Understand how a model instance uses the Composition values from its Fluid Type

A model instance can access the Composition values from a Fluid Type only when its corresponding Model Type uses a certain configuration for its parameters and variables. Specifically, the Model Type must include a Fluid Type type of parameter that indicates which Fluid Type to use. Then, any variables that you want to initialize with the Composition values from the Fluid Type must use the [Fluid Type parameter].Composition variable as their default value.

Most Model Types from the standard and example Model Libraries that are shipped with AVEVA Process Simulation use the FluidType parameter to set their Fluid Type and initialize their fluid composition variables (such as z and zm) based on the Composition values from that Fluid Type.

The default value of the FluidType parameter is the default Fluid Type for any model instances that you add to the simulation. When you first add a model instance to your simulation, the model instance uses the Composition values from its default Fluid Type to initialize any variables that use the FluidType.Composition variable as their default values.

After you attach a feed stream to a model instance, the model instance inherits the Fluid Type and composition of the equipment model immediately upstream as its feed conditions. Therefore, most model instances very briefly use the initial compositions from their default Fluid Type before the feed conditions override the Fluid Type compositions.

However, a model instance that represents an initial feed to the system, such as the Source model from the Process Library or the Supply model from the Cooling Water Library (CWLib), always uses the compositions from its default or current Fluid Type until you manually change the composition values in the model instance.

Changes to a model instance do not propagate back to the Fluid Type.

Normalization

When all the following conditions are true, the model instance uses the normalized Composition values from the Fluid Type as the default values for a variable instead of the actual Composition values.

• The variable uses the FluidType.Composition variable as its default value.

• The Type for the variable is Mass composition or Molar composition.

• The Fluid Type does not include any solid components or ions.

We normalize the Composition values to ensure that any initial fluid compositions (such as z[]) in the model instance always sum to one, which is a built-in physical constraint for all fractional fluid compositions. Some complicated model instances may not converge if their initial compositions are too far from a solution that satisfies this constraint. Therefore, whenever possible, we restrict the initial fluid compositions to values that sum to one, which are typically robust enough to solve without convergence issues.

Fluid Types that include solid components or ions are exceptions to the normalization rule. If the corresponding Fluid Type includes solid components or ions, a model instance uses the equivalent Composition values from its Fluid Type as its default composition instead of the normalized Composition values.

Depending on the configuration of your Fluid Type, the solid components may not participate in the vapor-liquid equilibrium (VLE) calculations. Therefore, the normalized Composition values may not represent the best initial starting point for the composition in the model instance.

Ions are closely tied to reaction equilibrium equations that link them to their corresponding non-ionic species in the fluid. You may want to keep the initial proportions of ions to their non-ionic species the same as what you specified in the Fluid Type. Therefore, like with solid components, the normalized Composition values may not represent the best initial starting point for the composition in the model instance.

If you use a Fluid Type that includes solid components or ions, you should ensure that the Composition values that you specify in the Fluid Type provide initial estimates that are close enough to a solution that they do not create any convergence issues in your simulations. Typically, Composition values that sum to one are robust enough to solve without convergence issues.

We recommend that you always specify the Composition values in a Fluid Type such that they sum to one to ensure the most robust behavior when you use them in your model instances.

Changes to the Fluid Type

If you change the Fluid Type that the model instance uses (that is, you change the FluidType parameter), the model instance uses the Composition values from the new Fluid Type to reinitialize its default composition values.

However, if the new Fluid Type has any of the same components as the original Fluid Type, any user-specified composition values remain the same for those components that appear in both Fluid Types. This may lead to fluid composition values in the model instance that do not sum to one.

Effects of the compositional basis on initialization

All source Model Types from the standard and example Model Libraries that are shipped with AVEVA Process Simulation have the following model characteristics:

• They specify the unnormalized compositional feed (M) by default.

• They calculate the molar and mass-based fluid compositions (z and zm, respectively) based on the M values.

• They include a CompBasis parameter that allows you to set the compositional basis for the model.

The value of the CompBasis parameter determines how the software initializes the source model instance. You can set the CompBasis parameter to the following values:

• Molar

  The M variable is a set of molar compositions that correspond to the z variable. Although the z variable is a calculated variable, the software sets the z values to the normalized Composition values from the Fluid Type specified in the FluidType parameter. The software then applies the Model Type's equations to calculate any remaining values that depend on the z variable, including the M and zm variables.

• Mass

  The M variable is a set of mass-based compositions that correspond to the zm variable. Although the zm variable is a calculated variable, the software sets the zm values to the normalized Composition values from the Fluid Type specified in the FluidType parameter. The software then applies the Model Type's equations to calculate any remaining values that depend on the zm variable, including the M and z variables.

Units of measure

The Composition values from the Fluid Type do not have any units of measure. When a variable in a model instance uses the Composition values as its default values, the model instance applies the variable's unit of measure to the Composition values without applying any conversion factors.

For source Model Types, if you switch the CompBasis parameter between Molar and Mass, the default composition (M) values do not change. Only the units of measure and the corresponding fluid composition variable (z or zm, respectively) change.