Parametric studies

To study how flowsheet input values (e.g. feed stream values, unit operation parameters) affect the solution of the flowsheet, Parametric Studies can be used. To start a Parametric Study, make sure that the flowsheet is solved. Then select Parametric Study from the Flowsheet menu (or hit F9).

Using parametric studies, one can perform a manual optimization of a flowsheet. Make sure to select the value which is to be optimized as an output, and the values that are to be modified as inputs. Once the study solves, the best input values can be selected from the study results.

Selection of input values

On the Inputs page, select the input values that you want to modify. To add an input value, click the Add button. Multiple Input values can be selected in the New Input Parameter dialog.

For the selected input, the current value will be shown in the Parametric Study dialog. Enter the lower and upper limit of the input value for your study. Expressions are accepted. Also enter the number of intervals that the input value should be modified over. By default, each interval is equally sized. For input values that are of logarithmic nature, such as pressures, it may be better to change to increments that are equal in the logarithmic domain. Zero intervals are allowed only if the minimum and maximum value for an input are equal.

Inputs can be renamed by clicking on the input name, or selecting the input and click the Rename button.

Each input value will be solved for (number of intervals + 1) values. For each combination of input values, a flowsheet job will be solved. The initial guess for each job will be taken from the closest solution available. It is assumed that the input values that appear first in the list have the least impact on the flowsheet solution; therefore it is best to keep values that allow for an easy solution when modified to be first in the list. The order of inputs can be modified by using the Move Up and Move Down buttons.

The unit of measure of input values can be changed by clicking on the displayed unit of measure.

Selection of output values

On the Outputs page, select the values of the solution that you are interested in. It is best to add all the outputs that you may be interested in, as adding values after the study has started requires re-solving the study.

Like the inputs, the name and unit of measure of output values can be changed. Also the order in which the outputs appear can be modified.

Solving the parametric study

One input and output values of interested have been configured, click OK to solve the parametric study.

Parametric studies will always solve in background threads. By default, up to 4 threads will be used. If you find that the document fails to solve unexpectedly, or if you find that COFE crashes, one of the models you are using may not be implemented in a thread-safe manner. In this case, please try again with the maximum numbers of threads set to 1. Multiple threads will allow maximum use of multiple nodes on a multi-node computer. If your computer is equipped with more than 4 nodes, please adjust the maximum number of concurrent jobs accordingly. The maximum number of concurrent solutions can be specified in the Application Preferences.

A Parametric Study view will open that initially lists all values of inputs. While the jobs are solved in the background, output values will appear in the table. The right-most column will show the status of each of the jobs. The second column will show which job is used as an initial guess for a given job (no entry in this column indicates that the current solution is used as initial guess).

To abort the solution of the parametric study, hit the Stop button on the toolbar. It may take a few seconds for all jobs to terminate.

Parametric study results

If a job has solved with warnings, the warnings can be inspected by clicking Status text for that job (which appears in the right-most column). If a job has failed to solve, the errors that were collected during the course of solving the job are available by clicking the status text of that job.

The results can be plotted; you can use one input value on the X-axis, and for all other inputs either select a particular value, or select that each value will create its own curve. The result plot can be created while the study is solving; points that become available when jobs finish are automatically added to the plot. For inputs for which a particular value has been selected, you can select a different value by opening the plot's properties dialog.

Multiple parametric study windows can be open, but not all may be up to date. As long as the flowsheet is unchanged since starting the study, the text up-to-date will appear in the study's title.

The results can be copied to other applications by using Copy from the Edit menu; the tab-delimited values will be copied to the clipboard, ready to paste in for example a spread sheet application. To save the study results in a text file, choose Export Parametric Study Results from the File menu.