A transfer line between the outlet of a steam cracker and the inlet for the quench system has metallic or ceramic inserts having a pore size from about 0.001 to about 0.5 microns inside the line forming a gas tight barrier with the inner surface of the line and having a vent for the resulting gas tight pocket are used to separate H2, CH4, CO and CO2 from cracked gases reducing the load on the down-stream separation train of the steam cracker.