Simulation and optimisation of AP-X process in a large-scale LNG plant

• AP-X process is optimized with GA and the unit power consumption is decreased to 4.337 kW h/kmol.
• The optimized AP-X process is identified as the most efficient process for large-scale LNG plant up to now.
• The efficiency could be further increased by improving the polytropic efficiency of compressors.

The AP-X process, designed by APCI, is regarded as a promising energy-efficient process for large-scale LNG plants. To decrease the power consumption further, a genetic algorithm (GA) was used to globally optimise the process and a suitable refrigerant for use in the sub-cooling cycle was studied. The optimised unit power consumption was 4.337 kW h/kmol, which was 15.56% less than that of the base case and 15.62% less than that of the C3MR process with its multi-throttling pre-cooling cycle. Results show that the optimised AP-X process is the most efficient liquefaction process for large-scale LNG plants to date. Exergy analysis was conducted to calculate the lost work for main equipment items used in the AP-X process. The polytropic efficiency of compressors is the most important item for potential further improvements in energy efficiency.