Controller Main Window

This window is used to specify a final value for a parameter (the Specification) in the flowsheet. In order to meet the specification, some other flowsheet parameter (the Variable or the Control Variable) must be allowed to change.

Specification

First, choose the stream parameter (for example a stream flowrate) or unit operation parameter (for example a duty) that should have a specified value in the final converged solution of the flowsheet. The final converged solution will meet the specification within the tolerance that is given here.

Variable

The flowsheet parameter chosen here will be varied automatically by the controller. For a list of available flowsheet parameters, see Generalized Variable Parameters for more details.

To set the minimum value or maximum value allowed for the control variable, or to set the step size of the change for the control variable on each iteration, push the Limits and Step Sizes.... button.

Parameters

You can change the maximum number of iterations from 10, the default value, to some other value.

To suppress printout of the results for each controller iteration, uncheck the appropriate check box. The default is to request the printout of the results for each iteration.

By default, if the control variable reaches a minimum or maximum limit, the simulation will continue the calculations using the limit value for the variable. However, you may request that the calculations stop if either limit is reached by clicking on the other radio button.

Usually, the next unit operation calculated after the control variable is changed should be the first unit operation in the calculation sequence that is affected by the change in the control variable. By default, PRO/II will determine the next unit operation to calculate, but you may wish to give this information directly. See Problem Specific Calculation Sequence for more details.

Controllers in Recycle Loops

If the controller is part of a Recycle Loop, the controller will be calculated as part of the recycle loop unless you change the calculation sequence. If the controller should check the specification and change the control variable only after the recycle loop is converged, then you must make sure that the calculation sequence is correct.

A controller 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 controller 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.

Examples

1. The specification is the flowrate of a product stream. The control variable is the temperature of a reactor.

2. The specification is the duty of a heat exchanger. The control variable is the flowrate of a feed stream containing superheated steam.

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