(544dz) Modeling the Kinetics of Ethane Oxidative Dehydrogenation Via Chemical Looping

Ethylene is commercially produced from ethane via high-temperature pyrolysis in the presence of diluting steam a.k.a. steam cracking. The process requires a high temperature (>1100 K) and a large steam generation load, making it energy intensive with an energy consumption of 17-21 GJ/tonne ethylene [1]. Significant NO_x and CO₂ emissions and periodic shutdowns to burn out coke are other concerns. Oxidative dehydrogenation (ODH) of ethane, where ethane is partially oxidized to ethylene and water, is an alternative route. The partial burning of the product hydrogen provides the required heat thereby reducing the parasitic energy consumptions and CO₂/NO_x emissions. In such processes, highly exothermic catalytic reactors are mostly used, making reactor design a challenging task. In addition, oxygen produced from air separation is energy-intensive. A chemical looping based ODH (CL-ODH) scheme addresses the abovementioned challenges and offers an alternative mode of ethylene production. In this process, the required oxygen is supplied by lattice oxygen from a redox catalyst composed of mixed metal oxides. The reduced catalyst is oxidized with air in a separate reactor, where it is regenerated and recirculated to the reactor. This allows built-in air separation with minimal parasitic energy loss and better temperature control. It transforms the upstream section of a traditional steam cracking process and has a potential to boost ethylene production with significant energy savings.

Mn-based oxides have been shown to be effective as redox catalysts for the ODH of ethane [2]. Process modeling with ASPEN Plus^® show that CL-ODH with 85% ethane conversion provides over 80% reduction in the overall energy demand with similar reduction in the overall CO₂ emissions [3]. In this work, existing kinetic models for ethane thermal cracking are tested and validated using CHEMKIN-PRO first. Selective combustion of hydrogen to water, over the redox catalyst, shifts the equilibrium towards ethylene and partially provides the heat required for the cracking reaction. In the latter part of this work, the hydrogen-consumption, occurring due to the presence of the oxide, is incorporated into the gas-phase cracking scheme. This addition is in the form of surface kinetics of the redox catalyst towards hydrogen combustion, which are experimentally obtained. Results from the kinetic model are compared with experimental observations from a lab scale reactor. The kinetic model, which couples ethane cracking with hydrogen combustion would aid in optimizing the redox catalyst and process conditions for ethylene production via CL-ODH.

References.

[1] T. Ren, M. Patel, and K. Blok, “Olefins from conventional and heavy feedstocks: Energy use in steam cracking and alternative processes,” Energy, vol. 31, no. 4, pp. 425–451, Mar. 2006.

[2] S. Yusuf, L. M. Neal, and F. Li, “Effect of Promoters on Manganese-Containing Mixed Metal Oxides for Oxidative Dehydrogenation of Ethane via a Cyclic Redox Scheme,” ACS Catal., vol. 7, no. 8, pp. 5163–5173, Aug. 2017.

[3] V. P. Haribal, L. M. Neal, and F. Li, “Oxidative dehydrogenation of ethane under a cyclic redox scheme – Process simulations and analysis,” Energy, vol. 119, pp. 1024–1035, Jan. 2017.