Tower models

The separation model determines how the flowsheet will perform separation into products. Select the separation model used throughout the flowsheet from the Separation Model combobox. The available choices of separation model include:

• Crude Independent: this is the recommended separation model to use when fitting test-run data. The cut points and separation efficiencies determined by the Flowsheet Wizard represent thermodynamic parameters describing the separation and are calculated from yield and product distillation data entered in the next stage of the wizard. The calculations used to determine the thermodynamic parameters used do not depend on the type of crude being distilled, only the conditions found within the distillation unit.

• Volume Yield: this method can be used for legacy approaches to tower-fitting in order to produce output equivalent to other software applications. Cut points are defined using the cumulative yield curve for the crude (i.e. the cut point specifies the yield for a given crude). As the yield curve is directly used to calculate the flowsheet parameters the final parameters depend on the crude; hence this method is crude-dependent. When selecting a volume based model the cut point of each stream is determined by using the volume-based cumulative yield curve. Any measurements entered using weight-based methods will be converted to volume-based measurements before they are used. Using this yield-based modelling option the yield of fractions does not change, even if the separation efficiencies in the model are changed.

• Weight Yield: this method is similar to the volume based method but the cut points are calculated using the weight-based cumulative yield curve. Volume-based measurements are converted to their weight-based equivalent before being used in any calculation.

• ECP/SI: another separation model using a slightly different terminology. Stripping and Rectifying FI are replaced by Bottom and Top SI, respectively.