Design and optimization considerations of advanced catalytic membrane reactors for efficient ammonia synthesis and decomposition

Abstract

The conversion of distributed renewable electricity into liquid ammonia is a promising vector for hydrogen transport and storage, decarbonizing ammonia production and enabling efficient hydrogen distribution. This thesis describes the development of the catalytic membrane reactor (CMR) technology as a compact, effective, and low-cost solution for decentralized and scalable processes of ammonia synthesis and decomposition. We introduce a CMR configuration where the membrane and catalyst are in intimate proximity, mitigating mass transport limitations, and realizing record hydrogen productivity from ammonia decomposition using less catalyst mass at reduced temperature (≤ 450°C). A numerical simulation of the CMR captures the experimental results with high fidelity, identifying rate-limiting regimes, and illustrating pathways for further improvement. A Ru/α-alumina catalyst was shown to be more active than Ru/YSZ and adding it to the interior of the CMR significantly improves the performance, including increased conversion and doubled hydrogen productivity. Clinoptilolite is identified as a low-cost, reusable adsorbent that reduces residual ammonia in the permeate stream from ∼1000 ppm exiting the CMR to the ultra-high purity standards (< 0.1 ppm) required by fuel cell electric vehicles. The second topic focused on the development of CMR configurations for ammonia synthesis reactions at mild temperature and pressure. Yttria-stabilized zirconia (YSZ) is demonstrated to be a highly active support for Ru catalysts. A number of alkali and alkaline-earth metal promoters were evaluated with Ba found to be stable and deliver the highest specific activity reported to date. A microkinetic model was developed that was capable of capturing the strong dependence on the H2/N2 ratio and temperature, highlighting the importance of surface coverage in controlling the kinetics. Alternatively, a low-cost Cs-Fe/γ-alumina catalyst was developed that reached ∼ 50% of the activity of the Ba-Ru/YSZ catalysts. Finally, a reactor simulation study was conducted to quantify the potential of the CMR configuration for the scalable synthesis of ammonia. The ammonia permeance controls conversion and recovery whereas purity scales with an effective selectivity. A thermally stable membrane could enable effective ammonia synthesis at much milder conditions (P < 30 bar, T < 400°C) than the conventional Haber Bosch process.