Add-on Activity Models

An activity-coefficient model supplied by an extension package cannot be recognised by the type-matching the built-in models use, because it lives in a different assembly. Such a model declares its own closed-form derivatives by implementing an interface that returns the activity coefficients together with \(\partial\ln\gamma_{i}/\partial T\) and \(\partial\ln\gamma_{i}/\partial n_{j}\) , the composition derivative being on the same total-moles basis as the built-in models return. The property package then obtains the flash and column derivatives with no further work, including the handling of the Poynting correction, the all-subcritical guard, and the ideal or Peng-Robinson vapour phase. The glycol dehydration package uses this route: its NRTL model carries a dedicated binary-parameter database, and its derivatives are the closed forms of the NRTL section applied to \(\tau_{ij}=a_{ij}+b_{ij}/T\) , for which \(\partial\tau_{ij}/\partial T=-b_{ij}/T^{2}\) .