Fractional Distillation

A Refinery. There's a few here, but this one's in England
Refinery in Hampshire UK

..is the process whereby crude oil  – the unprocessed stuff that comes out of the ground – containing many different hydrocarbons is separated into usable fractions because they boil at different temperatures. You basically heat crude oil up, let it vaporise and then condense the vapour.

  1. Heat the crude indirectly with high pressure steam to temperatures of about 600°C.
  2. The mixture boils, forming vapour (gases); almost all substances go into the vapour phase.
  3. The vapour enters the bottom of a long column (fractional distillation column) that is filled with trays or plates.
    • The trays have many holes or bubble caps (like a loosened cap on a soda bottle) in them to allow the vapour to pass through.
    • The trays increase the contact time between the vapour and the liquids in the column.
    • The trays help to collect liquids that form at various heights in the column.
    • There is a temperature difference across the column (hot at the bottom, cool at the top).
  4. The vapour rises in the column.
  5. As the vapour rises through the trays in the column, it cools.
  6. When a substance in the vapour reaches a height where the temperature of the column is equal to that substance’s boiling point, it will condense to form a liquid. (The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column)
  7. The trays collect the various liquid fractions.
  8. The collected liquid fractions may:
    • pass to condensers, which cool them further, and then go to storage tanks
    • go to other areas for further chemical processing

Fractional distillation is useful for separating a mixture of substances with narrow differences in boiling points, and is the most important step in the refining process.

Refineries must further treat the fractions to remove impurities. They combine the various fractions (processed and  unprocessed) into mixtures to make desired products.  For example, different mixtures of chains can create petrol with different octane ratings.

Print out a copy of this Word file “Crude at a Glance” and learn it carefully – it gives all the detail you need to know

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