Optimisation of a Haber-Bosch Synthesis Loop for PtA

This work presents a plantwide model of a Haber-Bosch ammonia synthesis loop (HB-loop) in a PtA plant, consisting of heat exchangers, compressors, steam turbines, flash separators and catalytic reactor beds. The total electrical power utility of the HB-loop is a combination of compressor power, refrigeration power, and steam turbine power. We optimise the HB-loop operating parameters, subject to constraints for maximum reactor temperatures, compressor choke and stall, minimum steam temperature, and maximum loop pressure. The loop features six degrees of freedom (DOFs) for the optimisation: three reactor temperatures, reactor N2/H2-ratio, separator temperature, and loop pressure. The optimisation minimises the total loop power utility for a given hydrogen make-up feed flow, with the PtA load varied by ranging the hydrogen make-up feed flow from 10 % to 120 % of the nominal. Across this load range, different constraints become active, with the compressor surge limit being particularly critical at low loads, significantly increasing HB-loop power consumption. To address this, we investigate configurations with two, three, and four compressor trains operating in parallel. Reductions in total power of 55%, 74%, and 84% are achieved at reduced plant loads, with two, three, and four parallel trains, respectively. In terms of total compressor capital and operating cost, we demonstrated savings of 8.06%, 8.29%, and 7.11% in total cost after ten years of operation with two, three, and four parallel compressor trains compared to a single train configuration.