Investigation of the limits of unconventional ammonia synthesis

Abstract

Intensified NH₃ synthesis is modeled in a Fe-based catalyst-coated, in-situ cooled microchannel reactor (MR) comprising a ZnCl₂-IMS (Immobilized Molten Salt) membrane. NH₃ produced in the reaction (R) channel is separated to the neighboring permeate (P) channel hosting sweep N₂ flow that regulates temperature to <623 K. A low-cost model is formulated and successfully benchmarked with the comprehensive counterpart and experimental data. MR configurations are modeled to elucidate the effects of catalytic activity, NH₃ permeance, sweep gas-to-reactive stream ratio (SR), and flow partitioning on the interplay between NH₃ production, separation, and heat transfer. At 613 K, 50 bar, H₂/N₂ = 3 and 1.5 × 10^–3 m³ kg_cat^-1 s^-1, the 47.4 % N₂ conversion exceeds the equilibrium limit (42 %) and the membraneless counterpart (14 %). Under identical conditions, a two-fold increase in the reaction rate and NH₃ permeance gives 91 % N₂ conversion. Increasing SR promotes N₂ conversion and NH₃ recovery, but decreases NH₃ fraction in the P channel, a metric for the post-NH₃ separation by a secondary membrane separator. While co-current dosing to the R and P channels offers slightly higher N₂ conversions, the identically operated counter-current mode gives higher NH₃ recovery. The findings provide guidelines for the opportunities and limitations of the membrane-separation driven NH₃ synthesis.