Potential of Sorption-Enhanced Ammonia Synthesis - An Equilibrium and Reactor Modeling Study

Ammonia production is one of the most important industrial chemical processes, but the synthesis reaction is strongly limited by chemical equilibrium. This is commonly compensated by applying high pressures, but large recycle ratios and purging losses are still unavoidable. Equilibrium limitations can alternatively be evaded by sorption enhancement, where NH3 is selectively removed from the reaction mixture by a solid sorbent material. One material class commonly applied in this approach are metal halides like MgCl2, as they typically show high NH3 capacity even at elevated temperatures. In this study, a thermodynamic equilibrium model based on Gibbs energy minimization is established that is able to predict the simultaneous NH3 synthesis and sorption equilibrium. After parametrization for metal chloride-based sorbents, the model is used to estimate the potential effect of sorption enhancement on the NH3 synthesis in equilibrium. For kinetic studies under realistic operating conditions, a reactor model was established using kinetics for both iron and ruthenium-based catalysts. Simulations reveal that near-full conversion is possible in sorption-enhanced NH3 synthesis under a wide range of realistic operating conditions. At thermodynamically unfavorable conditions, the process benefits from overstoichiometric amounts of sorbent as this keeps the sorbent saturation low and thus increases the sorption driving force. The integration of a sorbent material into the NH3 synthesis reaction was shown to result in increased conversion, but at the same time also allows for a higher NH3 formation rate. An increase in H2 conversion by up to 550% was found at 350 °C, 100 bar, 15,000 h-1 for twice the stoichiometrically required sorbent. While it has been demonstrated experimentally before, these findings quantify and emphasize the vast potential of sorption-enhanced NH3 synthesis under a wide range of conditions.