A team of Idaho National Laboratory (INL) researchers, with colleagues at Georgia Tech, has pioneered an electrochemical process that could eliminate the need for high-energy steam cracking. In an open-access paper in the RSC journal Energy and Environmental Science, the researchers suggest that their study provides a disruptive approach for petrochemical manufacturing, shifting the paradigm from thermal chemical practice to a clean energy regime.

Ethylene is one of the largest building blocks in the petrochemical industry. Ethylene production is energy-intensive and uses steam-cracking—the most energy-consuming single process in chemical industry. Typically the steam cracking of ethane has a conversion rate of 70%, with ethylene yields of about 50%.

Ethane steam cracking consumes typically 17-21 GJ of process energy per ton of ethylene, of which 65% is used in high temperature pyrolysis, 15% in fractionation and compression, and 20% in product separation. Estimates put the product cost contribution of steam cracking at 60% and the manufacturing carbon footprint at two-thirds of the total.

Ethylene is predominantly produced from naphtha or ethane. The current oversupply of ethane, a major component of natural gas liquids, due to the shale gas revolution has stimulated the shift to ethane from naphtha for ethylene production.

Specifically, the INL team developed a novel, pure proton-conducting electrochemical cell for the co-production of ethylene and hydrogen via electrochemical non-oxidative deprotonation (NDP) of ethane (400-500 ˚C). The electrochemical cell consisted of a superior proton-conducting electrolyte thin film, a porous anode support and a porous cathode.

Ethane was fed to the anode and electrochemically deprotonated into ethylene and protons when an electrical field was applied. The generated protons transferred through the dense proton-conducting membrane to the cathode where they combined with electrons and formed high-purity hydrogen.

Schematic drawing of the reaction principle and the configuration of the electrochemical cell. The rate of the reaction was controlled by the flux of protons passing through the electrolyte, the kinetics of ethane oxidation reaction (e.g., deprotonation), and hydrogen evolution reaction. DIng et al. Click to enlarge.

The electrolyte of the electrochemical NDP cells is acceptor-doped barium zirconate cerate (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ, BZCYYb), which exhibits ionic conductivity as high as 6.2×10^-3 S cm^-1 at 400 ˚C with a small activation energy. This type of material has a very high proton transfer number at temperatures lower than 550 C,37 allowing pure proton conduction at high flux under reduced operating temperatures, where coking is restrained thermodynamically.

A fully assembled cell consisted of a dense 10 μm-thick BZCYYb electrolyte thin film on a porous BZCYYb-Ni anode support (300 μm), and a porous double perovskite PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) layer (30 μm) as a cathode.

At a constant current density of 1 A cm^-2, corresponding to a hydrogen production rate of 0.448 mol cm^-2 per day, and 400 ˚C, a close to 100% ethylene selectivity was achieved under an electrochemical overpotential of 140 mV. Compared to the industrial ethane steam cracker, the electrochemical deprotonation process can achieve a 65% in process energy saving and reduce the carbon footprint by as much as 72% or even more if renewable electricity and heat are used.

Comparison of process energies and carbon footprint in NDP and steam cracking. (a) A comparison of the process energies for ethylene production from ethane. (b) A comparison of the carbon footprint for ethylene production from ethane. The NDP was carried out at 400 °C whereas the steam cracking was performed at 850 °C. Ding et al. Click to enlarge.

The INL research, being conducted in conjunction with Massachusetts Institute of Technology and the University of Wyoming, was selected under an Office of Energy Efficiency and Renewable Energy (EERE) Advanced Manufacturing Office funding opportunity focused on advanced materials, advanced processes, and modeling and analysis tools for materials and manufacturing.

Taking the estimated energy manufactured and serviced in the United States in 2016 as an example, 34% of the manufactured energy and 39% of the serviced energy were associated with industrial applications, of which the petrochemical industry consumed 42%. Given the intensity of energy consumption in this industry and relevant carbon footprint, as much as 6.4 quadrillion BTU of energy could be saved (65%) if such low-thermal-budget technologies can be widely deployed. Clearly, enabling advanced process innovation in the thermodynamic and electrical domains can be disruptive for changing the manufacturing infrastructure and in establishing new businesses that drive economic prosperity.

As an emerging technology, there exists opportunities to modify electrode catalysts and proton conduction in electrolytes to further reduce overpotential, i.e. the electrical energy consumption. Scaling-up of the electrochemical cells into the real reactor is ongoing to determine production and operation durability.

—Ding et al.

DOE announced $35 million for 24 projects to support early-stage, innovative technologies and solutions in advanced manufacturing. These projects will work to reduce technical uncertainty and develop new knowledge associated with potential breakthrough materials, processes, and tools for U.S. manufacturers that could improve their competitiveness and enhance their energy efficiency.

The INL project will receive $1.25 million over two years for emerging research to address manufacturing challenges.

Resources

• Dong Ding, Yunya Zhang, Wei Wu, Dongchang Chen, Meilin Liu and Ting He (2018) “A Novel Low-Thermal-Budget Approach for Co-Production of Ethylene and Hydrogen via Electrochemical Non-Oxidative Deprotonation of Ethane” Energy & Environmental Science doi: 10.1039/C8EE00645H