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Acid split glycerol's water content?

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September 09, 2014, 07:24 AM
Acid split glycerol's water content?
For a long time now, i have been burning crude glycerol as a fuel, by mixing it lightly with sawdust. The fuel burns well, but there is a slight smell (less than wood smoke, but some people dont like what they dont understand)

So, in light of that, im considering splitting off the salts / FFAs to give me a greater purity of glycerol, and, hopefully, less discernible smell.

I understand that the calorific value will be greatly reduced, once the FFAs are removed, however, that's not of a whole load of concern as its not like im short of glycerol... However, i am concerned that the acid splitting is, essentially, an Acid + Base neutralisation reaction, and will therefore give me wet glycerol.

I have been trying to do some very rough calculations, however, its been years since ive tried this stuff. So far i have -

100ltrs feedstock
20ltrs glycerol (this is a pretty loose figure)
700g naoh (assuming titration of 2)
1190ml phosphoric (1.7ml @ 85% / g of naoh - figures from JTF)

H3PO4 + NaOH > 2 H2O + NaPO2H2

700g NaOh / 40 (gramms per mol) = 17.5 mol


17.5 H3PO4 + 17.5 NaOH > 35 H2O + 17.5 NaPO2H2

1mol H20 @ 25c is 18g / 18ml

35 mol H20 is 630ml / 0.63 litres

Again, its been a long time since ive balanced equations like this, so i can see that the above may all be very wrong, so im wondering if someone with better understanding than myself might look over the above, and tell me if im on the right track, or far from it?

Thank you,
September 09, 2014, 07:43 PM
H3PO4 + 3NaOH---> Na3PO4 + 3H2O trisodium phosphate. This might be the reaction. I believe all three hydrogen in phosphoric acid react with sodium hydroxide to form water. Acid plus base makes salt plus water. I might be able to do the stoiciometric calculation. I'll see.
September 09, 2014, 09:06 PM
Sodium hydroxide neutralises phosphoric acid to produce salt and water. 3 moles of NaOH produces 3 moles of water, so 700 grams of NaOH is 17.5 moles which produces an equimolar amount of water. 17.5 moles of water is 315 grams of water. I think it is at 20 degrees centigrade, grams of water equals millilitres of water, so 700 grams of NaOH produces 315 millilitres of water upon neutralisation. Also a weak acid neutralised by a strong base produces a basic pH aqueous solution (if that matters to you).
September 10, 2014, 05:29 AM
I cant help you with the reaction but I experimented a little last year with splitting glycerol and found the ffa layer (top layer in the 3 way split) a great fuel that would make excellent logs. The glycerol layer has a lot of methanol in it I didnt find it overly wet but I wasn't trying to burn this layer as I had another use for it. IMB did some testing on the ffa layer and here's a link to the results, might be of some use to you.
September 10, 2014, 05:59 AM
I found on various places on the internet information on o-phosphoric acid (H3PO4). If the phosphoric acid has a density of 1.71 grams per millilitre and is 85.5% then the molarity is 14.8 moles per litre, or since 1 mole provides 3 hydrogen upon neutralisation, 44.4 equivalents per litre is a strength of 85.5%. Then 700 grams of sodium hydroxide (100%) would be neutralised by about 394.1 millilitres of 85.5% o-phosphoric acid, to produce trisodium phosphate (Na3PO4). I would think sulfuric acid for car batteries would cost less for making free fatty acids from soap.
September 10, 2014, 01:46 PM
WesleyB Hi,
Would you be able to calculate for me the amount of water liberated during the neutralisation reaction with an oil that say titrates at 10 KOH.
A am trying to compile a chart of the minimum amount of glycerol to add to oil for the "dry" caustic stripping process in order to have the resultant water level below 500ppm.
Thanks in advance.
September 10, 2014, 02:55 PM
A titration of KOH 10 grams per litre to neutralise the free fatty acids present, translates to about 0.178571429 moles of KOH. When this much KOH reacts with the free fatty acids in a neutralisation reaction, about 3.214 millilitres or grams of water is produced per litre of oil.
September 10, 2014, 03:48 PM
Thats Wonderful Wesley thanks very much.
September 15, 2014, 07:05 AM
Thank you for the replies, and im sorry it has taken me time to come back to this.

I used Phosphoric acid in my initial numbers, as that's what i came across data for... : )
Sulfuric would be much more sensible.

Using im also getting (for my theoretical 700g of NaOH) aprox 17.5mol / 315ml of water with H2SO4. Which i expect to be acceptable, given that i generally mix the crude glycerol to sawdust in about a 1:5 (ish) ratio. (i cant remember if all acids would give the same mols of water or not!)

Anyhow, I think that 315ml of water in that sort of volume of fuel is negligible.

One last point of confusion:

The reaction above is taken as being -

H2SO4 + 2 NaOH = 2 H2O + Na2SO4

However, this, surely, is not what is actually happening at all? The base is tied up in the products of the saponification reaction, and not 'floating free' at all. So im thinking the actual equations are more complex, but im guessing, for the above, the extras can just be ignored?

Again, thank you for the assistance.
September 15, 2014, 11:14 PM
If I understand, the question was adding acid to glycerine derived from a single stage base reaction to get products. I did that, if that was the question, even if it wasn't the question I did that one. It has made a four or five layer product in my clear glass storage bottle. One formula that wasn't mentioned yet is action of the strong acid on soap to produce a salt and free fatty acids. Is this what you're studying, in part?
September 16, 2014, 10:48 AM
That was what i was trying to say -

The equations discussed above are neutralization of the total volume of catalyst added.

However, if any saponification at all has happened (which it will have) then its not neutralisation of the total added catalyst. Indeed, im unsure how much catalyst is even left in the crude glycerol, assuming the feedstock was dosed sparingly to begin with.

I realized this in my preceding post - as you say, the soap equation is not the one being discussed (and i understand that it was my initial post, clutching at data from the JTF site, which introduced the issue) however, i was not sure if it would make a difference to the volume of water produced (which is what i am ultimately interested in).

I tried looking at the saponification reaction some more, but have realised i then need to know better what, exactly, the term "free fatty acid" really means...

I understand that this is the start of the process by which some chemists produce methylesters from crude glycerol, but it is not something ive looked into before.
September 17, 2014, 12:03 AM
A free fatty acid is a carboxylic acid with an even number of carbon atoms numbering 4 to 28 carbon atoms in length that is not attached (bonded) to another molecule. It is similar to glacial acetic acid, which is concentrated vinegar, except acetic acid only has 2 carbons in it's chain.
September 18, 2014, 06:27 AM
julianf; I google searched for a news article and found it. "Glycerine, Sulfuric Acid Mix Sparked Blast" Friday, July 17,2009. "The explosion was set off when a worker improperly mixed glycerine and sulfuric acid... the explosion caused extensive damage."
September 21, 2014, 12:29 PM
Methyl esters made from crude glycerol, is not from the glycerol, it is from the soap present in the crude glycerol. I've made free fatty acids from soap before by putting soap into a beaker with muriatic acid in it. I think it was lye soap, so the sodium bonded with the chlorine in the muriatic acid to make sodium chloride and free fatty acids. My point is that biodiesel is made from fatty acid methyl esters, not glycerol.
September 22, 2014, 06:12 AM
Biodiesel is usually fatty acid methyl esters made from transesterification of triglycerides or esterification of free fatty acids.
September 24, 2014, 06:23 AM
I wouldn't worry about the water content of the glycerol, its an advantage to have up to 20% water in it, I’ll go into that more a bit later on, I’d be more concerned whether you could actually get it to burn without the free fatty acids present despite having sawdust in it. Glycerol requires a very high temperature to get it to combust properly, in itself it contains a relatively low amount of energy compared to FFA or biodiesel. To get complete combustion it needs to be sprayed into an incandescent recirculating combustion chamber in order to get a retention time long enough for it to fully combust. with my burner experiments When running on pure Glycerol the energy contained in glycerol wasn’t sufficient to keep the combustion chamber hot enough for complete combustion and it self-extinguished after 15 minutes of operation without the FFA present. Analysis of the flue gasses showed that dry glycerol burned with a high level of carbon monoxide, Excess air cooled the mixture stream to much, but with the water present the level of CO dropped to barley measurable levels, because the carbon monoxide combined with the oxygen molecules present in the water/steam. Acrolein production wasn't a problem with the burner until the point it self-extinguished. The burner uses a jet that combines compressed air with the heated glycerol under pressure to obtain a mist of glycerol, indecently the mist won’t even ignite with a with a blow touch directed at it, it needs to be sprayed into a chamber with a lot of radiant heat coming off the walls to get it to combust.

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