NOVELTY - The method involves the removal of acetylene by solvent extraction and selective hydrogenation. A final gaseous current enriched in hydrogen and methane and a liquid fraction enriched in ethylene and ethane are collected.    USE - To produce a product enriched in H2 and methane, a product enriched in ethylene and depleted in acetylene and a product enriched in propylene from an anhydrous gas obtained by the pyrolysis of a gas containing H2 and 1 - 3C hydrocarbons. The process can also be applied to a gas obtained from the pyrolysis of naphtha and heavier hydrocarbons.    ADVANTAGE - The process has a number of advantages compared to prior methods; a reduction in total volumetric flow rate, hence a reduced volume of catalyst required, because of the low proportion of H2(2 - 10%), compared with 30 - 40% in traditional processes; a reduced risk of a race-away exothermic hydrogenation of ethylene; poisoning of the catalyst is partly eliminated; the low quantity of CO present has a beneficial moderating effect in the hydrogenation reaction.    DETAILED DESCRIPTION - A process for the fractionation of an anhydrous gas from hydrocarbons pyrolysis containing H2 and 1 - 3C hydrocarbons, including ethylene, propylene and acetylene into a current enriched in H2 and methane, a current enriched in ethylene and depleted in acetylene and a current enriched in propylene, involves:    (a) cooling the pyrolysis gas and progressively liquefying it by passing through a succession of heat exchangers at decreasing temperatures; condensates are obtained one enriched in propylene and another enriched in ethylene and ethane with a proportion of H2, methane and acetylene in solution. A residual H2-rich gas is collected;    (b) evaporating the condensates by lowering the pressure, and reheating to provide two fractions;    (c) introducing the partially evaporated fractions from (b) into a distillation column (de-ethanizer), the ethylene/ethane -enriched fraction entering at a point higher than the propylene-rich fraction. A primary gaseous current is separated at the head and a primary liquid current, enriched in propylene, from the base, which is collected;    (d) directing the primary gaseous current to a zone for elimination of acetylene by solvent extraction and selective hydrogenation by means of the H2 contained in the gaseous head current, giving an effluent free from acetylene;    (e) cooling and fractionating the effluent in a demethanizer column giving a head fraction enriched in H2 and methane, which is collected, and a base fraction enriched in ethylene and ethane, which is also collected.    An INDEPENDENT CLAIM is included for the installation to effect the process.    DESCRIPTION OF DRAWING(S) - The drawing shows an installation for treatment of a gas from the pyrolysis of ethane/propane and LPG.    Supply of pyrolysis gas (1)    Heat exchangers (2, 6, 9, 16, 20, 26)    Separators (4, 7, 11)    Deethanizer (C1)    Catalytic hydrogenation reactor (R1)    Demethaniser (C2)    Propylene-enriched effluent (18)    Ethylene/ethane enriched effluent (43)    H2 and methane-enriched effluent (44)