The stoichiometric quantity of methanol is the amount needed to convert triglycerides (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.
Good process completion depends on several different factors, not just the excess methanol: the type of oil, its condition, the catalyst, the type, size and shape of the processor, the type and duration of agitation, the temperature of the process -- and the arguments don'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 of methanol. So, for fresh (new, uncooked) soy or canola, you can try 60%, though 67% or more would be better. For palm kernel or coconut, try higher excesses.
For used oil, WVO -- waste vegetable oil, as it's called, though it often contains animal fats from the cooking, use 60% 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 and you're having difficulties with it, 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.
Most people use 20% or 22% methanol by volume, 200-220 ml methanol to 1 litre of oil. This usually gives good results. Difficulties with washing and the quality checks are more often due to errors with titration and inaccurate measurements or to poor processing than to not enough methanol.
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. Fresh oil is better -- with used 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
Biofuels at Journey to Forever
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