Figure 8.18 Flowsheet with control structure.

1-min deadtime in this loop and a temperature transmitter range of 300-400 K, the resulting controller tuning constants are KC = 0.535 and tj = 33 min. These numbers indicate that control will be fairly slow, which is to be expected in this highly interactive system.

The only remaining issue is how to handle reflux flow in both columns. Steady-state runs in Aspen Plus with product purities fixed were made over a range of feed compositions. Results showed that the constant reflux-to-feed structure is better than constant reflux ratio. Therefore, these R/F ratios are installed on both columns. The R/F mass ratio in the first column is 0.42557 and in the second is 0.67045.

8.3.2 Response of Single-End Temperature Control Structure

The responses of this highly interactive system to 20% disturbances in total feed flowrate are shown in Figure 8.19. The control structure provides stable base-level regulatory control. Figure 8.19a shows that flow and temperature responses take about 2h to settle out.

The effect of the step increase in the setpoint of the total feedflow controller is an immediate increase in F2. This causes a decrease in temperature in column C2, and the temperature controller increases reboiler heat input. This increases the vapor boilup in column C1, which increases stage 26 temperature. The TC1 temperature controller then brings in more feed F1.


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