(544g) Dynamic Simulation of an Ammonia Synthesis Plant Fed By Stranded Natural Gas in Aspen Hysys

The Haber Bosch process is one of the most important industrial processes to modern society. In 2015, 160 million tons of ammonia was produced mainly from the Haber Bosch process, with a global value of $250 billion [1,2]. The process has benefited from over 100 years of improvements and modifications. Today, the Haber Bosch process is being modified to operate on alternative feedstocks in an attempt to preserve the world’s natural gas reserves [2]. Modifying the Haber Bosch process to use Stranded Natural Gas (SNG) as a feedstock, can allow for the monetization of an unexploited resource [3]. SNG, is natural gas that is in a location that is uneconomical to recover due to its quantity, composition or both. SNG sources are intermittent and the quality of natural gas from the source can fluctuate dramatically. Previously a Haber Bosch process fed by SNG was studied at steady state [3], which does not accurately capture the transient nature of the feed source. Dynamic simulation of a Haber Bosch process fed by SNG will allow for accurate: operational, economic and environmental assessments of the modification.

This presentation will communicate the development, performance and validation of a dynamic model of a 100 tNH₃/day plant in Aspen Hysys [4]. The process of Fig. 1 uses feed streams of natural gas, water, and air to synthesize ammonia in a three-stage fixed bed reactor. Steam reforming of methane is performed to generate hydrogen from a variable stranded natural gas feed modeled after natural gas wells in the Bakken Basin of North Dakota. A water gas shift reactor is employed to convert CO from the steam reformer to CO₂. The CO₂ is removed ahead of the ammonia synthesis reactor using a Monoethanolamine stripping unit. Ammonia is synthesized at 200 atm and 450^oC, and is then refrigerated to -20^oC before being stored. The model includes heat integration of wastewater streams to reduce water consumption and pressure relief systems to prevent over-pressurization of system components. The model also includes controllers programmed to control system temperature, pressure, and flows. The steady state results of the model are compared and agree with the performance of previous works [4]. The following case studies are presented in this paper: startup under variable natural gas composition and flow, and normal operation under variable natural gas composition and flow. The model is found in good agreement with prior work and, therefore, it can be used as a benchmark virtual testbed for evaluation of intensified schemes of ammonia synthesis such as those presented in [4].

Acknowledgements
This work was partially sponsored by the United Technologies Corporation Institute for Advanced Systems Engineering (UTC-IASE) of the University of Connecticut. Any opinions expressed herein are those of the authors and do not represent those of the sponsor.

References

[1] Fao, World fertilizer trends and outlook to 2018; Annual Report 14; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 2015; ISBN 978-92-5-108692-6.

[2] L. Wang et al., “Greening Ammonia toward the Solar Ammonia Refinery,” Joule, vol. 2, no. 6, pp. 1055–1074, 2018.

[3] Z. Gou, Monetization of Stranded Gas through Ammonia and Urea Production, Texas A&M University 2017

[4]. L. Burrows et al., (In Review) “Dynamic Simulation of an Ammonia Synthesis Plant fed by Stranded Natural Gas in Aspen Hysys^â,” Chem. Eng. J.