Modeling of a packed bed membrane reactor for ammonia synthesis: on the role of membrane performance and operating conditions

Ammonia (NH 3 ) has gained attention as a potential hydrogen carrier for its high hydrogen content and absence of carbon atoms. However, the main route for ammonia production is still the energy intensive Haber-Bosch process. In this study, the membrane reactor technology is proposed as an alternative to integrate ammonia synthesis reaction and its simultaneous separation within a single unit. A non-isothermal 1-D packed bed membrane reactor model was developed, in which ammonia synthesis is promoted by a ruthenium-based catalyst, and its in situ separation takes place by means of NH 3 -selective membranes. The mathematical model was first validated with experimental data in a packed bed reactor, and afterward used to study the feasibility of a packed bed membrane reactor, aiming to identify the optimal membrane performance required to enhance the reactor efficiency, in terms of H 2 conversion, NH 3 purity and NH 3 recovery. Initial results under isothermal conditions, demonstrate that a P N H 3 > 10 − 7 m o l P a − 1 m − 2 s − 1 and S N H 3 / H 2 = 50 are required to significantly boost H 2 conversion. On the other hand a S N H 3 / N 2 = 100 was selected as a trade-off value influencing NH 3 purity and NH 3 recovery. The operating conditions study revealed that both sweep gas-to-feed flow ratio and pressure difference across the membrane are crucial for enhancing reactor performance, with optimal conditions identified at SW = 6 and ΔP = 20 bar. Furthermore, in the adiabatic case study, the investigation of the heat exchange integration between feed gas and sweep gas streams, highlights the importance of the latter as cooling fluid, achieving the best results at inlet permeate temperature of 200 °C. This enabled to achieve an H 2 conversion of 93 %, along with NH 3 recovery and purity of 99.14 % and 5.65 % respectively. Hence, the mathematical model demonstrates that an adiabatic packed bed membrane reactor with the integration of heat exchange has the potential to attain greater H 2 conversion compared to the equilibrium constraint observed in a conventional packed bed reactor. • The current study investigated the impact of a membrane-enhanced process on ammonia production. • The results demonstrated a substantial enhancement in H 2 conversion in the membrane reactor. • NH 3 selectivity of 50 towards H 2 and 100 towards N 2 , are required as optimal membrane properties.