Convergence issues with nearly pure streams

For fluids that include equilibrium reactions, you may encounter numerical convergence issues when you try to model streams of a single pure component. As the concentrations for the other components in the reaction approach zero, the calculation for the equilibrium reaction constant becomes unstable. This is especially true when you use the Electrolyte NRTL (eNRTL) method or other liquid activity coefficient (LACT) methods for your thermodynamic calculations.

To avoid these numerical convergence issues, you can:

• Ignore reactions in the nearly pure streams.

• Add finite trace amounts of another component either by modifying the lower bound of that component composition or by adding an applicable side reaction to the reaction equation set. This allows the other component compositions to more closely approach zero without destabilizing the equilibrium reaction constant calculations.

See Logarithmic form v. standard form for more information on modifying the lower bound.

If you choose to add a side reaction, the side reaction must decompose the nearly pure component and introduce trace amounts of another component in the main reaction. For example, electrolyte systems often involve aqueous solutions and include hydronium ion production in the main reactions. If you want to model a pure water stream for this case, you can add the water auto-ionization reaction to your equilibrium reaction submodel to improve the convergence. See Water auto-ionization reaction for electrolyte systems for more information.