Introduction•Separations are crucial in chemical engineering. A typicalchemical plant is a chemical reactor surrounded by separators,where raw materials are prepurified in separation devices and fedto the reactor; unreacted feed is separated from the reactionproducts and recycled back to the reactor.•Chemical plants commonly have from 40% to 70% of both capitaland operating costs in separations.•Separation processes can involve two phases (gas and liquid,liquid and solid, gas and solid or two immiscible liquids)•Relatively easy to separate two phases due to density difference.•Difficult to separate components from a single phase.