Engineering task and calculation objective
The RKS module calculates vapour-liquid phase equilibria of multi-component mixtures based on the cubic Soave-Redlich-Kwong (SRK) equation of state. For a given number of components, the bubble point line and the dew point line are generated over a selectable pressure range; the results can be stored as bubble point and dew point files for further evaluation.
For binary mixtures, the module additionally supports graphical column design: from the inlet concentration of the second component and the required concentrations at the top and in the bottom, a McCabe-Thiele diagram is constructed and the number of theoretical trays is determined. The calculation thus links the thermodynamic equilibrium determination directly with the process separation task.
Such a tool is needed in the design of distillation and rectification columns, in flash calculations and wherever the real behaviour of hydrocarbon and gas mixtures has to be represented. The SRK equation has been a proven standard in the process industries for decades, particularly for non-polar and weakly polar systems at moderate to high pressures.
Calculation workflow
- Define the mixture: The number of components is fixed and each component is described by its pure-component data (critical properties, acentric factor) and the composition of the mixture.
- Calculate the equilibrium with the SRK equation of state: For both phases, the fugacity coefficients of the components are determined with the Soave-Redlich-Kwong equation; the equilibrium condition of equal fugacities iteratively yields the K-values and the phase compositions.
- Generate the bubble point and dew point lines: Over the selected pressure range, the bubble point line and the dew point line are calculated point by point and, if desired, stored in bubble point and dew point files – the basis for phase diagrams and downstream column calculations.
- McCabe-Thiele evaluation for binary separation: For two-component mixtures, the McCabe-Thiele diagram is constructed from the inlet concentration and the top and bottom concentrations of the second component; stepping off stages between the equilibrium and operating lines yields the number of theoretical trays.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| No. of pure components | (Max. 10) | - |
| 1.Komponente | 1 : | - |
| 2.Komponente | 2 : | - |
| 3.Komponente | 3 : | - |
| 4.Komponente | 4 : | - |
| 5.Komponente | 5 : | - |
| 6.Komponente | 6 : | - |
| 7.Komponente | 7 : | - |
| 8.Komponente | 8 : | - |
| 9.Komponente | 9 : | - |
| 10.Komponente | 10 : | - |
| Komp. | 1 : | % |
| Komp. | 2 : | % |
| Komp. | 3 : | % |
| Komp. | 4 : | % |
| Komp. | 5 : | % |
| Komp. | 6 : | % |
| Komp. | 7 : | % |
| Komp. | 8 : | % |
| Komp. | 9 : | % |
| Komp. | 10 : | % |
| Komponente | 1 0.0 | - |
| Komponente | 1 0.0 | - |
| Komponente | 1 0.0 | - |
Frequently asked questions
For which systems is the SRK equation of state suitable?
Very well suited for non-polar and weakly polar systems such as hydrocarbons, natural gas components, nitrogen or CO2, also at high pressures up to the vicinity of the critical point. For strongly polar or associating substances (water, alcohols, acids), the plain SRK equation is inaccurate; there, fitted binary interaction parameters or activity coefficient models (NRTL, UNIQUAC) are preferable.
What do bubble point line and dew point line mean for a mixture?
Unlike a pure substance, a mixture boils over a temperature range: the bubble point line marks the onset of evaporation (first vapour bubble), the dew point line the onset of condensation (first liquid droplet). Between the two lies the two-phase region; the width of this boiling range largely determines how easily the mixture can be separated by distillation.
What are the limits of the McCabe-Thiele method?
It is strictly valid only for binary mixtures and assumes approximately constant molar overflow, i.e. similar molar heats of evaporation of the components. With strongly differing heats of evaporation, large heat effects or side draws, the tray count becomes inaccurate; azeotropic points fundamentally limit the achievable concentrations. For multi-component mixtures, shortcut methods or rigorous tray-by-tray calculations are required.
Why do calculated equilibria deviate from measured data?
The main cause is missing or unsuitable binary interaction parameters kij: with kij = 0, the mixing behaviour is estimated from the pure-component data alone. For reliable column designs, the parameters should be fitted to measured equilibrium data of the specific system, and the calculated bubble point and dew point lines should be spot-checked against literature data.