Peng-Robinson Isentropic Expansion and Compression Calculator

Calculation sequence

1. Enter a fully defined inlet PT feed and an outlet absolute pressure. The pressure may increase for a reversible compression reference or decrease for expansion. The fluid remains the same fixed overall composition; no separated vapor is removed along the entropy path.
2. Compute the inlet equilibrium entropy, then bracket the outlet entropy residual over 180-450 K and check an increasing 25-point grid. Solve with Brent and require residual below 1e-6 J/(mol.K). A single-point entropy constraint does not imply a real machine is reversible, dry or mechanically appropriate.
3. Calculate ideal shaft work from the checked equilibrium enthalpy difference. This work convention is positive into the fluid. Compare with the heat-duty flash only after recognizing that equal entropy and equal enthalpy are different processes; a throttling valve is not an isentropic expander.
4. At every returned state independently re-evaluate classical PR pressure, log fugacity and residual enthalpy/entropy. Require component balance <=2e-7, log-fugacity mismatch <=2e-5 and relative pressure residual <=2e-6. Reject vanishing phase fractions below 1e-6 and density separation below 1.1 for ordinary splits. These are numerical checks, not measured-PVT validation.
5. Calculations run locally in a dedicated WebAssembly worker. The advanced Python/scientific engine is a roughly 33 MB first download, separate from ordinary calculators; it is self-hosted and version pinned. No composition or measurements are sent to a calculation server. Save and file backups retain canonical inputs, while CSV/PDF include returned tables and source notes. The worker is released after idle time and failures never return partial results.

Worked check

For the default C1-C6 feed at 5 MPa and 330 K expanding reversibly to 2 MPa, the PR outlet is about 299.140628 K with work into the fluid -47.415138 kJ/kg and vapor mole fraction 0.745643. This is an ideal reference calculation, not a prediction of actual turbine performance.

Synthetic examples for reproducibility, not measured field results.

Assumptions and limitations

• Six components C1-C6, normal C4-C6, classical PR and all kij=0. No water, isomers, sour gas, heavy fractions, reactions, solids or volume translation. Use the expanded-mixture tools for their explicitly supported alternatives, not by inserting a new species in these fixed feed slots.
• Every flash trial must remain at 180-450 K and 0.1-10 MPa absolute, with consistent mole fractions summing to one. Near-critical, vanishing-phase or failed closure states are rejected. Single-phase states are not given an invented vapor quality. Independent engine agreement is not experimental validation.
• Instantaneous equilibrium and the documented steady balance only. No machinery efficiency, transient behavior, separator sizing, phase residence time, mechanical safety approval or claimed field calibration. A rejected or unbracketed root is not extrapolated into an unsupported range.

Sources and maintenance

Published by PetroCalcHub Team. Implementation and source checks: 2026-10-22. This does not claim independent expert review.

• Thermo: FlashVL/FlashVLN stability and phase splitting (runtime pinned to 0.6.0)
• CoolProp v8.0.0: pinned cubic pure-component parameters
• IDAES: isentropic pressure changer entropy equality and work sign

Related tools and references