[[Flash Separation - Mixture Properties]] **Fugacity**: The fugacity of one component in a mixture is defined as $f_i = x_i\phi_iP_i$. Here, $x_i$ is the mole fraction of the component, but in most VLE equations $x_i$ is used specifically for the mole fraction in liquid, while $y_i$ is the mol fraction in vapor. At phase equilibrium, $f_i^L = f_i^V$ and $x_i\phi_i^L = y_i\phi_i^V$ If you assume the vapor phase is ideal, $\phi_i^V = 1$ and $\phi_i^L = \frac{y_i}{x_i}$. Fugacity coefficients can be derived from equations of state. Using the SRK for a binary mixture, $ln(\phi_i) = \frac{b_i}{b}(Z-1) - ln(Z-B) - C_iln(\frac{Z+B}{Z})$ with $C_1 = \frac{A}{B}(\frac{-b_1}{b}+ \frac{2}{a}(x_1a_1 + x_2 \sqrt {a_1a_2}(1-k_{12}))$