The alcohol ethanol, C2H5OH, can be produced in two ways, by fermentation and from crude oil. In ethanol fermentation, yeast is used to break down glucose and other sugars into ethanol and carbon dioxide. The glucose and other sugars used in this process come from plants, mainly sugarcane. The sugars are produced during photosynthesis, and come from the carbon dioxide that enters the plant. The plant is then harvested and broken down into these sugars. The juice this produces then has yeast added to it, and is fermented under anaerobic conditions (without oxygen) to produce ethanol and carbon dioxide. The equation for this reaction is as follows:
C6H12O6 -> 2 C2H5OH + 2 CO2
Producing ethanol using crude oil is a completely different process. The ethanol is made by hydrolysing ethene, in the reaction C2H4 + H2O ->C2H5OH. The ethene comes from cracking long chain hydrocarbons found in crude oil. This cracking requires high temperatures and a catalyst of phosphoric acid in order to work.
Both of these two methods have advantages and disadvantages over the other. The first thing to consider when evaluating and comparing the two are the starting materials required for each; sugars from plants and crude oil. Sugar is a renewable energy source, as plants grow quite quickly. Crude oil is not. When we have used up all the oil, it will take millions of years for more to be produced. Therefore if our main goal is to protect our resources for use in the future, sugar is better to use than the little crude oil we have left.
The second thing to consider is the type and rate of the reaction. Producing ethanol using sugarcane and other plants is a very slow process, as it takes quite a while for the yeast to ferment all the alcohol. Ethanol produced in this way is a batch process, and can’t really be very efficient. The effect ethanol has on yeast has to be considered when using this method, and the yeast can only survive in a solution of up to 15% alcohol. This means for every batch of solution made this way, 15% is the wanted ethanol and 85% is waste products. Producing ethanol from crude oil on the other hand, is a fast and continuous process, and it produces pure ethanol rather than an aqueous solution of ethanol like fermentation does. It seems that producing ethanol from crude oil is a much faster, more efficient method than producing it by fermentation.
Finally, there are a few other conditions to consider when comparing the reactions. Fermentation of ethanol needs to take place under conditions of no air, and around 35C. These conditions are quite easy to acquire and would cost very little. The cracking of crude oil however, would require temperatures of 570K, a phosphoric acid catalyst and atmospheric pressure of 6500 KPa. These conditions would be much harder to produce, and would also be very expensive, and quite possibly dangerous to work around. So producing ethanol via fermentation seems to be the cheaper method.
Of course, all of these things must be taken into consideration. To produce one batch of 15% ethanol solution by fermentation the cost is much lower than making ethanol continuously by cracking crude oil and hydrolysing the ethene. But would it still be cheaper to make the same amount? The crude oil method would produce a lot more ethanol a lot more quickly, and the alcohol this method produces is pure, and does not have to be distilled, unlike its fermented counterpart. Then again fermentation would not use up finite energy resources, whereas cracking crude oil would.
One concern, which may affect the future of ethanol production, is the increase in global warming due to the releasing of carbon dioxide into the atmosphere. There is now a need for fuels to be carbon neutral, so that the amount of carbon dioxide produced in burning the fuel is the same amount that is being taken in during the production of the fuel. Out of the two methods of production discussed above, producing ethanol by fermentation is the only carbon neutral method. This suggests that in the future more people will start using bioethanol (the ethanol produced by fermentation) rather than ethanol made from crude oil.
