Simulation and optimization of a novel Rankine power cycle for recovering cold energy from liquefied natural gas using a mixed working fluid

A novel Rankine power cycle which uses a mixture of three hydrocarbons as working fluid is proposed to utilize the cold energy in LNG (liquefied natural gas). Process simulations and composition optimization of four schemes for the cycle are carried out. The results show that while the cycle is relatively simple, a high efficiency can be achieved. Ethylene is most appropriate for application in the mixed working fluid. The cycle, without LNG expansion, can output 1.023 kWh of work for 1 kmol of LNG after composition optimization. The cycle with LNG expansion can output 1.346 kWh of work for 1 kmol of LNG. An exergy analysis is also conducted. The results indicate that the normal heat exchanger and expander produce most of the exergy loss. A parameter analysis of the inlet and outlet pressures of the expander is carried out and the most appropriate values of the pressures are deduced.