A column the size of a building
Crude oil is a mixture of thousands of hydrocarbons, from gases with one carbon atom to tars with dozens. A refinery heats it to about 350–400 °C in a furnace and feeds it, mostly vapour, into the base of a tall tower full of plates, exactly like the column of chapter 4. The tower is hot at the bottom and cool at the top. Each kind of molecule rises until it reaches plates cool enough for it to condense, and there it is drawn off: gases at the top, then petrol, kerosene, diesel, and the heavy residue at the bottom.
The residue is not heated further at normal pressure, because it would start to crack into tar and coke. Instead it goes to a second tower under vacuum, where lower pressure lets it boil at lower temperatures and yield lubricating oils, fuel oil and finally bitumen for roads.
Roses and steam
Perfume oils are too delicate to boil directly. In steam distillation, steam is passed through the plant material; the fragrant oils, though they boil far above water, are carried over with it at under 100 °C. In the receiver the oil and water separate again because they do not mix. A Florentine flask does this continuously: the lighter oil floats and overflows at the top, the water leaves through a tube from the bottom. That water, still scented, is sold as a hydrosol: rose water, orange-flower water.
The yields are tiny. It takes something like three and a half to four tonnes of damask rose petals to make a single kilogram of rose oil, which is why it is among the most expensive materials in perfumery. Lavender, peppermint, eucalyptus and many other essential oils are made the same way, in stills that look very like the gin still on the gin page.
Oxygen from air
Cool air far enough and it becomes a liquid. Distil that liquid in a column and it separates by boiling point, just as alcohol and water do: nitrogen boils at −196 °C, argon at −186 °C, oxygen at −183 °C. Carl von Linde built the first commercial plant for the job in 1902. The oxygen in a hospital cylinder and the argon in a welder's gas were very likely made by distillation.
| What is separated | From | Kind of distillation |
|---|---|---|
| Spirit | a fermented wash | pot or column |
| Essential oils, rose water | petals, leaves, peel | steam distillation |
| Petrol, kerosene, diesel | crude oil | atmospheric, then vacuum columns |
| Oxygen, nitrogen, argon | liquid air | cryogenic column |
| Fresh water | seawater | multi-stage flash (now mostly replaced by membranes) |
Seen from here, the copper still is one small, beautiful example of a very old and very general idea: things that boil differently can be parted. The rest is the craft of deciding what to keep.