Process and apparatus for cryogenic fractionation of synthesis gas

The invention relates to a process and an apparatus for obtaining pure hydrogen (4) and a carbon monoxide product (31) from a predominantly hydrogen- and carbon monoxide-comprising input gas (1) which is partially condensed by cooling to obtain a first carbon monoxide-rich, hydrogen-containing liquid phase (6) and a first hydrogen-rich, carbon monoxide-containing gas phase (3) which via a membrane (M) is fractionated into pure hydrogen (4) and a carbon monoxide-rich, hydrogen-containing retentate (5) from which subsequently with the aid of a H2-separating column (T1) also employed for fractionating the first carbon monoxide-rich, hydrogen-containing liquid phase (6) hydrogen is removed. It is characteristic here that the retentate (5) is partially condensed by cooling to obtain a second carbon monoxide-rich, hydrogen-containing liquid phase (11) and a second hydrogen-rich, carbon monoxide-containing gas phase (12), of which merely the liquid phase (11) is introduced into the H2 separating column (T1) for remo