Feed Composition Sensitivity Analysis ZSA

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. 

If we change the design feed flow rate, all the column internal and external flow rates and heat-exchanger duties simply scale directly with the feed flow rate. In addition, the resulting temperature and compositions profiles are exactly the same at any flow rate. This occurs because the column pressure and tray pressure drops are specified in the design program and do not change with flow rates. Therefore, in theory, any conttol structure that incorporates any flow ratio (refiux-to-feed, refiux-to-distillate, etc.) will drive the column to the desired product compositions (at steady state) for feed flow rate disturbances. 

However, the flow ratios usually have to change, as do the temperature and composition profiles, when feed compositions change. Therefore, the dismrbance that must be examined is feed composition. 

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 

Run several cases in which the feed compositions of the light and heavy-key components are varied over a realistic range around the design feed compositions. 

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