Getting Value from Steam Cracker C_4/C_5 Olefin Streams

摘要

An innovative catalytic cracking route to convert low-value C_4/C_5 olefin streams into a higher-value effluent stream, where propylene and ethylene are the main products, is described. The proposed process is simple in principle, since it can be performed in a fixed bed reactor, without steam co-feeding. The process is also flexible, regarding both feed and products. It can convert feeds derived not only from steam cracking, but also from a refinery FCC unit. Furthermore, it allows to a certain extent modification of the propylene/ethylene ratio, in favour of propylene in the products. This paper describes the catalytic testing work carried on in a lab-scale fixed bed reactor, aimed at developing a catalyst, which is highly selective towards light olefins (especially propylene), and does not suffer from deactivation, even in the absence of steam. High silica ZSM-5 was modified to increase its original selectivity towards propylene and ethylene, without reducing its excellent stability over time. This was accomplished by mitigating the activity of ZSM-5, resulting in a higher propylene yield. Different synthetic feeds were employed for testing, in order to simulate C_4 and C_5 streams from a steam cracker. These feed-stocks contain linear, branched and cyclic olefins as well as paraffins, with a total olefin content ranging from 60 to 77 wt%. The ZSM-5 modified catalyst has proved to be stable for at least 150 h on stream, at 525℃, WHSV = 2 h~(-1.) The use of this catalyst increases the yields of both ethylene and propylene, as well as the propylene/ethylene ratio, compared with the use of unmodified ZSM-5, and compared with the propylene/ethylene ratio from a naphtha steam cracker. Furthermore, the C_6/C_9 and BTEX production are reduced, which means a more stable catalytic activity, since the formation of the coke precursors is restricted.