Chemistry - Ethanol Production

Chemistry - Ethanol Production

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Ethanol Production
Ethanol,C2H5OH, is an important alcohol. It is used as an intermediate in the production of other
organic compounds, and as a solvent in cosmetic products.
Making ethanol from crude oil
Ethanol is made by the hydration (adding water) of ethene. This reaction is reversible and is sped by
a catalyst of phosphoric acid absorbed on silica.
H2C=CH2 (g) + H2O (g) CH3CH2OH(g) H = -46kJ/mol
Applying Le Chatalier's principle-
Maximum yield produced with:
High pressure (reaction will try and counter the change by favouring forward reaction as there are
fewer gaseous moles produced)
Low temperature (the reaction looks to counter the change by favouring the forward, exothermic,
reaction to increases the temperature again)
Excess steam (the reaction counters this change by favouring the forward reaction to reduce the
amount of steam)
However, there are practical problems. A lower temperature slows the reaction rate and therefore
how quickly equilibrium is reached. This is compensated by the use of a catalyst. Also, a high pressure
can cause the ethene to polymerise to poly(ethene). Finally, excess steam dilutes the catalyst, which
reduces the rate of reaction.
In practice, a temperature of about 570k and a pressure of 6500kPa is used. These give a conversion
to ethanol of about 5%, but the unreacted ethene is separated and recycled over the catalyst again
until about 95% is converted.
Making ethanol by fermentation
During fermentation, carbohydrates from plants are broken down into sugars and then converted
into ethanol by the action of enzymes from yeast. The carbohydrates come from crops (sugar cane
etc.). The key step is anaerobic respiration.
Enzymes from yeast
C6H12O6 (aq) -> 2C2H5OH (aq) + 2CO2 (g)
The rate of reaction is affected by temperature. It is slow at low temperatures, but the enzymes are
denatured if the temperature is too high. A compromise temperature of 35 degrees is used.
Air is kept out to stop the oxidation of ethanol to ethanoic acid.
Once the fermenting solution contains about 15% ethanol the enzymes are unable to function and
fermentation stops.
Ethanol as a fuel
Ethanol is useful as a motor fuel. It is mixed with petrol. It can be a biofuel when made from crops.
When this is burned as a fuel, the same amount of carbon is released as was absorbed when the crop
was growing. In this sense, this means ethanol as a fuel is carbon neutral. However, there are more
factors to consider. Growing the crop uses lots of land and the crop has to be harvested and
transported. The essential point is that ethanol made by fermentation is a renewable source of fuel.
Carbon Neutrality

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Ethanol Production
Many conventional petrol engines will run on ethanol, or mixtures of petrol & ethanol, with little
modification. Only ethanol made by fermentation is renewable (plants can be replaced, hydration of
ethene - ethene from crude oil which is non renewable).
Almost all current fuels are carbon based. They release carbon into the atmosphere when
burnt/used. Rising carbon dioxide levels are associated with global warming and climate change.
Ethanol produced by fermentation is often called a carbon neutral fuel.…read more

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