- Ethanol is an important organic compound with various lab and industrial applications.
- At industrial level, it is produced by hydration of ethene.
- Ethanol is also prepared by fermentation of starch.
- Due to recent usage of ethanol as fuel. Its demand is increasing globally.
- Both ethene and fermentation method compete against each other for cost and environmental sustainability.
Ethanol or ethyl alcohol is an important organic compound. In general, it is also referred to as alcohol spirit, spirit of wine, grain alcohol, absolute alcohol, and ethyl hydrate. Its chemical formula is C2H5OH and has molecular mass 46.07g/mol. It is colourless liquid with a characteristic odour. It is a psychoactive substance and is the main component of alcoholic beverages.
Ethanol is used as a solvent in the lab and has many industrial uses. Recently ethanol has been in the news headlines highlighting the economic value of ethanol usage. This has attracted new energies in industrial production of ethanol. It has been used as car fuel as an alternative to gasoline. Certain countries such as Brazil have been using it as light vehicle fuel. In the USA, ethanol is either used alone or mixed with gasoline.
1.1 Production of ethanol
Ethanol can be produced synthetically using ethene or can also be made by fermentation of sugar using microorganisms. The cost of either procedure depends upon the cost of raw material. Ethene is obtained from petroleum while corn is the main source of sugar for fermentation process. Petroleum ethene being cheaper offer cost benefit for synthetic industrial scale production of ethanol.
1.2 From ethene to manufacture ethanol
Ethanol is made by hydration of ethene (CH2=CH2). This is the easies and cost effective method to manufacture ethanol. Ethene is a product of petroleum industry and thus is easily available raw material. The reaction is carried out in the presence of phosphoric acid (H3PO4) used as a catalyst. Phosphoric acid is coated over silicone dioxide. Ethene is mixed with steam in a fixed molar ratio of ethene/water (1:0.60). The mixture is heated to 300 oC and the gases react over the catalyst to form ethanol. The gaseous mixture of unreacted ethene and product ethanol is cooled down which liquefies ethanol. The product is separated and unreacted ethene is recycled back into the reaction chamber. The productivity per cycle (or per pass) is 5.0 – 25.0% depending upon the activity of catalyst. High yield is achieved by recycling ethene and by removing ethanol from the reaction system. The chemical equation can written as
Another method to convert ethene into ethanol is via ethyl sulfate. Ethene reacts with sulfuric acid to make ethyl sulfate which on hydrolysis gives ethanol. A common by-product is ether that is separated.
1.3 Fermentation procedure for ethanol production
Fermentation is a biochemical process carried out by bacteria, yeast or other organisms converting sugar such as glucose and fructose into cellular energy and producing ethanol and carbon dioxide as by-products. The whole process also releases heat energy. Alcoholic fermentation is one the oldest and most important industrial process known to mankind. Humans learnt this process thousands of years ago. Traditionally fermentation has been used to make alcoholic beverages, but today it adds a lot of economic value to global economy. As mentioned earlier, ethanol today is used as a source of energy for vehicles. Thus its demand has increased in recent years. Other than ethanol, scientists have developed various other techniques to make other chemicals of pharmaceutical and biological importance using fermentation. Today, vitamins, various antibiotics, enzymes and other industrial chemicals are also made by fermentation at industrial scale.
To make alcoholic beverages, traditionally, we have been using cereals, barely and grapes. Basically we need a source of starch that can be cereals, corn or even some suitable wood stocks. However, with its production now at large industrial scale, using barely and grapes is not economically viable. A new feedstock in USA market has been corn. The government agencies have been proposing to use wood stock or corn to make ethanol. Though even this process is not cost effective compared to ethene based industrial synthesis, still, government subsidies and incentives are encouraging industrialists to use fermentation process as a green and sustainable operation for ethanol production.
Starch is a complex carbohydrate and we need to break this complex structure into smaller sugar units such as maltose, fructose and glucose for ethanol production. The whole process for corn fermentation involves series of steps. It starts with cleaning and washing of raw corn to remove dirt and other impurities. The raw material is crushed and heated in warm water to extract their starch content. The material is transferred to fermentation tank where it is mixed with yeast. Yeast as we know is source of enzymes for this biochemical process. Enzymes convert complex starch molecule into maltose that in further biochemical steps converted into simple glucose units. This chemical reaction looks simple written in a chemical equation, however, the fact is, it is a complex chemical reaction involving various intermediate steps. The material is kept at warm temperature (30-35 oC) in air tight environment. Oxygen interferes into this process and may oxidise ethanol to acetic acid (vinegar). Yeast microorganisms start eating sugar and producing ethanol. Their activity continues until the system gets to 8-12% ethanol level. Above this, their activity diminishes and reaction comes to a halt.
The fermented material known as mash is further processed using one single step distillation to separate liquid from solid. The liquid is impure and contains only 8-12% ethanol. The solid is in a slurry form and contains significant level of organic mass. This can be used additive for animal feed or is burned as an energy source.
The distillation of liquid increases the purity level of ethanol and the final product is 96% ethanol. This is used for various lab scale or industrial applications. It still contains some water and hence is not suitable for gasoline mixture.
In the end, a 25kg of corn gives approximately 9 to 10 liters of ethanol.
1.4 Denatured alcohol
Alcoholic beverages are a major and important industry that contributes significantly to tax revenue. In certain countries, these beverages are heavily taxed. As ethanol has many other uses, to reduced tax burden over ethanol, it is made unfit for drinking purposes. This is called denatured alcohol. Ethanol is mixed with some bittering or toxins such as methanol, denatonium benzoate or pyridine to denature alcohol.
1.5 Absolute Alcohol
A complete removal of water gives us absolute or anhydrous ethanol. It is almost 99.6% ethanol with trace amount of other liquid such as benzene added to give a azeotropic mixture. Dehydration of ethanol can also be carried out using molecular sieves. Absolute alcohol is used as solvent where water is undesired or water can react with other chemicals. In spectroscopy, water strongly absorbs in ultraviolet region and hence absolute ethanol is a preferred choice as a solvent. Also, this purity grade is used as a fuel mixed with gasoline. Ethanol containing water will not mix with gasoline and thus water must be removed completely.
Books for further study
- Austin GT. Shreve’s chemical process industries: 5th Ed. 2012. McGraw Hill.
- Ethanol: in Ullmann's encyclopedia of industrial chemistry. 7th Ed. 2016.
- Ali MF, Ali BME, speight JG. Handbook of industrial chemistry. 2005. McGraw Hill.