Referencing Data Using the Define System
Unit operation parameters are normally given fixed numeric values in the data entry windows. The Define system provides an alternative method of setting a unit operation parameter in terms of other unit or stream parameters in the flowsheet. The defined parameter may be set equal to another flowsheet parameter or it may be the sum, difference, product or quotient of any two flowsheet parameters or constants. The parameters on which the defined parameter is based are called Reference Parameters. If the values of the reference parameters change, the defined parameter will automatically be changed as well.
For correct simulation results, the reference parameters must be available, i.e., previously computed or supplied, when the unit for which the operating condition is being defined is calculated. Note that only the current value of the referenced parameter is retrieved when the referencing unit is solved. It is possible for the retrieved values to differ from final flowsheet solution values if they are updated by another unit operation after being retrieved.
For examples refer to Define System - Examples.
How to Define Data
From any data entry window, select the data entry field for the parameter you wish to define. If the parameter can be defined, the Define button on the Toolbar will become available.
Push the Define button. This brings up the Define - Definition Window. You check the "Set up Definition for ..." option and select the required type of expression - sum, difference, product or quotient. Then click on the "Parameter" text to select the required reference unit or stream in the Define - Parameter Window.
From the Define - Parameter Window, you can select any unit operation or stream that is present in the current simulation or specify a constant value for the reference parameter. If a stream or unit is selected, click on the "Parameter..." text to select the required reference parameter in the Parameter Selection Window.
When you close the Define - Definition Window after making the desired selections, the data entry field for the parameter that you just defined displays "DEFINED" instead of a numeric value. PRO/II then automatically retrieves the defined value whenever the unit is calculated.
To delete a defined value, simply uncheck the "Set up Definition for ..." selection in the Define - Definition Window and push the OK button.
Related Topics
Define - Column Parameter Selection
Define - Stream Parameter Selection
Define - Unit Operation Parameter Selection