Flash Distillation Calculator

Compute phase split and equilibrium compositions for a binary flash drum using Raoult’s law and the Rachford–Rice equation. Supports isothermal and isobaric (pressure-specified) flashes.

1. Feed & Operating Conditions

kmol/h

Binary system: z₂ = 1 − z₁.

2. Component Data (Antoine Parameters)

Enter Antoine constants for vapor pressure: log10(Psat/bar) = A − B/(T + C), T in °C.

Component 1 (more volatile)

Component 2 (less volatile)

Results

How the flash distillation calculator works

This tool models a single-stage, equilibrium flash of a binary mixture using Raoult’s law. You specify the feed flow rate and composition, operating pressure and temperature, and Antoine constants for each component. The calculator then:

  1. Computes saturation pressures \(P_{i}^{sat}(T)\) from the Antoine equation.
  2. Computes K-values \(K_i = P_{i}^{sat}/P\).
  3. Solves the Rachford–Rice equation for the vapor fraction \(V/F\).
  4. Calculates liquid and vapor compositions from overall material balances.

1. Antoine equation for vapor pressure

\[ \log_{10}\left(\frac{P_i^{sat}}{\text{bar}}\right) = A_i - \frac{B_i}{T + C_i} \] where \(T\) is in °C and \(P_i^{sat}\) is in bar.

From this, the K-value for component \(i\) at pressure \(P\) is:

\[ K_i = \frac{y_i}{x_i} = \frac{P_i^{sat}(T)}{P} \]

2. Rachford–Rice equation

For a binary feed with overall mole fractions \(z_1\) and \(z_2 = 1 - z_1\), the Rachford–Rice equation for the vapor fraction \(\beta = V/F\) is:

\[ f(\beta) = \sum_{i=1}^{2} \frac{z_i (K_i - 1)}{1 + \beta (K_i - 1)} = 0 \]

The calculator finds \(\beta\) numerically in the physically meaningful range where both vapor and liquid exist.

3. Phase compositions

Once \(\beta\) is known, liquid and vapor compositions follow from:

\[ x_i = \frac{z_i}{1 + \beta (K_i - 1)}, \qquad y_i = K_i x_i \]

Phase flow rates are \(V = \beta F\) and \(L = (1 - \beta) F\). The tool reports the phase fractions \(V/F\) and \(L/F\) along with \(x_i\) and \(y_i\).

Assumptions and limitations

  • Binary mixture only (two components).
  • Ideal vapor and liquid phases (Raoult’s law is valid).
  • Constant pressure and temperature (isothermal/isobaric flash).
  • No heat losses or pressure drop within the flash drum.

For multi-component, highly non-ideal, or high-pressure systems, an equation of state or activity-coefficient model (e.g., NRTL, UNIQUAC) is required; use a process simulator for design-grade work.

Worked example

Consider a 100 kmol/h feed of a binary mixture with:

  • \(z_1 = 0.5\), \(z_2 = 0.5\)
  • Flash conditions: \(T = 80^\circ\text{C}\), \(P = 1\ \text{bar}\)
  • Antoine constants as in the default inputs above.

The calculator will:

  1. Compute \(P_1^{sat}\) and \(P_2^{sat}\) at 80 °C.
  2. Compute \(K_1\) and \(K_2\).
  3. Find a vapor fraction \(\beta\) between 0 and 1 that satisfies Rachford–Rice.
  4. Return \(V/F\), \(L/F\), and the phase compositions \(x_i\), \(y_i\).

FAQ

What is flash distillation?

Flash distillation is a single-stage separation where a liquid feed is partially vaporized by a sudden change in pressure or temperature. The resulting vapor and liquid phases are allowed to reach equilibrium and are then separated, producing a vapor richer in light components and a liquid richer in heavy components.

How do I choose Antoine constants?

Antoine parameters are tabulated for many pure components in handbooks and databases. Make sure the constants you use are valid over the temperature range of interest and that the units (especially for pressure) match the form used in this calculator.

Why do I sometimes get “no two-phase solution”?

For some combinations of T, P, and composition, the mixture is either all liquid or all vapor. In those cases, the Rachford–Rice equation has no solution with \(0 < \beta < 1\), and the calculator reports that no two-phase flash is possible under the specified conditions.