Hydrogen and Ethylene Production through Water-Splitting and Ethane Dehydrogenation Using BaFe0.9Zr0.1O3- δ Mixed-Conductors

Hydrogen production from water-splitting has attracted significant interest because of its use in refining, chemicals’ production and as an alternative fuel. A promising technology for hydrogen production through water-splitting at moderate temperatures is the use of mixed ionic-electronic conducting (MIEC) membranes [1-2]. Using an inert gas on the oxygen-lean side, oxygen permeation rates are slow unless vacuum is used (or higher pressure on the feed side). Fuel addition in the oxygen-lean stream raises the oxygen chemical potential difference and hence the oxygen permeation and hydrogen production rate increase significantly [1-5]. One such fuel is ethane whose partial dehydrogenation leads to a valuable chemical, namely ethylene. Coupling water-splitting and ethane dehydrogenation using a MIEC membrane can reduce the complexity and capital cost of producing both (process intensification). This study investigated the co-production of hydrogen and ethylene using BaFe 0.9 Zr 0.1 O 3- δ membranes. Experimental measurements performed in a button-cell reactor showed significant oxygen permeation, ethane conversion and selectivity to ethylene. The performance of a 1.1 mm thick membrane operating at inlet X H2O =50% at the steam side (balance is nitrogen) was investigated as a function of temperature and inlet ethane mole fraction at the oxygen-lean side (balance is helium). At T=900 °C and X C2H6 =10%, the oxygen permeation flux (J O2 ) was ≈ 2.0 μmole/cm 2 /sec, while the ethane conversion and selectivity to ethylene were 95% and 83%, respectively. At these conditions, the combination of gas-phase and surface reactions lead to the production of other products, such as hydrogen, methane, acetylene, carbon monoxide and carbon dioxide. When using ethane, the oxygen permeation through BaFe 0.9 Zr 0.1 O 3- δ increases due to electrochemical reactions of these products with oxygen ions on the membrane surface, while the electron transfer process takes place through a redox mechanism that involves iron and its different oxidation states [5]. Lowering the temperature to T=850 °C decreased the oxygen permeation flux to ≈ 1.0 μmole/cm 2 /sec and conversion of ethane to 79% while ethylene selectivity increased to 93%. Under long-term operation, BaFe 0.9 Zr 0.1 O 3- δ shows good stability. To further increase the performance of the material, we investigated the limitations imposed by surface reactions and charged species diffusion in an effort to identify the rate-limiting step in the overall oxygen permeation process. References: [1]: X. Wu, L. Chang, M. Uddi, P. Kirchen, A. F. Ghoniem, Toward enhanced hydrogen generation from water using oxygen permeating LCF membranes, Phys. Chem. Chem. Phys. 17 (2015) 10093–10107. [2]: X. Wu, A. F. Ghoniem, M. Uddi, Enhancing co-production of H 2 and syngas via water splitting and POM on surface-modified oxygen permeable membranes, AIChE J. 62 (2016) 4427–4435. [3]: G. Dimitrakopoulos, A. F. Ghoniem, A two-step surface exchange mechanism and detailed defect transport to model oxygen permeation through the La 0.9 Ca 0.1 FeO 3- δ mixed-conductor, J. Membr. Sci. 510 (2016) 209–219. [4]: G. Dimitrakopoulos, A. F. Ghoniem, Role of gas-phase and surface chemistry in methane reforming using a La 0.9 Ca 0.1 FeO 3- δ oxygen transport membrane, Proc. Combust. Inst. 36 (2017) 4347–4354. [5]: G. Dimitrakopoulos, A. F. Ghoniem, Developing a multistep surface reaction mechanism to model the impact of H 2 and CO on the performance and defect chemistry of La 0.9 Ca 0.1 FeO 3- δ mixed-conductors, J. Membr. Sci. 529 (2017) 114-132.