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

Abstract

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 BaFe0.9Zr0.1O3- 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 XH2O=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 XC2H6=10%, the oxygen permeation flux (JO2) 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 BaFe0.9Zr0.1O3- 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, BaFe0.9Zr0.1O3- 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.