Optimal Operation of a Reaction-Absorption Process for Ammonia Production at Low Pressure

Renewable ammonia from wind and solar power has significant potential for decarbonization efforts, despite technical and economic challenges. A promising low-pressure alternative to the high-pressure Haber-Bosch process is replacing condensation with absorption. This enables distributed small-scale production near renewable sources. However, the reduced pressure results in low single-pass conversion, emphasizing the importance of efficient operation for competitiveness with fossil fuel methods. In this study, we focus on optimizing the operation of an ammonia production system that includes a three-bed reactor (filled with a wustite-based iron catalyst), one absorption column (loaded with MgBr2-Si), and three heat exchangers. Our goal is to maximize ammonia production by simultaneously optimizing six key process variables including five flow rates and one temperature. A dynamic optimization problem based on a first-principles model of the process system is solved using a direct method. Different constraints on the feed gas are considered. Our results indicate that the optimal operating profiles lead to maximum ammonia production rates that remain nearly constant for most of the time horizon, despite limitations on the feed gas availability. Moreover, the optimal solution exhibits a turnpike behavior, which is characterized by spending most of the time close to an optimal steady-state value. This finding suggests that efficient and steady ammonia production at low pressure can be achieved in a dynamic environment such as intermittent feedstock supply.