Column Specifications and Variables

Specifications may be imposed on a column such that product stream flows or properties, column internal flows, column tray temperatures, etc. are at desired values in the solution. For each specification a degree of freedom, or variable, is calculated. Variables may include column feed stream rates, column heat duties, and/or the draw rates for column "fixed rate draw" products. There must be a calculated variable corresponding to each specification. Furthermore, the collective variables defined for a column must have a direct effect on all of the specifications for convergence to be achieved.

Note that for Inside-Out or Enhanced I/O columns with sidestrippers the number of specifications and variables should be equal across the entire system of columns. Each column or sidestripper may have an unequal number of specifications and variables, but the their total sums should be equal for the merged column system.

As an example, consider a conventional splitter column. Reasonable specifications are the recovery of the light key in the overhead product and the recovery of the heavy key in the bottoms product. Logical variables are the condenser and reboiler duties. Note that two variables are necessary for the two specifications. Collectively, the reboiler and condenser duties affect both of the specifications; therefore the problem is solvable provided that a sufficient number of theoretical trays are used in the model.

Usage

From the Column Main Window, push the Performance Specifications button.

On clicking Performance Specifications button, "Transfer to Specification" Message Box pops up.

On accepting "Yes" transfer is accepted, "No" rejects it, and pressing "Cancel" returns to the Column Specifications and Variables Window.

From the Column Specifications and Variables Window, check the Add Specifications and Variables option.

Specifications and Variables are entered in their corresponding tables by clicking on the highlighted texts, which are described below:

SPEC1, VARY1, etc.,

The default reference name for the specification or variable which may be replaced with a unique name.

Unit or stream parameter

The specification or column variable selected from the option list.

Value

The numeric value for the specification in the column solution.

Tolerance

The basis and value for the tolerance in meeting the specification. The tolerance is defaulted to the bases and values selected in the Default Unit Specification Tolerances Window, which can be accessed using Input/Flowsheet Tolerances menu item. This is indicated by the text "default tolerance".

You can change the tolerance to either an Absolute or Relative tolerance value by clicking on the tolerance text and displaying the Specification Tolerance Window.

See the Generalized Performance Specifications and Column Internal Variable Parameters for more information on specification and variable options for columns, respectively.

Specifications/variables may be added to the problem by selecting a row in the appropriate table and clicking the Insert button.

Specifications/variables may be deleted by selecting a Specification/Variable and clicking the Cut button.

All supplied information for a Specification\Variable may be erased by selecting the Specification\Variable and pushing the Reset button.

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 in the Specifications & Variables window saying, "The number of active specifications, n, equals (or, does not equal) the number of variables". 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.

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