Dynamic model of the reactor HB (gPROMS)

     In this research line, we now are focusing on the Green Haber-Bosch process, which means:

• The synthesis section of the Haber-Bosch remains unchanged, with the following synthesis loop configuration:
• After the compression stage, the synthesis gas enters the ammonia converter. Ammonia is then separated by condensation, and the unreacted gas is recycled back to the reactor.
• Interestingly, this configuration is that used in the conventional Haber-Bosch process when the make-up gas is both pure and dry. Other configurations are also employed in the industry depending on the conditions of the make-up gas.
• Hydrogen is produced by water electrolysis.

     This process, entirely powered by green electricity, results in ammonia production. Therefore, the Green Haber-Bosch process can be conceived as a Power-to-Ammonia system.

     In a Power-to-Ammonia system, the fluctuating nature of renewable energy means that hydrogen production is not constant over time. As a result, the synthesis loop must be operated dynamically, as the hydrogen content in the makeup gas to the synthesis loop varies according to the availability of renewables. This presents a major operational challenge, particularly at the reactor level, where process conditions have been optimized over the years for steady-state operation. Current industry practice is to have ammonia synthesis plants designed for a particular steady-state point with relatively constant throughput. Moreover, typical steady-state operation of NH₃ converters occurs at the point of highest stability, not at the optimal operation point, as the latter is close to an unstable operation region, which can ultimately lead to reaction extinction and/or temperature oscillations–both of which are undesirable, as they can severely damage the catalyst and cause material fatigue. Additionally, reaction extinction requires a significant external heat load to be provided to the converter, so that the reaction is reignited, and the converter can resume its autothermal operation.

     Dynamic operation of the synthesis loop, and thus of NH₃ converters, is, of course, associated with these very operational issues, since the introduction of variable hydrogen loads over time will necessarily shift the reactor operation away from that well-defined steady-state point. As so, it increases the risk of operation slipping into an unstable region, where either the reaction is extinguished, or temperature oscillations arise. To address such a problem, strategies that allow for the accommodation of variable hydrogen loads must be developed so that the converter (and, more broadly, the synthesis loop) can continue their normal operation in a flexible manner. This is the focus of this work.

     Team GrAPHy has selected gPROMS® software to carry out dynamic studies of the NH₃ converter and the synthesis loop. Utilizing the fixed-bed catalytic reactor models available in the gPROMS® library, we are working with rigorous partial differential-algebraic equation (PDAE) models that reproduce fixed-bed radial-flow reactors, which are those encountered in the ammonia industry.

     Currently, several dynamic tests are being performed to gain a deeper understanding of the intricate dynamics of an ammonia converter. We are investigating the effect of variables such as inlet reactor temperature, feed flow rate, H₂:N₂ molar ratio, reactor operating pressure on the reactor performance.

     In the near future, we expect to propose feasible operating strategies for this system to make it more robust and flexible, enabling its operation under a wider range of conditions. These strategies may involve developing control strategies (such as model predictive control) or simply relying on the additional equipment to dampen fluctuations in the hydrogen supply.