Crate vle_thermo 

VLE Engine — Vapor-Liquid Equilibrium Thermodynamic Calculator

This crate is the computational core of the VLE project, a modernization of two legacy thermodynamic codebases (VB6 ~15,000 lines + Pascal ~2,500 lines) into a high-performance Rust library with Python bindings via PyO3.

What this library does

VLE (Vapor-Liquid Equilibrium) calculations predict how chemical mixtures split between liquid and vapor phases at given temperature and pressure conditions. This is fundamental to chemical engineering — every distillation column, flash drum, and separation process relies on VLE predictions.

The library supports:

• 22+ cubic equations of state (EOS) for modeling PVT behavior of gases and liquids
• 5 activity coefficient models for liquid-phase non-ideality (van Laar, Wilson, etc.)
• 11 mixing rules for combining pure-component parameters into mixture parameters
• 6 saturation pressure correlations for estimating boiling points
• 6 flash calculation types: bubble point (T/P), dew point (T/P), isothermal flash, adiabatic flash
• Parameter regression: kij (binary interaction) and Aij (activity model) fitting

Why these modules exist

The module structure mirrors the mathematical layers of a VLE calculation:

1. eos — Equation of state definitions. An EOS is a mathematical model (e.g., Peng-Robinson, Soave-Redlich-Kwong) that relates pressure, volume, and temperature for a substance. Each variant uses a different α(T) function to capture how molecular attractions change with temperature. The 22+ variants come from merging 19 VB6 models with 3 additional Pascal models (Schmidt-Wenzel, Patel-Teja) that handle polar and asymmetric molecules better via a third parameter.

2. activity — Activity coefficient models. These capture liquid-phase non-ideality (deviations from Raoult’s law) using empirical correlations fit to experimental data. Essential for polar/associating mixtures (e.g., water + alcohol) where cubic EOS alone gives poor liquid-phase predictions.

3. mixing — Mixing rules. When applying a pure-component EOS to a mixture, you need rules for combining the individual a, b (and sometimes c) parameters into mixture-average values. Classical rules (quadratic in composition) work for simple mixtures; advanced rules like Wong-Sandler incorporate activity coefficient information for better accuracy with non-ideal systems.

4. saturation — Saturation pressure models. These correlations (Antoine, Riedel, etc.) estimate pure-component boiling pressure at a given temperature. Flash calculations use these for initial K-value estimates to seed iterative convergence.

5. types — Core data structures shared across all modules. Component holds pure-component properties (critical T/P, acentric factor, etc.), Mixture groups components with their compositions, Flow represents a process stream with phase-specific data, Tolerances controls convergence criteria, and ReferenceState defines the thermodynamic reference for enthalpy/entropy calculations.

Origin and academic references

Based on the thesis: “Desarrollo de un Programa Computacional para el Cálculo del Equilibrio Líquido Vapor de Mezclas Multicomponentes bajo el Ambiente Windows” (Jackson & Mendible, USB, 1999). Additional models from Reference (4): Da Silva & Báez (1989) — Mac Pascal program contributing Schmidt-Wenzel, Patel-Teja, and Chao-Seader models. All code derived from (4) is annotated with citation comments.

Internal units

All calculations use these canonical units (matching the legacy codebases):

• Temperature: K (Kelvin, absolute)
• Pressure: kPa (absolute — never gauge)
• Molar energy: kJ/kmol
• Molar entropy: kJ/(kmol·K)
• Molar volume: cm³/mol
• Amount: kmol
• Gas constant R: 8.31451 kJ/(kmol·K)

Re-exports

pub use activity::ActivityModel;

pub use eos::CubicEos;

pub use mixing::MixingRule;

pub use saturation::SatPressureModel;

pub use types::*;

Modules

activity

Activity coefficient models for liquid-phase non-ideality.

energy

Mixture energy properties — enthalpy and entropy (Milestone 8.4).

eos

Cubic equation of state variants.

flash

Flash, bubble/dew, critical-point, and parameter-regression algorithms (Milestone 9, Phase 15 of MODERNIZATION_PLAN.md).

liquid_volume

Saturated-liquid molar volume correlations.

mixing

Mixing rules for cubic equations of state.

mixture

Multicomponent mixture core: mixing rules, mixture Z-factor, partial fugacity coefficients, and the exact composition-derivative layer.

numerics

Numerical primitives used throughout the VLE engine.

petroleum

Petroleum characterization — turning a crude assay into pseudocomponents.

refinery

Refinery thermodynamics — the methods a crude column is validated against.

saturation

Saturation pressure models.

types

Core data structures shared across all VLE engine modules.

virial

Truncated virial equation of state (second coefficient only).