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Production of Ethanol

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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.

Displayed formula

Production of alcohols ethanol displayed formula StudySmarter

Molecular weight46.07
Molecular formulaC 2 H 5 OH
Structural formulaCH 3 CH 2 OH

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:

C 6 H 12 O 6 → 2C 2 H 5 OH + 2CO 2

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.

ConditionReasoning
Optimum temperature of 35o CMaximise product yield without denaturing the enzymes involved in anaerobic respiration.
Atmospheric air kept out Prevent ethanol from oxidising into ethanoic acid (ie., Vinegar).

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

You may ask here - what about spirits like gin or whiskey, whose ethanol concentration is much greater than 15 percent? How can this high percentage of alcohol be achieved? In the case of spirits, the fermented solution is concentrated via fractional distillation.

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.

Production of Ethanol Fractional distillation StudySmarterApparatus 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

The lower boiling point of ethanol (75o C) compared to water (100o C) allows ethanol vapours to escape from the fractionating (reflux) column quicker and thus condense quicker as the distillate.

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.

If you picture a reaction where water is involved, you're on the right track! Essentially, hydration involves adding steam to ethene in the presence of a phosphoric (V) acid catalyst, resulting in ethanol. The environmental conditions for hydration include high temperatures (300o C) and pressures (60-70atm) .

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

Production of Ethanol ethene hydration StudySmarterChemical 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.

Study tip: Do you notice here that the hydration reaction is reversible? This explains the importance of the reaction conditions. By changing the conditions, we can maximise the equilibrium shift to the right-hand side. Also, be sure to read the question carefully to see the type of formula asked for when writing equations.

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.

Starting materialsCrude oilPlant carbohydrates
MethodCracking into alkenes, then hydrationfermentation
Rate of reactionNearlySlow
Continuous or batch processingContinuous Batch
Reaction conditionsHigh temperature and pressureLow temperature (35℃) and atmospheric pressure
Resource typeNon-renewableRenewable
Pure or impureEssentially pureImpure (aqueous solution)

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

Study tip: Complete combustion always involves oxygen and releases carbon dioxide and water. What differs is the starting molecule (i.e., octane or ethanol), as well as the number of moles of oxygen, carbon dioxide and water required to ensure a balanced chemical equation.

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.

Study tip: You are expected to give equations to justify the highlighted argument in the exam.

Don’t forget to ensure that all the equations above are balanced!

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.

Production of Ethanol - Key takeaways

  • Ethanol is used as a solvent for cosmetics, as an intermediate in the production of organic compounds, and as a biofuel. It is also the main component of alcoholic beverages.

  • Ethanol is produced in two main ways: fermentation and hydration. Fermentation involves anaerobically fermenting biomass using yeast, whereas hydration involves reacting ethene with steam and a phosphoric acid catalyst.

  • The arguments for ethanol as biofuel are the use of renewable plant crops, and no net release of carbon dioxide. However, the arguments ignore the use of non-renewable energy in the harvesting and transport of plant crops, as well as the imbalance between the rate of crop growth and ethanol combustion.

  • The environmental issues linked to the use of ethanol as a biofuel include deforestation, air pollution from the mass burning of rainforests, global famine from reduced food agriculture, and the high costs associated with distilling the fermented mixture.

Frequently Asked Questions about Production of Ethanol

Ethanol and isopropyl alcohol are two different alcohols. Ethanol is a primary alcohol, contains two carbons and has the structural formula CH3CH2OH. In contrast, isopropyl alcohol is branched, contains three carbons, and has the structural formula CH3CH(OH)CH3.

It is through anaerobic respiration of glucose that ethanol is produced as a byproduct.

Ethanol produced by fermentation consists of two liquids, whereas simple distillation is used when purifying a liquid containing a dissolved solvent (for example, seawater).

Strong acids, commonly phosphoric acid (and sometimes sulfuric acid) are used in the hydration process.

Yes. For instance, sugar cane used in the production of biofuel is commonly grown in Brazil.

Final Production of Ethanol Quiz

Question

State four uses of ethanol.

Show answer

Answer

As a solvent in cosmetics, intermediates in the making of organic compounds, biofuel, alcoholic beverages.

Show question

Question

What are the two main ways of producing ethanol?

Show answer

Answer

Fermentation, and hydration of ethene.

Show question

Question

What are the conditions required for successful fermentation?


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Answer

Yeast - provides the necessary enzymes. 

Optimum temperature (35C).

Air kept out - prevent further oxidation of ethanol into ethanoic acid.

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Question

How do you produce spirits (highly concentrated solution of ethanol)?


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Answer

Fractional distillation of the ferment, as the yeast enzymes are denatured once the ethanol content reaches 15 percent. Without any distillation, the maximum alcohol content is 15 percent.

Show question

Question

What are the conditions required for the effective hydration of ethene into ethanol?


Show answer

Answer

high temperature (300C), high pressure (60-70atm)

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Question

How would you justify the argument that ethanol is NOT a biofuel?


Show answer

Answer

Exclusion of non-renewable energy sources used in harvesting and transportation, rate of crop growth much slower than the rate of fuel combustion.

Show question

Question

What are the environmental issues linked to decision making about biofuel use?


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Answer

Hint: think about how do they get the land to grow crops, as well as the distillation process post-fermentation

Show question

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