Equation Of State Selection

Equation of State Selection

Overview

A structured decision tool that takes a component list and an application context (gas processing, refinery LPG, sour gas, amine sweetening, glycol dehydration, hydrocarbon polymerisation, steam utility, etc.) and returns:

• A primary EOS / property method recommendation.
• The Python backend (thermo class, chemicals correlation, or external).
• Rationale and the public-source basis for the recommendation.
• Specific caveats (binary interaction parameters, volume translation,

validity range).

The logic is original and based on publicly available documentation from DWSIM, Carlson's Don't gamble with physical properties for simulations (CEP, 1996), and the Aspen / HYSYS / PRO/II training materials. No proprietary text, table, or default is reproduced.

Prerequisites

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

underlying library citations.

When to Use

• Starting a flow-sheet and the property method has not been chosen.
• The current method gives results that disagree with lab or plant data.
• A reviewer asks "why did you pick PR?" and you need a defensible answer.

Don't use for

• Running the actual flash — that is vle-flash-calculations.
• Steam tables — that is steam-tables-iapws.
• Tuning kij values (this skill recommends a family, not parameter values).

Utility Scripts

• uv run scripts/select_eos.py --components propane,n-butane,h2s --application sour_lpg --pressure-pa 1200000 --output /tmp/select.json
• uv run scripts/select_eos.py --components methanol,water --application polar_low_pressure --pressure-pa 200000 --output /tmp/select_mw.json
• uv run scripts/select_eos.py --components dea,water,h2s,methane --application amine_sweetening --pressure-pa 5000000 --output /tmp/select_amine.json

Procedure

1. Enumerate components with their real names. Note any polar, associating,

electrolyte, or reactive species.

1. State the application context: LPG storage, refinery off-gas, sour gas,

amine, glycol, electrolyte, polymer, hydrate, supercritical extraction.

1. Note the pressure and temperature range you care about (skip extremes

only if you are sure the system stays away from them).

1. Run the recommender; it returns a primary pick and an alternative. The

alternative is the screening "second opinion" — run a flash with both and compare densities and K-values.

1. Validate: against lab data, regressed VLE, or a trusted simulator. If the

primary EOS misses, switch to the alternative and document the swap.

1. Re-run the recommender whenever components, pressure regime, or

application change materially.

Pitfalls

• Using PR for amine sweetening because it is the "default". Amine systems

need rate-based reactive packages; cubic EOS is screening only.

• Using IAPWS for a water-containing hydrocarbon flash. IAPWS handles only

pure water/steam; for water + hydrocarbon you need a method that handles immiscibility.

• Reaching for SRK as the "industry default". PR has the larger binary

parameter database in thermo / chemicals and is usually a better starting point for hydrocarbons.

• Picking a cubic EOS for a system containing alcohols, organic acids, or

water. These associate; CPA (cubic-plus-association) or activity-coefficient

• Henry's law are usually better.
• Treating the result as final without doing the second-opinion flash.
• Picking a method without naming it in the final report; the property method

is part of the calculation, not an implementation detail.

• Using one EOS for the whole flowsheet without re-validating at zones where

the regime changes (e.g. compressor inlet vs cryogenic exchanger outlet).

• Ignoring the model validity envelope when integrating very far in T or P.
• Forgetting to revisit the choice when a new component is added that breaks

the original system class (e.g. adding water to a hydrocarbon stream).

Fallback Strategies

• If you cannot identify a clean class for the system, run two flashes with

PR and SRK and compare against any available data. If they disagree by more than 5% in liquid density or 10% in K-value, escalate to an experimental data check or a more advanced method.

• If components include known associating species (water, alcohols, organic

acids), and a CPA or SAFT implementation is not available, fall back to Raoult + Antoine for low-pressure screening with a warning.

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/decision_matrix.md — full decision matrix.
• references/method_validity_ranges.md — published validity envelopes for

PR / SRK / PRSV / PR-Translated.

• DWSIM Property Package Selection: https://dwsim.org/wiki/index.php?title=PropertyPackageSelection
• Carlson, E.C., "Don't gamble with physical properties for simulations,"

Chemical Engineering Progress, October 1996.

Anti-Patterns

• "Aspen uses PR-Peneloux by default, so it must be right."
• "We always use SRK at this site."
• "The simulator converged, so the method is fine."

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.