Co-production of hydrogen and ethylene in an oxygen permeable membrane reactor
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Date
2022-04-08
Authors
Elmanakhly, Faris
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
The demand for low-carbon hydrogen keeps increasing. Hydrogen production from water splitting attracts attention due to the easiness of hydrogen purification from hydrogen-water mixtures and the flexibility of renewable energy integration. A potential technology is oxygen permeable membrane-supported water splitting. The membrane separates oxygen from hydrogen and pushes the thermodynamic equilibrium for higher water conversion ratios. Meanwhile, the call for a more sustainable and less energy-intensive process for ethylene production has always been there. Integrating oxidative coupling of methane (OCM) to membrane-supported water-splitting technology can utilize the oxygen from water splitting to co-produce higher value products (e.g., ethylene).
The technology investigated uses catalysts to increase the number of active sites on the membrane surface, which facilities the production rates and selectivity. On the feed side, the oxygen incorporation process is through the gaseous oxygen and oxygen vacancies at the membrane surface to form lattice oxygen. Then the lattice oxygen diffuses through the membrane driven by potential chemical gradients. Once the lattice oxygen reaches the sweep side, a reaction between lattice oxygen and electron holes at the membrane surface releases gases oxygen. The final step includes the mass transfer of gases oxygen from the membrane surface (sweep side) to the gas (methane) stream, which provides the necessary oxygen molecule for OCM reactions to convert methane to higher hydrocarbons such as ethane and ethylene. The entire process can be driven by renewable energy to co-produce hydrogen and ethylene with limited CO2 production, thanks to the high selectivity catalysts.
This research develops a high-fidelity membrane reactor model that combines the microkinetic of water splitting, catalytic OCM reactions on the membrane surface, and the charged species diffusion across the membrane. The model helps evaluate the effect of using an oxygen-permeable catalytic membrane reactor on the co-production of ethylene and hydrogen. The results show that using a membrane reactor for this process provides a more controlled oxygen inlet concentration (or partial pressure), increasing ethane and ethylene production rates while enhancing the water conversion ratio. The membrane reactor achieved a C2+ yield of 25.64 %, which lies in the industrial range for the C2+ yield estimated in this research. This achieved C2+ yield promotes this technology to be industrially applicable.
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Keywords
Oxidative coupling of Methane, Membrane reactor, oxygen permeable membranes, water thermolysis, hydrogen, hydrocarbons