Alcohols

Ethanol is made on an industrial scale by two completely different routes; one is fast and continuous, the other slow and batch-based. Both are important in different parts of the world.

Route A — Hydration of ethene

• Ethene from cracking is reacted with water (as steam) in the presence of a catalyst:

C₂H₄(g) + H₂O(g) → C₂H₅OH(g)

• An addition reaction across the C=C: H and OH from a water molecule attach to the two carbons of the double bond, leaving an ethanol molecule
• Conditions:
  • Temperature about 300 °C
  • Pressure about 60–70 atm
  • Catalyst: concentrated phosphoric acid, H₃PO₄
• The reaction does not finish in a single pass: the gas mixture leaving the reactor still contains some ethene along with the ethanol and the water. The unreacted ethene condenses at a much lower temperature (boiling point −104 °C) than the other two and is separated off and pumped back into the reactor; the ethanol-and-water mixture is then split by fractional distillation
• Hydration is a continuous industrial process — feedstock flows in, products flow out, around the clock
• The route depends on having a cheap supply of ethene from cracking oil, so it dominates ethanol manufacture in oil-rich countries

Route B — Fermentation of sugar

• Glucose or sucrose (in fruit juice, sugar-cane syrup or molasses) is dissolved in water with yeast and left to react in the absence of air:

C₆H₁₂O₆(aq) → 2 C₂H₅OH(aq) + 2 CO₂(g)

• The yeast contains enzymes that catalyse the anaerobic breakdown of glucose into ethanol and carbon dioxide
• Conditions:
  • Temperature kept at about 25–35 °C (optimum near 30 °C)
    • Below 25 °C the enzymes work too slowly
    • Above 35 °C the enzymes start to denature (lose their shape and stop functioning)
  • No oxygen — fermentation is an anaerobic process. If air is allowed in, Acetobacter would oxidise the ethanol back to vinegar (see section 2 part b)
• The yeast keeps fermenting until the ethanol concentration reaches around 14–15%, at which point the alcohol kills the yeast and the reaction stops
• The fermented mixture is then fractionally distilled to give purer ethanol (up to about 96%)
• Fermentation is a batch process — each fermenter is filled, run, drained and refilled separately
• The route depends only on plant material (a renewable source), so it dominates ethanol manufacture in countries with large agricultural surpluses such as Brazil

Comparison