Switch between liquid-liquid equilibrium algorithms

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. All Fluid Types initially use the Original algorithm by default. Because the different liquid-liquid equilibrium (LLE) algorithms can produce significantly different results, we highly recommend that you capture a snapshot of the simulation before you change the selected LLE algorithm for a Fluid Type.

Because the open-form equations for vapor-liquid-liquid equilibrium (VLLE) are the same for the Original and Modified Original algorithms, you should see a comparable convergence in the simulation when you switch from the Original algorithm to the Modified Original algorithm. However, the initialization improvements for the Modified Original algorithm may provide a better overall convergence for the simulation. If your simulation has convergence issues with the Original algorithm, you can try switching to the Modified Original algorithm to see if the convergence improves.

If you switch from the Modified Original algorithm to the Original algorithm, we recommend that you revert the simulation to the snapshot that you captured before you switched from the Original algorithm to the Modified Original algorithm. Because the Modified Original algorithm uses a different initialization method than the Original algorithm, you may compound convergence issues if you continue with the initialization point from the Modified Original algorithm instead of using the results from the saved snapshot.

Because the VLLE open-form equations are completely different for the Embedded K algorithm, you may experience convergence issues and significant re-solve times if you directly switch from the Original or Modified Original algorithm to the Embedded K algorithm. For example, if you have a simulation that previously converged to a wrong solution, the simulation may fail to solve due to the drastic changes to the equations and variables.

If you want to switch from the Original or Modified Original algorithm to the Embedded K algorithm, we highly recommend that you do the following:

1. Capture a snapshot of the simulation while it is using the Original or Modified Original algorithm.

2. Create a Fluid Type with the same configuration as the Fluid Type that the simulation uses except that the Liquid/Liquid Algorithm is set to Embedded K.

3. Isolate portions of the simulation into small, manageable model blocks by doing either of the following:

   • Insert Recycle models with their Operation parameter set to Source.

   • Move the model blocks into subflowsheets and then set the Operation parameter for the corresponding Flag models to Isolate.

4. Gradually switch the Fluid Type for the smaller model blocks until each model block uses the Fluid Type with the Embedded K algorithm.

   If you have any problems converging Column or Extractor models, you can try using the phase override options for those models to initially limit the phase calculations to vapor-liquid equilibrium (VLE) only. This change makes the most complicated models solve with VLE while the simpler models solve with VLLE that uses the Embedded K algorithm. After the simpler models solve with the Embedded K algorithm and the Column and Extractor models solve with VLE, you can gradually change the phase override options for each Column and Extractor model back to using the phase calculations from the Fluid Type until all the Column and Extractor models also solve with VLLE that uses the Embedded K algorithm.

5. After each model block converges, change the Operation parameter for the Recycle or Flag models to Tear and then to Normal.

   After you set the Operation parameter to Normal for all Recycle and Flag models, the simulation solves simultaneously, and you have successfully switched the simulation to the Embedded K algorithm.

If you want to switch from the Embedded K algorithm to the Original or Modified Original algorithm, you should not change the Fluid Type. Instead, you should capture a snapshot of the simulation while it is using the Embedded K algorithm, and then revert the simulation to the snapshot that you captured for the Original or Modified Original algorithm.