Additional configuration for distillation units

Note: If a process model has been imported from a password-protected file, model details are unavailable.

View distillation unit outputs

The Outputs page is only shown for distillation type units. It lists the streams output by the model and the properties calculated for each stream.

Add and edit properties on output streams

Add and edit individual properties on an output stream using the Stream Property Editor. Right-click and choose Add Stream Property or Edit Stream Property from the context menu.

Configure swing cuts for distillation unit

Swing cuts are used to optimize the cut points between streams during optimization by inserting an imaginary cut between streams and then allocating parts of these streams to the upper and lower cut depending on the constraints in the solution. See the In Depth box at the bottom of this section for more information.

The Swing Cuts page in the Model Structure tab of the Process Units page lists the cuts in a distillation model and lets you configure swing cuts.

Select the correct process model via the Process Unit, Mode and Process Model drop-down lists at the top of the tab.

Tip: Filter the list via the Filter menu and configure which columns are shown, and in what order, via the View menu. See the Grids topic for more information.

For each cut, the table displays the ECP (effective cut point) temperature, and the Stripping FI and Rectifying FI parameters. The stripping FI is a factor describing how much light type material is in the heavier stream of the cut. A larger number means less light material has been carried over and the separation is better. Instead, the rectifying FI describes how much heavy type material is in the lighter stream of the cut. A larger number means less heavy material has been carried over and the separation is better.

To enable a swing cut for a specific cut:

1. Select the check box in the Swing column for the relevant cut.

2. Enter the Start point, that is, the lowest temperature the swing cut can move to.

3. Enter the End point, that is, the highest temperature the swing cut can move to.

Swing cuts are modeled as operating parameters for your distillation unit. When the Swing Cut check box is selected, an operating parameter is added for each mode of the distillation unit. Both the swing cut limits and calculated value can be entered and reviewed using the Operating Parameters tab.

Swing cuts between two distillation units

Swing cuts can move between two distillation units. To do this, click on the swing check box for the cut which can move in the first distillation unit in the sequence. For example, if a CDU has an AGO/Atm Res cut, and a vacuum distillation unit has an LVGO/HVGO cut, to swing between the atmospheric and vacuum unit, select the AGO/Atm Res cut in the CDU as a swing cut, and enter the range of this swing. This cut is essentially the cut between the higher boiling point of the AGO and the lower boiling point of the LVGO.

View swing cut temperatures after optimization

After you have optimized your case, go to the Operating Parameters tab of the Process Units page. The Solution column shows the optimized temperature for each swing cut.

In depth: The separation point between two products is known as the cut point. This is a temperature at which the lower boiling product is imagined to end and the higher boiling product begins. For example, heavy naphtha and kerosene might have a cut point of 170°C. This means that heavy naphtha contains hydrocarbon molecules with a boiling point below 170°C (down until the end of the previous product) and kerosene contains hydrocarbon molecules with a boiling point above 170°C (up until the start of the next product).
This example implies a sharp transition between these cuts and no mixing of hydrocarbons. This is known as a perfect cut.
In reality we can never achieve a perfect cut in a refinery, because the flow of material through the crude unit is too great to allow the time for efficient separation of the hydrocarbon species. Instead, when we remove atmospheric residue from the tower it will contain molecules that have a lower boiling point (perhaps down to 300°C) and would ideally be in the AGO fraction. Conversely, the AGO will contain some molecules that have a higher boiling point (perhaps up to 400°C) and would ideally be in the atmospheric residue. The AGO will also contain some low boiling molecules that would ideally be in kerosene, and kerosene high boiling molecules that ideally would be in AGO, and so on. Thus the different products do not have sharp cuts between them and contain molecules that ideally would be in a different product. The cuts are imperfect or sloppy.
This small amount of cross-over between different product streams can have a significant effect on the properties of the cut. For example, if the AGO cut has a significant amount of atmospheric residue in it, the pour point of this cut could be severely affected.
The cut point is often worked out by measuring the yield of each particular product produced from distilling a crude, and then looking back at the crude's yield distribution to find the cut point which matches that yield. For example, when distilling Arab Light we might find that it produces 45% atmospheric residue. If we look back at the original assay's yield distribution we might find this correlates to a boiling point of 345°C. Therefore we would say the cut point between AGO and atmospheric residue for Arab Light is 345°C.
To calculate the properties of the atmospheric residue of Arab Light, we use this cut point to recut the original crude assay. So by knowing that atmospheric residue is composed of material from 345°C to the final boiling point of the crude oil, we can calculate the other crude properties such as density, sulfur content and pour point by using the original measurements for the crude in the assay.
Thus to simulate the distillation of the crude, we need to know the set cut points that the refinery operates with. We can then apply these cut points to all the assays of the crudes we process, to calculate the properties of the products derived from these crudes. So, if we know that our refinery cuts atmospheric residue at 345°C, we can then calculate the properties of atmospheric residue deriving from Nemba, Dalia, Brent or any other crude.
Depending on which crude oils we are running and the products we are making, we might choose to cut the crudes at a slightly different temperature. To calculate the effect of moving the cut points we use swing cuts. Swing cuts are imaginary cuts placed between different fractions that represent the cut point flexibility that we have in a refinery. During optimization, a portion of the swing cut can be added to each of the neighboring streams to find the optimal cut point. As a result, a portion of the swing cut's properties will also be added to the neighboring streams.
For example, the normal cut point for atmospheric residue might be 345°C, but it may be possible to move the cut point to the equivalent of 330°C or 350°C. When recutting the crude, the AGO is cut to end at 330°C, there is a swing cut between 330 - 350°C and the atmospheric residue begins at 350°C. During optimization the swing cut is portioned between the AGO and atmospheric residue to find the optimal point in the overall solution. For example, if the normal atmospheric residue had a pour point too high to use for fuel oil blending, it could be possible to move the cut point lower to add more light material to the atmospheric residue and reduce the pour point. A result of moving the cut point lower would be that the yield of AGO would be reduced (as this material is now present in the atmospheric residue). Therefore the optimizer would calculate the optimal position of the swing cut to maximize profitability. It might be found, if the price of AGO-derived products such as diesel was high, that the swing cut would move to 350°C, increasing the AGO yield and therefore the amount of diesel produced.