Design and optimisation of novel cascade mixed refrigerant cycles for LNG production – Part I: Benchmark cascade cycles

Abstract

Complex cascade refrigeration cycles are typically used for production of liquefied natural gas (LNG). These include the propane precooled mixed refrigerant (C3MR) cycle, the dual mixed refrigerant (DMR) cycle, and the Phillips Cascade cycle. These are highly energy-intensive processes, resulting in many studies focusing on improving their energy efficiency by reducing their shaft power demand. However, their relative performance remains unclear. Modelling assumptions and natural gas feed differ across the published literature. This work models and evaluates these three commercial cascade cycles on a common basis of modelling assumptions, natural gas feed and process complexity (e.g. number of compression stages). However, the performance of the cycles cannot be compared meaningfully without first optimising the designs. The optimisation strategy proposed combines a genetic algorithm, which includes regeneration of the population to improve diversity, with a nonlinear optimisation. A case study demonstrates the effectiveness of this strategy. This work provides a meaningful comparison of the relative efficiencies of these commercial cascade cycles, to provide a basis for benchmarking novel cascade cycle options in a study to follow.