Beyond equilibrium ammonia synthesis in a membrane and heat exchange integrated microreactor: A modeling study

Abstract

Ammonia (NH3) synthesis is modeled in a micro–structured membrane reactor (MR) comprising reaction, cooling and NH3 separation functions in the same volume. The proposed MR involves permeate and reaction channels segregated by layers of zirconia supported ZnCl2 immobilized molten–salt (IMS) membrane selective to NH3 transport. While H2 + N2 is fed to reaction channels washcoated with an iron–based catalyst, permeate channels host N2 as the sweep gas which also regulates reaction temperature. The in–situ cooled MR is modeled by considering mass, momentum and energy conservation in the fluid phases of the reaction and permeate channels, reaction in the catalyst layer and NH3 transport across the membrane, whose thermal stability limit of 623 K is set as the maximum reactor temperature. Upon N2 sweep dosing equal to 50 × the molar H2 + N2 input at H2/N2 = 3, 613 K, 50 bar and 1.5 × 103 m3 kgcat–1 s−1, MR can deliver ∼47 % N2 conversion which exceeds 40 and 13.5 % of the pertinent thermodynamic barrier and the membraneless case, respectively. Despite the exothermic heat release, co–current partitioning of the streams ensures operability below 623 K. Increasing space velocity and H2/N2 ratio, and decreasing inlet temperature and pressure inhibit reactor performance. Using molar sweep–to–reactive mixture ratios < 50 is penalized by the violation of the specified maximum temperature. Membrane integration can improve NH3 production by ∼3.5 times per unit amount of H2O consumed in its electrocatalytic conversion to H2.