Swing Cuts

The CDU and most process units offer flexibility to vary the cut points of the various products produced. The swing cut is a hypothetical stream, at the interface between two products, representing material that can go into either the light or the heavy product, by varying the cutpoint. In the figure below, there are four products, with a swing cut between the second and third products; the swing cut represents cutpoint variations between 160 and 180 degrees.

In the planning tools, a portion of the swing cut material is allocated to each product stream to simulate a change in cut point. The yields are additive and the qualities are determined by blending the properties of the swing with the properties of each of the basic products. This is only an approximation since the same property values are used for the swing regardless of how much material is swung up or down. The larger the cut point change encompassed in the swing, the greater the error from this approach.

The technique used by the DGS to calculate the swing for any process unit is the same. When the swing is defined, the operating condition, i.e. cut point, is also identified together with the depth of cut. For each swing, the model is executed first with the standard cut point then again with the cut point changed by the full depth of swing. The yield and properties of the swing are then determined by back blending. For consistency, the heavier product is used as the basis for the swing calculation.

Swing cuts represent the potential to move the cut point in the distillation column to affect the final yields and properties of the output streams.