(189a) Modeling and Optimization of Industrial Ammonia Synthesis Process

Ammonia synthesis is a major process in agricultural fertilizers production. It constitutes an intermediate step in the production of urea and other nitrogenous fertilizers. Ammonia synthesis bears enormous importance in Canada considering significant contribution towards agricultural sector, which constitutes 6.7% of national GDP of Canadian economy [1].

Taking advantage of proper mathematical modelling it is possible to study process behaviour and explore feasible engineering ways of its improvement. Thus, optimization of unit operation is a way to boost up process performance. Even minor improvements allow for considerable savings or production gain, as even 1-2% change in large scale brings significant benefits.

Conventional large scale ammonia synthesis, known as Haber process, is carried out in fixed-bed catalytic reactor under elevated temperature and pressure [2]. The processâ?? feedstock is mainly comprised of stoichiometric hydrogen/nitrogen mixture received from steam reforming upstream. In this work, authors perform modelling and optimization study of the industrial ammonia synthesis converter located in Canada. Converterâ??s modelling is done from the first principle: mass and energy balance account for heterogeneous gas-solid catalytic reaction with intra-particle diffusion. The approach is based on the one found in the literature [3] and extended in order to adapt for the converter design. The model was validated with industrial data and has provided authors with satisfactory prediction in order to perform further systematic optimization study. Due to exothermic nature of ammonia synthesis process, a vast amount of heat is generated within the converter. Hence, the excess heat is used to (a) supply converter with heat to maintain synthesis reaction, and (b) to generate steam to be used elsewhere on the facility. Using the heat regeneration as optimization objectives, the modelling and simulation study revealed conflicting behavior towards the objective functions. Moreover, heat regeneration affects ammonia production, which imposes additional constraints onto the optimization problem. Bearing the fact that one cannot satisfy all the objectives simultaneously, it was necessary to provide optimal solution where neither of objectives cannot be improved simultaneously. In order to do so, authors obtained a set of Pareto-optimal solutions representing a possible ways of heat integration, heat regeneration around the ammonia converter while keeping target product production rate high.

[1] â??An Overview of the Canadian Agriculture and Agri-Food System 2015,â? 2015. [Online]. Available: http://www.agr.gc.ca/eng/about-us/publications/economic-publications/alp....

[2] M. Appl, Ammoniaâ?¯: principles and industrial practice. Weinheimâ?¯;; New York: Wiley-VCH, 1999.

[3] S. S. Elnashaie, M. E. Abashar, and A. S. Alubaid, â??Simulation and Optimization of an Industrial Ammonia Reactor,â? Ind. Eng. Chem. Res., vol. 27, no. 11, pp. 2015â??2022, 1988.