Property packages¶

A process simulator needs two things from its thermodynamics: what splits (fugacities, K-values, flashes) and how much energy (enthalpy, entropy, volume). Fugacio has long answered the first question for several method classes, but until this release every energy balance in fugacio.sim was wired to a cubic equation of state alone: a heater, a mixer, an isentropic compressor, or a column reboiler could only be evaluated on Peng-Robinson or SRK, whatever model had been used to flash the stream.

A property package closes that gap. fugacio.thermo.PropertyPackage is one object that owns both phase equilibrium and energy for a set of components, and it's the single interface every energy-balanced unit, the rigorous column, the two-sided heat exchanger, and the equation-oriented engine consume through their model argument. It's implemented for all four method classes Fugacio carries:

Package	Route	Energy side
`CubicPackage`	phi-phi on PR, SRK, RK, or vdW	residual functions of the cubic (the historical default)
`GammaPhiPackage`	activity-coefficient liquid (NRTL, UNIQUAC, Wilson, UNIFAC, Dortmund) with an ideal or EOS vapor	pure-liquid enthalpies plus the excess enthalpy from autodiff of (g^E)
`SAFTPackage`	PC-SAFT with Wertheim association	temperature derivatives of the residual Helmholtz energy
`HelmholtzPackage`	one pure reference fluid (IAPWS-95 water, Span-Wagner CO2, and the other vendored formulations)	the published multiparameter equation itself

Build one¶

fugacio.sim.package_for is the constructor a flowsheet author reaches for. It takes component names and a method keyword from fugacio.sim.METHODS:

from fugacio.sim import package_for

pr = package_for(("propane", "n-butane", "n-pentane"))            # Peng-Robinson, the default
srk = package_for(("methane", "ethane"), "srk", use_database_kij=True)
nrtl = package_for(("ethanol", "water"), "nrtl")                   # curated binaries
unifac = package_for(("acetone", "chloroform"), "unifac")          # predictive
saft = package_for(("methanol", "n-hexane"), "pcsaft")
steam = package_for(("water",), "iapws")                           # reference fluid

Method-specific options (kij, alpha_default, vapor="eos", poynting, and so on) are forwarded to the underlying model factory; see package_for in the API reference. If you already hold a bare model object (an EOSModel, a GammaPhiModel, or a SAFTModel), any model= argument in fugacio.sim accepts it directly and upgrades it to the matching package.

One interface¶

Every package exposes the same calls:

Single-phase properties at ((T, P, x)) with an explicit phase: ln_phi, enthalpy, entropy, volume, and gibbs.
Phase equilibrium: flash_pt, bubble_pressure, dew_pressure, bubble_temperature, dew_temperature, k_values, and k_seed (the method's natural initial K-value estimate, which the column solver uses).
Two-phase-aware bulk properties, implemented once on top of the flash: mixture_enthalpy, mixture_entropy, mixture_volume.
Energy-specified flashes, also generic: flash_ph, flash_ps, flash_tv.

import jax.numpy as jnp

pkg = package_for(("ethanol", "water"), "nrtl")
x = jnp.array([0.4, 0.6])

h_liq = pkg.enthalpy(350.0, 1.013e5, x, phase="liquid")   # J/mol, ideal-gas reference
res = pkg.flash_pt(360.0, 1.013e5, x)                      # beta, x, y, K
state = pkg.flash_ph(1.013e5, h_liq + 20_000.0, x)         # T and phase split at fixed P, H

Enthalpy and entropy are relative to the ideal-gas reference at T_REF and P_REF from fugacio.thermo.properties, except for HelmholtzPackage, which keeps the reference of the published formulation. Only differences are physical and any consistent reference cancels in a balance, so don't mix packages with different references across one energy balance (a HelmholtzPackage steam side and a cubic process side inside one exchanger is fine, because each side is balanced separately).

Heat of mixing for free¶

The gamma-phi package doesn't need a separate heat-of-mixing correlation. The excess enthalpy follows from the Gibbs-Helmholtz relation,

[ h^E = -R T^2 \,\frac{\partial}{\partial T}!\left(\frac{g^E}{RT}\right) = -R T^2 \sum_i x_i \,\frac{\partial \ln\gamma_i}{\partial T}, ]

and the temperature derivative is taken by automatic differentiation of the very ln_gamma the activity model already provides. The heat of mixing is therefore exactly consistent with the activity coefficients that decide the VLE, and it inherits any temperature dependence the parameters carry. excess_enthalpy and excess_entropy are exported from fugacio.thermo for direct use:

from fugacio.thermo import excess_enthalpy

h_e = excess_enthalpy(nrtl.activity, jnp.array([0.3, 0.7]), 320.0)  # J/mol

Everything takes `model=`¶

The energy-balanced units in fugacio.sim.units (heater, valve, pump, compressor, turbine, mix, flash_drum), the rigorous column, the two-sided heat_exchanger, and the EOFlowsheet all accept model=. Omitting it keeps the historical Peng-Robinson default, so existing flowsheets are unchanged.

from fugacio.sim import Stream, heater, compressor

feed = Stream.from_fractions(("ethanol", "water"), jnp.array([0.1, 0.9]), 100.0, 300.0, 1.5e5)
warm = heater(feed, t_out=350.0, model=nrtl)        # NRTL liquid enthalpy, heat of mixing included

A package is a registered JAX pytree whose model parameters are leaves, so a flowsheet built on any of them is differentiable with respect to the thermodynamic parameters as well as the operating conditions: jax.grad of a column duty with respect to a kij or an NRTL (\tau_{ij}) works the same way as a gradient with respect to a reflux ratio.

64-bit arithmetic is on by default¶

Cubic-root selection, Newton iterations to (10^{-10}) residuals, and the implicit-function-theorem gradients throughout Fugacio assume double precision. Importing fugacio.thermo now enables jax_enable_x64 automatically. Set the environment variable FUGACIO_X64=0 before importing if you deliberately want single precision and accept the loss of digits.