How does OLI predict which liquid phase is aqueous or organic?

Updated October 08, 2024 17:52

Understanding the OLI Engine’s Phase Determination Process

When working with complex chemical systems, understanding how phases are predicted and balanced is crucial. The OLI Engine, a powerful computational tool, goes through a careful process to determine which phases are present in a given system and how different chemical species are distributed among them. This blog post simplifies that process for non-chemical engineers, providing a clearer picture of how the Engine works.

Setting the Stage: Initializing the Engine

Before any calculations begin, the OLI Engine must decide which phases might be present in the system. These phases include:

Liquid-1 (aqueous phase): The water-based phase where most salts dissolve.
Liquid-2 (organic phase): A phase where organic compounds typically reside.
Vapor: The gaseous phase.
Solids: Any precipitated materials (though this post will focus on the first three phases).

The Engine starts by predicting which phases could exist in the system. It uses different models, such as the MSE model and the AQ model, to make these predictions. Depending on the model, the Engine calculates how likely each phase is to be present, based on the properties of the species involved and the system conditions like temperature and pressure.

SRK Equation of State: A Key Tool for Vapor Phase Calculation

One of the critical tools the Engine uses is the Soave-Redlich-Kwong (SRK) equation of state, a mathematical model that helps predict how gases (vapor phase) behave under different conditions. The SRK equation can handle complex systems, including those with multiple phases.

In the MSE model, the SRK equation is mainly used to predict how the vapor phase behaves. In contrast, in the AQ model, it is used for both the vapor phase and the liquid-2 phase. This dual application is crucial for systems where both organic liquids and gases are present.

However, just because the SRK equation predicts that a vapor or liquid-2 phase could exist doesn’t mean it will actually be part of the final system. The Engine must still check if these phases fit within the overall balance of the system—a process we’ll explore further.

The Initializer: The Engine’s Starting Point

Before final calculations, the initializer in the OLI Engine makes educated guesses about which phases will be present and how much of each species will be in each phase. Here’s how it works:

Starting with Liquids: The initializer begins by assuming only the liquid phases (Liquid-1 and Liquid-2) are present. It calculates how species might distribute between these liquids.
Adding the Vapor Phase: If the liquid phases seem stable, the initializer tests if a vapor phase should be added by calculating vapor species concentrations. If the vapor phase is likely to form (indicated by the sum of the vapor mole fractions exceeding a certain threshold), it is included in further calculations.
Re-evaluating Phases: If, after including all phases, the liquid-2 or vapor phases are found to be insignificant (i.e., their amounts are below a certain tolerance), they might be excluded from the final system.

This step-by-step approach ensures that only the most relevant phases are considered in the final equilibrium calculations, leading to more accurate results.

Fig 1. Initializer block diagram

The Super-Critical Phase: A Special Case

Sometimes, a phase doesn’t neatly fit into the categories of liquid or vapor. This is known as a super-critical phase, which occurs under extreme conditions where the distinction between liquid and gas blurs. The Engine treats these phases carefully, categorizing them based on their behavior during the calculations. Whether a phase is considered liquid or vapor in the final results depends on how the system converges rather than on simple properties like density.

Fig 2. a) Density VS. [P,T] for carbon dioxide

Fig 2. b) Density VS. [P,T] for ethane

From Fig 2., a clear distinction between the calculated liquid-2 and vapor phases can be observed. In the subcritical region, the difference in densities is clear between the two phases; however, in the super-critical region, this becomes less apparent. In any case, density cannot be an indicator for phase assignment in any region; the determinant factor is the phase equilibrium calculation convergence.

Conclusion: The Engine’s Flexible Approach to Phases

The OLI Engine’s approach to phase determination is both systematic and flexible. It doesn’t strictly categorize phases as aqueous, organic, or vapor but rather considers them as Phase-1, Phase-2, and Phase-3. This flexibility allows the Engine to handle real-world chemical systems' complex and sometimes ambiguous nature.

By understanding these processes, users of the OLI Engine can appreciate the depth of the calculations performed and the care taken to ensure accurate, reliable predictions in even the most challenging chemical environments.

References

Wang, P.; Anderko, A. Fluid Phase Equilib. 2002, 203, 141–176.
Soave, G. Chem. Eng. Sci. 1972, 27, 1197–1203.
Zemaitis, J. F., Jr. Predicting Vapor-Liquid-Solid Equilibria in Multicomponent Aqueous Solutions of Electrolytes. In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S. A., Barner, H. E., Klein, M., Sandler, S. I., Eds.; American Institute of Chemical Engineers: New York, 1979; pp 227–246.