Production of Ethanol

What is ethanol?

Ethanol (C₂H₅OH; structural formula CH₃CH₂OH) is an organic molecule with the hydroxyl functional group (-OH). It belongs to the alcohol homologous series. In particular, ethanol is a primary alcohol with two carbon atoms, containing just one hydroxyl group. Its structure is shown below.

Fig. 1: The structure of ethanol, with its hydroxyl group highlighted.StudySmarter Originals

Ethanol has many uses in our daily lives. We don't just find it in the wide variety of alcoholic drinks available in shops, bars, and restaurants, but in disinfectants, solvents, and as a fuel additive. Believe it or not, ethanol is also used as an antidote to highly-dangerous methanol poisoning! As a result, the production of ethanol is important to understand. Let's now consider the two main ways ethanol is produced industrially.

Production of ethanol process

There are two main ways in which ethanol is produced:

• The fermentation of glucose.
• The hydration of ethene.

The two processes have a few key differences in terms of their reactants, conditions, and byproducts. They also both have their advantages and disadvantages. We'll start with an overview of the two methods before comparing them in more detail.

Production of ethanol: Fermentation of glucose

Ethanol can be produced through the fermentation of glucose. In this process, specific strains of yeast convert glucose (C₆H₁₂O₆) from plant carbohydrates into ethanol inside a fermenter with set environmental conditions. Most alcoholic beverages made in breweries undergo this fermentation process.

Fermentation process

The plant carbohydrates, the starting materials in the production of ethanol, typically come from crops such as sugar cane or sugar beet. The yeast, which could be a species such as Saccharomyces cerevisiae, contains enzymes responsible for converting the glucose in plant carbohydrates into ethanol via anaerobic respiration. The reaction also produces carbon dioxide (CO₂).

Ethanol is toxic to yeast in high concentrations. Consequently, the yeast dies out when the solution reaches levels of about 15% ethanol. This means that the production of ethanol by fermentation is a batch process. Thanks to ethanol's boiling point, which is lower than that of water, the ethanol can then be separated from the rest of the solution by fractional distillation. You should also note that fermentation has a slow rate of reaction, especially when compared to the hydration of ethene (which we'll look at in just a second).

Fermentation word equation

The chemical equation for fermentation is shown below, alongside the word equation.

$$C_{6}H_{12}O_{6}\rightarrow 2C_{2}H_{5}OH+2CO_{2}$$

$$Glucose \rightarrow ethanol + carbon\ dioxide$$

Fermentation starting materials and by-products

We mentioned that fermentation requires plant carbohydrates. These frequently come from crops such as sugar beet or corn. Crops are quick and easy to grow, and often just the discarded part of the plant (such as the stem or husk) is used in fermentation, leaving the edible portion for human or animal consumption.

You can see from the chemical equation that fermentation releases carbon dioxide (CO₂), a greenhouse gas. This may seem harmful from an environmental point of view. However, fermentation itself is technically carbon neutral - all of the carbon released was taken in via photosynthesis during the crop's lifetime, and we'll support this statement with chemical equations later on in the article.

Fermentation doesn't break down all of the plant material and so we are left with a residual byproduct. The leftover matter makes a more-than-suitable cattle feed.

Fermentation conditions

Effective fermentation requires certain conditions. These are shown below alongside the reasoning behind them.

Production of ethanol: Hydration of ethene

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.

Hydration process

The term hydration gives you a clue about this way of ethanol production. If you picture a reaction where water is involved, you're on the right track!

Hydration is an example of an electrophilic addition reaction. Essentially, it involves adding steam to ethene in the presence of a phosphoric (V) acid catalyst, resulting in ethanol. Ethene (CH₂CH₂) is the simplest alkene, an unsaturated hydrocarbon with a C=C double bond. Adding steam to ethene turns this alkene into a saturated molecule and adds the hydroxyl (-OH) group to one of its carbon atoms. The reaction is reversible, but we can alter the conditions to increase the yield considerably.

Fig. 2: Ethene to ethanol. Both molecules are shown with their functional groups highlighted.StudySmarter Originals

We separate the ethanol from the unreacted gases by cooling the mixture. Although a small amount of steam condenses as well, the ethanol collected is essentially pure. Fractional distillation takes care of the steam impurities. Any unreacted gases are continuously recycled over the catalyst to achieve an overall conversion rate of around 95%. You should also note that this process has a fast rate of reaction.

Hydration equation

Here's the chemical equation for the hydration of ethene:

$$C_{2}H_{4(g)} + H_{2}O_{(g)} \rightleftharpoons C_{2}H_{5}OH_{(g)} $$ ΔHº = -45 kJ mol^-1

Note the use of the equilibrium arrow (⇌). It shows us that this reaction is reversible and exists in a state of dynamic equilibrium.

Hydration starting materials and by-products

The chemical equation above tells us that hydration doesn't produce any by-products. This might make it look like an environmentally-friendly process, but we must consider the bigger picture. Hydration requires ethene, which is produced by cracking hydrocarbons found in crude oil. Crude oil is non-renewable - it is a finite resource, meaning that it can't be replaced naturally at the rate that we are using it. Therefore, producing ethanol by hydrating ethene isn't sustainable.

Hydration conditions

Because the hydration of ethene is reversible, the reaction's conditions are vital when it comes to determining yield. The table below summarises the conditions used in ethene hydration and the reasoning behind them.

Comparing methods of ethanol production

Deciding on the best way to produce ethanol isn't an easy task. There are advantages and disadvantages to both methods that must all be taken into consideration. The table below summarises the differences between ethanol production via the fermentation of glucose and the hydration of ethene.

Despite hydration's improved product purity and faster rate of reaction, fermentation is used more widely for ethanol production. This is because of its lower costs and environmental impact.

Production of ethanol: Using ethanol as a biofuel

In the last 50 years or so, ethanol has established itself as a handy alternative to petrol or diesel. It burns just like any other fuel and is a useful way of powering our vehicles. For example, E10 petrol (containing up to 10% ethanol) is the standard fuel-grade in the UK, whilst in France you can find E85 petrol stations. Brazil even owns an extensive fleet of vehicles that run on 100% ethanol!

The ethanol used in this way is produced using fermentation, and so is a type of biofuel.

Like all biofuels, so-called bioethanol is derived from renewable resources. On the other hand, petrol and diesel come from crude oil, which we've already learned is non-renewable. Other popular fuel sources, like coal and gas, are also non-renewable - they are types of fossil fuels. The renewability of biofuels gives them an advantage over coal, gas, diesel, and petrol, and is partly why they are increasing in popularity so rapidly.

However, there are negatives to using biofuels, including bioethanol. We'll now discuss both the pros and cons of their use.

Advantages of biofuels

Biofuels manage to one-up traditional fossil fuels or crude oil derivatives in many ways. Here are a few:

• Biofuels are renewable. They are made from fast-growing plant or animal matter - often from crops that can be planted and harvested in less than a year. Further crops can then be replanted in the same area in subsequent years. On the other hand, crude oil and fossil fuels take millennia to form and so are non-renewable.
• Biofuels have a lower carbon footprint than traditional fuels. All carbon-based fuels (including biofuels) release carbon dioxide (CO₂) when burned. However, biofuels are theoretically carbon-neutral because all the carbon they release was absorbed from the atmosphere during the original plant's lifespan.
• Biofuels are cheap, easy to process, and can be locally made. Sourcing biofuels doesn't require drilling in deep-sea mines or cutting down forests to access coal deposits.
• Biofuels can be made from waste material, such as crop stalks or husks, that would otherwise be discarded.
• Biofuels release fewer additional harmful pollutants, such as sulphur dioxide and nitrous oxides, which are associated with the combustion of traditional fuels.

Disadvantages of biofuels

For your exams, you could be expected to argue both for and against the use of biofuels. At the very least, it is important to understand why some people aren't convinced by their use. Here are some of the disadvantages of biofuels.

• Although biofuels are carbon neutral themselves, they require energy to harvest, process, and transport. This energy frequently comes from non-renewable sources and thus releases carbon dioxide into the atmosphere. However, the same could be said about traditional fuels - they too require specific processing, handling, and transportation.
• Growing the plant matter used to produce biofuels needs a large water input.
• Growing plant matter for biofuels also takes up a lot of land. It is argued that this land could be used more efficiently to grow crops for human or animal consumption, and that making space for biofuel plantations is a cause of deforestation and habitat loss.
• Government incentives to produce biofuels have led to a decrease in agriculture in certain countries, resulting in food shortages and climbing grocery bills.