(69a) Novel Energy-Efficient Configurations of LNG Cascade Refrigeration Cycles Based on Structural Modifications

Complex and energy-intensive cascade refrigeration cycles are employed for the production of liquefied natural gas (LNG). The cost of the shaft work energy required for refrigerant compression usually dominates the overall operating costs of the LNG plant. Three novel cascade cycle configurations are developed in this work to be competitive – in terms of energy-efficiency – against relevant commercial processes, i.e. the Propane Precooled Mixed Refrigerant (C₃MR) cycle, the Dual Mixed Refrigerant (DMR) cycle, and the Phillips Optimized Cascade cycle. The novel cascade cycle configurations are developed from structural modifications based on the CryoMan SMR process.

To evaluate the competitiveness of the novel configurations, the operating variables of both the commercial and the novel cascade configurations are optimised to minimise the specific shaft work energy demand for refrigerant compression (i.e. total shaft work demand per unit production of LNG, MW·MTPA^-1­). The optimisation strategy consists in applying a stochastic algorithm (using Genetic Algorithm) to search the solution space, and the best solution obtained is then used as the initial guess for a deterministic optimisation using SQP. Moreover, the complexity of all the cascade configurations is kept to a similar level (by fixing the total number of compression stages, major contributors to capital cost) to have a ‘fair’ comparison of their corresponding energy performance.

The three novel CryoMan options are demonstrated to be more energy-efficient than the commercial cascade cycles in a case study for large scale production of LNG. The novel configurations achieved up to 15.5% savings in shaft work energy demand compared to the commercially dominant cascade cycle (i.e. the C₃MR cycle). Compared to the most energy-efficient commercial cascade cycle (i.e. the DMR cycle), the performance of the novel configurations is equivalent to around 6.9% savings in shaft work energy demand. Furthermore, the energy savings achieved by the novel configurations come at the expense of only relatively minor structural modifications compared to the configuration of the DMR cycle.