Abstract: Apparatus and process for an energy saving system for the fractionation of a C.sub.4 stream containing predominately n-butane and isobutane using an open heat pump cycle in which the n-butane bottoms from the fractionation are the compressing medium and isobutane overhead is used to vaporize the n-butane by indirect heat exchange. A problem which would be expected to prevent this system is the unusual characteristic of n-butane (also isobutane) that on compression a liquid-vapor phase results, which is damaging to the compressor. This is overcome by passing the vaporized n-butane prior to compression, through an apparatus that removes any entrained liquids and heats the vapors to a temperature sufficient to prevent the formation of the liquid phase under compression. It has been found that this process results in the use of only about 9% of the outside energy previously used in conventional operation for a specific unit.