Ethanol and Biodiesel Production
Chemistry (Year 12) - Chemical Synthesis
Previously, we discussed how ethanol is produced via the hydration of ethene. Remember this reaction:
This method is quite popular as it is a fast production process, however it is not the only method by which ethanol is produced. There are greener methods to ethanol production, for example fermentation.
Fermentation is a reversible reaction between fructose and glucose molecules, that are obtained by the breakdown of starchy plants such as corn, sugar cane, wheat and barley.
These molecules ferment over a couple of days in the presence of yeast enzymes. The result is the production of ethanol and carbon dioxide gas.
As there are enzymes present in this reaction, it must occur under biological conditions in order to prevent the denaturing of these enzymes. The biological conditions include:
A temperature of 37°C
A concentration of ethanol that does not exceed 8-15%
An excess concentration results in a change in pH that denatures the enzyme and lowers the yield.
Once the reaction is complete the yeast enzymes are then separated from the mixture. Distillation is then used to boil off the ethanol for collection.
Let's have a look at the advantages and disadvantages that exist between the production of ethanol through hydration and fermentation:
Biodiesel is a fuel-form that is increasing in popularity due to its greater sustainability than fossil fuels. It is extracted from triglycerides through transesterification (For revision please refer to the content page 'Soaps and Detergents'). Triglycerides such as canola oil, soybean oil, palm oil and animal fats are used.
Biodiesel is a methyl or ethyl ester produced from the naturally occurring triglycerides and free fatty acids (FFA) in these oils and fats.
Biodiesel production occurs via a reversible transesterification reaction between the triglyceride and methanol in the presence of either a lipase enzyme or base catalyst (e.g. NaOH) to produce biodiesel and glycerol:
If FFAs are used to form biodiesel, their concentration must be kept low, as if they are catalysed by a base, unwanted soap might be formed.
Industrial Biodiesel Production
Production occurs across 2 chambers.
In the first chamber: the triglyceride and methanol are added with either a lipase or base catalyst to produce biodiesel and glycerol:
Biological temperatures are used for the lipase method to avoid denaturing the enzyme.
For the base-catalysed method, a moderate temperature of 60°C is used.
In the second chamber: The less dense biodiesel rises to the top and the glycerol sinks to the bottom. This forms two distinct layers, and the biodiesel and glycerol can then be collected.
Unreacted methanol is also distilled out of the mixture and recycled back into the reactor.