NOVELTY - The process for separating a mixture of carbon monoxide (1), nitrogen, hydrogen and methane by cryogenic distillation in a system of separation unit, comprises separating the mixture in a column for obtaining a fluid enriched in carbon monoxide and nitrogen content, separating the fluid in a denitrogenation column, compressing a flow of carbon monoxide using a high pressure unit, cooling a part of the carbon monoxide with high pressure, passing a variable quantity of other part (57) of carbon monoxide through a valve (59), and passing the carbon monoxide mixture in a tub.    USE - The process is useful for separating a mixture of carbon monoxide, nitrogen, hydrogen and methane by cryogenic distillation in a system of separation unit (claimed).    ADVANTAGE - The process ensures efficient and fast separation of a mixture of carbon monoxide, nitrogen, hydrogen and methane, and allows a variation of reboiled flow of the carbon monoxide without destabilizing the column thus avoiding the energy loss.    DETAILED DESCRIPTION - The process for separating a mixture of carbon monoxide (1), nitrogen, hydrogen and methane by cryogenic distillation in a system of separation unit, comprises separating the mixture in a column for obtaining a fluid enriched in carbon monoxide and nitrogen content, separating the fluid in a denitrogenation column, compressing a flow of carbon monoxide using a high pressure unit, cooling a part of the carbon monoxide with high pressure, passing a variable quantity of other part (57) of carbon monoxide through a valve (59), passing the carbon monoxide mixture in a tub of the denitrogenation column, and varying the carbon monoxide flow in the valve in function of the reboiling of the denitrogenation column. The column comprises a methane washing column, a depletion column, carbon monoxide or methane column, the denitrogenation column in down stream or in upstream of the carbon monoxide or methane column, a phase separator in upstream of the denitrogenation column, and unit for passing the fluid to the separator. A fraction of the carbon monoxide is passed to a vaporizer of a tub of the depletion column and/or carbon monoxide or methane column. A fluid of high pressure carbon monoxide placed in the compressor is passed to a turbine using a unit. An INDEPENDENT CLAIM is included for an installation for separating a mixture of carbon monoxide, nitrogen, hydrogen and methane by cryogenic distillation in a system of separation unit.    DESCRIPTION OF DRAWING(S) - The figure shows a mixture separation process.    Carbon monoxide (1)    Fraction (7)    Exchanger (9)    Other part of carbon monoxide (57)    Valve. (59)