April 9, 2012
Make monoaromatics from natural gas. Because of the ever-increasing supply of inexpensive shale gas in the United States, there is interest in developing new technologies for converting this low-cost resource to chemicals normally made from more expensive petroleum-based hydrocarbons such as naphtha. Specifically, manufacturers conventionally produce monoaromatics (benzene, toluene, and the xylenes [BTX]) by reforming naphtha in a refinery or by extracting them from ethylene steam cracker pyrolysis gas. A new route to BTX from inexpensive methane instead of expensive naphtha would give the developers a significant economic advantage.
L. L. Iaccino and co-inventors disclose a process for making aromatics largely from methane. In an example, they fed natural gas containing 30.5% CO2 to a methanation reactor loaded with a Cu–Zn catalyst. The reactor operating conditions were 300 °C and 2070 kPa. The CO2 in the feed reacted with hydrogen from a recycle stream to increase the CH4 content. The inventors then sent the feed to a CH4 aromatization reactor that contained a MoZSM-5 catalyst and was maintained at 900 °C and 50 kPa.
The aromatic products consisted of 90 wt% of a benzene fraction and 10 wt% of a heavy fraction that contained naphthalene. The inventors combined 100 kg of the benzene fraction with synthesis gas (81 kg CO and 11 kg H2) and fed it to an alkylation reactor that contained a mixed catalyst of supported Cr2O3–ZnO and Si-selectivated PZSM-5 at 500 °C and 2760 kPa. They separated the effluent to remove 136 kg of a xylene fraction with a p-xylene concentration of >90%. Finally, they recycled the hydrocarbons from the xylene-depleted effluent to the alkylation reactor. (ExxonMobil Chemical Patents [Houston]. US Patent 8,138,384, March 20, 2012; Jeffrey S. Plotkin)