Exergy Assessment of Single and Dual Pressure Industrial Ammonia Synthesis Units

In exothermic, equilibrium-limited processes such as ammonia synthesis, higher per-pass conversions are often achieved by withdrawing the enthalpy of reaction before the conversion has been completed. However, although inter-bed cooling may help controlling the bed feed temperatures and generates high pressure steam, it also shifts the reacting mixture away from equilibrium (i.e. by increasing the reacting driving force, -G), thus increasing the process irreversibilities. In order to offset the unfavorable effects of the bed intercooling in the decreasing-volume reactive system as well as to reduce the power consumption, a catalytic once-through conversion section is introduced in a 1000 metric tNH3/day ammonia synthesis unit. Three unit configurations are analyzed: two are based on single pressure loops (SP150, SP200), whereas the other (DP) operates at two incremental levels of pressure (83/200bar). The dual pressure process aims to show the relevance of the Counteraction Principle for driving the system irreversibilities down. The plant-wide and main components' performance are also compared in terms of exergy efficiency, economic revenues and utilities consumption. As a result, the syngas compressor, ammonia converter, waste heat recovery and ammonia refrigeration systems are found to be responsible for about 80-86% of total irreversibilities in the ammonia loop, which varies from 23.8MW for DP and 27.2MW for SP150. A cryogenic purge gas treatment unit allows improving the loop performance in 9-13% if compared to non-hydrogen-recovery systems.