Flowsheet Optimizer Main Window

This window is used to specify the objective function and one or more optimization variables. The objective function may be either a stream parameter (such as a stream flowrate or component composition) or a unit operation parameter (such as a duty or a calculator result) or a mathematical expression which relates two flowsheet parameters. You must choose to "maximize" or "minimize" the objective function. See Optimizer Objective Function for more information.

The optimizer is converged when the relative change in each variable is less than 0.001 (0.1%); use the Flowsheet Optimizer Step Sizes Window to change this default tolerance. The optimizer is also converged when the relative change in the objective function from one calculational cycle to the next is less than 0.005 (0.5%). This tolerance can be changed in the Flowsheet Optimizer Options Window. Finally, convergence is also achieved when the scaled error at the is less than 1.0E-7; you may enter a different value on the Flowsheet Optimizer Options Window.

Optimization variables are flowsheet values that are varied to maximize or minimize the objective function. Optimization variables are entered in a table; you must enter at least one optimization variable. For each variable you must also input minimum and maximum values. The user may either use the default step sizes, or enter data by selecting the linked text "user defined step sizes".

You may enter values to replace the default values for overall Flowsheet Optimizer parameters, such as the number of iterations and the calculation sequence. This information can be entered by pushing the Options... button. Pushing the Print Options... button will allow you to specify options for the final print-out.

Variables

The flowsheet parameters chosen here will be varied automatically in order to maximize or minimize the objective function. Optimization variables may be stream rates or properties, unit operating conditions, calculator results, and thermodynamic properties. For each variable, you must also input minimum and maximum values. For a complete list of flowsheet parameters available for use as optimizer variables, see Generalized Variable Parameters. You may enter limits on the calculation step size for each optimization variable on the Flowsheet Optimizer Step Sizes Window

Flowsheet Optimizers in Recycle Loops

If the Flowsheet Optimizer is part of a Recycle Loop, the Flowsheet Optimizer will be calculated as part of the recycle loop unless you change the calculation sequence. If the Flowsheet Optimizer should perform the optimization calculations only after the recycle loop is converged, then you must make sure that the calculation sequence is correct.

A Flowsheet Optimizer always causes a recycle loop in the simulation calculations, even if there are no recycle streams in the flowsheet. If the recycle loop created by the Flowsheet Optimizer contains a flowsheet recycle then there will be nested recycle loops in the calculations. If there are nested recycle loops, the inner recycle loops must have a tighter tolerance than the outside loops.

Specifications

You may enter specifications to define specific values for flowsheet parameters that must be met (within a given tolerance) to obtain an acceptable solution to the optimization problem. Specifications entered on this window must be for flowsheet parameters other than the optimization variables. Specifications may be provided for process streams, internal column streams, and unit operations. The specification may be a single parameter or a mathematical expression which relates two flowsheet parameters. The final converged solution will meet the specifications within the tolerance specified for each specification. You may enter a value for the tolerance to be used instead of the default tolerance value.

Constraints

Constraints limit the value of an expression to a specified range during the optimization. The constraint expression may be a single flowsheet parameter or it may be the sum, difference, product or quotient of any two flowsheet parameters. In the latter case, the second parameter may be a constant value.

How to Specify a Constraint

The constraint is set up in the Optimizer Constraints Window which is opened by pressing the Constraints button on the Optimizer Main Window.

$image\EDIT03.gif$ Check the "Use Constraints" option to make the constraints table available.

$image\EDIT03.gif$ By default, the constraint will apply to a single flowsheet parameter. To specify an expression, click on the "=" sign and select from the list.

$image\EDIT03.gif$ Click on the "Parameter" text to open the Optimizer - Parameter Window in order to select the required parameter. After a selection is made, the "Parameter" text will be replaced by a text describing the selected parameter.

$image\EDIT03.gif$ If you have specified an expression, click on the second "Parameter" text in order to select the other parameter or constant value.

Active (check box):

Check ON/OFF the Active check box to activate/deactivate the Variable/Specification/Constraint respectively.

Complete technical details may be found in topics under Flowsheet Solution Algorithms in the PRO/II Reference Manual.

$image\SML_LINE.gif$