Calculation Sequence Dialog

PRO/II performs a simulation by solving one unit operation at a time, following a certain calculation sequence to reach the problem solution. This window is used to select the method to determine this calculation sequence for the current problem. This window has the following options to modify the calculation sequence:

• Sequence Map: All the unit operations, recycle loops, and control loops are displayed in the Sequence Map. The sequence map has a tree view.

• Show Units List Box: Select from one of the following options: All Units, Only Included Units, or Only Excluded Units. The default option is All Units.

• Include or Exclude Unit Operations: In All Units view, the loops and unit operations have check boxes to include or exclude them from the calculation sequence.

• : These buttons expand or collapse the tree view. When the window is launched, the tree view is expanded. When no node is selected, these buttons affect full tree view. If a node is selected, these buttons affect the selected node and its sub-tree.

• Search Box:  When you type a string of characters in the Search box, its matching unit operations are highlighted in yellow in the Sequence map.

• : These buttons help in navigating through the Sequence map.

• : Selecting this option enables you to search only those unit operations in the Sequence map whose initial part matches the string in the Search box.

• Cross Loops: If there are any crossings among the recycle or control loops, they are highlighted in red in the Sequence map.

Usage

Select one of the following calculation sequence methods:

Minimum Tear Streams (SimSci Method)

This default sequencing method uses improved algorithms developed by SimSci to determine the best sequence for calculation purposes. However, this method has a few limitations when recycle loops are present. These limitations and ways to circumvent them are discussed below.

Alternate Method (Process Method)

This method determines the sequence based partially on the order in which the unit operations were placed during the construction of the flowsheet. Units placed first are likely to be solved earlier than the units placed later. If you want to select this method, make sure that the units are added to the flowsheet in approximately the same sequence in which you want the flowsheet to solve.

Explicitly Defined by User

You can explicitly define the order in which the unit operations should be executed by manipulating the unit operations in the Sequence map. Initially, this view includes all the unit operations in the flowsheet.

Reorder Available Units

You can rearrange the units, block diagram, and full loop by dragging them to the required place. Multiple nodes can also be rearranged together. At any time, clicking the Reorder by MTS button rearranges the Sequence map in the order determined by the Minimum Tear Stream method.

Exclude or Include Units

In PRO/II 9.3 or later versions, you can exclude the unit operations by clearing their corresponding check boxes in the Sequence map. After you finish tuning the sequence, click the Run button. PRO/II solves the flowsheet by executing the unit operations selected in the Sequence map, from top to bottom. Excluded unit operations are neither calculated nor displayed in the output report.

Conversely, excluded unit operations may be moved back to the sequence by selecting the corresponding check boxes in the Sequence map.

In PRO/II 9.2.1 or later versions, for the sequence Explicitly Defined by User, you can retain the sequencing order of a unit operation during the include operation. For example, if a unit operation is excluded and re-included, it reverts back to the previous position in the sequence.

When the flowsheet is solved, all units included in the sequence have legitimate solutions. Subsequently, unit operations that are excluded do not compromise this data. Their results are retained. When the flowsheet is re-run after a solved unit has been excluded, the products of the excluded units act as flow sources into the available units. The excluded units are not recalculated and their results do not change. To re-run excluded units, include them in the calculation sequence and re-run the problem.

Output Reports for Excluded Units

When an output report is generated, it includes results of all the solved units included in the Sequence map. To report the results of a solved unit that is excluded, include it in the sequence map and generate the report. It is not necessary to rerun the flowsheet unless the unit has never solved. Units that have never solved in any flowsheet run are always excluded from the output reports.

Limitations of the SimSci Method

If your flowsheet has no recycle streams:

If there are no recycle streams in your flowsheet, the SimSci sequence algorithm will determine the correct calculation sequence automatically.

If the flowsheet has recycle streams:

First, the sequence algorithm is driven by reducing the number of tear streams in your flowsheet. Once the minimum number of recycle streams is reached, the choice of a tear stream is arbitrary.

Secondly, the sequence algorithm is driven by the recycle stream estimates provided by you. If no recycle estimates are given and the resulting number of tear streams is the same for each choice of recycle streams used as tear streams, then the actual tear streams chosen will be arbitrary.

We recommend that you always provide a recycle stream estimate for each recycle loop. These estimates will cause the calculation sequence algorithm to select the recycle streams as tear streams.

If your flowsheet has a controller inside a recycle loop:

The calculation sequence algorithm always puts controller units outside recycle loops. If you specifically want a controller unit to be inside a recycle loop, you must supply the calculation sequence explicitly.