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_0.9Zr_0.1O_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/cm²/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_0.9Zr_0.1O_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/cm²/sec and conversion of ethane to 79% while ethylene selectivity increased to 93%. Under long-term operation, BaFe_0.9Zr_0.1O_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 H₂ 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_0.9Ca_0.1FeO_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_0.9Ca_0.1FeO_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 H₂ and CO on the performance and defect chemistry of La_0.9Ca_0.1FeO_3-δ mixed-conductors, J. Membr. Sci. 529 (2017) 114-132.