(453e) Non-Catalytic Direct Partial Oxidation of Methane to Methanol in a Microreactor

This project uses an integrated mixer-reactor-heat exchanger microtube scalable module for the non-catalytic direct partial oxidation of methane in natural gas (NG) to methanol at elevated pressures (70-95 bar), reaction temperatures of 380-460°C, reactant CH₄:air ratios of 1.5-2.9 and reaction zone residence times on the order of 1 min.

Currently, methane in remote locations is often flared in large quantities rather than transported or converted to the more easily transportable methanol. However, if this gaseous methane were converted to liquid methanol at the wellhead, it could be transported in ways other than pipeline, or used onsite. For example, methanol is added to pipeline gas at remote locations to prevent hydrate formation in pipelines, and is a component used in water purification packages.

The microtube reactor/heat exchanger system for this process consists of a methane/air mixing, preheating and reaction section and a quenching section. In the quenching section, the heat transfer is from tubes to shell. The shell-side flow rate is controlled to attain the desired reaction and final (~250°C) process fluid temperatures. The entire reactor, tubes and shell, are made of Inonel 625 alloy. Alloy I625 is a high Ni-Cr steel (60 wt% Ni, 22 wt% Cr). To passivate the metal and reduce rates of total oxidation, the tubes are carbide-coated in a process based on the decomposition of propane at ~700°C. Several other coating schemes were tested along with several other potential tube metals (e.g., sulfiding instead of carbiding, and also metals C1010 mild steel, 304L, 904L, Nichrome and pure Ni) in coating experiments using coupons. The reasons for the choice of I625 and the carbiding process will be discussed.

For the current microreactor system, methane conversions of 12.5% have been achieved with a methanol selectivity of 80% at 417°C, 80 bar, 0.82 min space time and a CH₄:air molar ratio of 2.9. However, at 0.50 min and molar ratio 1.5 the methanol selectivity is only 16% (CH₄ conversion 14.5%, 417°C, 83 bar). So while methanol can be made selectively in the microtube reactor system, its selectivity can vary over a wide range depending upon certain reaction conditions.