Startup of an olefins production plant

1. In an olefin cracking process wherein a natural hydrocarbonaceous feed is employed in a furnace zone to cause thermal cracking of said natural feed to form a gaseous cracked feed product that contains molecular hydrogen, methane, and other hydrocarbonaceous compounds, said gaseous cracked feed product is passed to a quench zone to cool same and separate a liquid fraction from a quenched gaseous product, said quenched gaseous product is passed to a compression zone to be compressed, the thus compressed quenched gaseous product is cooled in a refrigeration zone to condense to a liquid state components of said natural feed that are desired to be recovered as a product of said cracking process thereby producing a mixture of said liquefied components and non-condensed gases including molecular hydrogen and methane, said mixture being passed to a demethanation zone for removal of said non-condensed gases, said liquefied components being subsequently processed in a fractionation zone to separately remove said liquefied components as final products of said cracking process, and wherein said cracking process is not in a normal operational mode as to temperature, pressure, and natural feed throughput and is in need of start up to achieve said normal operational mode, the improvement comprising 1) maintaining said furnace and quench zones in said non-operational mode, 2) maintaining said fractionation zone in an operating mode as to temperature and pressure and on total reflux, 3) introducing artificial feed gas into said compression zone for subsequent flow into said refrigeration and demethanation zones, said artificial feed having a chemical composition different from said natural feed and being of a composition that approximates the composition of said natural feed to an extent that minimizes condensation of gaseous components to a liquid state in said compression, refrigeration, and demethanation zones when compared to the condensation that would normally occur with said natural feed in said normal operational mode of said cracking process, 4) starting compression of said artificial feed in said compression zone while continuing the introduction of artificial feed until said compression zone is at a pressure of at least about 50% of its normal operational pressure, 5) after reaching at least about 50% of said normal operational pressure in said compression zone starting cooling in said refrigeration zone while continuing the introduction of artificial feed to said compression zone, 6) recycling essentially all gases from said refrigeration zone to the input of said compression zone with essentially no venting of same, 7) continuing said compression and refrigeration zone start up with said gas recycling to said compression zone until said zones reach essentially their normal operating conditions, 8) while said compression and refrigeration zones are progressing toward their normal operating conditions, removing condensed liquid formed in said refrigeration zone from that zone and passing said liquid to said demethanation zone, and starting operation of said demethanation zone, 9) after said compression, refrigeration, and demethanation zones have started up, starting up said furnace and quench zones while introducing natural feed into said furnace zone, and 10) when said quenched gaseous product from said quench zone approaches normal operating conditions while employing natural feed, terminating said introduction of artificial feed.

2. The method of claim 1 including removing gaseous components from said demethanation zone during start up of same and returning same to at least one of said compression zone and said refrigeration zone, and when said demethanation zone reaches normal operating conditions passing liquid there from to said fractionation zone.

3. The method of claim 1 wherein said natural feed and said quenched gaseous product obtained from said natural feed contains at least naphthenic, paraffinic, and aromatic hydrocarbons having from 1 to 35, inclusive, carbon atoms per molecule, and said artificial feed contains at least two of molecular hydrogen, methane, ethane, and propane in approximately the same proportions as those compounds are present in said quenched gaseous product when produced from said natural feed.

4. The method of claim 3 wherein said quenched gaseous product from said natural feed contains at least two of molecular hydrogen, methane, ethane, and propane, and said artificial feed contains at least the same components in approximately the same proportions as they are present in said quenched gaseous product obtained from said natural feed.

5. The method of claim 3 wherein said proportion of each of said components present in said artificial feed is within about 10 wt. % of the amount of each of those components in said quenched gaseous product obtained from said natural feed.