Key components for the liquid phases

In the Fluid Editor, in the Component List section, in the LLE Key column, you can specify a component as the key component in the Liquid 1 (L1) or Liquid 2 (L2) phase. You can specify only one key component for each liquid phase. For example, if you expect the L2 phase to be mostly water, you can specify water as the key component for the L2 phase.

If you choose to specify key components, you should specify both the L1 and L2 key components. Although you can specify only a single key component in the Fluid Editor, this configuration results in inconsistent behavior in the key component identification depending on the stream conditions. We highly recommend that you specify either no key components or both key components.

You can use the key components to adjust which liquid phase AVEVA Process Simulation identifies as the L1 phase and the L2 phase. This is especially useful when your simulation contains model instances with L1 and L2 product ports, such as the Separator from the Process Library. In these model instances, the L1 phase always exits the model from the L1 port, and the L2 phase always exits the model from the L2 port. You can use key components to prevent your liquid phases from exiting a model instance from an unexpected port.

You can use one of the following algorithms to characterize the liquid-liquid equilibrium (LLE) phases, including the key component identification:

• Original — A default algorithm that provides accurate and robust LLE identification.

• Modified Original — An improved version of the Original algorithm that includes better initialization logic. This algorithm is currently in the experimental phase.

• Embedded K — An algorithm that uses different open-form equations for the vapor-liquid-liquid equilibrium (VLLE) than the Original algorithm. This algorithm maintains the K-values by rigorously solving the VLLE flash calculations and providing the corresponding sensitivities. This algorithm is currently in the experimental phase.

You specify the LLE algorithm in the Liquid/Liquid Algorithm list in the System section of the Fluid Editor. If you plan to switch between the different algorithms, you should capture a snapshot of the simulation before you change the LLE algorithm selection. See Switch between liquid-liquid equilibrium algorithms for more information.

If you do not specify any key components in the Fluid Editor, then the software does the following for all LLE algorithms:

• If water is present above trace amounts, then the software uses water as the L2 key component. It also automatically identifies the L1 key component.

• If water is not present (or is present in trace amounts), then the software automatically identifies both the L1 key component and the L2 key component.

  Important: In some cases, your component may have a custom name that differs from the default component name in the thermodynamic library. Therefore, the software treats any component with a molecular weight between 17.5 and 18.5 g/mol as water.

The key component detection logic is different between the Original algorithm and the Modified Original and Embedded K algorithms. Therefore, you might see different key components when you use the Original algorithm compared to when you use the Modified Original or Embedded K algorithms.

When you specify both key components and only one liquid phase is present, the software identifies the liquid phase according to the compositions of the L1 and L2 key components. If the composition of the L1 key component is greater than the L2 key component (xL1Key > xL2Key), then the software identifies the liquid phase as the L1 phase. Otherwise, it identifies the liquid phase as the L2 phase.

When you specify both key components and two liquid phases are present, the software identifies the liquid phases according to the composition ratios of the key components in each liquid phase, r1 and r2:

where

xIL1Key is the composition of the L1 key component in the first liquid phase

xIL2Key is the composition of the L2 key component in the first liquid phase

xIIL1Key is the composition of the L1 key component in the second liquid phase

xIIL2Key is the composition of the L2 key component in the second liquid phase

When r1 is greater than r2 (r1 > r2), the software identifies the first liquid phase as the L1 phase. Otherwise, it identifies the first liquid phase as the L2 phase. This ensures that the L1 phase always contains more of the L1 key component compared to the L2 phase.