(14b) Distributed and on-Demand Ammonia Synthesis By Inorganic Membrane Reactor

The world’s primary energy sources are currently fossil fuels, including coal, oil and natural gas. Although society has benefited from the steady use of energy, continuing to use fossil fuels at such rates could lead to dire consequences. This motivates the development of new processes and technologies to produce carbon-neutral energy that breaks linear scaling relations while critically addressing environmental needs.

Ammonia (NH₃) is an important raw material for fertilizer production and today it is gaining attention as a new energy carrier for hydrogen (H₂). Conventionally, ammonia is synthesized through the well-known Haber-Bosch process at 400-500 °C and P~150 bar. Both critical reaction conditions and massive production (176 million metric ton of NH₃ in 2019 globally) make it one of the most energy extensive processes, consuming 1-2% of the world’s total energy.

Here, an intensified process using a ‘smart’ reactor (aka membrane reactor) is proposed to produce H₂ from methane steam reforming and NH₃ in the same system at lower operating conditions than the traditional process. In addition, the system allows for the separation of a highly concentrated stream of carbon dioxide, which can be captured to address environmental issues. This alternative system allows to synthesize NH₃ on demand with near zero emissions, without the necessity of centralized facilities.

The smart reactor consists of a stainless-steel module, equipped with two C-based gaskets at both ends of the tubular membrane to prevent the mixing of the permeate and retentate flow streams. The membrane has a thin Pd-Au layer deposited onto a porous ceramic support via electroless plating. The physical membrane characteristics are as follows: total length of 75 mm, active Pd length of 50 mm, and a 10 mm O.D.

Two commercial catalysts were packed into the smart reactor. The Ni-based catalyst – selective towards methane steam reforming reaction - was packed in the annulus region of the membrane reactor, while the Fe-based one - selective towards ammonia synthesis - was packed in the core of the membrane. Prior to the reaction tests, the Pd-based membrane was tested in pure gas permeation tests in order to evaluate the hydrogen/other gases ideal selectivity. Successively, natural gas steam reforming reaction tests have been performed at T = 400 °C, reaction pressure = 30 bar, and various permeate pressure and sweep-gas flow rate. Ammonia synthesis was then performed in the same system at the best operating conditions, in terms of hydrogen permeated flow rate, achieved in the previous experimental campaign.

Laboratory data suggests that this technology is promising for innovative NH₃ synthesis processes operating at reduced conditions while paying attention to the environment.