Determination of the Optimal Operating Condition of the Dual Mixed Refrigerant Cycle at the Pre-FEED stage of the LNG FPSO Topside Liquefaction Process

摘要

The dual mixed refrigerant (DMR) cycle, which pre-cools natural gasrnwith the mixed refrigerants and then liquefies the natural gas withrnanother set of mixed refrigerants, is well known for having the highestrnefficiency among the liquefaction cycles. Its possible application tornLNG FPSO is also extensively investigated.rnIn this study, the optimal operating conditions, such as the equipment’srnflow rate, pressure, temperature, and refrigerant composition per flowrnof the DMR cycle, were determined in consideration of powerrnefficiency. To determine the optimal operating conditions, arnmathematical model was formulated based on the mathematical modelrnof the single mixed refrigerant (SMR) cycle with additionalrnconsideration of tees, phase separators, and common headers. In thernmathematical model, the 227 operating conditions were defined asrnunknowns and 196 equality constraints related to the thermodynamicsrnfor the equipment were formulated. Further, the 15 given variablesrnrelated to the natural gas, compressor efficiency and seawater coolersrnwere used. This mathematical model can be regarded as thernoptimization problem, in which the number of unknowns is larger thanrnthe total number of equality constraints and given variables. To obtainrnthe optimal operating conditions, the minimization of the requiredrnpower for the compressors at the cycle was defined as an objectivernfunction. Moreover, set as design variables were the 16 operatingrnconditions, such as flow rate, pressure, temperature and composition ofrnthe main and precooling refrigerant, natural gas temperature afterrnprecooling, and flow rate ratio for the tee. In addition, the temperaturernconditions for the equipment’s operation were used as inequalityrnconstraints. The optimal operating conditions were then obtained byrnusing a hybrid optimization method that consists of the geneticrnalgorithm (GA) and sequential quadratic programming (SQP). Therncalculation results show that the required power at the obtainedrnconditions was decreased by 34.5% compared with that of the relevantrnpatent obtained in 2001, and 1.2% compared with the correspondingrnvalue from the past relevant study carried out in 2008.