Override the alpha form coefficients for the equations of state

When you use an equation of state (EOS) for your Fluid Type, you can locally override the alpha form coefficients that determine how the software calculates alpha (a) for the vapor-liquid equilibrium calculations. See the following sections for more information on the specific calculations for each EOS:

• PR — Peng-Robinson equation of state

• PRM — Peng-Robinson Modified Panagiotopolous-Reid method

• PSRK — Predictive Soave-Redlich-Kwong

• SRK — Soave-Redlich-Kwong equation of state

• SRKM — Soave-Redlich-Kwong Modified Panagiotopoulos-Reid method

A Fluid Type can use the preceding equations of state (EOSs) as:

• The main thermodynamic system method.

• The vapor equilibrium method in combination with a liquid activity coefficient (LACT) system method.

You can override the alpha form coefficients regardless of how the Fluid Type uses the EOS.

The <EOS>_Alpha tab in the Method Data section (where <EOS> is the EOS that you are using in the Fluid Type) contains a table of components and the corresponding alpha form coefficients that you can view and override. When you locally override the values for an alpha form coefficient, a blue badge appears at the start of the row for that component in the list. This allows you to quickly locate the overridden alpha form coefficients on the <EOS>_Alpha tab.

You can override the alpha form coefficients whether the Alpha Selection is Acentric Factor Formulation or Alpha Databanks. This means that you can use a combination of alpha formulations and the acentric factor formulation to calculate the alpha values for the components in your Fluid Type.

Override the alpha form coefficients

1. In the Fluid Editor, expand the Method Data section, and then select the <EOS>_Alpha tab, where <EOS> is the EOS that you are using in the Fluid Type.

2. In the table, locate the row for the component that you want to change the parameters for.

3. (Optional) In the Alpha Form list, select the equation form that you want to use for the alpha formulation.

   The values in the remaining table columns (Coeffs, c1, c2, and c3) update to match the Alpha Form selection.

   See Equations forms for alpha formulations for a list of the available equation forms.

   You can set the Alpha Form to 0 to use the acentric factor formulation instead of an alpha formulation.

   You can set the Alpha Form to 12 to use the Redlich-Kwong formulation or to 13 to use a generalized formulation instead of one of the alpha formulations available in AVEVA Thermodynamic Data Manager. You should use the generalized formulation only for petro components. See Additional equation forms for alpha formulationsfor more information.

   The default selection for Alpha Form depends on the Alpha Selection for the Fluid Type. When the Alpha Selection is Acentric Factor Formulation, the default selection is 0. When the Alpha Selection is Alpha Databanks, the default selection matches the alpha formulation for the related EOS family (SRK or PR) that is specified in the ALPHA data banks that you are using in your Fluid Type.

   A blue badge appears at the start of any row whose data you modify.

   

4. Change the alpha form coefficients (c1, c2, and c3) as you desire.

   Tip: To return an alpha form coefficient to the value from the ALPHA data banks, delete the value in the corresponding cell, and then press Enter.

   

5. Repeat steps 2 to 4 until you have updated all the desired alpha form coefficients.