Notes for GL's Biodiesel Quiz #1
True biodiesel can be made from all animal or plant oils and fats (lipids or glycerides), but not from mineral oils derived from crude oil.
Although petroleum oil is derived from fossilised plant and animal material, it has a completely different chemical structure from edible oils, which prevents it from being reacted with methanol and lye, so we can't make biodiesel from it.
Vegetable oil (and animal fat) is made of these components:-
- Three Long chain fatty acid molecules, joined to ...
- One glycerol molecule
The 3 fatty acids are bonded to the glycerol molecule, to make what is known as a tri-glyceride. This is an ester. This is quite thick and viscous, compared to petro diesel or biodiesel. To reduce its thickness, we can strip the fatty acids off the glycerol molecule, using lye, and add a methanol molecule to each fatty acid, to make a methyl ester, or biodiesel. (We have converted one ester to another - this is known as trans-esterification)
The stripped glycerol is much denser and thicker than the biodiesel, so will drop to the bottom under gravity. We can then drain off the glycerol.
If the transesterification has been complete, in other words if all the oil or fat has been converted to biodiesel, no glycerol esters will remain, and we will have 'full conversion' of the oil or fat to biodiesel.
Apart from tri-glycerides, partial conversion may leave di-glycerides and mono-glycerides. Di-glycerides are oil or fat molecules which have 2 fatty acid chains joined to the glycerol molecule, mono-glycerides have only 1 fatty acid chain joined to the glycerol molecule. In other words, we haven't finished stripping off all the fatty acid molecules.
Di-glycerides and mono-glycerides bond well to water and bond well to oils, fats and biodiesel. This is bad for us, because it means they can create emulsions by holding water to biodiesel and fat. In fact margarine is made by mixing oil and water. Mono and di-glycerieds are added to the margarine to stop the oil from floating to the top.
There are two tests which determine how much glycerol remains in biodiesel.
One is called the 'bound-glycerol' test, this looks at how much glycerol is in the form of mono- and di- and tri-glycerides (bound to fatty acid chains), and is a good guide to conversion completeness. Ideally there will be no bound gycerol remaining.
The second test is the 'total glycerol' test. This measures boung glycerol plus free glycerol, that which has been completely stripped of fatty acids. If you subtract the bound glycerol from free glycerol amount, you get an idea of how well washed your biodiesel is.
Free glycerol can drop out to form a solid or gel, which can easily block filters and lower bends in pipework.
Lye's main function is to help strip the long chain fatty acids from the glycerol backbone of the tri-glyceride ester (oil or fat), so that methanol can be joined to the free fatty acids which result. Because the lye simply aids this separation, and does not form part of the resulting biodiesel molecule, it is classed as a catalyst.
If the oil is acidic, because it contains free fatty acids, the lye will first act to neutralise these free fatty acids, and in so doing will form soap and water. This is a wasteful reaction, which produces no biodiesel, it only uses up lye and creates soap and water.
This is why we titrate - we want to know how much free fatty acid exists, so we know how much of our lye will be used up in this initial wasteful reaction. If we don't add extra lye, there won't be enough left to strip the glycerol off, so our reaction will be incomplete.
We need to know how much free fatty acid will wastefully use up our catalyst, as explained above. This allows us to add extra catalyst to make up for this loss.
We measure this using a titration.
When we titrate, we add a known strength of dilute lye to a known amount of our oil, and measure how much of the lye we need to add before the acidic oil is neutralised.
When oil is used in frying, it is heated, and there is usually water present as the food gives off steam. Over time, this action of heat and steam strips off some of the long chain fatty acids from the tri-glyceride (fat / oil) molecule. The longer the oil is used, and the hotter the oil and steam, the more of these fatty acids are freed from the oil or fat. These freed fatty acids are usually known as FFA. We can measure how much FFA is present by doing a titration.
When we make biodiesel from oils or fats, we split off the long chain fatty acids from the glycerol backbone of the oil molecule, which is a tri-glyceride. Some of the lye combines with some of the freed fatty acids to form soap, especially if there is not enough methanol present.
In forming soap, some water is given off. When the glycerol backbone has been stripped of all its long chain fatty acids, it will drop out under gravity. So, in the reaction, we start with tri-glycerides (oils / fats) and conver these to biodiesel, soap, glycerol and water.
We usually use 1 gram of lye in 1 litre of water, which is 1 part per 1000, commonly expressed as 0.1 percent. See here for an in-depth explanation on how to titrate.
There are many substances which can be used as indicators in titrations, to show when a pH change has occurred.
The most commonly stated indicator for oil titrations is phenolphthalein, but other options exist, such as turmeric (see here and here.)
Other pH change indicators include red cabbage and beetroot.
pH indicators in a titration change colour as they go through a certain pH range. They do not tell us the actual pH, because most often they only have 2 colours, one when above the changeover pH, one when below the changeover pH. So if we know at which pH the colour change will occur, and we gradually add an acid or base to change the pH of the material we are titrating , we can detect at which point the pH goes through the changeover point. At that point, we make a note of how much titrant we added.
Color blindness is not a barrier to titrating - you can perform a titration just as easily as someone with standard color vision, because you are looking for a 'change' in color, not an absolute color. See here
Methanol is dangerous.
- It is flammable
- It is poisonous and damages the nervous system
- It can be absorbed through your skin
- It can damage your optic nerve and blind you
- It can be absorbed through your lungs by breathing the vapour
It has a weak smell, by the time you can smell it, the concentration of methanol in air is above the safe working level. There are NO breathing filters available which are practical for filtering out methanol. (Those which do exist are expensive and have a very short service life, not long enough for our processing).
You must ensure that any methanol vapours are vented outside, away from people, and you should use a fresh-air breathing system if your workplace has methanol vapours. Do not use an industrial air compressor for a breathing supply - the oil mist it creates will be damaging to your lungs.
A simple, practical, low cost method is shown here.