eos-cpp

Overview

eos-cpp is a header-only library for cubic equations of state (EoS) written in c++.

This library provides templated classes for three types of cubic EoS: Van der Waals, Soave-Redlich-Kwong, and Peng-Robinson EoS. These are respectively defined as the following classes:

• eos::VanDerWaalsEos<Scalar>
• eos::SoaveRedlichKwongEos<Scalar>
• eos::PengRobinsonEos<Scalar>

You can customize and create a new cubic EoS by inheriting CubicEosBase<Scalar, EosPolicy, CorrectionFactor>. CubicEosBase is a template base class for general two-parameter cubic EoS. CubicEosBase requires Scalar, EosPoicy, and CorrectionFactor types. While EosPolicy class implements functions to calculate thermodynamic properties such as pressure and compressibility, CorrectionFactor class provides the calculation method of the correction factor for attraction parameter.

An EosPolicy class must provide the following static functions:

• pressure : Compute pressure.
• zfactorCubicEq : Compute coefficients of the cubic equation of Z-factor.
• lnFugacityCoeff : Compute the natural log of fugacity coefficient.
• residualEnthalpy : Compute residual enthalpy.
• residualEntropy : Compute residual entropy.
• residualHelmholzEnergy : Compute residual Helmholtz energy.

A CorrectionFactor class must provide the following member functions:

• value : Compute the correction factor.
• derivative : Compute the derivative of the correction factor.

Helper functions are defined for each EoS to easily create EoS instances:

• eos::makeVanDerWaalsEos
• eos::makeSoaveRedlichKwongEos
• eos::makePengRobinsonEos

Dependencies

This library only depends on the STL library.

Unit tests depend on GNU Scientific Library and Googletest. While Googletest will be automatically downloaded when unit tests are enabled, GSL must be installed manually. You can install GSL by using a package manager or vspkg to install GSL.

An Example of Usage

#include <gsl/gsl_poly.h> // gsl_poly_solve_cubic

#include "eos/PengRobinsonEos.hpp"

// A solver for cubic equation using GSL library
struct CubicEquationSolver {
  std::vector<double> operator()(const std::array<double, 3>& a) {
    std::vector<double> x(3);
    const auto n = gsl_poly_solve_cubic(a[0], a[1], a[2], &x[0], &x[1], &x[2]);
    x.resize(n);
    return x;
  }
};

int main() {
  // Methane
  const double pc = 4.6e6;    // Critical pressure [Pa]
  const double tc = 190.6;    // Critical temperature [K]
  const double omega = 0.008; // Acentric factor
  
  // Create an instance of Peng-Robinson EoS
  const auto eos = eos::makePengRobinsonEos(pc, tc, omega);
  
  {
    const double p = 3e6;    // Pressure [Pa]
    const double t = 180.0;  // Temperature [K]

    // Compute z-factor at the pressure and temperature.
    // Please note that there can be multile values for Z-factor.
    // `params` contains parameters required for calculating thermodynamic properties,
    // e.g. fugacity coeffcients.
    const auto [z, params] = eos.zfactor(p, t, CubicEquationSolver{});

    // Compute the fugacity coefficient of a Z-factor.
    const auto phi = eos.fugacityCoeff(z[0], params);
  }

  {
    const double t = 180.0;  // Temperature [K]
    const double v = 0.001;  // Volume [m3]

    // Compute pressure at given temperature and volume.
    const auto p = eos.pressure(t, v);
  }
}