Aleks Kac referred to "backsplitting" of the esters if the acid stage was run too long, apparently judged by loss of yield. While it is possible that the reaction could be reversed if methanol was allowed to evaporate, his remark should be used as a warning that one should be on the lookout for unwanted side reactions.

I recently ran an acid stage monitoring the loss of acidity. The acidity eventually fell lower than I expected. The reaction produces water which must compete with the methanol so will approach an equilibrium. I estimated that this equilibrium would be proportional to the ratio of water and methanol at the end. This may not be quite true of course, but is a starting point. I found the acidity fell lower than the combined estimated FFA and the added acid but was still a little higher than expected from the added acid alone.

The eventual washed and dried yield was 91%, substantialy better than possible with base only with the particular waste oil used, but not as good as one might have expected.

The sulphuric acid is described in the literature as a catalyst in this reaction so should not disappear.

Can anyone offer an explanation for this loss of acidity other than that the acid reacted with the oil?

[This message was edited by neutral on 10 July 2003 at 04:49 AM.]

I know you know the answer already.
Any way the acid reaction does produce water thus diluting the concentration of the acid hence the higher ph.
Another possibility is the acid reacted with the methanol forming an ether.
Even further you know that time is very important on the acid catalised reaction as it will attack triglycerides forming mono and di glycerides.
I am no sure what type of veg. oil you are using and how must FFA is present.
I come across a veg oil with 67% FFA that required over 35% methanol and 1.5% H2SO4 before the reaction could reach any equilibrium.(ph 8)
Water formed in the reaction was diluting the catalyst.Required higher levels of methanol and acid to dihydrate the mixture.
After the second stage using a base catalyst I got about 96% conversation.

[This message was edited by ColinS on 15 July 2003 at 12:55 PM.]

No I don't know the answer.

I agree that the acid will slowly catalyse the transesterification of triglyceride to methyl esters. I also agree that ether formation is possible. These two reaction are like the desired esterification in being true catalytic reactions and none of these three reactions should consume sulphuric acid.

One thing which is very obvious when doing the acid/base reaction in small glassware is that the separation of the layers is very different from base-only. A base-only reaction produces good separation in an hour or two while acid/base takes a day and even then there is not a good separation. The top of the glycerine layer is ill defined and the upper portion is paler indicating a higher proportion of esters trapped there.

The manner of separation is also different. In the base-only reaction the glycerine can be seen forming in globules which settle. Quite early in the settling process the settled layer shows a well defined upper surface which gradually rises as the globules fall and join with it. In the acid/base process when the layers become reasonably defined the interface can be seen moving downward.

In the base-only process we wait for the last of the glycerine to settle down. In the acid/base process we wait, and wait, for the last of the ester to settle up.

The only way you can get such different physical behaviour is to have different chemical compounds present.

Clearly there is every indication that one or more side reactions are occurring in the acid stage to a significant degree.

Does anyone really know what is happening?

[This message was edited by neutral on 16 July 2003 at 01:12 AM.]

However, I believe that during the acid catalysis stage, the amount of alcohol used affects the phase separation perhaps more than in the base transesterification. I do not keep good notes, but I seem to recall that with a heavy dose of methanol, the glycerol/catalyst mixture can actually separate to the top. It is a matter of SG.
Neutral please indulge me with a bit of your patience, I studied art, not chemistry. My perception is that since the water produced in acid esterification stops the reaction, and since that water is contained in the glycerol layer, if after reaching equilibreum, the glycerol is drained, the acid catalysis process can be employed repeatedly until the FFA content is below 1%, after which the base transesterification process can be administered.

Neutral, As I said at the outset, I do not know what is going on, but I do know some BD research chemists who know what is going on. Although they have declined to join this forum, citing time constraints, if Neutral can contact me via email, I will pass their contact information on to you.
Although they seem quite willing to talk with me, I am sure they would much prefer to communicate with you.
Diff

The Diff

You said: "...with a heavy dose of methanol, the glycerol/catalyst mixture can actually separate to the top. It is a matter of SG."

Yes this is certainly true and it also depends on how long the reaction is run. In the early stages even with a small amount of methanol it will form an upper layer.

"... since that water is contained in the glycerol layer, if after reaching equilibreum, the glycerol is drained, the acid catalysis process can be employed repeatedly until the FFA content is below 1%, after which the base transesterification process can be administered."

I think this is right though I have never tried it. I have only worked with oil up to about 8% FFA and in this situation you can just add a little extra sulphuric to push the reaction further. This works with moderate FFA content presumably because of the dehydrating nature of sulphuric acid.

Draining of the glycerine layer from acid reaction is not very satisfactory as it does not appear to separate as cleanly as base reaction. If you are only dealing with moderate FFA it seems better to just leave it there and use enough caustic to neutralize it in the base stage. This also avoids the need for a messy recovery of methanol from the acid glycerine.

My concern about side reactions remains however. Whether you add extra acid or repeat the process it would seem that side reactions must increase. I would be very interested to know the sulphur content of biodiesel produced with heavy sulphuric treatment.

[This message was edited by neutral on 04 September 2003 at 12:04 AM.]

Some of the grease (I hesitate to call it WVO) that I have collected and experimented with, has a FFA content of 80+%.
Diff

The Diff.
You noticed the methanol on the top layer for your WVO this is also true for any veg. oil. when an amount above 25% of methanol is added it floats above the veg oil or ester formed.
This is more aparent in the acid stage were very little glycerol is formed with the high FFA's.
When high levels of methanol are added to the base reaction again methanol floats to the top.

In the acid reaction the water diluting the methanol increase the SG of methanol hence it be at the bottom layer if the correct amount of methanol is added.

neutral

In the question would it increase suphur content in the biodiesel the answer is no if the biodisel is washed and if base cat. is used after acid cat.
In my original and current tests the sulphur content has being well under the specified allowable standard.

Side reactions do occur with acid catalyst if the temp. is high and long reaction time ie. breaking of double bonds on the FA, forming a sulphur linkage at the double bond site(this could also explain the loss of the acid), reaction with free glycerol forming cyclic compounds(even acrolein can form), reaction of methanol with methanol forming ether, given the right conditions methanol and glycerol will react, and worst of all carbon.

We must not forget the proteins in the veg,oil or WVO under acid conditions they will react also mainly breaking down to smaller molecules and in turn take part in the main reaction path reducing the yield.

In a previous posting I did say that the reaction with an acid cat. must be stopped after a period of time this avoids many side reactions.

IN the base cat. reaction the side reactions are far less but they take place again the type and quatity depend on temp. and time.

Another interesting investigation VU decreases the reaction time in acid cat so does microwaves.

An acid cat. reaction takes 3 to 6 hours with normal heating. When heated in a microwave (750watts) at defrost 10-20 minutes. Warning higher settings ignite the samples.