This is a working chemistry primer designed to give you the basic chemistry knowledge you need to understand what is happening when you make biodiesel. It has been written by a non-chemist for non-chemists. You can easily make biodiesel without understanding any of this, but it definitely helps to have some theoretical background.
The building blocks
Biodiesel chemistry is essentially organic chemistry, which is chemistry based around carbon. Carbon can combine with other elements in many ways to make different molecules, all of which have different properties.
The basis of organic chemistry is that elements can connect together to achieve a more stable configuration. Each element is trying to get to the point where it has 8 electrons in its outer shell. It can achieve this by sharing electrons with other elements. This sharing binds the elements together into a molecule. The connections formed between the elements are known as bonds.
To get an idea of how the elements combine, we can imagine elements as balls with different numbers of holes in them. For example, carbon is a ball with 4 holes, Oxygen has 2 holes, Hydrogen has 1 hole. If we connect the balls together with rods that slot into the holes, we can create simple or more complicated molecules. It’s just like meccano! Reactions between chemicals tend to produce stable molecules. In our ball and stick model this means that all the holes are filled.
An example of a stable molecule is methane:
Carbon is the larger ball in the middle; the balls around the edge are hydrogen. Notice that carbon has 4 sticks attached to it, while each hydrogen has 1 stick attached to it. This structure is represented in 3-D to show more accurately how the molecule fills space, but is easier to draw in 2 dimensions:
This is the style we will use for the rest of the tutorial. When writing about common organic molecules chemists often don’t bother to use pictures and will use text abbreviations. In this style, methane is known as
CH4 (carbon with 4 hydrogens). It’s a less informative notation but it’s easier to type!
There are a relatively small number of elements involved in basic biodiesel chemistry. Here are the most important ones
|
element |
symbol |
valency |
|
Carbon |
C |
4 |
|
Hydrogen |
H |
1 |
|
Oxygen |
O |
2 |
Valency is the technical name for how many electrons the element has to share (how many holes in the ball).
With these simple elements as building blocks we can make many different molecules with different properties.
Chains
The simplest types of larger molecules are hydrocarbon chains: chains made out of hydrogen and carbon only. Starting with methane we can make longer chains by connecting 2 or more carbons together:
Alkanes
These molecules all have similar but not identical properties. Together they are known as alkanes. The principle can be extended to make very long chains, which are known by the number of carbons in the chains. So a C20 alkane has 20 carbons in a chain and would be known as eicosane (eicos is Greek for 20). Its shorthand notation would be C20H42 and it would look like this:
Petrodiesel consists of a mixture of different long-chain alkanes with smaller amounts of other things. The average chain length is about C20.
In alkanes, the carbons are held together by one single bond (one shared electron). Because each carbon has 4 electrons to share, 2 carbons could also share 2 electrons between them and each one still have 2 electrons to share. This kind of bond between the carbons is known as a double bond and the simplest chains using double bonds are the alkenes:
Alkenes
Equally, the carbons could share 3 electrons and have one electron to share. This is known as a triple bond. The simplest chains using a triple bond are the alkynes:
Alkynes
Compounds containing single bonds are known as saturated. Compounds containing double or triple bonds are known as unsaturated. Saturated compounds are more stable than unsaturated compounds: double or triple bonds can be broken allowing another bond to be made.
These hydrocarbon chains are a fundamental building block of organic chemistry and form the backbone of most of the compounds we are interested in.
Functional groups
A functional group is an atom or group of atoms in an organic compound that gives the compound some of its characteristic properties. A typical organic molecule consists of a hydrocarbon chain with one or more functional groups attached. The characteristics of the overall molecule depend more on the functional group than on the length of the chain. One on the simplest functional groups is the hydroxyl group OH. When this is attached to different length chains it forms a series known as the alcohols:
Alcohols
The alcohols all have similar properties because they all have the same functional group: the hydroxyl group (OH).
This is an important functional group in biodiesel chemistry. The other important group is the carbonyl group COOH containing a double bond between carbon and oxygen. This gives rise to two important series.
Fatty acids
Fatty acids terminate in a hydrogen attached to the single-bonded oxygen (i.e. the one on the right in these diagrams). The other side of the carbonyl group is attached to a hydrocarbon chain except in the simplest fatty acid (methanoic acid). Used vegetable oils contain fatty acids.
Esters
Esters have a hydrocarbon chain attached to the single-bonded oxygen. The other side of the carbonyl group is also attached to a hydrocarbon chain except in the simplest ester (methyl methanoate). The first two esters above are called methyl esters because they have a methyl group (CH3) attached to the single-bonded oxygen.
Esters can be formed from fatty acids and alcohols by a process known as esterification.
This process needs a catalyst to take place, usually a concentrated acid.
A useful way of thinking of an ester is as an alcohol attached to a fatty acid.
Both vegetable oil and biodiesel are largely esters. The biodiesel we make is a type of methyl ester.
Putting it all together
The worst part is over! We have now got the basic knowledge to understand the structure of vegetable oil and the process of transesterification. The next diagram shows a vegetable oil molecule or triglyceride. It looks rather complicated but by breaking it down into recognisable parts we will make more sense of it.
The first thing we should notice is the functional group. It is the carbonyl group, and we can see that this molecule is an ester. In fact it’s a triester: there are three places where the carbonyl group’s single-bonded oxygen is connected to a hydrocarbon chain. So what about those chains? We can see that some are saturated whereas others contain one or more double bonds. In fact the second chain is mono-unsaturated and the third chain is poly-unsaturated. In a real-life triglyceride the chains might be of different lengths depending on the type of vegetable oil. They would also vary in their degree of saturation: palm oil chains would be more saturated than sunflower oil chains for example. The fact that the chains can be quite different without changing the basic chemistry means we can simplify the diagram by representing them with the shorthand term ‘R’. ‘R’ means a hydrocarbon chain. We can number them R1 R2 R3 to show they are different.
That already looks a lot more manageable. One thing we can still see is that the molecule is rather big: in fact it’s a lot bigger than the esters we looked at before. The size of the molecule is what gives this triglyceride its viscosity, and that viscosity is, as we have already seen, the major problem facing us if we want to use vegetable oil as a fuel in conventional Diesel engines.
As we saw before, esters can be thought of as an alcohol attached to a fatty acid. In this case we have 3 different fatty acids attached to a large alcohol molecule. The alcohol in question is glycerol, which is a triol (it has 3 OH functional groups).
We can see that this glycerol backbone holding together the three fatty acids is the key to the large size of the molecule. What we want to do is replace this with a smaller alcohol.
Transesterification
This brings us to the heart of biodiesel chemistry: transforming the vegetable oil esters into the biodiesel esters. This is known as transesterification, turning one ester (glyceryl esters) into another ester (methyl esters). We do this by adding methanol to the triglyceride. This removes the glycerol backbone and replaces it with a smaller methyl group, thus splitting apart the large molecule. We are left with methyl esters and glycerol as separate products.
Transesterification is a reversible reaction. This means that as the reaction progresses from left to right creating methyl esters and glycerol the opposite is also happening: some of the products are re-combining to form trigylcerides. At some point the reaction will reach equilibrium where there is a certain proportion of unreacted vegetable oil to biodiesel. We want to make sure that the reaction proceeds as far as possible, which can be achieved in two main ways. The first is to remove the products as they are formed. In large-scale industrial plants this is done by removing the glycerol using separators. This is impractical in smaller set-ups. Instead we use an excess of methanol to ‘push’ the reaction towards completion; there’s always enough methanol for the left-to-right reaction to happen more readily than the reverse reaction.
Like esterification, transesterification needs a catalyst, in this case normally an alkaline catalyst such as Potassium or Sodium Hydroxide (also known as lye).
Both the catalyst and the excess methanol are left over after the reaction mixed with the products. In the second section of this manual we will explore the practicalities of separating them back out.
Free fatty acids
When we make biodiesel from ‘fresh’ vegetable oil, we can assume that the oil is in the form of triglycerides only. Waste vegetable oil is a different matter however. When the oil is repeatedly heated, the triglyceride bonds are broken and some of the fatty acid chains break off. They are then known as free fatty acids (FFAs). Free fatty acids are a problem because they make the fuel acidic, which will damage the engine. There are various ways to deal with them. The most thorough method is to esterify the FFAs using the esterification reaction we saw earlier, giving methyl esters. This uses a concentrated acid catalyst. We can then transesterify using a basic (alkaline) catalyst as for fresh vegetable oil. However for everyday simple biodiesel making this two-stage acid/base reaction is a little complicated. Instead, we deal with the FFAs by turning them to soap.
The soaps end up mixed with the glycerol by-product. This reaction has the advantage that we can do it as part of our transesterification by adding more of the lye we are already using as a catalyst. We need to measure how much lye to add to turn the FFAs to soap and add that to the amount we need to catalyse the transesterification. To do this we titrate the oil (see how to make simple biodiesel).
