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Triacetin From glycerol

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March 15, 2013, 06:39 AM
imakebiodiesel
Triacetin From glycerol
Ive done quite a bit of AE over the years but with high titration oil. If the oil contains very high levels of FFAs then a very acidic water is produced as a byproduct. As this is heavier than biodiesel it settles out and can be drained off. The biodiesel/veg oil above is only very slightly acidic ( 2,5ml KOH) I assume that any excess sulphuric acid is dissolved in the water and not in the upper layer.
March 17, 2013, 01:08 AM
WesleyB
I think the water wash of pure biodiesel is neutral pH or very close to 7 pH. pH is the concentration of hydrogen ions in water. If it takes 2.5 grams per litre KOH to neutralise the product biodiesel then the biodiesel is acidic. Maybe the acidic character is the result of using sulfuric acid in processing. Maybe the acidic character of the product biodiesel shows at least qualitatively that sulfuric acid is present.
March 17, 2013, 06:52 AM
imakebiodiesel
The veg oil that I start with is very acidic 14-18ml KOH and the aim of the AE is to reduce that figure to 2.5. Biodiesel made from that oil is not acidic, Acid number 0.5 ( equivalent to 7 ph)
March 19, 2013, 02:07 AM
WesleyB
Earlier you wrote "The simplest method is to mix glycerol and acetic acid in a 1:8 ratio, heat to 120C, add sulphuric acid and mix for 4 hours. This yielded about 90% triacetin." I haven't seen the paper this came from, but I expect who ever wrote it might have left something out. I mean, that as the esterification reaction proceeds, water is produced. Water being added pushes the reaction to the left (starting materials side). So, by distilling out the water as it is formed, a more complete reaction can be done. Possibly better results than 90% product might be achieved. Stoiciometrically 1 mole of glycerol reacts with 3 moles of acetic acid + sulphuric acid (catalyst) to yield 1 mole triacetin plus 3 moles of water. Since water is one of the products distilling it out by fractional distillation, will push the reaction to the right (products side). Anyway stoiciometrically one mole glycerol (73 millilitres) + three moles acetic acid (171 millilitres) + sulphuric acid catalyst (enough) yields one mole triacetin (188 millilitres) + water (about 54 millilitres). You get a larger volume of triacetin than the volume of glycerine that you started with. I'm still waiting to get a three neck one litre boiling flask to do this in. It might be a week or a month.
March 19, 2013, 05:06 AM
imakebiodiesel
The article I took that from was a comparison of different methods and their kinetics. So yes they may have left something out and also I might have missed something. The article was copyright so I didnt want to post it here.
Water removal seems to be a key element in the process.
Im going to be very busy in the next few weeks and wont be able to devote any time to this project. I have just completed my latest waste plastic to fuel processor and intend to do at least 2 batches a week for a while to iron out any bugs. This is something my son and I have been working on for 2 years now. In fact we will be in Houston TX in May competing in the ISWEEP science competition with the project.
March 19, 2013, 08:07 AM
WesleyB
You wrote "Water removal seems to be a key element in the process." I think that's right. But there's another consideration when removing water from the still pot by fractional distillation. 1 mole of glycerine reacts with 3 moles of acetic acid + sulphuric acid catalyst to yield 1 mole triacetin plus 3 moles of water. That's what simple stoiciometry indicates. If you can distill out the water as it is formed, by fractional distillation, remember you put in 8 moles acetic acid in an earlier post. There's no indication that I know of, of exactly when to stop the fractional distillation. Distill out about 54 grams-milliliters of water per 1 mole of glycerine then you're done? Well With 5 unreacted moles of acetic acid left in the still pot and sulphuric acid catalyst present, the water keeps boiling off. 2 1/2 moles more of water will boil off or 45 grams-millilitres, leaving acetic anhydride + triacetin + sulphuric acid catalyst in the still pot. Acetic anhydride can be distilled off at a higher temperature. With the acetic anhydride removed, there's still the sulphuric acid catalyst left in the triacetin. I read in a chemical dictionary here that, triacetin was purified by vacuum distillation. Good vacuum distillation equipment is expensive. I think you mentioned vacuum fractional distillation in de-watering acetic acid with the other contents of the still pot, to save energy. I'm not sure how a mechanical vacuum pump will react to acetic acid vapors.
March 24, 2013, 05:26 PM
WesleyB
I've found a scholarly paper that comes up on the internet titled "Chemical Kinetics for Synthesis of Triacetin from Biodiesel Byproduct". It says its in the International Journal of Chemistry, Volume 4, Number 2; April 2012. Maybe you already found it. Thanks
March 24, 2013, 11:11 PM
WesleyB
imakebiodiesel; There's a general chemical principle stated in this paper "Chemical Kinetics for Synthesis of Triacetin from Biodiesel Byproduct", in short it says "taking one of the products so that the equilibrium shifting toward product formation." Maybe you recall I said something like slowly distilling water out of the reaction solution as the water is formed (fractional distillation)? Water is one of the products of the glycerol+ acetic acid + catalyst reaction. If most of the water is removed, that shifts the equilibrium to the right , products side. This paper says the same thing just in a little different way. Also in the paper it seems the glycerol that they used to do the kinetic study was only 93% pure. Why wouldn't they use the pure stuff to do an expensive study? Will wonders never cease? Using the purer stuff as starting material I expect a higher conversion to the desired product than these Indonesian chemists got. They left that part out of the paper. Hello, Is there anybody out there?
March 26, 2013, 02:33 PM
WesleyB
Making the glycerol triacetate proceeds by Fischer-Speier esterification probably. This can be looked up on Wikipedia. It even gives the reaction mechanism, and more. Thanks imakebiodiesel
March 27, 2013, 02:32 PM
WesleyB
Triacetin in contact with water gradually decomposes. If I distil wet triacetin to purify it, at the higher temperature it would decompose more quickly. thanks When I tried to vacuum distil methyl biodiesel, what I got looked like free fatty acids, not liquid biodiesel. The higher heat (170 degrees centigrade about) decomposed the biodiesel somehow.
March 28, 2013, 01:36 PM
WesleyB
By doing a google search for the words; triacetin acetic acid heat hours; I found an exerpt from; A Dictionary of Applied Chemistry; A.C. Geitel (J. pr. Chem. 1897 [ii.]55,417). It describes how a chemist made triacetin without a catalyst to speed up the reaction. I think its worth reading. One thing to note in the paper is that triacetin distils off, without decomposition at 172-172.5 degrees centigrade at a pressure of 40 millimeters Hg.
July 08, 2013, 11:45 PM
WesleyB
Imakebiodiesel; Are you still interested in glycerol triacetate?
October 04, 2013, 01:10 PM
Robert Goodman
Any chance of starting with vegetable oil & an excess of methyl or ethyl acetate and doing a double replacement at each ester linkage to wind up with biodiesel + triacetyl glycerol?

These days as an alternative to partial hydrogenation, margarine makers are mixing triglycerides and getting individual fatty acids to migrate between them. Any idea what conditions they're using?