Modelling the feasibility of membrane integration into periodic open cellular structures for ammonia decomposition

Abstract

Ammonia is one of the leading carriers for the storage and transport of renewable hydrogen, but its deployment requires compact and scalable technologies for efficient decomposition and purification. This study investigates how operating conditions and design factors impact external mass transfer in Porous Open Cellular Structures (POCS) interfaced with Pd-based membranes. To achieve this, a dip/spin coating method was optimized to deposit Ru based catalytic layer onto nickel alloy POCS produced via Selective Laser Melting (SLM), and kinetic activity was tested providing validation basis for CFD modelling activities. Numerical permeation tests highlighted the influence of packing type and porosity, revealing that the Kelvin 3-0.6 with baffles performed best at a Gas Hourly Space Velocity (GHSV) below 1211 h⁻¹, achieving higher hydrogen recovery and minimized concentration polarization. At higher GHSV, baffles improved the Concentration Polarization Coefficient (CPC) but resulted in slightly lower hydrogen recovery compared to baffle-free configurations. The study of ammonia decomposition in a Kelvin cell POCS membrane reactor revealed that optimizing POCS membrane reactors requires balancing hydrogen production kinetics with the extraction driving force. Hydrogen production increased with GHSV, peaking at 1850 h⁻¹ before declining due to non-permeating gas accumulation, and a similar trade-off was observed with porosity, where optimal performance occurred at 0.8 porosity before kinetic limitations caused hydrogen recovery to decline. Overall, optimizing POCS membrane reactors involves a balance of hydrogen production and extraction, and the integration of baffles has the potential further boost performance. Certainly, POCS could yield economic benefits by protecting the membrane and reducing mass transfer limitations, requiring less membrane area for a given separation.