Methane Aromatization in a Membrane Reactor
The goal of this project was to develop a catalytic reactor process that can continuously generate aromatic hydrocarbon liquids from stranded natural gas deposits. The originally proposed idea was to use a fluidized bed to perform this reaction while cycling the catalyst between reaction and regeneration conditions at an optimal temperature to prolong catalyst and membrane life. Regeneration conditions (including H2, possibly with small amounts of H2O, and CO22 added to aid in coke removal) would remove coke from the catalyst before damaging quantities can accumulate and irreversibly deactivate the catalyst (Xu at al. 2017 used H2) In our original design, a Mo2C/V (or TiC/V) membrane module would be located in a fluidized bed MDA reactor to extract H2 as it is produced during the reaction. To minimize erosion of the membrane, it would be suspended above the bubbling bed of catalyst particles in the freeboard of the reactor. As long as the membrane itself was not deactivated by the reaction environment (gas composition, temperature, etc.) then the catalyst produces benzene via the MDA reaction, which is removed as vapor, while the equilibrium is shifted to higher benzene yields by real-time removal of hydrogen produced by the MDA reaction. Coke is hydrogenated back to CH4 (and/or other light hydrocarbon gases) or oxidized in the catalyst regenerator (a separate fluidized bed) and recycled back to the MDA reactor. Unfortunately, in Phase I, we found two problems: 1) At moderate temperatures (ca. 500°C), the H2 permeability of the Mo2C/V and TiC/V membranes falls to zero when there is methane added to the feed gas. Even when the gas composition was 10% CH4 + 90% H2, the permeability of the membranes was drastically reduced. The flux experiment was repeated (without catalyst) at the MDA temperature (700°C) and the membrane irreversibly coked due to methane pyrolysis. Even if there were a method to expose the membrane to pure H2 in the absence of CH4 (in an attempt to regenerate it), the membrane does not completely regenerate in H2. The fact that any other gas besides hydrogen or a noble gas (Ar, He) poisons the membrane severely limits the applicability of Mo2C/V and TiC/V membranes when separating H2 from other gases, especially with methane dehydroaromatization. Because of the unanticipated (and unfortunately unsatisfactory) performance of the Mo2C/V and TiC/V membranes, we tried several palladium-based membranes, as they are standards for H2 separation. Specifically, we tried a membrane that contained 60% Pd and 40% Au. Adding gold to the Pd membrane makes it much more resistant to carbon deposition (coking) compared to pure Pd membranes. We also used a 100 μm thick membrane to counteract the problems that Pd membranes have at temperatures above about 600°C. Unfortunately, while more resistant to coking than pure Pd or Mo2C/V membranes, the Pd/Au membrane material still coked, losing its H2 permeability. Finally, we tested three MDA catalysts for their activity and selectivity for benzene formation in a fixed bed reactor (no membrane). Two were laboratory samples from Clariant that consisted of Mo/MCM-22 zeolite and one was a Mo/HZSM-5 catalyst made at TDA. In both cases, smaller particles were found to exhibit higher conversions to benzene, which is consistent with a decrease in mass transport limitations as the particle size is decreased. In both cases, benzene conversions on the order of 10-15% (close to the equilibrium limit) were observed with smaller catalyst particles. The catalysts were not tested in a membrane reactor because the membranes are irreversibly coked by methane pyrolysis when tested at 700°C in mixtures containing as little as 10% CH4 in H2. Methane dehydroaromatization operates with a feed that is essentially pure CH4. Unfortunately, it is not possible to de coke the membranes without permanently destroying their H2 permeability. Nevertheless, if a high-permeability, high-selectivity H2 membrane material could be found that does not coke at 700°C during MDA, it should be possible to use a membrane reactor to shift the equilibrium to obtain higher yields of benzene and other light aromatics. This in turn would make the MDA reaction more economically attractive as a method for converting natural gas into more valuable petrochemical streams.
- Research Organization:
- TDA Research, Inc.
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0018485
- OSTI ID:
- 1491580
- Type / Phase:
- SBIR (Phase I)
- Report Number(s):
- TDA-2101-014-G; TDA-2101-014-F
- Country of Publication:
- United States
- Language:
- English
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