Vle Flash Calculations

Vapor-Liquid Equilibrium Flash Calculations

Overview

Solves VLE flashes for multicomponent mixtures using thermo.FlashVL backed by a chosen cubic equation of state (PR, SRK, PRSV, PR-Translated variants). Also provides a pure-Python Rachford-Rice solver for screening when only K-values are available.

Prerequisites

1. uv available on PATH (see the uv skill).
2. On first use, the script writes LICENSE_NOTIFICATION.txt into this

skill directory listing the upstream terms for the thermo and chemicals libraries; review them before any regulated or commercial use.

When to Use

• A flash split, K-values, or dew/bubble curve is needed for a defined hydrocarbon

or sour mixture.

• The user wants to evaluate phase behaviour at multiple T,P points.
• A quick Rachford-Rice solve with pre-computed K-values is needed for screening.

Don't use for

• Pure water / steam utility states — use steam-tables-iapws.
• Picking the right property method in the first place — use

equation-of-state-selection.

• Reactive systems (amine-CO2-H2S kinetics, ester hydrolysis, etc.) where the

rate-based reaction matters more than phase equilibrium alone.

• Polymer or electrolyte systems — cubic EOS are unsuitable.

Core Rules

• Always specify component identifiers in a form chemicals.search_chemical

can resolve (IUPAC name, common name, or CAS number).

• Always declare which EOS is used; the default is PR but the right choice

depends on the system (see equation-of-state-selection).

• Binary interaction parameters (kij) default to zero. For sour-gas, polar,

or amine systems, pass realistic kijs (sources: Sandler, Whitson, vendor databases) — using zeros silently is a common error.

• Report the JSON result envelope; never paraphrase numerical values without

reference to the JSON file written by the script.

Utility Scripts

• uv run scripts/flash.py tp --components propane,n-butane --zs 0.4,0.6 --T-K 300 --P-Pa 800000 --eos PR --output /tmp/flash.json
• uv run scripts/flash.py bubble --components propane,n-butane --zs 0.4,0.6 --T-K 300 --eos PR --output /tmp/bubble.json
• uv run scripts/flash.py dew --components propane,n-butane --zs 0.4,0.6 --T-K 320 --eos PR --output /tmp/dew.json
• uv run scripts/flash.py rr --Ks 4.2,0.6 --zs 0.4,0.6 --output /tmp/rr.json
• uv run scripts/flash.py envelope --components propane,n-butane --zs 0.5,0.5 --T-K-list 280,290,300,310,320 --P-Pa 800000 --eos PR --output /tmp/envelope.json

Procedure

1. Identify components and resolve their CAS numbers via

chemicals.search_chemical (do this once, paste resolved names into the call).

1. Choose the EOS family (PR for most hydrocarbon work; PRSV / translated

variants for sour or polar systems; SRK as a sanity check).

1. Decide on kij values: zero for chemically similar hydrocarbons; ~0.05-0.08

for H2S in light hydrocarbon; ~0.10-0.25 for H2S/amine. Document the source.

1. Run flash tp for a specified (T,P,zs); inspect vapor_fraction and

the phase classification.

1. For phase envelope work, repeat at a list of T or P values.
2. Validate the result: density vs lab data, dew/bubble vs operating data, or

trusted simulator. Adjust kij if needed.

Pitfalls

• Using PR (default) for systems containing H2S, CO2, or amines without

supplying non-zero kij values. Mixture properties (especially density and loading) will be wrong.

• Confusing PR with PRMIXTranslatedPPJP — both are PR family, but the

translated/consistent versions give markedly different liquid densities (5-15% closer to experimental).

• Reporting K-values from an in-phase region (single-phase) as meaningful;

K-values are only physically defined when both phases exist.

• Passing zs that sum to something other than 1 without normalizing. The

script normalizes for you but logs a warning — don't ignore it.

• Using flash tp with a T that is above the mixture critical temperature

and expecting two phases; thermo may converge to a "vapor" with a phase classification that needs interpretation.

• Treating volumetric flow as molar flow when computing absolute mass rates

downstream; the JSON gives mole-basis K-values and compositions.

• Picking SRK because it is faster; the binary-parameter database for PR is

larger, so PR usually gives better results for hydrocarbon work.

• Forgetting that PRMIXTranslatedPPJP needs volume-translation constants

cs. The script defaults them to zero (a documented assumption); supply real values for production work.

• Calling Rachford-Rice with one phase or with all K > 1 or all K < 1; the

script returns the trivial single-phase answer with a regime flag rather than failing.

• Treating the dew/bubble pressure as exact at the second decimal place; the

underlying solver converges to a tolerance of about 1e-6 in mole fractions, which corresponds to wider uncertainty in pressure for near-azeotropic mixes.

Fallback Strategies

• If thermo.FlashVL fails to converge (rare for hydrocarbon systems), drop

to Rachford-Rice with K-values from Antoine vapor pressures and a modified-Raoult assumption (flash rr). Document the assumption.

• If chemicals.search_chemical cannot resolve a component (e.g. exotic

amine or carbazole), supply explicit --Tcs, --Pcs, --omegas arrays using literature values.

• If the system contains water + hydrocarbon and the result is

obviously wrong (e.g. water dissolves completely into the hydrocarbon phase at low T), switch to an activity-coefficient model — but those are outside this skill; surface that to the user.

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/eos_choice.md — quick decision matrix for PR/SRK/PRSV/translated.
• references/kij_starting_points.md — sourced kij starting values for

H2S/CO2/N2 with light hydrocarbons.

• Caleb Bell, thermo documentation: https://thermo.readthedocs.io/
• Caleb Bell, chemicals documentation: https://chemicals.readthedocs.io/

Anti-Patterns

• Reporting flash results as exact without stating the EOS, kijs, and zs basis.
• Using flash output as a final design basis for a relief case or for safety

classification without independent validation.

• Hiding the difference between PR and PRMIXTranslatedPPJP when reporting

liquid density.

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.