A crystallization separation process for separating an at least partially liquid mixture containing two volatile components (A and B) the two components forming a binary system in which the vapor pressure ratio of A:B is 0.1 to 10 at the B-triple point and is less than the ratio of the heat of sublimation to the heat of melting of the B-component at the B-triple point, the constant pressure phase diagram having a B-enriched solid-vapor two phase region covering a substantial concentration range above the three phase state temperature and a B-enriched solid and liquid region below such temperature wherein a first vapor mixture and first condensed mass are formed by vaporizing the two components in a first zone at a temperature and pressure lower than the triple point temperature and pressure, the first condensed mass comprising a mother liquor and a solid phase which is B-enriched, the heat released in the formation of the solid being utilized to vaporize the volatile components.
Such process can be extended to a gas mixture containing two volatile and condensable components (A and B) into substantially pure B and a B-lean component, in which heat is removed from the gas to cause part of the B to desublime, the A remaining in the vapor stream, the vapor stream then having heat removed therefrom while the vapor is at a temperature and pressure below the B-triple point values thereby causing formation of a further condensed mass.