Gas Mixture Properties Calculator

Calculate density, phase equilibrium, specific heats, transport properties, and flow conversions using real gas Equations of State with Binary Interaction Parameters.

US CUSTOMARY METRIC / SI

1. Project Data

Description

Ambient pressure ($P_a$)

bara

Ambient temperature ($T_a$)

°C

Case

Engineer

OPERATING CONDITIONS

Equation of State

Pressure ($P_1$)

barg

Temperature ($T_1$)

°C

Flow basis

kg/h

2. Gas composition

Mole % Mass %

Total

Assume composition as 100%

Calculated properties

$M$ Molar mass -

$Q_1$ Volumetric flow -

$Q_{st}$ Standard volumetric flow -

$Y_v$ Vapour mole fraction 1.000

$\phi$ Relative humidity -

$Cp$ Specific heat (Cp) -

$Cv$ Specific heat (Cv) -

$\kappa$ Specific heat ratio -

$R$ Specific gas constant -

$SG$ Specific gravity -

$\rho$ Density -

$Z$ Compressibility factor -

$c$ Speed of sound -

$\mu$ Viscosity (Gas phase) -

$k$ Thermal Cond. (Gas) -

$Td$ Dew point (Water) -

$Td_{HC}$ HC Dew point -

$\Delta T$ Superheat margin -

Engineering Reference & Technical Basis

1. Cubic Equations of State (EOS)

The calculator determines volumetric and thermodynamic properties using generic solutions to cubic Equations of State:

P = \frac{RT}{v - b} - \frac{a(T)}{v^2 + ubv + wb^2}

By adjusting constants $u$ and $w$, the engine adapts to:

Van der Waals (VdW): $u = 0, w = 0$
Redlich-Kwong (RK): $u = 1, w = 0$
Soave-Redlich-Kwong (SRK): $u = 1, w = 0$ with distinct $a(T)$
Peng-Robinson (PR): $u = 2, w = -1$

2. Thermodynamic Departure Functions & BIPs

Ideal gas heat capacities ($C_{p,ideal}$) are adjusted to real gas conditions using Residual Properties derived mathematically from the exact chosen EOS:

C_v^{real} = C_{v,ideal} + T \int_{v}^{\infty} \left( \frac{\partial^2 P}{\partial T^2} \right)_v dv

C_p^{real} = C_v^{real} - T \frac{\left(\frac{\partial P}{\partial T}\right)_v^2}{\left(\frac{\partial P}{\partial v}\right)_T}

Mixture attraction parameters ($a_{mix}$) utilize built-in Binary Interaction Parameters ($k_{ij}$) for common light hydrocarbon and acid gas pairs ($CO_2-CH_4$, $H_2S-CH_4$, etc.).

3. Real Gas Speed of Sound

Instead of assuming an ideal fluid behavior ($c = \sqrt{kRT}$), this application calculates the true speed of sound directly from the density variations and rigorous partial derivatives:

c = \sqrt{ -v^2 \left( \frac{C_p}{C_v} \right) \frac{1}{MW} \left( \frac{\partial P}{\partial v} \right)_T }

This formulation is essential for dense phase fluids and compressor calculations near the critical point.

4. Transport Properties ($\mu$ and $k$)

Gas mixture Viscosity ($\mu$) is estimated using the Lucas corresponding states method for pure components ($\xi_i = T_{ci}^{1/6} M_i^{-1/2} P_{ci}^{-2/3}$) blended via the Herning-Zipperer semi-empirical rule:

\mu_{mix} = \frac{\sum y_i \mu_i \sqrt{M_i}}{\sum y_i \sqrt{M_i}}

Thermal Conductivity ($k$) utilizes Eucken's modified relation mixed via the Wassiljewa-Mason-Saxena equation.

5. Moisture and Dew Point

If water ($H_2O$) is present in the mixture, the partial pressure of the vapor is evaluated against the Saturation Pressure computed via the Antoine Equation.

\log_{10}(P_{sat}) = A - \frac{B}{T + C}

Relative humidity is calculated as $\phi = P_{H_2O}/P_{sat}$. If the mixture is oversaturated ($\phi > 100\%$), the calculator estimates the resulting Vapour phase mole fraction ($Y_v$).

References

Peng, D.-Y., & Robinson, D. B. (1976): A New Two-Constant Equation of State. Industrial & Engineering Chemistry Fundamentals, 15(1), 59-64.
Smith, J. M., Van Ness, H. C., & Abbott, M. M. (2005): Introduction to Chemical Engineering Thermodynamics (7th Ed.). McGraw-Hill.
Poling, B. E., Prausnitz, J. M., & O'Connell, J. P. (2001): The Properties of Gases and Liquids (5th Ed.). McGraw-Hill.