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Degrees of Freedom Analysis

A degrees of freedom analysis must be made before unit operations can be successfully modeled with a process simulator.  While the simulator wizards will usually guide you through the steps of defining unit operations when they are placed in the process flow diagram, the wizards will generally suggest typically chosen options.

Fixed Variables

Existing distillation columns such as in Unit Operations Laboratory, have variables that are fixed that we cannot manipulate to alter the output. These variables are for instance: the height of the column , the number of trays, the plate efficiency, the tray holdups, the material of the tower, the thickness of the walls and so on. 

Independent Variables

On the other hand there are a number of variables we will have control over so we can vary the column output depending on our project goals:

• Feed F (that is feed temperature and pressure as well as feed concentrations and mass flowrate)
• Steam flowrate and conditions (thus the reboiler duty Qr in the simulation)(
• Reflux ratio R
• Feed plate location

These variables are the ones that are available for planning experiments on the distillation columns.

Dependent Variables

With the fixed and independent variables defined for HYSYS, we can now ask HYSYS to solve for not only the exit concentrations at the top and the bottom of the column but also all variables in the column.  HYSYS will allow us to adjust any of these independent variables to either maximize or minimize the product component concentrations. 

Design mode is also possible. HYSYS will allow us to input the desired concentrations and then determine the necessary conditions to be applied. We will illustrate this point with case studies after building the model for the column.

Continuous Distillation

Continuous steady state operation of a distillation column will be modeled and simulated as the first example.