MSE-SRK

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MSE-SRK Thermodynamic Model in the OLI Software

Introduction

Starting with version 9.6.1, OLI is introducing a third thermodynamic model in addition to the previously available Aqueous (AQ) and Mixed-Solvent Electrolyte (MSE) models. This new model, called MSE-SRK, is based on the MSE model for electrolyte systems but utilizes the Soave-Redlich-Kwong (SRK) equation of state for both the gas phase and the second liquid (or nonelectrolyte) liquid phase. The MSE-SRK model is targeted at upstream oil and gas and related chemistries and is designed to eliminate the deficiencies that hampered the use of MSE for these applications.

The purpose of this document is to answer the following questions:

  1. Why was MSE-SRK developed?
  2. How does it work?
  3. What are the advantages of MSE-SRK?
  4. What are its disadvantages?


Why was MSE-SRK developed?

Since 2000, OLI has been intensively developing the MSE thermodynamic framework. Simultaneously, the older AQ model has been maintained and upgraded on an as-needed basis. The MSE model offers major advantages over the AQ model including:

  1. No limitations with respect to electrolyte concentration, making it possible to simulate electrolyte systems from infinite dilution to pure solute limits;
  2. Accuracy in predicting the properties of multicomponent systems with multiple competing solid phases;
  3. Simulating the effects of important organic components (e.g., glycol or methanol) on the behavior of electrolytes;
  4. Reproducing the properties of systems with two ionic liquid phases;
  5. Including the computation of solid-gas (or sublimation) equilibria in addition to vapor-liquid, solid-liquid, and liquid-liquid equilibria

Thus, the MSE model represents the state-of-the-art of electrolyte modeling. However, MSE has significant limitations for calculations that include supercritical components at elevated pressures. These limitations are:

  1. Inability to reproduce the critical behavior of nonelectrolyte mixtures;
  2. Spurious discontinuities in the phase behavior of systems containing supercritical components at transition points between vapor-liquid and liquid-liquid equilibria arising from the use of different formulations for the gas phase and second liquid phase

Since accurate modeling of systems containing supercritical components at elevated pressures is essential for upstream oil and gas applications, MSE has found limited use in this area. For upstream oil and gas, the AQ model has been generally preferred even though it is clearly inferior to MSE with respect to predicting the behavior of electrolytes. To remedy this situation, the MSE-SRK model has been developed. It retains the state-of-the-art treatment of electrolytes that is a key feature of MSE and, at the same time, introduces the same formulation for the gas phase and second liquid phase to eliminate the above deficiencies of MSE.


How does MSE-SRK work?

Figure 1 compares the structure of the MSE and MSE-SRK models. The key difference lies in the treatment of the second liquid phase in liquid-liquid equilibrium computations.

In both the MSE and MSE-SRK models, the electrolyte-containing (usually aqueous) liquid phase is represented in the same way, i.e., by a combination of the Helgeson-Kirkham-Flowers (HKF) equation of state for standard-state properties and the MSE activity coefficient model for solution nonideality. Accordingly, the chemical potential of a species i in a liquid phase is calculated as

μiL = μiL,O,x(T,P) + RT ln xiγiX* (T,P,x) (equation 1)