Thermo Process Properties

Thermodynamic Property Calculations

Overview

Caleb Bell library wrapper utilities. Implements:

• recommend: property-method recommendation given components and

application context.

• costald-density: COSTALD (saturated) or COSTALD_compressed mixture

liquid density for light hydrocarbons.

• pr-translated-eos: instantiate thermo.eos_mix.PRMIXTranslatedPPJP

with reported molar volumes, Z-factors, and departure functions.

• iapws-state / iapws-saturation: pure water / steam properties.

Heuristics for method selection are transparent and built from public sources (DWSIM, Carlson, AVEVA / Aspen training material). No proprietary defaults are reproduced.

Prerequisites

1. uv available.
2. On first use, the script writes LICENSE_NOTIFICATION.txt listing

the upstream library terms.

When to Use

• Picking a property method for a flowsheet zone.
• Estimating LPG liquid density at storage / vessel conditions (COSTALD).
• Getting a Translated-PR liquid molar volume for an LPG mixture.
• Pure-water / steam state at known T, P.

Don't use for

• Multi-component flash (vle-flash-calculations).
• EOS decision tree by itself (equation-of-state-selection).
• Steam-table-only calculations (steam-tables-iapws).
• Non-LPG hydrocarbon systems where COSTALD is outside its accuracy

envelope.

Utility Scripts

• uv run scripts/property_methods.py recommend --components propane,n-butane,isobutane --application LPG --pressure-pa 1200000 --output /tmp/method.json
• uv run scripts/property_methods.py costald-density --components propane,n-butane --zs 0.5,0.5 --temperature-k 300 --pressure-pa 1000000 --output /tmp/lpg_density.json
• uv run scripts/property_methods.py iapws-state --temperature-k 373.15 --pressure-pa 101325 --output /tmp/steam.json
• uv run scripts/property_methods.py pr-translated-eos --components propane,n-butane --zs 0.5,0.5 --temperature-k 300 --pressure-pa 1000000 --output /tmp/pr.json

Procedure

1. Identify components and the operating envelope (T, P, composition).
2. Run recommend for a property-method check.
3. For LPG liquid density at vessel conditions, prefer COSTALD compressed.
4. For pure water/steam, prefer IAPWS.
5. For full flash, hand off to vle-flash-calculations with the selected

EOS.

Pitfalls

• Using PR (default) for LPG liquid density at moderate pressure; PR

underpredicts liquid density by 5-15%. Use Translated-PR or COSTALD.

• Using COSTALD for a non-light-hydrocarbon (water, alcohol, amine);

COSTALD is accurate only for nonpolar light hydrocarbons.

• Using IAPWS for water-in-hydrocarbon systems; IAPWS is pure water only.
• Passing the kij matrix as zeros for a sour LPG mixture; H2S / mercaptan

/ amine interactions need real kij.

• Confusing mass density (kg/m³) with molar volume (m³/mol).
• Treating the PRMIXTranslatedPPJP molar volume as exact without

providing the per-component translation constants cs.

• Using recommend output as a final decision without doing the

second-opinion flash recommended by the decision-tree.

• Treating water-content trace as negligible without checking with a

hydrate / glycol skill.

Fallback Strategies

• If thermo / chemicals import fails for an exotic component, fall

back to built-in LPG defaults via engg_skills_common.property_backends.

• For density that is wildly off, switch from COSTALD to

PRMIXTranslatedPPJP (or vice versa) and report both.

Verification

• Run the listed script with representative inputs and an --output file when a deterministic calculation is available.
• Confirm the JSON result contains ok: true, expected units, and no unhandled warnings.
• Check result magnitudes against the stated assumptions, references, and a hand calculation or known operating range before reporting them.

References

• references/property_method_selection.md — full method matrix.
• references/caleb_bell_libraries.md — library overview and links.
• thermo docs: https://thermo.readthedocs.io/
• chemicals docs: https://chemicals.readthedocs.io/
• DWSIM property package selection wiki.

Anti-Patterns

• Reporting properties without naming the EOS or correlation.
• Using cubic EOS for steam.
• Reporting LPG liquid density without naming whether COSTALD or PR was

used.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jskherman
• Source: jskherman/engg-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.