Optimisation of an Ammonia Synthesis Loop

A Hysys model representing the steady state behaviour of a Haber-Bosch ammonia plant was developed based on a dynamic model and plant data from Norwegian fertiliser company Yara International ASA. The plant model was optimised using matlabs fmincon function with an interior-point method for economical optimisation with the aim to study the performance of different strategies. The Haber-Bosch ammonia plant represents an interconnected complex system with many nested mass and energy streams that make for a challenging optimisation. Therefore this thesis studies the effect of separating the plant model into units that are optimised separately to reduce the computational effort needed for the optimisation. These units consists of the synthesis gas makeup, reaction, separation and refrigeration sections. Two strategies where proposed, one were the plant refrigeration section was separated out from the plant model, which greatly reduced the amount of nested loops in the problem. The 2nd strategy replaced the separation and refrigeration section of the plant with a simplified model in the first optimisation set. The individual sections of both strategies were independently optimised There were several difficulties with developing a model that allowed for a wide range of operating configurations of the ammonia plant in Hysys. In addition a few unphysical aspects of the Hysys model were uncovered, such as the compressor lacking an upstream flow effect and poor methods for evaluating pressure drop for varying conditions. These challenges meant that the optimisation problem was only posed for a limited window of operation for the process. Thus, for application to a real world plant the results should be scrutinised. The optimisation strategies were successful in improving the plant profitability from the nominal operating point. In addition, as expected, the division decreased the computational effort needed for each optimisation. Further work is required to develop the separation strategy and identify possible limitations.