Superstructure Optimization of Low Temperature Organic Rankine Cycle with Multi Component Working Fluid

Abstract

Liquefied natural gas (LNG) has been receiving attention as energy source because of its high-energy density and low emission of greenhouse gas problems. Typically, LNG is evaporated by sea water in LNG terminal without using its cryogenic energy. The cryogenic energy of LNG can be utilized for power generation using organic Rankine cycle (ORC). In this thesis, an optimal ORC process utilizing LNG cold energy is proposed. The ORC process is modeled using commercial process simulator. The working fluid of the ORC is composed of normal pentane, trifluoromethane, and tetrafluoromethane. The optimization of the process to minimize total annualized cost (TAC) is performed using superstructure based approach. The developed superstructure includes four process alternatives, which are MSCHE, vapor flash process, 2-stage expansion, and VRP. The optimum solution is attained using the process simulator-interface-optimizer structure. As a result of optimization, the optimum ORC process configuration including MSCHE and 2-stage expansion is obtained. The optimal process shows the net power generation of 409.6 GJ/h, and the power generation per unit kilogram of LNG is increased by 68.2 %.