December 16, 2013
Make aromatics from shale gas–derived methane. The advent of plentiful shale gas is rejuvenating the US petrochemical industry. The sudden availability of methane, ethane, and propane, at very low prices compared with those in Western Europe and Asia, is making the United States one of the world’s lowest cost ethylene-producing regions. Several world-scale ethane-based steam crackers are under construction.
There is, however, a downside to inexpensive ethane: Ethane-based crackers do not spin off valuable coproducts such as propylene, butadiene, and benzene. These products may be in short supply, and their prices may rise. This situation presents opportunities for innovation in developing “on-purpose” routes to these building-block chemicals from low-cost natural gas.
Inventor L. L. Iaccino is developing a route for making aromatics from methane. His patent discloses catalysts and process conditions for carrying out this process. The preferred catalyst is an aluminosilicate zeolite with an active metal, preferably molybdenum, deposited in its pores. Modifying metals such as gallium, iron, and cobalt may also be used.
Because the methane dehydrocyclization reaction is endothermic, the catalyst enters the reaction zone at ≈900–1100 ºC and exits the zone at ≈600–700 ºC. Reaction pressure is maintained between 50 and 200 kPa, and the weight-hourly space velocity is 1–20 h–1.
The dehydrocyclization reaction tends to deposit coke, so the catalyst must be continuously regenerated. The major components of the reactor effluent are hydrogen, benzene, naphthalene, CO, ethylene, and unreacted methane. The amount of aromatic rings in the effluent ranges between 5 and 30% more than in the feed. (ExxonMobil Chemical Patents [Houston]. US Patent 8,552,236, Oct 8. 2013; Jeffrey S. Plotkin)