Methodology for selection of a controller structure

The economic performance of a distillation system is linked to its steady-state degrees of freedom. In other words, the economic benefits of a column control scheme depend on how well it controls composition, recovery, or yield and not on how well it holds integrating variables such as levels and pressures. The integrating variables (i.e. inventories of level and pressure) must obviously be controlled, but their control 

1 Count the control valves in the process to determine the overall degrees of freedom for control. 

2 Determine from a steady-state analysis the steady-state degrees of freedom. 

3 Subtract the steady-state degrees of freedom from the overall degrees of freedom to determine how many inventory loops can be closed with available control valves. 

4 Design pressure and level controls and then test for disturbance rejection. 

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