Generalized Performance Specification

Generalized Performance Specification Overview

The generalized performance specification is a powerful tool with which you can calculate the values for flowsheet operating conditions needed for a desired result. The performance of any unit operation can be controlled by a specification using a controller. Additionally, for unit operations like the flash, column/sidestripper and splitter the performance can be controlled by internal specifications.

PRO/II allows numerous stream and unit operation parameters to be selected for specifications. All specifications may simply set a flowsheet parameter at a specified value. In data entry windows, this usually is displayed as follows:

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Optionally, the specification can be entered as a mathematical expression (sum, difference, product or quotient) between two flowsheet parameters. This is useful when you want to fix the result of a relationship between parameters within the same unit or across different units. To specify an expression, click on the "=" sign and select the operator from the list. The specification will be displayed in the form:

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Click on each "Parameter" text to select the required parameters and enter the specification "value". A default tolerance basis and value is supplied automatically. PRO/II then tries to meet the specified value for the unit or stream parameter(s) within the given tolerance by varying the appropriate parameters. For examples of different kinds of performance specifications, see Specifications - Examples.

Degrees of Freedom

There is always a one-to-one relationship between the number of specifications and degrees of freedom (number of parameters that can be varied to achieve the desired result). Degrees of freedom are either identified with the Generalized Parameter Variables or, in some cases, are inherent in the particular unit operation.

A controller has exactly one degree of freedom and, hence, there is exactly one upstream variable that can be varied and one corresponding parameter value that can be specified. Columns and Sidestrippers have an unlimited number of degrees of freedom. However, PRO/II requires one specification for every column parameter that is varied. For Inside-Out columns with sidestrippers, the total number of specifications and variables should be equal across the entire system of columns.

The splitter has one degree of freedom for the first separation into two products and adds one additional degree for each additional product. This means that you must provide separation specifications for 'N-1' of the 'N' product streams leaving the splitter. Flashes inherently have two degrees of freedom that establish a thermodynamic state. For the flash "Product Specification", only one thermodynamic variable is set and hence the other specification should be given. For all other flash types, both the variables are automatically set.

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