A Multi-Physics Model of Hydrogen Permeation through Pd-Ag Membranes: Quantifying Ammonia Inhibition in Catalytic Membrane Reactors

Ammonia is a promising carrier for storing and transporting hydrogen, but its efficient decomposition into high-purity hydrogen remains a challenge. Catalytic membrane reactors (CMRs) equipped with palladium-based membranes offer an elegant solution by integrating reaction and separation, yet their performance is critically influenced by complex transport phenomena and potential inhibition by ammonia. This thesis develops and rigorously validates a predictive, multi-physics model for hydrogen permeation through a supported Pd-Ag membrane to de-risk its application in ammonia cracking environments. The model is constructed incrementally, coupling external gas-film mass transfer, a modified Sieverts' law for the dense Pd-Ag layer, and dusty gas model (DGM) for the porous support. A sequential validation strategy was employed using permeation datasets from Cechetto et al. (2021). First, using non-inhibiting H2/N2 mixtures, a robust baseline model was established, demonstrating excellent agreement with experimental data (R² = 0.97, MAPE = 3.2%). This analysis revealed that hydrogen transport is co-limited by permeation through the dense metal and external mass transfer resistance, with the support playing a minor role. When applied to H2/NH3 mixtures, the baseline model systematically overpredicted the hydrogen flux, providing clear evidence of inhibition. The introduction of a physically-grounded Langmuir-Hinshelwood site-blocking term successfully eliminated this bias. The fitting procedure quantified ammonia as a weak, reversible inhibitor at typical operating temperatures (>400 °C), with a fitted enthalpy of adsorption (ΔHads = -40 kJ/mol) consistent with weak molecular chemisorption. Furthermore, the validated mechanistic model was compared with a permeation model based on modified Sieverts' law, demonstrating consistently higher H2 recovery of approximately 6--12 percentage points across the studied temperature range. Ultimately, this work delivers a validated, physics-based tool that not only accurately describes the permeation process but also provides crucial parameters for the design and optimization of ammonia decomposition membrane reactors. The findings confirm the viability of Pd-Ag membranes for this application by quantitatively demonstrating that ammonia inhibition is a manageable phenomenon, thereby advancing a key technology for the hydrogen economy.