Fermentation

1. Fermentation makes ethanol from aqueous glucose using yeast, at $25–35{}^{\circ }C$ and in the absence of oxygen.
2. Advantages of fermentation
   1. The glucose comes from crops such as sugar cane and sugar beet, which can be grown again every year, so the raw material is renewable.
   2. The low temperature and normal atmospheric pressure mean that little energy is needed to keep the reaction going.
   3. The equipment is simple and cheap, because the tanks do not have to withstand high temperatures or pressures.
   4. Fermentation does not depend on supplies of petroleum, so it suits countries that grow large amounts of sugar crops but have little crude oil.
3. Disadvantages of fermentation
   1. Fermentation is slow, taking several days to make each batch of ethanol.
   2. It is a batch process, so the vessel has to be emptied, cleaned and refilled between batches, which wastes time.
   3. The product is a dilute solution, because the yeast dies when the ethanol concentration reaches about $15\%$.
   4. The ethanol is impure, mixed with water, yeast and unused glucose, so it must be filtered and then purified by fractional distillation, which needs extra energy.
   5. Carbon dioxide forms alongside the ethanol, so not all of the atoms in the glucose end up in the useful product.
   6. Growing the crops needs large areas of land, which could otherwise be used to grow food.

Steam addition to ethene

1. Catalytic addition of steam to ethene makes ethanol at about $300{}^{\circ }C$ and $6000kPa$, using an acid catalyst.
2. Advantages of steam addition
   1. The reaction is fast, so a large amount of ethanol is made in a short time.
   2. It is a continuous process, so ethanol is produced all the time without stopping to empty and refill the reactor.
   3. Ethanol is the only product, so all the atoms of the reactants end up in the ethanol and there is no waste product.
   4. The product is pure ethanol, so it does not need the long purification that fermentation needs.
   5. Unreacted ethene and steam are recycled, so a high overall yield is obtained.
3. Disadvantages of steam addition
   1. The ethene is made by cracking fractions from petroleum, which is non-renewable and will eventually run out.
   2. The high temperature and high pressure need a lot of energy, which is often obtained by burning fossil fuels.
   3. The reactor and pipes must be strong enough to withstand $6000kPa$, so the plant is expensive to build and maintain.
   4. The reaction is reversible, so only a small fraction of the ethene is converted on each pass and the gases must be separated and recycled.

Comparing the two methods

1. Raw material
   1. Fermentation uses glucose from crops, which is renewable.
   2. Steam addition uses ethene from petroleum, which is non-renewable.
2. Rate of production
   1. Fermentation is slow, while steam addition is fast.
3. Type of process
   1. Fermentation is a batch process, while steam addition is a continuous process.
4. Purity of the product
   1. Fermentation gives a dilute, impure solution that needs fractional distillation, while steam addition gives pure ethanol.
5. Energy and conditions
   1. Fermentation uses a low temperature and atmospheric pressure, so its energy needs are low.
   2. Steam addition uses a high temperature and high pressure, so its energy needs are high.
6. Equipment and cost
   1. Fermentation tanks are cheap, while a high-pressure steam addition plant is expensive.
7. Products formed
   1. Fermentation also produces carbon dioxide, while steam addition produces ethanol only.
8. The best method for a country depends on whether it has more farmland and sugar crops or more petroleum and money for an industrial plant.