May 21, 2012
Use methane-derived syngas to make liquid motor fuels. The availability of low-priced methane from gas production in the United States is stimulating interest in all types of C1 chemistry—for example, shuttered methanol plants are being restarted. The availability of inexpensive methanol is prompting interest in converting it to ethylene and propylene via methanol-to-olefin (MTO) processes. Methanol conversion to dimethyl ether, a low-cost diesel fuel substitute, is also under consideration.
Finally, gas-to-liquids (GTL) technology, in which methane is converted to liquid fuels via the Fischer–Tropsch (FT) conversion of synthesis gas (syngas) to long-chain hydrocarbons, is regaining interest. This technology is used on a large scale in Qatar to add value to gas produced from the world’s largest conventional gas well. This idea is under consideration in the United States as a way to upgrade inexpensive methane to transportation fuels.
One of the problems with FT technology is that it produces a wide range of carbon chains: from one to 100 or more carbon atoms. Only carbon numbers in the C5–C20 range are useful for transportation fuels; hydrocarbons with >20-carbon chains are waxy and must undergo secondary hydrocracking if they are to be used for liquid fuels. C. L. Kibby and colleagues found that adding small amounts of acetylene to the syngas significantly limits carbon chain growth and gives greater yields of useful liquid hydrocarbons.
In a control run, no acetylene was added to the syngas feedstock. The reactor temperature was 210 °C, the pressure was 5 atm, and the H2/CO mol ratio was 2.0:1. The throughput rate was 144 mmol/h per gram of catalyst, and the total reaction time was 5 h. Under these conditions, CO conversion was 60% and H2 conversion 65%. The reaction product was cloudy and opaque, and the yield of the C15+ cut was >46%.
A second run was made with 1.61 vol% acetylene in the syngas feed. Under the same conditions as the control run, CO conversion was 55%, H2 conversion was 70%, and acetylene conversion was 100%. The reaction product was clear and nonviscous, and the C15–30 cut yield was ≈40%, significantly lower than without acetylene in the feed. (Chevron U.S.A [San Ramon, CA] and Commonwealth Scientific and Industrial Research Organization [Campbell, Australia]. US Patent 8,163,808, April 24, 2012; Jeffrey S. Plotkin)