Energetic and Kinetic Analysis of Ammonia Decomposition for Hydrogen Production in Packed Bed Reactors

• A novel heterogeneous 2D model was developed to simulate ammonia decomposition. • Radial gradients in concentration and temperature in the reactor were significant. • Transfer resistances internal to the catalyst were observed. • The influence of several operating parameters on reaction performance was studied. In this work, a novel heterogeneous two-dimensional (2D) model was developed to simulate ammonia decomposition in packed bed reactors for hydrogen production. Preliminary results showed significant axial and radial gradients in ammonia concentration and fluid temperature. Moreover, external mass and heat transfer resistances were negligible, but internal concentration and temperature gradients became significant under certain conditions. These results justified the need for a heterogenous 2D model to depict the reaction under study. The influence of several parameters was evaluated using ammonia conversion and the overall energy efficiency of the process as performance indicators. Decreasing the inlet ammonia molar flow rate, reactor inner diameter, and catalyst particle size, and using Ni-based catalysts enhanced ammonia conversion and energy efficiency. Furthermore, increasing the inlet pressure reduced ammonia conversion but improved energy efficiency. Moreover, increasing the reactor length under a constant space-time enhanced ammonia conversion, while having a negligible effect on energy efficiency. Additionally, raising the external wall temperature consistently increased ammonia conversion. However, at high conversions, further increases in temperature led to reduced energy efficiency. After an evaluation of these parameters, a 6 m reactor with a 7.5 cm inner diameter was proposed, packed with 1 mm diameter Ni/Al 2 O 3 catalyst particles, operating at an external wall temperature of 1013 K, an inlet pressure of 30 atm, and an ammonia inlet molar flow rate to reactor volume ratio of 20 mol∙s -1 ∙m -3 . Under these conditions, an ammonia conversion of 98.9 % and an energy efficiency of 91.4 % were achieved.