Property package options

General options

The general options include the name and the description of the property package. They are found at the General page (select General from the Configure menu).

The name of a property package template will appear as a set of choices when adding a property package to a simulation. Depending on the simulation environment, the name and description of the property package are shown in the configuration.

Numerical options

The numeric options include all the options that reflect on numeric methods that are applied by the property calculations and equilibrium calculations. They are found at the Options page.

For all property derivatives that are determined by perturbation, the relative temperature, relative pressure and composition perturbation magnitude are adjustable. All perturbation distances default to 0.001. If you have trouble with the precision of perturbed derivatives, try adjusting these values to be smaller.

The default relative tolerance on iterative processes (cubic equations of state, equilibrium calculations) is 10^-6. This might be too strict if models are imported that use their own tolerance, and use a lower value. Lowering the tolerance might cause invalid bubble and dew points.

The maximum number of iterations (default: 75) applies to equilibrium calculations mainly.

In a number of models as well in equilibrium calculations by internal methods, you can adjust below which a compound is considered absent from the mixture. Make sure this value (10^-15 by default) is lower in magnitude than the tolerance value, or mass balances will likely fail.

For the values of estimation method for equilibrium calculations, see the description of the internal VLE equilibrium server.

When using cubic equations of state, and when evaluating properties of a phase outside of its region of existence, properties are returned that belong to the other phase. I.e. gas phase properties are returned for the liquid phase in regions where liquid does not exist and vice versa. This may lead to unwanted solutions. To prevent this from happening, pseudo property calculations can be enabled (default). In this case, pseudo properties are returned for the cubic equation of state dependent variables, as described in Mathias, Boston and Watanasiri, "Effective utilisation of Equations of State for Thermodynamic Properties in Process Simulations", AIChEJ, vol 30, no.2, pp 182-186 (1984)

The pseudo property calculations for cubic equations of state are only applied during:

• equilibrium calculations performed by an external equilibrium routine
• equilibrium calculations performed by the internal equilibrium routine in case of multiple compounds

By default, trivial solutions are not allowed. This means that azeotrope calculations will likely fail. When allowing trivial solutions, it is best to use the pseudo properties for cubic equations of state (which does not apply to fugacity calculations imported from external property calculation routines).