Flash Calculation

The isothermal flash calculation is one of the most important and most common calculations in any PVT calculation. The term flash is nothing more than taking some mixture ($z_i$) and equilibrating it at a specified pressure ($p$) and temperature ($T$). The resulting single or multi-phase mixtures (vapor $y_i$ and liquid $x_i$), K-values ($K_i=y_i/x_i$) and the resulting volume(s) (or Z-factors) are the main outputs of the flash calculation.

The flash calculation is divided into three main parts: (1) the material balance, (2) calculating the component fugacities and (3) updating the K-values to try and reach thermodynamic equilibrium. The first part requires that the molar balance of each component is concerved. This is solved using the Rachford-Rice or Muskat-MacDowell equation. Part (2) requires the fugacities for each component in each phase to be calculated. This calculation is EOS dependent. The goal is to find a set of K-values that result in the equal fugacity constraint: $f_{Vi}=f_{Li}$. The last part of the flash calculation considers how to update the estimate of K-values based on the current set of K-values and the fugacities.

The material balance equation, for which the Rachford-Rice and Muskat-MacDowell equations are based, can be written as $z_i=y_i \cdot F_V + x_i \cdot (1-F_V)$, where $z_i$ is the total composition, $y_i$ is the vapor composition, $x_i$ is the liquid composition, and $F_V$ is the vapor molar fraction ($F_V = \frac{n_G}{n_G+n_L}$). The solution space for the flash calculation is divided into three regions: positive flash where $F_V$ is between 0 and 1, negative flash where $F_V$ is between 0 and $-\infty$ or 1 and $\infty$, and the third region is the trivial region where all K-values are equal to 1 and the value of $F_V$ is arbitrary.

1. Generating a Flash Calculation

To perform a Flash Calculation, enter the temperature and pressure at which the reaction will occur, along with the composition of each relevant component. The complete process is shown in the GIF below.

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The Flash Calculation feature provides a comprehensive breakdown of a multicomponent mixture at specified temperature and pressure conditions. The output includes:

Component Composition
- Total mole fraction ( $z_i$ ): The overall fraction of each component in the feed.
- Liquid mole fraction ( $x_i$ ): The fraction of each component present in the liquid phase.
- Vapor mole fraction ( $y_i$ ): The fraction of each component present in the vapor phase.
- Equilibrium K-value ( $K_i$ ): The phase equilibrium ratio of each component, indicating its relative preference for the vapor phase compared to the liquid phase.
Overall Phase Properties
- Relative moles: The distribution of moles among phases, showing how the feed splits between vapor and liquid.
- Molecular weight: Average molecular weight of the total mixture and individual phases.
- Z-factor: Indicates deviation from ideal gas behavior for the mixture and each phase.
- Density: The calculated density for the liquid, vapor, and overall mixture, useful for further process calculations.