Other separations

Distillation isn't always possible or cheapest. If the volatilities are too close, the chemical breaks down when heated, or the amounts are small, engineers reach for another method. Each one uses a different difference between the substances.

Absorption dissolves a gas into a liquid: flue gas is washed with an amine solution to capture CO₂, and air with water to remove ammonia. Stripping is the reverse: blowing gas or steam through a liquid to drive a dissolved gas out. Both run in columns like distillation, with the liquid flowing down and gas up.

Liquid–liquid extraction uses a second liquid that doesn't mix with the first and dissolves the wanted substance better. It's used to recover penicillin from fermentation broth and to take aromatics out of petrol streams. The solvent is then recovered, often by distillation, and reused.

Crystallization cools or concentrates a solution until the product comes out as pure crystals, which are then filtered and dried: sugar, salt and most drug ingredients are made this way. Filtration and centrifuges separate solids from liquids. Membranes let some molecules through and hold others back: reverse osmosis pushes seawater through a membrane at high pressure to make fresh water.

Choosing comes from the properties: big boiling point gap, distil; gas to capture, absorb; heat-sensitive or close-boiling liquids, extract; a solid product, crystallize; very dilute or a pure water stream, membranes. Many processes use several in a row.