Ethanol production

​​In a nutshell

Ethanol can be formed via hydration and fermentation. It can be used as a biofuel as an alternative to fossil fuel. There are advantages and potential disadvantages of using biofuels.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

​$$alkene \space + \space steam \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} alcohol \\ \ \\sugar \xrightarrow[30-40 \degree C]{yeast} ethanol \ + \ carbon \ dioxide$$​​

These are equations for specific chemical reactions. You are required to know them to prove ethanol as a biofuel is carbon neutral.

​$$6CO_2 \ + \ 6H_2O \rightarrow C_6H_{12}O_6 \ + 6O_2 \\ \ \\C_6H_{12}O_6 \rightarrow 2C_2H_5OH \ + \ 2CO_2 \\ \ \\2C_2H_5OH \ + \ 6O_2 \rightarrow 4CO_2 \ + \ 6H_2O$$​​

Alcohol formation

Alcohols can be formed via hydration. Hydration is the addition of a gaseous water molecule (steam) across the $$C=C$$ bond of an alkene, forming an alcohol. This is done in the presence of a catalyst, such as solid phosphoric acid $$(H_3PO_4)$$​​.

​$$alkene \space + \space steam \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} alcohol$$​​

Example

​$$ethene \space + \space steam \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} ethanol$$

$$C_2H_4(g)\space + \space H_2O (g) \xleftrightharpoons[300\degree C, \space 60atm]{H_3PO_4} C_2H_5OH(g)$$

Fermentation

Ethanol can be produced via fermentation in the presence of yeast.  It is a low cost method which uses renewable resources.

$$sugar \xrightarrow[30-40 \degree C]{yeast} ethanol \ + \ carbon \ dioxide$$​​

​Yeast contains an enzyme which converts sugars into ethanol and carbon dioxide.

Example 

​$$glucose \xrightarrow[30-40 \degree C]{yeast} ethanol \ + \ carbon \ dioxide \\ \ \\C_6H_{12}O_6 \xrightarrow[30-40 \degree C]{yeast} 2C_2H_5OH \ + \ 2CO_2$$​​

The yeast enzymes work at their optimum between $$30-40 \degree C$$ hence why the reaction is carried out at this temperature. If the temperature is below optimum, the reaction occurs slowly and if it exceeds the optimum the enzymes denature.

The ethanol can be separated out from the mixture via fractional distillation. Although fermentation costs are low, this purification step adds to the cost and requires further time and energy.

Biofuel

Alternatives to fossil fuels are being looked into which are renewable. One candidate is the use of biofuels. Biofuels are fuels made from biological material. Ethanol can be made via fermenting sugar cane and added to petrol, therefore classifying it as a biofuel.

Advantages of biofuel

There are a few advantages of using biofuels instead of fossil fuels. Biofuels are:

• Renewable
  
• Sustainable
• Classed as carbon neutral

​​

Drawbacks of biofuel

Although biofuels may seem as though they are the way forward they do have drawbacks. Vehicles will need to be manufactured differently to accommodate the use of biofuels and handling high concentrations of ethanol. 

Developing countries may also suffer, as less land may be available to grow food as there may be a preference to grow crops for fuel.

It seems as though biofuels such as bioethanol are carbon neutral because the carbon dioxide released into the atmosphere will be the same carbon dioxide absorbed whilst growing. This can be supported by the following chemical equations.

Plants absorb $$6$$​ moles of carbon dioxide from the atmosphere during photosynthesis.

​$$6CO_2 \ + \ 6H_2O \rightarrow C_6H_{12}O_6 \ + 6O_2$$​​
​

During fermentation $$2$$ moles of carbon dioxide is given out.

​$$C_6H_{12}O_6 \rightarrow 2C_2H_5OH \ + \ 2CO_2$$​​

Ethanol is then burned, releasing $$4$$ moles of carbon dioxide.

$$2C_2H_5OH \ + \ 6O_2 \rightarrow 4CO_2 \ + \ 6H_2O$$​​

Combining the three equations, you see $$6$$ moles or carbon dioxide absorbed from the atmosphere and $$6$$​ moles released.​

However, fossil fuels are required to power machines which make fertilisers, harvest, refine and transport the crops. So, using ethanol as a biofuel isn't carbon neutral.