Design and Optimisation of Novel Cascade Refrigeration Cycles for LNG Production

LNG production involves the use of large, complex and energy-intensive refrigeration cycles. The costs associated with the energy for refrigerant compression (shaft work energy) dominates the overall operating costs of the refrigeration cycles. In this work, a novel cascade refrigeration cycle is presented, as an alternative option for large scale LNG production, which is competitive in teens of energy-efficiency with the commercial cascade cycles: propane precooled mixed refrigerant (C3MR) cycle, dual mixed refrigerant (DMR) cycle and Phillips Cascade cycle. In a case study, the novel and the commercial cascade cycles are optimised to minimise shaft work consumption for a given liquefaction duty. A stochastic search optimisation (Genetic Algorithm) is firstly applied in order to avoid local optima, and the best solution found with the Genetic Algorithm is used as the starting point for a deterministic optimisation (SQP). The optimised novel cascade cycle ('CryoMan Cascade' cycle) is able to achieve energy savings of around 5 % when compared to the optimised DMR cycle, which is the most energy-efficient commercial cascade cycle.