(171c) Optimal Renewable Production of Ammonia from Water and Air

Authors: 
Martin, M., University of Salamanca
Sánchez, A., University of Salamanca
Wind and solar energy are plentiful natural resources; however they are intermittent and difficult to handle. Chemical processes are capable of storing this energy in the form of useful products. Lately methane and methanol have been produced from CO2 and electrolytic hydrogen [1,2]. Both are easy to transport and to deal with within the chemical industry. Ammonia is typically produced from fossil sources. However, it can also be produced using air and water. It is not entire new idea; a process was discussed in the early 1925’s [3]. Nowadays, we can evaluate the use of solar and wind energy to produce the power required to produce the basic species for the fertilizer industry.

In this work we have proposed the optimal design and operation of a production facility of ammonia from air and water. The separation of air as well as water electrolysis are energy intense processes. Air is filtered and compressed before dehydration. Next, it is further compressed and cooled down before entering Linde’s double column. The product streams are used in the cooling stage of compressed air. In parallel, water is splitted into hydrogen and oxygen. Both streams must be dehydrated and compressed. Finally, hydrogen is mixed with nitrogen and fed to the reactor. Two reactor designs are considered. Both are three packed bed reactors. Each bed operates adiabatically. The first design is a three packed bed reactor that uses indirect cooling between beds. The second alternative is also a three packed bed reactor. However, in this case the streams exiting each bed are cooled down by mixing with fresh syngas before being fed to the next. All the units are modelled using mass and energy balances, thermodynamic principles such as chemical equilibrium and surrogate models based on rigorous simulation to predict the liquefaction performance, the distillation column operation and the ammonia recovery. In particular, ammonia converters are modeled in detail in MATLAB based on industrial case studies [4,5] to determine the conversion reached at each bed since typically equilibrium is not reached.

The optimization of the process selects the use of solar energy. Furthermore, a direct cooling reactor results in better performance. However, the production cost for ammonia is high, 0.9€/kg, as well as the investment, 1050MM€ for an average production of 320t/day. Half of the investment cost is due to the solar panels and the rest is the chemical plant as such. Further analysis is required to include the operation of these facilities with variable solar and wind availability under uncertainty.

References

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2.- Martin, M. Grossmann I.E: (2017) Optimal integration of a self sustained algae based facility with solar and/or wind energy J Clean Prod. 10.1016/j.jclepro.2017.01.051

3.- Ernest, F.A., Reed, F.C:, Edwards, W.L., (1925) A direct synthetic ammonia Plant. Ind.Eng. Chem. 17 (8) 775--788

4.-Elnashaie, S.S., Abash, M.E:, Al-Ubaid, A.S. (1988) Simulation and optimization of an industrial ammonia reactor. Ind. Eng. Chem., Res, 27, 2015-2022

5.-Galnes L.D., 1977 Optimal Temperatures for Ammonia synthesis converters. Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 381-389