Combined Exergy Analysis and Energy Integration for the Optimization of Nitrogen Fertilizer Plants

Modern ammonia production plants are equipped with efficient energy integration networks able to recover an important fraction of the enthalpy of reaction released by the exothermic chemical systems. However, to fully supply the exergy demands of the highly endothermic reforming reaction, the syngas purification and compression systems, an additional energy consumption, typically provided by means of costly non-renewable resources, is still required. Consequently, an optimal energy integration of the reactive components to the remaining systems of the plant might allow not only reducing the amount of fuel consumed, but also minimizing the process irreversibility by pursuing enhanced heat recovery and power generation. Furthermore, the valorization of the byproducts (e.g. CO2) may also increase the overall efficiency of the process, whereas the reduction of wastes ensures a minimum degradation of valuable feedstock. On the other hand, the choice of a carbon capture unit based on either physical or chemical absorption drastically affects the way in which the waste heat recovery (e.g. combustion air preheating) must be performed, and whether one or more energy technologies should or not be integrated (e.g. heat pump). Furthermore, the selection among various energy resources, such as the import of electricity over the autonomous combined heat and power production (CHP), strongly depends on the ratio between the prices of electricity and fuels consumed, as well as on the extent of the energy integration. Thus, evidently, a simple trial and error approach falls short in efficiently determining the most suitable energy technologies and the operating conditions that enable the chemical plant to operate under minimum cost. Accordingly, in this work, a systematic methodology is used to identify the most suitable utility systems (cooling, refrigeration, and cogeneration) that satisfy the minimum energy requirement (MER) with the lowest energy consumption and operating cost. In addition, the exergy analysis is used to identifying potential improvements that may remain hidden to conventional energy integration analyses, regarding the minimization of the avoidable exergy losses and the integration of reactive, CHP and syngas purification systems. By applying this methodology, the best operating condition and size of such systems are identified, as well as the opportunities for producing surplus electricity in complex ammonia production plants, typically associated to urea and nitric acid production facilities in SNF complexes.