Ammonia Plant Analysis

The efficient operation of existing plants in the process industry requires their analysis through simulation together with power and heat integration by retrofitting. Pinch Analysis (PA) is a classical procedure to achieve this goal. Yet, a further integration can be obtained reducing the entropy generated in the chemical processes. Ammonia, important as raw material in many process industries, represents the primary feedstock for the nitrogenous fertilizer industry, being produced from its elements through a very exothermic reaction. The ammonia synthesis process is characterized by high reaction temperatures and pressures, thus needing high power requirements. It represents a typical reaction-separation system with a recycle stream (“ammonia synthesis loop”) and a cold separation train for the main product. The recycle is the consequence of the thermodynamic equilibrium limitations, implying low degrees of hydrogen-nitrogen mixture conversion to ammonia. The chemical driving force is improved removing the ammonia at low temperatures and recycling the unreacted species back to the reactor. Thought in cascade with pinch analysis, the chemical reactors energy integration (CREI) concept focuses on, simultaneously, the entropy generation reduction of both chemical reactor network (CRN) and heat exchanger networks (HEN). It replaces the chemical reactors with virtual heat exchangers generating the same amount of entropy. CREI analysis aims to improve the reaction heat utilization and to reduce the operating costs. The energy savings lead to an important emission reduction, in the ammonia plant (less fuel has to be burnt), in the energy production sector, since less energy is needed for compression and refrigeration and, also, in the utilities generation area. The ammonia synthesis loop was modelled using Aspen Plus® as process simulation software. Cascaded Pinch and CREI analysis of the process generated a suitable network configuration and operating parameters that minimize the entropy generation