July 7, 2014
If you can’t get higher olefins from steam cracking, how can you get them? Steam cracking is the “heart and soul” of the petrochemical industry: Depending on the nature of the feedstock, it produces key building blocks such as ethylene, propylene, butadiene, and benzene. As cracker feedstocks become heavier, more and more of these products are made.
For example, an ethane cracker mostly makes ethylene, with very little propylene, butadiene, and benzene. But a naphtha cracker gives, in addition to the main product ethylene, substantial quantities of the other valuable building blocks.
The economic performance of steam crackers is usually measured in terms of the cost of producing ethylene; the coproducts are given credit based on their valuations. The lowest cost ethylene in the world is produced in ethane-based steam crackers in Saudi Arabia because the ethane is priced very low (US$0.75 per million Btu [MMBtu], as set by royal decree). The next lowest cost ethylene is produced in ethane crackers in North America because abundant shale gas–based ethane is priced relatively low (US$3.50–4.50/MMBtu). Therefore, in both regions, ethylene crackers produce very little coproduct propylene, butadiene, and benzene compared with heavier-feed crackers in other parts of the world. This situation has prompted the petrochemical industry in North America and the Middle East to look for alternative supply sources.
n-Butenes are readily available from refinery-based fluid catalytic cracking (FCC) streams and from raffinate-2 streams in naphtha crackers. J. W. van Hal and co-inventors disclose a scheme to metathesize n-butenes to a mixture of ethylene, propylene, pentenes, and hexenes. Ethylene and propylene are valuable petrochemical building blocks, but pentenes and hexenes have relatively little value. To remedy this situation, the inventors show that the hexenes can be aromatized to give benzene and higher aromatics. The pentenes can be recycled to the front end of the metathesis reactor.
The question that must be answered before this strategy becomes commercial is: What is the cost of the capital equipment needed to carry out these conversions and various separations? This cost must be balanced against the increase in value created. (Saudi Basic Industries Corp. [Riyadh, Saudi Arabia]. US Patent 8,722,950, May 13, 2014; Jeffrey S. Plotkin)