There has been increasing attention to optimal design and operation of natural gas liquefaction processes. This paper examines the operation optimization of propane precooled mixed refrigerant (C3MR) and C3MR with split propane (C3MR-SP) processes. The optimization problem is formulated using four objective functions including shaft work consumption, two different exergy efficiency expressions, and operating expenditure (OPEX) to identify process performance improvements. The best performing objective function for C3MR process is found to be exergy efficiency (considering power and cooling duty) followed by OPEX. For C3MR-SP, however, shaft work is found to be the best objective function, OPEX being the second best performer. The findings indicate that for performance optimization of an installed LNG train, achieving the lowest specific shaft work together with the highest exergy efficiency is not possible for fixed feed natural gas flowrate and a fixed value of heat transfer coefficient and exchanger area, UA. The results of this work are then compared with numerous available literature studies; comparisons of optimal shaft work or optimal exergy efficiency across literature studies are impractical because of the dissimilar process conditions used, such as natural gas composition and pressure, pressure of LNG product, and UA of main cryogenic heat exchanger (MCHE).
- Exergy efficiency
- Objective function