Equation of State Fitting Process

The Equation of State (EOS) Fitting Process

Highlighted items are required for fitting an EOS. All other items are optional.

Select a Compound

Complete a TDE Evaluation (without EOS generation): This process normalizes closely related properties, generates required critically evaluated T_c and d_c values, adds predicted property values (if needed), and filters bad data.

Select an EOS for fitting. The EOS Control Center opens automatically after selection.

Fit the EOS to the available experimental and predicted property values.

Review results: View deviation plots for experimental and predicted data.

Save results for re-opening (Span-Wagner type only): Fit results can be saved for future re-opening to continue the fitting process or serve as initial equation parameters for a related compound.

View EOS parameters

View calculated data: View data calculated with the EOS.

Save evaluation: Includes EOS parameters and calculated values

Select an Equation of State (EOS) for fitting

Fit the Equation of State (EOS)

User actions that can affect the results.

Review results of EOS fit: Deviation Plots

The example shows application of the Modified Sanchez-Lacombe EOS to data for biphenyl. The deviation plot shown is for vapor pressures (i.e., pressures for the L-G phase boundary)

Save Equation Parameters for Re-Opening (Span-Wagner type only)

View EOS Parameters

View data calculated with the EOS

The example shows application of the Modified Sanchez-Lacombe EOS to data for biphenyl. The deviation plot shown is for vapor pressures (i.e., pressures for the L-G phase boundary)

View EOS Parameters

Save Evaluation Results: Includes EOS parameters and calculated values

User Actions to Affect the Fitting Process

See HELP for the EOS Control Center for descriptions of the actions listed here.

The fitting process consists of fitting and data rejction operations until convergence is achieved.

User actions affecting this process are:

Selection/De-selection of properties and constraints using check boxes. It is not recommended to select additional properties for the Peng-Robinson or PC-SAFT equation.

Change of relative weights for properties and constraints by right-clicking and increasing or decreasing the relative weight.

Editing data.

Adding data.

Fitting technique

The user can simply click the Fit button and wait for the result. If the default procedure does not produce a satisfactory result, (which may happen for the Span-Wagner equation), the reason may be inconsistent raw data or a local minimum in the error function. It is recommended that properties with inadequate represenation (high adequacy criterion values) be reviewed, and to reject or correct erroneous experimental values, if any.

As a remedy against achieving a local minimum, the user can try the following:

Select additional fitting constraints.
Increase/decrease relative weight of different properties by right-clicking.
Temporarily exclude, then again include some properties.
Try another fitting method.
Start from a different approximation. For that purpose Open EOS (for Fitting) a previously saved successful EOS for a similar compound and try fitting it.