Design and optimisation of novel cascade mixed refrigerant cycles for LNG production – Part II: Novel cascade configurations

Large scale production of liquefied natural gas (LNG) typically employs energy-intensive cascade refrigeration cycles. Three well-established cascade cycles in the LNG industry are the Propane Pre-cooled Mixed Refrigerant (C3MR), Dual Mixed Refrigerant (DMR) and Phillips Cascade cycles. Previous work has focused on improving the energy efficiency of these cascade cycles with various methods (e.g. exergy analysis), as this can bring significant economic benefits. However, potential benefits associated with changes in the configuration of a cascade cycle are usually neglected. This work develops three cascade cycle options, based on structural modifications, to be competitive with the commercial processes. The novel configurations are the CryoMan Cascade cycle, the Bypass Cascade cycle and the Double CryoMan Cascade cycle. The performance of these novel cascade cycles is evaluated with a case study previously used to assess the commercial processes. The optimisation strategy consists of a genetic algorithm with regeneration of the population followed by a nonlinear optimisation. The results obtained demonstrate that the structural changes enable the novel cascade cycles to achieve energy savings of up to 15.6% compared to the C3MR cycle, and up to 5.9% compared to the DMR cycle. Moreover, these energy savings come at the expense of only minor added process complexity, as the number of compression stages and multi-stream heat exchangers are kept the same as in the commercial processes.