Modeling of ammonia synthesis reaction in membraneintegrated microreactors for small-scale applications

The cascade of packed–bed reactors (PBRs) and microchannel membrane heat exchangers (micro–HExs) is modeled using ANSYS and MATLAB under 2D and 1D, non-isothermal, steady-state conditions. Permeate and reaction channels are physically segregated by layers of ZnCl2-immobilized molten salt (IMS) membrane that is selective for NH3 transport. Each Fe-based catalyst packed adiabatic PBR effluent, composed of H2-N2-NH3 mixture, is supplied into the reaction channels while N2 is supplied in the permeate channels as the sweep gas that regulates the inlet temperature of the PBR. Modeling considers the conservation of mass, momentum, and energy in the PBR and the micro-HExs. Membrane separation is modeled by Fick's' law. Output of the reaction channel is dosed to the next PBR where synthesis takes place. Due to the laminar flow conditions in the micro-HEx units, heat transfer and separation are solved simultaneously in ANSYS (v. 19.2) for both co- and counter-current flow. The solutions are compared with MATLAB, which is also used in simulating adiabatic catalytic reaction in the PBRs by a 1D pseudohomogeneus model. Upon validating the similarity of two models, MATLAB is used, then PBR and micro-HEx units are integrated under a single code. It is observed that the counter-current mode yielded better results. Later, two different configurations are designed to improve the system. The first involves adding a fresh nitrogen flow at 573K to the output of the reaction channel. In this arrangement, the temperature profile is observed to be as desired. Additionally, there is a significant increase in the converted nitrogen ratio. The other configuration involves feeding the outlet of the sweep channel into the next one. This prevents the issue of additional fresh N2 feed and helps increase the fraction of ammonia in the sweep channel, thereby facilitating the necessary conditions for obtaining pure ammonia in the later stages. As a result, a system consisting of 5 units operating in a counter-current mode with additional N2 feed and interconnected heat exchangers is optimal based on these calculations