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Triacetin From glycerol

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February 27, 2013, 07:16 AM
Triacetin From glycerol
Has anyone here ever tried producing triacetin (Glycerol triacetate) from glycerol. Ive been doing some research on the subject and the process seems to be with in the scope of a home brew outfit.
Triacetin is a valuable fuel additive that improves the cold performance of diesel and also improves the antiknock quality of petrol.
Its has a lot of other uses including a sweet food additive for animal feed.
The process involves reacting glycerol with acetic acid (vinegar) in the presence of a catalyst made from sulphated silica gel.
I would like to have a go at making some but dont want to have to reinvent the wheel so if some one else has already done it and is willing to share please post here.
February 27, 2013, 02:32 PM
An old formula for making aspirin was, salicylic acid+acetic anhydride+ concentrated sulfuric acid catalyst plus heat yields the ester. I think that was the formula. So; maybe glycerine+acetic anhydride+sulfuric acid catalyst plus heat might make your desired chemical on a small scale. But it sounds expensive to me. I didn't find the glycerine + acetic anhydride + H2SO4 catalyst in a book. You might be able to verify it. But there may be a steric hinderance problem with three R-O-H reaction sites close to each other. Thanks
February 28, 2013, 09:31 AM
Thanks Wesley, the reaction is certainly not easy to initiate. The temperature has to be 110C and the catalyst has to be strongly acid.

My idea is to incorporate a reflux still into the design so that methanol can be removed at about 70C first. Then the temp is turned up to 110C and the catalyst introduced. The still would continue to work removing any water produced as a side reaction.
I have several catalyst in mind to try, sulphated charcoal, sulphated silica gel and zeolite. There are several others but I can prepare these 3 myself without any investment in equipment.
Here is a sketch of the reactor based on a stainless steel oildrum with steel pipework.
What do you think?

Triacetin is sold here and in the UK as a cold flow improver for diesel and biodiesel at 14.5GBP per litre so there is no problem about it being expensive to make.
February 28, 2013, 10:47 AM
In your proceedure, when is the acetic acid added to the glycerine + catalyst mix?
February 28, 2013, 12:22 PM
This will depend on how pure the acetic acid is. If it contains water it will have to be dried off before the catalyst is introduced. So my idea is at the moment..

1 dry off methanol from glycerol
2 add acetic acid
3 dry off water from both glycerol and acid.
4 divert circulation flow through the catalyst.
5 continue to dry while reacting.

A light vacuum might speed up the early drying stages and save some energy.

If acetic anhydride is available at a realistic cost then the glycerol would be dried first and then the acetic would be added. We shall see.
February 28, 2013, 01:00 PM
I make small quantities, less than a litre. I make biodiesel in laboratory glassware. I took organic chemistry in college, with some organic research in a university. Anyway, I would suggest making a litre or so of your chemical to be sure your proceedure works, before up-sizing to multiple gallon batches. The lower glycerine layer from biodiesel production also has soap in it , which is a basic pH. If you put a base with an acid(the catalyst) it might be a problem. I was assuming you were starting the proceedure with pure glycerine.
February 28, 2013, 11:47 PM
One point that I noticed, is the purity of the glycerol. If you simply demeth the glycerine layer obtained from biodiesel production, then there's soap in the glycerine. I put potassium soap that I made into vinegar in a small beaker and heated it with a cigarette lighter. I obtained some free fatty acids floating on the surface of the reaction liquid. Acetic acid is a stronger acid than the fatty acids we make biodiesel and soap from. So adding acetic acid to the demethed impure glycerine layer will produce some free fatty acids. I think you should first figure out how to purify the glycerine that you're going to use to make the anti-knock chemical.
March 01, 2013, 05:12 AM
I take your point about doing experiments on a small scale before constructing a full size reactor.
The point of this venture is to make use of the crude glycerol from biodiesel production so that is what we will have to work with. Triacetin made from crude glycerol would probably never be pure enough for the more demanding uses , food supplements,pharma and cosmetics but may be good enough for fuel additives. I do have access to gas chromatography and a mass spectrometer but I want to save that for a possible finished product.
I had thought that we could do a soap titration to the glycerol and then add extra acetic acid to compensate for the soap and residual KOH, in the same ( but opposite) way we do for making biodiesel. That way the correct proportions would be present when the mixture was diverted through the catalyst vessel.
March 01, 2013, 02:07 PM
It might complicate the proceedure to use the impure glycerol. Which brings up a question can we purify the glycerine by product some, in a cost effective manner? Vacuum distillation of the glycerol might be one energy expensive method that would work. Thanks
March 02, 2013, 01:26 AM
imakebiodiesel; You might look at "glycerol data page" on the internet. It looks like at really low pressure that glycerol can be distilled to purify it at a reasonably low temperature. My best vacuum pump only pulls down to about 100 millimeters Hg above zero pressure. A vacuum pump that would pull a vacuum to say 10 millimeters Hg above zero allows glycerol to boil at around/about 167.2 degrees centigrade.
March 02, 2013, 06:25 AM
A vacuum pump and boiler that could withstand such low pressure would probably be very expensive. And distilled glycerine has probably as much value as triacetin. The triacetin I have seen here as fuel additive has a deep yellow colour and may have been made from crude glycerol. I believe pure (pharma grade) triacetin is perfectly clear.
If we can work out what contaminants will cause problems and neutralize them we might be able to avoid expensive purification.
Soap and KOH/NaOH are obvious ones and titration plus additional acetic acid should deal with those.
Water can be boiled off via the still. Is water really a contaminant? Will it interfere with the reaction or simply dilute the end product?
FFAs are likely to be present, how do we deal with them? Would a strongly acidic catalyst convert FFAs into biodiesel in the same way as sulphuric acid does?
There are three possible catalysts that I am considering, sulphated carbon, sulphated silica gel and finally zeolite. It would be great if zeolite worked because it requires no preparation.
I have some journal articles on the subject. Some of them are copyright so I would have to email them to you privately.
March 02, 2013, 04:55 PM
Generally, when water is one of the products of a reaction, then the presence of water in the starting materials can be a problem that hinders production of products. It can shift the equilibrium to the left (the starting materials side of the equation). Aspirin can be made from acetic acid, salicylic acid and concentrated sulfuric acid as catalyst. One of the products of the reaction is water. I put aspirin in distilled water added sulfuric acid heated a while, let cool, filtered out hydrolysed aspirin which is salicylic acid. I hydrolysed the aspirin (ester) on purpose to get some salicylic acid, which I then used to make methyl salicylate (oil of wintergreen). The presence of water in your glycerine will probably hinder and diminish production of triacetin. Adding acetic acid to soap and heating appeared to me to make free fatty acids and potassium acetate. I think the potassium atom moves from the soap molecule to acetic acid a stronger acid than the long chain free fatty acids. So you've got free fatty acids and sodium acetate in your glycerine. I don't know if you can make the triacetin from that mixture as starting material, even if all the water is removed. In my chemical dictionary it says industrially, distillation and Ion Exchange are used to purify glycerine. Maybe there's another way though. Thanks
March 04, 2013, 07:14 AM
I have a very good article examining the kinetics of various different versions of the reaction. The simplest method is to mix glycerol and acetic acid in a 1:8 molar ratio. heat to 120C , add sulphuric acid and mix for 4 hours. This yeilded about 90% triacetin. Of course this was using high purity, anhydrous reactants. Im afraid my organic chemistry is very patchy, how would 1:8 molar ratio translate into volume measurements assuming 100% purity.
March 04, 2013, 09:07 AM
Glycerol to acetic acetic acid in a 1 to 8 molar ratio would be 73 millilitres glycerine to 458 millilitres acetic acid. Which would produce about 188.7 millilitres of triacetin if the reaction went 100%. A small problem after the reaction is you've still got your sulfuric acid catalyst in the mix which you wouldn't want to run through an engine. So how do you remove the sulfuric acid, prior to adding the triacetin to the biodiesel fuel? When doing research it helps to know all the names of the chemical I'm trying to make. In the 1975-1976 CRC Handbook of Chemistry and Physics Triacetin is listed under Glycerol Triacetate, another place it was called 1,2,3-triacetoxypropane. If I'm looking in old or foreign literature for information on a chemical, they might list it under another name, causing me to repeat research that was already done. In making banana oil (ester) the sulfuric acid catalyst was neutralised with sodium bicarbonate. I tried to make benzyl acetate with benzyl alcohol and acetic acid+sulfuric acid catalyst then neutralized the sulfuric acid with sodium bicarbonate. When I was distilling it to purify it, the bicarbonate or carbonate ion in the mix exploded, in a low power explosion, It didn't shatter the apparatus but it made a very nice polymer of some kind that I couldn't get out of my boiling flask, the experiment didn't turn out as I thought it might. Your acetic acid+glycerol + sulfuric acid catalyst reaction also produces 3 moles of water if it goes 100%. 3 moles of water weighs about 54 grams or is 54 millilitres.

This message has been edited. Last edited by: WesleyB,
March 04, 2013, 06:02 PM
I mentioned using acetic anhydride to make triacetin. Later I looked up acetic anhydride toxicity, and the tolerance of it in the air was 5 parts per million. It is nasty stuff. If you use it, avoid breathing the fumes. Thanks
March 05, 2013, 01:09 AM
imakebiodiesel;So I think I can make the triacetin by reacting acetic acid + glycerol + sulfuric acid catalyst + Heat. Does the paper you found saying they got 90% reaction product say how they purified it? I am wondering how they got the sulfuric acid catalyst out of the triacetin product. In the USA ultra low sulfur diesel fuel is sold. I wouldn't want the sulfuric acid in a biodiesel anti-knock additive. There is probably an inexpensive simple way to get it out.
March 05, 2013, 05:37 AM
Water is mentioned as a secondary product of the reaction and I assume this water would be acidic from the sulphuric acid. This may be why solid acid catalysts are preferred by some authors. With these catalysts the triacetin would not be contaminated with sulphuric acid. If you send me your email address I could forward you some articles. They are copyrighted so I dont want to post them on a public forum.
March 05, 2013, 10:42 AM
I try not to give out the Email address I receive messages on. Its my father's not mine. I don't have my own Email address. Anyway, my last organic chemistry professor retired. I sent a message to Dr. Turner PhD chemist asking him how to remove the sulfuric acid catalyst from the product. I am awaiting his reply. He does not always answer. Thanks
March 07, 2013, 12:55 AM
I remember in organic chemistry class the professor said, acetic acid + sulfuric acid + heat produces acetic anhydride plus water. If fractional distillation is used on the reaction solution, I think the water can be removed. Also glycerol + acetic acid + sulfuric acid catalyst + heat makes triacetin + water, fractional distillation might remove most of the water. Acetic acid boils at 117.9 degrees centigrade. Acetic anhydride boils at 139.8 degrees centigrade. Also acetic anhydride + glycerol + sulfuric acid catalyst probably makes triacetin + acetic acid + sulfuric acid catalyst. I've applied for a permit to buy a proper size boiling flask to try to make it (triacetin). The state government is slow to issue permits to buy "precursor apparatus".
March 07, 2013, 04:08 AM
That makes sense, the optimum temperature for the reaction seems to be 120C, just above the boiling temp of acetic acid but well below the boiling temp of acetic anhydride. Seveal of the articles mention that the reaction is exothermic so care must be taken to control the temperature.
I quite understand about your email address. I will try to post it to my drop box and you could access it that way.
It sounds as if it is easier to buy a gun than a boiling flask where you come from. Mind you, I suppose you could do more harm with a boiling flask. "Breaking Bad" is shown here as well.