Snohvit mixed-fluid-cascade LNG liquefaction

A simplified Statoil/Linde MFC train: three independent, cascaded closed refrigerant loops (propane precool, an N2/methane/ethane liquefaction MR, and an N2/methane subcooling MR) each condense/precool through the previous stage's cold box before their own JT expansion -- the same cross-stage coupling TEALARC uses, one level deeper. Natural gas is cooled 300 K -> 255 K -> 175 K -> 112 K across the three multi-stream exchangers before letdown to LNG storage (~114 K). All three circulations are set via tear_specs. From the ChemSep casebook (Snohvit MFC).