The stoichiometric
quantity of methanol is the amount needed to convert triclycerides (fats and
oils) into esters (biodiesel) -- the "methyl" portion of methyl
esters.
You also need an excess of methanol to push the conversion process
towards completion -- without the excess the process runs out (reaches
equilibrium) before all the triglycerides are converted to esters, resulting
in poor fuel that doesn't combust well and can be corrosive.
The excess methanol acts more like a catalyst: it encourages the process but
does not become a part of the final product and can be recovered afterwards.
Stoichiometric
quantity
The stoichiometric quantity is
usually said to be 12.5% methanol by volume -- that is, 125 millilitres of
methanol per litre of oil. Some people put it at 13%, or 13.5%, or even as low
as 8%.
In fact it depends on the amounts of the various fatty acids in the oil, and
varies from one oil to another. Biofuel mailing list member Christian Lenoir
figured it out, and here are the results. Christian provided the average
proportions of the different fatty acids in each of the more common fats and
oils, calculated their total molecular weights, and from this was able to
calculate the stoichiometric amount of methanol required to convert them. The
amount varies from 11.3% for rapeseed oil (canola) to 16.3% for coconut oil.
These figures are averages -- fatty acid quantities vary somewhat when oil
crops are grown in different conditions in different parts of the world. But
they're close enough for our purposes, and a lot more accurate than the
general figure of 12.5%.
If you have an analysis of the fatty acid content of your oil, you can
calculate the correct stoichiometric ratio from the figures provided in these
tables.
| Fats
and oils |
Fatty
acids % |
C4:0
Butyric |
C6:0
Caproic |
C8:0
Caprylic |
C10:0
Capric |
C12:0
Lauric |
C14:0
Myristic |
C16:0
Palmitic |
C16:1
Palmitoleic |
| Molecular
wt. |
88 |
116 |
144 |
172 |
200 |
228 |
256 |
254 |
| Tallow |
- |
- |
- |
- |
0.2 |
3 |
27 |
2 |
| Lard |
- |
- |
- |
- |
- |
1 |
26 |
2 |
| Butter |
3.5 |
1.5 |
- |
2.5 |
3 |
11 |
30 |
3.5 |
| Coconut |
- |
- |
8 |
8 |
48 |
16 |
8.5 |
- |
| Palm
kernel |
- |
- |
3 |
5 |
48.5 |
17 |
7.5 |
0.5 |
| Palm |
- |
- |
- |
- |
- |
3.5 |
39.5 |
- |
| Safflower |
- |
- |
- |
- |
- |
- |
5.2 |
- |
| Peanut |
- |
- |
- |
- |
- |
0.5 |
7 |
1.5 |
| Cottonseed |
- |
- |
- |
- |
- |
1.5 |
19 |
- |
| Maize |
- |
- |
- |
- |
- |
1 |
9 |
1.5 |
| Olive |
- |
- |
- |
- |
0.5 |
1 |
13 |
2 |
| Sunflower |
- |
- |
- |
- |
- |
- |
6 |
- |
| Soy |
- |
- |
- |
- |
- |
0.3 |
7.8 |
0.4 |
| Rapeseed/Canola |
- |
- |
- |
- |
- |
- |
3.5 |
0.2 |
| Mustard |
- |
- |
- |
- |
- |
- |
3 |
- |
| Cod
liver oil |
- |
- |
- |
- |
- |
4 |
10 |
14.5 |
| Linseed |
- |
- |
- |
- |
- |
0.2 |
6 |
- |
| Tung |
- |
- |
- |
- |
- |
- |
- |
- |
Fats
and oils
(continued) |
Fatty acids % |
| C18:0 Stearic |
C18:1 Oleic |
C18:2 Linoleic |
C18:3
Linolenic |
C20:0 C22:0
Arachydic - Behenic & others |
Mono-
unsaturated acids <C16:1 |
C20:1 C22:1
Arachidonic - Erucic & others |
| Molecular
wt. |
284 |
282 |
280 |
278 |
326 |
226 |
324 |
| Tallow |
24.1 |
40.7 |
2 |
- |
0.7 |
- |
0.3 |
| Lard |
13 |
45.2 |
10.3 |
- |
- |
- |
2.5 |
| Butter |
12 |
26 |
3 |
- |
1.65 |
1.5 |
0.85 |
| Coconut |
2.5 |
6.5 |
2 |
- |
- |
- |
0.5 |
| Palm
kernel |
2 |
14 |
1 |
- |
1.5 |
- |
- |
| Palm |
3.5 |
46 |
7.5 |
- |
- |
- |
- |
| Safflower |
2.2 |
76.4 |
16.2 |
- |
- |
- |
- |
| Peanut |
4.5 |
52 |
27 |
- |
7.5 |
- |
- |
| Cottonseed |
2 |
31 |
44 |
- |
- |
- |
2.5 |
| Maize |
2.5 |
40 |
45 |
- |
- |
- |
1 |
| Olive |
2 |
68 |
12 |
- |
0.5 |
- |
1 |
| Sunflower |
4.2 |
18.7 |
69.4 |
0.3 |
1.4 |
- |
- |
| Soy |
2.5 |
26 |
51 |
5 |
7 |
- |
- |
| Rapeseed/Canola |
2 |
13.5 |
17 |
7.5 |
0.9 |
- |
56.3 |
| Mustard |
1.5 |
39.5 |
12 |
8 |
- |
- |
36 |
| Cod liver
oil |
0.5 |
28 |
- |
- |
- |
1 |
42 |
| Linseed |
5 |
17.3 |
16 |
55 |
0.5 |
- |
- |
| Tung |
- |
8 |
12 |
80 |
- |
- |
- |
| Oils and
fats |
Total
molecular weight |
Density |
Density
@ 50ºC |
Volume
oil (ml) |
Volume
methanol (ml) |
Stoich.
ratio
methanol : oil % |
| Tallow |
858.54 |
0.895 |
0.88 |
981.18 |
121.52 |
12.4 |
| Lard |
863.73 |
0.92* |
0.9* |
959.7* |
121.52 |
12.7 |
| Butter |
797.64 |
0.91 |
0.89 |
896.73 |
121.52 |
13.6 |
| Coconut |
674.51 |
0.926 |
0.91 |
744.57 |
121.52 |
16.3 |
| Palm
kernel |
704 |
0.912 |
0.89 |
789.33 |
121.52 |
15.4 |
| Palm |
847.28 |
0.923 |
0.9 |
938.29 |
121.52 |
13 |
| Safflower |
879.1 |
0.927 |
0.91 |
966.44 |
121.52 |
12.6 |
| Peanut |
885.02 |
0.919 |
0.9 |
984.45 |
121.52 |
12.3 |
| Cottonseed |
867.38 |
0.918 |
0.9 |
963.76 |
121.52 |
12.6 |
| Maize |
872.81 |
0.923 |
0.9 |
966.57 |
121.52 |
12.6 |
| Olive |
870.65 |
0.923 |
0.9 |
964.17 |
121.52 |
12.6 |
| Sunflower |
877.22 |
0.925 |
0.91 |
969.3 |
121.52 |
12.5 |
| Soy |
882.82 |
0.925 |
0.91 |
975.5 |
121.52 |
12.5 |
| Rapeseed/Canola |
959.04 |
0.914 |
0.89 |
1072.75 |
121.52 |
11.3 |
| Mustard |
925.43 |
0.916 |
0.9 |
1032.85 |
121.52 |
11.8 |
| Cod
liver oil |
908.81 |
0.929 |
0.91 |
1000.34 |
121.52 |
12.1 |
| Linseed |
872.4 |
0.934 |
0.91 |
954.48 |
121.52 |
12.7 |
| Tung |
873.68 |
0.944 |
0.92 |
945.54 |
121.52 |
12.9 |
| *
Approximate |
Excess
Further arguments rage over how
much excess is needed, with figures quoted claiming that this much
will achieve 98% conversion but that much only 95% or less.
It depends on several different factors: the type of oil, its condition, the
type, size and shape of the processor, the type and duration of agitation, the
temperature of the process -- and it doesn't make much sense anyway if the
stoichiometric ratio is wrong in the first place.
However, excess is usually between 60% and 100% of the stoichiometric amount.
So if the stoichiometric ratio of the oil you're using is 12.5%, that is 125
ml of methanol per litre of oil, the excess would range between 75 ml and 125
ml, for a total amount of methanol of 200-250 ml per litre of oil.
Oils with higher stoichiometric ratios seem to need higher excesses. So, for
fresh (virgin, uncooked) soy or canola, you can try 60%, though 67% or more
would be better. For palm kernel or coconut, closer to 100% excess would be
better. For tallow and lard, use higher excesses.
For used oil, WVO -- waste vegetable oil, as it's called, though it often
contains animal fats from the cooking, use 67% minimum excess. For heavily
used oils with high titration levels, use higher excesses, up to 100%.
If you don't know what kind of oil your WVO is, try using 25% methanol -- 250
ml methanol to 1 litre of oil. If you've taken care with the titration, used
accurate measurements and followed the instructions carefully, you should get
a good, clean "split", with esters on top and the glycerine and free
fatty acids cleanly separated at the bottom. If you have trouble washing it,
with a lot of frothing, that could be because the process didn't go far enough
and unconverted material is forming emulsions -- try using more methanol next
time. If everything works well, try using less methanol. You'll soon figure
out what's best for you.
With the "Foolproof"
acid-base two-stage method, don't worry about it, just follow the
instructions.
Ethyl esters
The same principles apply for
making ethyl esters instead of methyl esters, using ethanol rather than
methanol -- with some differences. Use 1.4 times more ethanol than methanol.
It won't work if there's any water in the ethanol or the oil. It works much
better with some methanol added, up to 3:1 ethanol:methanol. Virgin oil is
better -- with waste oil (WVO) it won't work with FFA content much more than
1ml by titration. Experiment -- there's more information here: Ethyl-esters
biodiesel. Please let us know your results.
References
Liberty
Vegetable Oil Company
lists the fatty acid composition of their oils as well as other details such
as the Iodine Value, SG, Flash point etc -- Sweet Almond Oil, Pecan Oil,
English Walnut Oil, Hazelnut Oil, Macadamia Nut Oil, Soybean Oil, Oleic
Sunflower Oil, Canola Oil, Peanut Oil, Sunflower Oil, Corn Oil, Safflower Oil,
Soybean Oil (Non-GMO), High Oleic Oils including Canola and Safflower. http://www.libertyvegetableoil.com/products.html
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