"Backsplitting" defined in Foolproof Method

A person asked the following question on another Forum:
"I believe I read on the infopop biodiesel forum that too much mixing in the first stage could cause back splitting.'

To which Mr Swearingen replied:
"What you've been led to believe is accurate, up to the point that the maximum amount of conversion has occured. Beyond that there would be some minor flip-flopping or "back-cracking," what most call an equilibrium state with a slight exchange back and forth.
The net gains of prolonged agitation in the acid stage more than counters what minimal number of esters might crack back to FFAs.

Do any chemists have any thoughts on this latest revelation?

Rev Tilly

The questioner on the other forum was clearly worried that stirring for too long would reverse the reaction and reduce yield. It is of course an equilibrium reaction and proceedes ever more slowly toward the equilibrium point. If one wants to be perfectly accurate it never will get there, but one could not tell this by experiment because the changes become so small that they are lost in experimental error.

If there is any loss in yield after very long stirring it will be because some as yet unexplained slow side reaction occurs, not because of "back-splitting".

Personally I think a small amount of side reaction may occur but it is not slow and occurs during the normal reaction time. My reason for thinking this is that the yield appears to be a little lower than expected.

Neutral, thank you for your quick response

I am also curious as to your thoughts about the "minor flip-flopping or "back-cracking.. to FFA's"

Tilly

I've found Alek's explanation of backsplitting

Someone else was confused with the term and asked him what it means.

"Sulphuric acid is a powerfull decomposer. It looks after the 'week link' in organic compounds and splits them there. The ester bond seems to be this kind of week link. So, if acid can split the link between glycerine and a fatty acid it will do the same with a methyl group and a fatty acid. Neutralization is essential in the E/TE process because all my work so far is to eliminate wasting valuable feedstock (I have quite a lot of trouble to get used oils and fat) and simplifying the process. Now, you don't need a degree in chemistry to make nice fuel. By the way, I also have no science degree. I can read, though. All knowledge of the world is written down somewhere,it's just hard to find what you neeed.
Cheers, and thanks for the support.
Aleks"

The person who asked the question then replies

"Thanks Aleks
Most of the micro transestifications seem to be done with huge excesses of alcohol which could explain the lack of concern for "Backsplitting" i guess if there is another free alcohol molecule available it will just go on linking until the splitting stops.
Regards
John"

To which Aleks replies

"I fear it's not that simple. Raction kinetcs of the acid based transesterification is somehow tricky. Huge alc excess can help 'till some grade, but further on it's just time. So, when you neutralize after cca. 1.5 hours of reaction time (speaking oils) you still got the reaction curve in rising (yield reasons). Acid based TE curve almost flattens just about under 50% conversion and there is less methanol available (some has reacted). Now, to speed thing up, the lye takes over, rises the convertion curve again and takes the raction to the end.

Cheers, Aleks"

Rev Tilly KE

This message has been edited. Last edited by: Tilly,

Backsplitting- a possible explanation

My good friend Mickey Mouse has been doing a bit of thinking about this phenominum and would like me to put this idea forward for all the chemists to think about.

"Backsplitting was "discovered" within 2 weeks of Aleks posting his Original "Foolproof" method. To stop backsplitting he decided that the Acid stage should be neutralized and transesterfication started after 2 hours.
Now it is a fact that the titration often goes up between the first hour and the second hour when doing the reaction at 35deg c, typically by 0.1 or 0.2. So, at the end of 2 hrs mixing the titration is often more than the WVO was before the start of mixing and more than at the end of the first hour. This happens over and over at 35deg c..
Also, if you add the H2SO4 to the WVO first, the titration of the oil goes up about 1.4
If you add the methanol to the WVO first and then the acid, the titration goes up by quite a bit less than 1.4
The original Foolproof method had the reaction at 55deg c. As most people were working with open top reactors back then, at the end of 2 hours most of the methanol would be gone through evaporation.

That would mean that if you checked the titration at the end of 2 hours there would be every likelyhood that the titration was the same or higher than when you started.

Also, any methanol left in the reactor would almost certainly evaporate by the next morning which would cause a rise in titration the next morning.
Kevin and Michael did point out that evaporation was a problem and they also pointed out the rise in titration when the oil was allowed to sit over time.
But they were not really concerned at the moment, it was something they would sort out once they got the chemistry sorted out.
When Aleks was dumming-down the Tidball/Fox method he evidently missed those parts of their reports or did not understand their importance.
Now, if Aleks were working with new/lightly used oil there would be a big jump in titration with the Acid Addition.
If he checked it at the end of the 2 hour mix and again in the morning there is every likelyhood the titration would be up in the morning because all the remaining methanol had evaporated.
His answer was to put half the remaining methoxide into the reactor after two hours acid esterfication which killed the esterfication stage while at the same time starting the Base Transesterfication.
Once Aleks reduced the reaction temp to 35deg C in his Update, the backsplitting problem apparently disappeared because he removed the neutralizing/start transesterfication after 2 hours requirement.

MM Legend"

Rev Tilly KE

This message has been edited. Last edited by: Tilly,