Design and comprehensive optimization of C3MR liquefaction natural gas cycle by considering operational constraints

Abstract

Liquefied natural gas (LNG) plays a key role in gas transits. Hence, attempts to improve the current liquefaction technology to increase its efficiency and economic advantages have earned a place in the master plan of many countries. Propane precooled mixed refrigerant process (C3MR) is one of the most successful cycle in the liquefaction industry. In this study, an enhanced process including a three stage propane precooling cycle and two cryogenic heat exchangers with mixed refrigerant (MR) has been modeled in ASPEN-HYSYS software by 1 MTPA production capacity. Saturated temperatures of Precooling stages, outlet temperatures of heat exchangers (HE) and aftercoolers will determine the quality and efficiency of the precooling stage. Minimization of energy consumption per 1 kg LNG production (specific energy consumption) has been defined as the main objective function. Optimization of the cycle variables has been performed, intended to minimize the specific energy consumption. Subsequently, comprehensive and integrated optimization has been developed. Mixed refrigerant composition has been recognized as the most effective parameter in the performance of the cycle. Therefore, mixed refrigerant composition has been optimized by two methods. HYSYS optimizer functions and a self-initiated method entitled as “Observation of governing trend in mixed refrigerant cooling curve behavior by an approach to maximization of possible fit in cryogenic heat exchangers composite curve”. Although these methods will cause a great amount of energy saving, their results are not practically achievable. Therefore the cycle has been optimized by considering operational constraints. The ultimate results of the optimization are not only theoretical, but also practical. The optimized point introduced here is obtainable in real performance, and cycle results are constant despite perturbation. Thus specific energy consumption decreases from 1028.94 kJ/kg at initial condition to 973.93 kJ/kg at sustainable optimized condition, by 5.35 percent of specific energy consumption saving.