Multi-objective optimization of the dual independent expander gas-phase refrigeration process for LNG

A multi-objective optimization study was conducted to optimize the gas phase refrigeration process. A 1000 kmole/hr natural gas stream at 5.5 MPa and ambient temperature consisting of 96.929 mole % methane, 2.938 mole % ethane, 0.059 mole % propane, 0.01 mole % n-butane, and 0.064 mole % nitrogen was liquefied using natural gas as a refrigerant, and subcooled using N₂ as a refrigerant. The two objective functions optimized simultaneously were the capital cost and the energy efficiency. An extended range of process parameters such as heat exchanger, pressure ratio in the natural gas as well as the nitrogen refrigeration loop, and the number of refrigeration stages was evaluated. Three natural gas stages were favored over one or two stages, because one or two stages often involved internal pinching in heat exchangers, resulting in very high heat transfer area. In the case of nitrogen refrigeration, one or two refrigeration stages were favored. This is an abstract of a paper presented at the AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).