methyl ester side.

Get some sort of a heater - an immersion heater, even a aquarium heater that goes hot enough, and use a water bath on top of the stirrer. Microwave heating is very likely to cause overheating, perhaps even some sulfanation of the unsaturated fatty acids. You're also going to boil off more methanol, hurting the conversion to methyl esters.

Some excess NaOH isn't going to hurt the conversion. In simple base catalysed transesterfication the NaOH would combine with any FFA first, if there's any base let over then it would promote the transesterfication as desired.


batch mode biodiesel is a wasteful a process. It's fun to try, but it consumes NaOH, H3PO4 (or other acid), and in this version H2SO4; plus it usually wastes the excess MeOH needed to force the reaction to methyl esters. It also wastes a lot of water, counter-flow washing is much more effective. Reactive distillation gives a better grade of esters and glycerol, doesn't consume acid and base, uses almost all the methanol putt into the system, and uses very little water.

Nixie - 13-6-2008 at 00:58

The oil was fresh vegetable oil, and clear and no lairs, so I assume it's dry. Unless virgin olive oil has moisture? I know it has lots of solids. The sulfuric acid was 95%. The lye is hygroscopic, but the amount of lye is rather small and it was in a sealed container, so I don't see that much moisture could have gotten in from there. The methanol is unknown. I got it from the hardware store and it's just a high density polyethylene bottle saying methyl hydrate. Could that be a source of water? Should I run it through some calcium sulfate next time? Or maybe other impurities...

I'm going to try next time with used fryer oil from a local store so I don't waste anymore fresh vegetable oil.

not_important - 13-6-2008 at 04:42

Worth following the site's instructions on drying the oil. If using used frying oil you need to filter and wash (and thene dry) it first.

The main thing would seem to be the heating and giving it enough time.

A decent grade of olive oil shouldn't be showing any solids when it's at body temperature.

Nixie - 13-6-2008 at 04:49

Well in my failed batch I used fresh oil. For the next one I'll use used so as not to waste any more food supply.

I understand filter and dry, but how do you wash oil?

Also, is the acid amount on that site right? One milliliter per liter of oil seems pretty tiny...

[Edited on 13-6-2008 by Nixie]

not_important - 13-6-2008 at 05:33

Wash with water, obviously done before drying. May not be needed, some used oil has a lot of water soluble gunk in it that's better washed out than having it eat up catalyst.

The acid is a catalyst, doesn't take much.