Correlations

The Correlations page shows the property correlations you can use in your case. Property correlations allow property values to be predicted based on the values for other properties. They are only calculated and applied at blenders. For a property correlation to work, all the input values to the correlation must be present on the input streams of the blended product.

Example: A blended product may have an API specification, but the API may not have been explicitly modeled on the blend components. Instead the API property correlation can be used, which calculates the API value based on the stream specific gravity. All the blend components must have a specific gravity. During optimization, the API is calculated based on the stream's specific gravities. This correlated value is then used in the property constraint.

Evaluation order

Correlations can be evaluated prior to blending (default) or after blending. You can set this behavior for each correlation by choosing either Pre-Blend or Post-Blend from the drop-down list in the Evaluation column.

With pre-blend evaluation, for each input to a blender, the correlated property value is determined based on the stream's properties and the correlation definition. After determining the value for each blend component, the correlated property is then blended linearly.

Note: Pre-blend correlations are not evaluated immediately upstream of a blender. They are evaluated at the point of origin of the corresponding properties. Examples of points of origin are a feed, a distillation tower or a process unit.
A property override is not considered a point of origin. Therefore, even if you override a property used in a pre-blend correlation, the correlation still uses the original value of its input property.

With post-blend evaluation, the correlated property value is instead determined according to the properties of the blender output stream.

Publish new correlations

Note: New correlations cannot be added within Plan and must be published by administrators from an external application, such as Assay or Process Model Manager. Several standard correlations are included by default.

To add a new correlation, ask your Plan administrator, or consult the documentation for the relevant product.

Unsupported correlations

Where a correlation contains an unsupported element, a warning is returned. Errors are generated if the correlation is used for optimization.