(597d) Non-Catalytic Direct Partial Oxidation of Methane to Methanol in a Wall-Coated 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 to methanol at elevated pressures (70-95 bar), reaction temperatures of 380-460C, reactant CH₄:air ratios of 0.5-4.4 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. In the case of hydrate formation, the direct methanol to methanol (DMTM) process could save ∼$500,000/year for one remote wellhead installation and reduce its carbon footprint by 10⁴ metric tons of CO₂.

Additionally, methane is currently converted to methanol commercially via an expensive, energy intensive two-step process. First, the methane is steam-reformed to synthesis gas (CO and H2), which then undergoes a high-pressure catalytic conversion to methanol.

The microtube reactor/heat exchanger system for this DMTM process consists of a methane/air mixing, preheating and reaction sections 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 Inconel 625 alloy, 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. Other coatings, including nitrided carbide layers, are begin explored. The reasons for the choice of I625 and the carbiding process will be discussed.

With this system, one can control the five key parameters (temperature, pressure, residence time, wall inertness and methane:air ratio) to selectively produce methanol at greater than 8% per-pass yield (product of conversion and selectivity), under optimal conditions. For this module, these conditions were 80 bar, ~420C outer wall temperature, a methane/air molar ratio of 2.9 and a residence time of 0.8 min. At lower methane/air molar ratios good yields were still possible, but at lower methanol selectivities. 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, and whille the carbide layer is not stable under long-term, repeated use, it is currently stable for about two days of operation.