Method for converting hydrocarbon feedstocks into olefinic product flows by means of thermal steam cracking

1. A process for converting feeds composed of hydrocarbons by thermal steamcracking to at least one olefin-containing product stream comprising at least ethylene and propylene, with at least partial conversion of the feeds in at least one first cracking furnace ( 1 ) and in at least one second cracking furnace ( 2 ), wherein a fresh feed (B) is fractionated into at least one first and one second fresh feed fraction (B 1 , B 2 ) of different composition and the first fresh feed fraction (B 1 ) is conducted at least partly into the first cracking furnace ( 1 ) and the second fresh feed fraction (B 2 ) at least partly into the second cracking furnace ( 2 ), characterized in that the second fresh feed fraction (B 2 ) comprises predominantly hydrocarbons having a maximum carbon number of 5, in that the first fresh feed fraction (B 1 ) comprises predominantly hydrocarbons having a carbon number of at least 6, in that cracking conditions that lead to a ratio of propylene to ethylene of 0.7 to 1.6 kg/kg at the cracking furnace exit exist in the second cracking furnace ( 2 ), and in that cracking conditions that lead to a ratio of propylene to ethylene of 0.25 to 0.85 kg/kg at the cracking furnace exit exist in the first cracking furnace ( 1 ), the values attained for the ratio of propylene to ethylene in the second cracking furnace ( 2 ) being above those in the first cracking furnace ( 1 ).

2. The process as claimed in claim 1 , characterized in that cracking conditions that lead to a ratio of propylene to ethylene of 0.8 to 1.4 kg/kg and preferably of 0.85 to 1.2 kg/kg at the cracking furnace exit exist in the second cracking furnace ( 2 ).

3. The process as claimed in claim 1 , characterized in that cracking conditions that lead to a ratio of propylene to ethylene of 0.3 to 0.75 kg/kg and preferably of 0.4 to 0.65 kg/kg at the cracking furnace exit exist in the first cracking furnace ( 1 ).

4. The process as claimed in claim 1 , in which the values attained for the ratio of propylene to ethylene in the second cracking furnace ( 2 ) are at least 0.1 kg/kg above, preferably at least 0.15 kg/kg above and more preferably at least 0.2 kg/kg above those in the first cracking furnace ( 1 ).

5. The process as claimed in claim 1 , characterized in that one or more fractions (P, T) which are obtained from the product stream and which comprise hydrocarbons having a maximum carbon number of 5 are recycled into the second cracking furnace ( 2 ).

6. The process as claimed in claim 1 , characterized in that one or more fractions (U) which are obtained from the product stream and which comprise hydrocarbons having a carbon number of at least 6 are recycled into the first cracking furnace ( 1 ).

7. The process as claimed in claim 1 , characterized in that the second cracking furnace ( 2 ) is supplied not only with the second fresh feed fraction (B 2 ) but also with a further fresh feed (BL) consisting predominantly of hydrocarbons having a maximum carbon number of 5.

8. The process as claimed in claim 1 , in which the cracking furnace exit temperature for the conversion in the second cracking furnace ( 2 ) is between 680° C. and 820° C., preferably between 700° C. and 800° C. and further preferably between 710° C. and 780° C. and more preferably between 720° C. and 760° C., and the cracking furnace exit temperature for the conversion in the first cracking furnace ( 1 ) is between 800° C. and 1000° C., preferably between 820° C. and 950° C. and more preferably between 840° C. and 900° C., the cracking furnace exit temperature of the first cracking furnace ( 1 ) being above that of the second cracking furnace ( 2 ).

9. The process as claimed in claim 8 , in which the cracking furnace exit temperature for the conversion in the first cracking furnace ( 1 ) is at least 10° C. above, preferably at least 15° C. above, more preferably at least 20° C. above, the cracking furnace exit temperature for the conversion in the second cracking furnace ( 2 ).

10. The process as claimed in claim 1 , in which 0.3 to 1.5 kg of steam per kg of hydrocarbon feed is used in the first cracking furnace ( 1 ), and 0.15 to 0.8kg of steam per kg of hydrocarbon feed in the second cracking furnace ( 2 ).

11. The process as claimed in claim 1 , in which at least one fraction (V) comprising predominantly hydrocarbons having a carbon number of 2 or 3 is obtained from the product stream and at least partly converted in a cracking furnace ( 3 ) for gaseous feed.

12. The process as claimed in any of claim 1 , characterized in that the fresh feed (B) used comprises natural gas condensates and/or crude oil fractions, especially naphtha, and/or synthetic and/or biogenic hydrocarbons and/or mixtures derived therefrom.