Design spec/vary function

In the steady-state design using Aspen Plus, two Design Spec/Vary functions are typically used to manipulate the flow rates of reflux and distillate to achieve the specified purities (or impurities) of the two products. These calculations are done at the design feed composition and the design feed flow rate. 

Set up the steady-state simulation at design feed composition with the two Design Spec/Vary functions active. Record the required reflux ratio RR and reflux-to-feed... 

StBady-State Design. The feed conditions, the pressure, the total stages, and the feed stage are specified in the steady-state design using Aspen Plus (version 7.3). Two Aspen Design Spec/Vary functions are set up to achieve the desired product specifications of 0.001 mol fraction water in the liquid distillate and 0.001 mol fraction methanol in the bottoms. 

The presence of a sidestream provides an additional degree of freedom. Three purities can be set. We use three Design Spec/Vary functions to achieve the following... 

The steady-state simulation is converged by first setting the RR equal to 0.5. Then two Design Spec/Vary functions were used the first adjusted distillate flow rate to achieve 0.01 mol% MeOH in the distillate, and the second adjusted sidestream flow rate to achieve 0.01 mol% MeOH in the bottoms. 

Using Design Spec/Vary Function in RadFrac... 

In order to obtain the same basic operation while design parameters are changed, two design spec/vary functions are set up in column block Cl. 

Once a converged simulation result is achieved, the Design Spec j Vary function can be used to achieve the exact bottom composition. Notice that only one Design Spec/Vary can be set up with a particular flowrate of the makeup stream. The optimum makeup flowrate is usually set by minimizing the reboiler duty while achieving a specific bottom composition specification. 

Figure 5 also shows the advanced features for design specifications. The DES-SPEC paragraphs show how variables in the problem can be defined, in a DEFINE statement, to be FORTRAN variables which can be used in any FORTRAN statements to define any arbitrary function. Then, any other problem variable can be varied, in a VARY statement, to drive the function to zero. 

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