Question

What technique is used to butane from a mixture of propane and butane

Answer 1

The answer is chilling. Look up the temperatures needed to liquefy the four components and then cool the mixture and the propane and butane will become liquid.

Answer 2

Depropanizer extracts propane from propane + and sends the remaining mixture, called butane cut +, to the debutanizer. The debutanizer extracts butane from butane + and the remaining products are called condensates or pentane +. The splitting is then complete.