Cryogenic distillation

Abstract

A cryogenic technique for recovering pure products from a mixture of at least three close-boiling components. A preferred process is provided for separating a hydrocarbon mixture containing an alkene (i.e. ethene or propene), corresponding alkane having the same number of carbon atoms and at least one heavier hydrocarbon component. The improved process comprises: feeding the hydrocarbon mixture to a first distillation tower having an upper reflux stage; recovering a first overhead vapor stream rich in alkene and alkane from the first distillation tower and passing the first overhead vapor stream to a middle distillation stage of a second multi-stage distillation tower; recovering a second overhead vapor stream rich in alkene from the second distillation tower; adiabatically compressing the alkene-rich vapor stream and passing the compressed vapor to a second distillation tower reboiler stage. This provides a heat pump for cooling and condensing the compressed vapor and heating a liquid reboiler stream. Pressure in the alkene stream is reduced by flashing cooled and condensed vapor from the reboiler stage to provide a partially vaporized flashed mixture stream rich in alkene, followed by recovering and separating the flashed mixture stream to provide recovering a liquid portion and vapor portion. The liquid portion is passed to a second distillation tower reflux stage and a pure alkene stream is recovered.