Patent Watch
July 21, 2008
Methanol-to-olefins—a new paradigm for olefins production? The methanol-to-olefins (MTO) process—discovered by Mobil in the mid-1980s—continues to receive much attention as a potential alternative for manufacturing ethylene and propylene. The conventional technology for producing olefins is steam cracking ethane and propane from natural gas or naphtha from crude oil. As natural gas and petroleum prices continue to increase, lower cost routes to olefins are being sought.
One approach, under active development in China, is gasifying coal to synthesis gas, converting synthesis gas to methanol, and producing olefins from methanol. The first two steps in this sequence have been commercially proven; but the final stage, the MTO process, has not been commercialized on a large scale and is still under development by R&D organizations worldwide.
T. Xu and co-inventors disclose a catalyst system and operating conditions that significantly boost the selectivity of MTO to the desired products, ethylene and propylene, and allow control of the ethylene/propylene ratio. Using a gallium-substituted bound zeolite, H(Ga)ZSM-5, and mixing methanol in various proportions with aromatic cofeeds maximizes olefin production and gives some control over the C2/C3 ratio.
In one example, when methanol alone is passed over H(Ga)ZSM-5 at 275 kPa and 450 °C, ethylene and propylene are made in 5.9 wt% and 49.8 wt%, respectively, at 100% methanol conversion. However, if the methanol is diluted with 50 wt% p-xylene, ethylene selectivity is boosted significantly to 44 wt% and propylene selectivity declines to 33.5 wt%. Overall, total ethylene–propylene production increases from 55.7 wt% to 77.5 wt%.
If the methanol/p-xylene weight ratio is 10:90, the total ethylene–propylene selectivity increases to 89.8 wt%, with 72.7 wt% ethylene. There is a tradeoff between maximizing olefin selectivity and increasing capital expense because diluting the methanol with an aromatic hydrocarbon increases the size of the equipment needed for a given output of ethylene and propylene. (ExxonMobil Chemical Patents [Houston]. U.S. Patent 7,393,990, July 1, 2008; Jeffrey S. Plotkin)
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