The projected increase in world demand for ammonia (for production of fertilizers and as a potential energy carrier) and the benefits of distributed manufacturing make imperative the use of membranes as part of energy-efficient methods for separating the NH3/N2/H2 gas mixture produced on smaller scales through the Haber-Bosch process. Recent advances in high-aspect-ratio zeolite MFI nanosheets synthesis have led to membranes with ultra-high ammonia selectivity at room temperature. Given their stability at high temperatures, these MFI membranes are promising candidates for small-scale ammonia separation required in distributed green ammonia production plants. However, the high temperature and pressure conditions required for the Haber-Bosch process make it hard to evaluate their performance through laboratory experiments. For this reason, molecular simulations are useful to understand separation processes through MFI membranes and predict their behavior under conditions not easily accessible experimentally. In this context, the present work has evaluated the performance of bulk MFI and MFI nanosheets for the separation of ammonia in binary and ternary mixtures with hydrogen and nitrogen at temperatures ranging 373 to 523 K and pressure ranges suitable for a given temperature. NpT-Gibbs ensemble Monte Carlo simulations were carried out to understand the selective-adsorption behavior of ammonia. Following this step, NVT molecular dynamics simulations were performed to examine the transport behavior. Our work shows that bulk MFI and MFI nanosheets are selective towards ammonia adsorption, but that hydrogen diffuses faster than nitrogen in their corresponding binary mixtures with ammonia. The overall membrane selectivities obtained from our simulations are within factors of 3 to 4 of the experimentally-observed separation factors. However, more complex model systems need to be developed to account for the complexity of the actual experimental system.

Molecular simulations to understand ammonia/nitrogen/hydrogen separation in MFI nanosheet membranes

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.