Degree

Doctor of Philosophy (PhD)

Department

Cain Department of Chemical Engineering

Document Type

Dissertation

Abstract

This project used 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 methane: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.

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. To passivate the metal and reduce rates of total oxidation, the tubes were either carbide-coated or carbonitride-coated in a process based on the decomposition of propane at ~700°C or the nitriding agent, ammonia, at ~700°C.

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, ~420°C 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 while the carbide layer is not stable under long-term, repeated use, the carbonitride layers can extend the usage. The nitriding of the carbide passivation layers inside the microreactor had little effect on the methanol selectivity in the DMTM reaction at similar conditions but did impact methane conversion.

Date

1-2-2025

Committee Chair

Dooley, Kerry

Share

COinS