Crude Assay and Distillation

Crude is not one thing

A crude assay characterises a particular crude: its density, sulphur content, the yield of each boiling range fraction, and the quality of those fractions — metals, nitrogen, acidity, wax and asphaltene content. Two crudes of the same density can behave entirely differently in a refinery.

This is the basis of refinery economics. A refinery is built to process a particular range of crudes, and the price difference between light sweet and heavy sour crude reflects how much processing is needed to turn each into saleable products. A complex refinery able to process cheap heavy sour crude earns the difference; a simple one cannot.

Desalting

Before distillation, salt and water are removed. Salt would hydrolyse to hydrochloric acid in the crude unit and corrode the overhead system severely, and metals poison downstream catalysts. Wash water is mixed with the crude and removed electrostatically — the same principle as field desalting but to a tighter specification.

Atmospheric distillation

The heart of every refinery. Crude is heated in a fired heater and fed to a fractionating column where it separates by boiling range:

• Gas and LPG — the lightest, off the top.
• Naphtha — gasoline blendstock and petrochemical feedstock.
• Kerosene — jet fuel.
• Gas oil — diesel and heating oil.
• Atmospheric residue — the bottoms, which cannot be distilled further without cracking.

Separation is achieved by counter-current contact between rising vapour and descending liquid across trays or packing, with heat removed by pumparounds and reflux. The cut points between products are adjusted to shift yield between them, which is a daily optimisation decision driven by product prices.

Vacuum distillation

The atmospheric residue still contains valuable material, but heating it further at atmospheric pressure would thermally crack it. Distilling under vacuum lowers the boiling points, allowing further separation at a temperature the molecules survive.

It yields vacuum gas oil — feed for the conversion units — and vacuum residue, which becomes bitumen, fuel oil, or feed to a coker or other bottom-of-the-barrel unit.

Why conversion is necessary

Straight-run distillation yields more heavy material than the market wants and less light material. Demand is concentrated in transport fuels, while crude contains a large heavy fraction of low value. Everything downstream of distillation exists to close that gap — converting heavy molecules into lighter ones and improving their quality. The measure of how much of this a refinery can do is its complexity, and complexity is what determines which crudes it can profitably buy.