Cascade refrigeration, solved as coupled loops
Propylene, ethylene and methane loops in series to −150 °C, each colder loop condensing against the next-warmer one's evaporator.
The actual dark-mode canvas: three closed refrigerant loops in series, each one rejecting its condensing duty into the evaporator of the loop above it, reaching −150 °C at the cold end.
The loops are coupled, so the duties compound
Cold-end duty propagates upward: every watt lifted at −150 °C has to be rejected by the methane loop into the ethylene loop, and again into the propylene loop. Solving all three together is what makes the compressor powers real rather than three independent textbook cycles added up.
Unit ops shipped for this vertical
Polytropic compression to pipeline/process pressure.
Isenthalpic Joule-Thomson expansion — the same relation cryogenic JT liquefaction relies on.
Duty- or outlet-condition-specified energy-balance stage.
Two-stream counter-current exchanger — the crude/residue preheat that pays for a CDU's fired duty.
Stream each stage's suction and discharge pressure plus the cascade-exchanger approaches from the plant's OPC-UA server into this flowsheet's twin comparison — a warm approach in one exchanger loads every compressor above it, which is exactly what a whole-cascade solve shows and a single-loop calculation cannot.
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- ✓Three genuinely coupled closed refrigerant loops
- ✓Cold-end duty propagates up through every stage above it
- ✓From the ChemSep casebook (Refrigeration_3-Stage-150C)