Production of Ethanol

Ethanol belongs to the alcohol homologous series. It is an alcohol made of two carbons, with many uses in our daily lives. Hence, the production of ethanol is important to understand. You'll recall from the IUPAC nomenclature of organic compounds that the root name 'eth-' shows it has two carbons, whereas the suffix '-ol' shows it has the hydroxyl functional group '-OH'.

The structural properties of ethanol are summarised in the table below.

What are the uses of ethanol?

The main uses of alcohol include:

• As a solvent in cosmetics.
• As an intermediate in the manufacture of drugs, detergents, inks, and coatings.
• As biofuel.

Let's move on to ethanol production.

What is the production process of ethanol?

There are two main ways in which ethanol is produced.

• Fermentation of glucose
• Hydration of ethene

Fermentation of glucose

During the fermentation of glucose, yeasts convert plant carbohydrates, the source of glucose, into ethanol inside a fermenter with set environmental conditions. Most alcoholic beverages made in breweries undergo this fermentation process.

Plant carbohydrates, the starting materials in the production of ethanol, can come from crops such as sugar cane or sugar beet. The 'magic ingredient', yeast, contains enzymes responsible for converting the glucose in plant carbohydrates into ethanol via anaerobic respiration. Anaerobic respiration is a process in which glucose is broken down to generate energy (ATP) in the absence of oxygen.

The chemical equation for fermentation is as follows:

Glucose ethanol carbon dioxide

As well as the starting conditions discussed above, effective fermentation also requires certain conditions. These are shown below alongside the reasoning behind them.

Once fermentation reaches 15 percent, the enzymes involved in anaerobic respiration are denatured, halting the fermentation process.

Fractional distillation separates the components of a solution using differences in their boiling points. In this case, ethanol has a much lower boiling point than water. Here's a simplified diagram of the fractional distillation apparatus.

Apparatus for fractional distillation that has two main components: a fractionating (reflux) column to separate the compounds, and a condenser to obtain the liquid compound known as the distillate. commons.wikimedia.org

Now that we've finished looking at the chemistry of fermentation, let's see how we can produce ethanol via the hydration of ethene. This is another method of alcohol production used in industry.

The chemical equation including the displayed formula for the hydration of ethene is as follows:

Chemical equation and displayed formula for the hydration of ethene. Notice how steam is being added, and how the carbon-carbon double bond becomes a single bond.

Even though the ethanol formed via hydration is pure in theory, fractional distillation of the product is necessary to obtain pure ethanol. This is because there is condensed steam inside the collecting vessel.

How do hydration and fermentation compare?

The table below summarises the differences between ethanol production via fermentation and hydration.

Despite the improved product purity and faster rate of reaction for the hydration method, the fermentation method is more widely used for ethanol production. This is because of its lower costs and environmental-friendliness, which we will discuss shortly.

Using ethanol as a biofuel

Ethanol is used as a biofuel, increasingly replacing petrol as an environmentally friendly resource. In fact, unleaded petrol in the UK now has 10 percent of ethanol added to it.

What is a biofuel?

Biofuels are fuels derived from biomass such as plants. Biofuel is advantageous over crude oil for its renewable properties - it is naturally replenished at the rate we use it. In terms of using ethanol as a renewable resource, the crops where the plant carbohydrates are sourced (sugarcane, for example) can be sowed again once the existing batches have been harvested.

Combustion of ethanol biofuel

You may wonder about the chemical reaction associated with ethanol acting as a fuel. Similar to that of petrol, the chemical reaction that allows ethanol to generate energy is combustion, whose equation is as follows:

2C₂H₅OH + 6O₂ -> 4CO₂ + 6H₂O

ethanol oxygen carbon dioxide water

Advantages of using ethanol as a biofuel

There are arguments for and against using ethanol as a biofuel, and you are expected to justify both sides of the argument at A-levels.

Burning fuels always releases carbon dioxide. This is something to avoid because carbon dioxide as a greenhouse gas causes global warming. Yet, the main benefit of using ethanol as a biofuel is its carbon neutrality. This means using ethanol from biomass as a fuel has no net release of carbon dioxide. In other words, the amount of carbon dioxide released equals the amount taken in by the plants fermented to make ethanol.

Plants take in carbon dioxide in photosynthesis. Let’s look at the equations involved, as well as the equations showing the release of carbon dioxide into the atmosphere.

Carbon dioxide taken in

Photosynthesis

6CO₂ + 6H₂O -> C₆H₁₂O₆ + 3O₂

Carbon dioxide water glucose oxygen

Carbon dioxide released

Fermentation

C₆H₁₂O₆ -> 2C₂H₅OH + 2CO₂

Glucose ethanol carbon dioxide

Combustion

2C₂H₅OH + 6O₂ -> 4CO₂ + 6H₂O

ethanol oxygen carbon dioxide water

From the equations above, we can see that the amount of carbon dioxide taken in (6mol) equals the amount released (6mol: 2mol from fermentation + 4 mol from combustion) when two moles of ethanol are used as a fuel. Hence there is no net release of carbon dioxide.

Another benefit of using ethanol as a fuel is that it releases fewer pollutants. Unlike the combustion of fossil fuels, the combustion of ethanol is ‘cleaner’ in that there are no end products containing sulfur or nitrogen oxides. Both sulfur and nitrogen oxides are pollutants that cause acid rain as well as respiratory conditions.

In terms of the production of ethanol biofuel, the fermentation of plant carbohydrates into ethanol is cheap, not to mention its easy storage and distribution.

Disadvantages of using ethanol as a biofuel

Even with equations given to justify the carbon-neutrality of ethanol, one may say that this argument is too theoretical. This is down to two main reasons.

• Exclusion of other, non-renewable energy sources - the energy used in the harvesting and transportation of crops may come from fossil fuels.

• An imbalance between the rate of crop growth and combustion. The burning of ethanol biofuel is a rapid process. However it takes time for the crops responsible for photosynthesis to grow.

Briefly, the carbon costs from the processing of plant crops are not considered.

In addition, the use of ethanol as a fuel poses some environmental issues.

• Deforestation to grow crops for biofuels, resulting in habitat loss and reduced biodiversity.
• The burning of rainforests for agriculture releases poisonous greenhouse gases that worsen global warming.
• The manpower required to grow biofuel may lead to reduced manpower in food agriculture, potentially causing global food shortages.
• The energy needed to distill the fermented mixture is not only costly but may come from non-renewable sources.