Model-based Optimisation of Mixed Refrigerant LNG Processes

Abstract

Natural gas liquefaction processes are energy and cost intensive. This thesis pursues the optimisation of propane precooled mixed refrigerant (C3MR) processes considering variations in upstream gas well conditions, in order to maximise gas well life. Four objective functions were selected for the design optimisation of the C3MR and dual mixed refrigerant (DMR) processes: 1) total shaft work (W), 2) total capital investment, 3) total annualised cost, and 4) total capital cost of both compressors and main cryogenic heat exchanger (MCHE). Optimisation results show that objective function 4 is more suitable than other objective functions for reducing both W and UA (MCHE design parameter). This leads to 15% reduction in specific power for C3MR and 27% for DMR, while achieving lower UA values relative to baseline. The operation optimisation of the C3MR process and its split propane version (C3MR-SP) was performed using four objective functions: 1) total shaft work, 2-3) two different exergy efficiency expressions, and 4) operating expenditure (OPEX). Objective function 3 results in the lowest specific shaft work 1469 MJ/tonne-LNG. For C3MR-SP, however, the lowest specific shaft work is found to be under objective function 1. A comparison of optimisation results across literature studies is impractical due to dissimilar process conditions, feed gas conditions, product quality, and equipment size. A sensitivity analysis highlights the effect of feed gas conditions on performance of the C3MR. For instance, as LNG production decreases from 3 MTPA to 2.4 MTPA over time, the specific OPEX increases from $128/tonne-LNG to $154/tonne-LNG. A subsequent study was conducted focusing on energy benefits of two configurations: integrating natural gas liquids (NGL) recovery unit with C3MR. An integrated NGL recovery within C3MR shows a 0.74% increase in energy consumption as methane concentration of the feed gas decreases, however a frontend NGL recovery unit only has a 0.18% decrease.