Comparison With Economic Optimum Design

Now that all the basic dynamic simulation tools have been developed and their application illustrated, the last issue to be covered in this chapter is the effect of column design on control performance. In Chapter 4 the economic optimum design for this system was shown to be a 44-stage column operating with a reflux ratio of 3. So far in this chapter a 32-stage column with a 3.46 reflux has been simulated and controllers developed. This section provides a comparison of these two alternative designs. Is the economic optimum design also the best design from the standpoint of dynamic controllability?

A dynamic simulation of the 44-stage column is developed. The smaller column diameter (5.22 vs. 5.91 m) and lower reboiler heat input (24.8 vs. 27.2 MW) gives a slightly smaller reflux drum (D = 4 m) and base (D = 5 m). Stage 9 is selected for temperature control (336 K at steady state).

A relay-feedback test gives an ultimate gain of 5.31 and an ultimate period of 4.8 min. The corresponding values for the 32-stage column are 5.9 and 4.5 min. These results

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