An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes

Four processes for small-scale liquefied natural gas (LNG) production are evaluated. These include a single-stage mixed refrigerant (SMR), a two-stage expander nitrogen refrigerant and two open-loop expander processes. Steady-state simulations were undertaken to ensure that each process was compared on an identical basis, was fully optimised and was in agreement with published results. Composite curves for the feed and recycle streams and the refrigerant or cold recycle stream showed the degree of optimisation available within each process. The full exergy analysis showed the relative contributions to the total shaft work requirements, with the lowest being the SMR process. The lower efficiency of the expander-driven compressors is the main difference between processes. A more general comparison suggested that the nitrogen refrigerant process and the New LNG open-loop process are the leading candidates for offshore compact LNG production.