Mixed refrigerant systems combine traditional refrigerants as a single fluid to extend the operating range of the refrigeration system and also match the temperature profile required by a given process. Mixed refrigerant processes are used for ethylene production and the liquefaction of natural gas and for LNG they are now arguably the dominant refrigeration technology. As well as the single mixed refrigerant process (SMR), there are numerous variations including the propane pre-cooled process C3MR and the dual mixed refrigerant (DMR) process.

In the conceptual design of a single mixed refrigerant LNG process, it is necessary to consider the usual process trade-offs between compressor shaftwork requirements, refrigerant flowrates, and heat exchange area. However, in addition to these parameters, the composition of the refrigerant system can be tuned to the overall process. Such a problem lends itself to multi-objective optimization, where both energy efficiency and capital cost can be optimized simultaneously to give a Pareto Optimal Front. At the conceptual design stage, the Pareto Optimal Front provides a lot of very useful information, which can aid in tailoring the design to a specific project. For example, the binding constraints are identified, which limit both objective functions. The parameters directly related to a given optimal solution are also determined. This paper reports new results for a previously published SMR case study. Of particular interest is the refrigerant composition and how this changes as the optimum moves from a low capital cost low efficiency process to a high capital cost high efficiency process.