(518g) Nonlinear Model Predictive Control of Flexible Ammonia Production

Ammonia is the second most produced chemical globally with an annual production amount of 176 million tons [1]. Virtually all ammonia is synthesized through the Haber-Bosch process, in which nitrogen is converted to ammonia through a reaction with hydrogen at a temperature of 300-500 ^◦C and a pressure around 200 bar [2]. Green ammonia, synthesized with green hydrogen from water electrolysis and nitrogen from air separation, plays an important role in decarbonizing the agriculture [3] and energy [4] sectors. Dynamic operation of the Haber-Bosch process over a wide range, i.e. operation that largely follows the intermittency of renewable energy resources, is crucial to produce green ammonia without significant storage capacity for hydrogen which is prohibitively expensive [5]. Such flexible operation is at the heart of recent advancements by major ammonia technology vendors [6]. Yet, the control of the Haber-Bosch process under widely fluctuating hydrogen loads has not been addressed in the open literature. Existing works have focused on either regulatory control of the process with little (<10%) variations in the reactor load [7], or flexible operation for only a surrogate ammonia synthesis model in which the physical dynamics of the process are neglected [8].

In this work, a nonlinear model predictive control (NMPC) scheme is proposed and implemented for the Haber-Bosch ammonia synthesis process with a varying hydrogen feed flowrate. The considered process consists of three interstage-cooled reactor beds and a flash separator, with a flexible reactor load varying between 50% and 100% of its nominal capacity. The proposed control scheme aims to control the reactor temperatures, the separation pressure, and the liquid volume in the flash tank during feed transitions. A simulation study is performed given an assumed 5-hour feed schedule. The results indicate that all controlled variables can be maintained in a safe operating range and tracked with small offsets in the nominal case as well as under disturbances. It takes approximately 10 minutes for the reactor temperatures and 40 minutes for the separation pressure to reach steady state after a large step change in the reactor load.

A comparison with the PID control scheme adopted from [9] has also been conducted. The results show that the proposed NMPC scheme tracks the feed temperatures and the separation pressure at least 3 times faster and exhibits significantly less oscillations in the production flowrate. This study therefore demonstrates the feasibility and effectiveness of the proposed control scheme in enabling flexible ammonia production, and potentially motivates further research on this control problem integrating realistic process measurements and wider operating ranges.

References:

[1] W. David et al. “Ammonia: zero-carbon fertiliser, fuel and energy store”. In: Policy Briefing (2020).

[2] J. W. Erisman, M. A. Sutton, J. Galloway, Z. Klimont, and W. Winiwarter. “How a century of ammonia synthesis changed the world”. In: Nature geoscience 1.10 (2008), pp. 636–639.

[3] M. J. Palys, H. Wang, Q. Zhang, and P. Daoutidis. “Renewable ammonia for sustainable energy and agriculture: vision and systems engineering opportunities”. In: Current Opinion in Chemical Engineering 31 (2021), p. 100667.

[4] H. Du, P. Shen, W. S. Chai, D. Nie, C. Shan, and L. Zhou. “Perspective and analysis of ammonia-based distributed energy system (DES) for achieving low carbon community in China”. In: Iscience 25.10 (2022).

[5] C. Smith and L. Torrente-Murciano. “The importance of dynamic operation and renewable energy source on the economic feasibility of green ammonia”. In: Joule (2023).

[6] R. Ostuni and F. Zardi. Method for load regulation of an ammonia plant. US Patent 9,463,983. Oct. 2016.

[7] J. W. Rosbo, T. K. Ritschel, S. Hørsholt, J. K. Huusom, and J. B. Jørgensen. “Flexible operation, optimisation and stabilising control of a quench cooled ammonia reactor for power-toammonia”. In: Computers & Chemical Engineering (2023), p. 108316.

[8] D. Wen and M. Aziz. “Flexible operation strategy of an integrated renewable multigeneration system for electricity, hydrogen, ammonia, and heating”. In: Energy Conversion and Management 253 (2022), p.115166.

[9] C. Zhang, S. Vasudevan, and G. Rangaiah. “Plantwide control system design and performance evaluation for ammonia synthesis process”. In: Industrial & engineering chemistry research 49.24 (2010), pp. 12538–12547.