Thermodynamic Model

 Mixed Solvent Electrolytes + Soave-Redlich-Kwong

OLI is introducing the MSE-SRK thermodynamic model.  This model is a combination of MSE for electrolyte systems and SRK for the gas phase and the second liquid (or nonelectrolyte) phase. The MSE-SRK model is targeted at oil and gas production. 

Why was MSE-SRK developed?

The MSE-SRK model was developed to in response to the increasingly HPHT conditions in upstream oil and gas, for limitations observed for supercritical components at elevated pressures. These limitations included:

  1. Inability to reproduce the critical behavior of non-electrolyte mixtures;

  2. Phase discontinuities for mixtures containing supercritical components at transition points between vapor-liquid and liquid-liquid equilibria. 

If you are working in upstream oil & gas, MSE SRK is the recommended method 

select a Thermodynamic model

The difference is in how to model the 2nd liquid phase

The key difference between MSE and MSE-SRK lies in the treatment of the second liquid phase.  MSE-SRK is used for systems containing one electrolyte (first liquid) and one non-electrolyte (second liquid) phase. The second liquid phase (e.g., hydrocarbon, supercritical CO2) is assumed to be non-ionic, and is modeled using the Soave-Redlich-Kwong (SRK) equation of state.

By comparison, the MSE model is used for systems containing two potential electrolyte phases.  It also predicts transitions from partially miscible to homogeneous liquid systems (i.e., the upper or lower critical solution temperature).  

The different treatment of the second liquid phase in the two models leads to a key difference in representing critical behavior. MSE can reproduce liquid-liquid, but not vapor-liquid critical end points. MSE-SRK can reproduce vapor-liquid, but not liquid-liquid critical behavior.  

The gas phase properties are obtained using the SRK equation and are the same in both the MSE-SRK and the MSE models. 

The solid phase properties are computed the same way in MSE and MSE-SRK.

Advantages of MSE-SRK

The use of the SRK equation of state for the gas and organic-rich liquid phase provides the MSE-SRK model with two key advantages:

  • Eliminate any "un-physical" discontinuities between the vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE) regions that have been found in MSE

  • Correctly reproduce vapor-liquid critical behavior for hydrocarbon and related systems.

Disadvantages of MSE-SRK

MSE-SRK is not suitable for:


  • Systems that show liquid phase splitting with upper or lower critical solution temperatures (UCST or LCST).  Such predictions require a single model so that they can merge at UCST or LCST. An example is the phase behavior of the phenol – water system.

  • Systems where the two coexisting liquid phases will contain ions. This applies to solutions of common salts of polyvalent ions (such as phosphates) at high temperatures, aqueous polymer (e.g., polyethylene glycol) – salt mixtures, and mixtures used in thermochemical cycles for hydrogen generation (e.g., HI – I2 – H2SO4 – H2O). When both liquid phases are ionic they must be modeled as such, using MSE. 

  • Systems containing thiols, amines, alkanolamines, disulfides, etc., have not been designed for use with MSE-SRK.  MSE-SRK could be used in principle, but MSE is more accurate.

Validation cases and example plots

OLI has been studying cases from upstream oil and gas clients where we have observed the need for MSE-SRK. As a result, we now have an extensive test set for MSE-SRK. Because these are client cases we cannot make them available directly to other clients, but we are experienced with the chemistries that have posed problems in the past and welcome submission of other cases for our analysis.

In addition, standard OLI validation plots for MSE-SRK are available to clients upon request.

Also, we have a technical paper by OLI thermodynamicists, "MSE-SRK Thermodynamic Model in OLI Software" that has a more in-depth description of the model, as well as several example MSE versus MSE-SRK plots. Please check the OLI wiki at for this article, or contact us for a copy of the paper.

MSE-SRK is available as of the OLI platform V9.6 

Request more information on MSE-SRK