Abstract
One of the current challenges for hydrogen-related technologies is its storage and transportation. The low volumetric density and low boiling point require high-pressure and low-temperature conditions for effective transport and storage. A potential solution to these challenges involves storing hydrogen in chemical compounds that can be easily transported and stored, with hydrogen being released through decomposition processes. Ammonia stands out as a promising hydrogen carrier due to its high hydrogen content (17.8% by weight), relatively mild liquefaction conditions (~10 bar at 25°C), and the availability of a well-established storage and transportation infrastructure. The objective of this study was to develop a mathematical model to analyze and design a membrane fixed-bed reactor (MFBR) for large-scale ammonia decomposition. The kinetic model for the Ru-K/CaO catalyst was obtained from the literature and validated using the experimental data reported in the original study. This catalyst was selected due to its effective performance under high-pressure conditions, which increases the drive force for hydrogen permeation in the membrane reactor. The model was developed in Aspen Custom Modeler (ACM) using a 1D pseudo-homogeneous approach. The governing equations for mass, energy, and momentum conservation were discretized via a first-order backward finite difference method. The effectiveness factor was incorporated to account for intraparticle mass transfer limitations, which are prevalent with the large particle sizes typically employed in industrial applications. The study further investigated the influence of sweep gas ratio, temperature, relative pressure, and space velocity on ammonia conversion and hydrogen recovery, employing response surface methodology generated through an ACM-Python interface. The proposed multi-tubular membrane reactor achieved approximately 94% ammonia conversion and 90% hydrogen recovery, operating at an inlet temperature of 380°C and a pressure of 40 bar. Under comparable conditions of temperature, pressure, and WHSV, the membrane reactor demonstrated an approximately 41.7% higher ammonia conversion compared to a conventional fixed-bed reactor. Furthermore, the developed model is easily transferable to Aspen Plus, facilitating subsequent process conceptual design and economic analyses.