Multivariable Controller Main Window

This window is used to specify final values for selected parameters (the Specifications) in the flowsheet and to select other flowsheet parameters (the Variables or the Control Variables) which are allowed to change in order to meet the Specifications. The number of variables must equal the number of specifications. The information entered on this window cannot be saved until there is the same number of specifications and variables.

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

Specifications

First, choose the stream parameters (such as a stream flowrate) or unit operation parameters (such as a duty) and specify their desired values in the final converged solution of the flowsheet. The final converged solution will meet the specifications within the tolerance specified for each specification.

Variables

The flowsheet parameters chosen here will be varied automatically by the Multivariable Controller. For a list of flowsheet parameters available for use as control variables, see Generalized Variable Parameters.

To set the minimum value or maximum value allowed for each 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.

Multivariable Controllers in Recycle Loops

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

An Multivariable 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 MVC 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.

Example

1. Heat Exchanger Control:

Specifications:

1. Molar flowrate of a process stream.

2. Weight flowrate of another process stream.

3. Specified log-mean temperature difference for heat exchanger.

Variables:

1. Vary temperature of flash unit over a specified range.

2. Vary pressure of compressor.

3. Vary flowrate of a third process stream over a specified range.

Active (check box):

Check ON/OFF the Active check box to activate/deactivate the Specification\Variable respectively. The number of active specifications should be equal to number of active variables.

There is a new line of text present above Specifications/Variables saying, "Specification: n, equals (or, does not equal) Variables: m". If the active specifications and variables are not equal, the "does not equal" text turns to red which is cascaded all the way up to the PFD level.

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

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