distillation

Distillation separates crude petroleum into usable products by heating it to around 350°C and allowing vapors to condense at different temperatures as they rise through a fractionating column. Lighter hydrocarbons (smaller molecules) condense high in the column where it is cooler; heavier ones (larger molecules) sink and condense lower down where it is hotter. This temperature gradient does the sorting work mechanically, without chemicals.

The crude enters as a liquid near the base of the column and is immediately vaporized by a furnace. As the hot vapor rises, it encounters trays or packing that cool it progressively. At each level, some vapor condenses into liquid and drains sideways into collection trays, while uncondensed vapor continues upward. The result is a series of distinct product streams: petroleum gas and liquefied petroleum gas at the top, then petrol (gasoline), naphtha, kerosene, diesel oil, fuel oil, and finally bitumen at the bottom.

The crude's journey through the column

Crude oil is a complex mixture of hydrocarbons ranging from methane (one carbon atom) to molecules with 50 or more carbons. Boiling point increases steeply with chain length: methane boils at minus 162°C, petrol components around 40 to 200°C, diesel around 250 to 350°C, and heavier fractions much higher. The column exploits these differences. Engineers control the column's base temperature and reflux (how much condensed liquid is recycled back down) to optimize the split between products.

Fractionating columns are the industrial heart of every refinery. A typical column might be 30 to 60 meters tall and 2 to 4 meters in diameter, operating continuously under pressures of 10 to 20 bar. Heat loss through the walls is minimized with insulation because maintaining the right internal temperature gradient is critical to product quality. Trays can be bubble cap trays (older, robust), valve trays (more efficient), or sieve trays (simple perforations that allow vapor to bubble through liquid).

The term 'fractionation' and 'distillation' are used almost interchangeably in refining, though fractionation technically emphasizes the separation into distinct fractions rather than the boiling mechanism itself. Problems in the column are usually related to fouling (heavy residues accumulating on trays), foaming (poor liquid drainage), or loss of temperature control. The crude must be desalted (water and mineral salts removed) before entry, or corrosion and equipment plugging become severe.