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For the low-pressure column C1, the SVD results suggest the use of stages 11 and 26. The temperature on stage 26 is controlled by manipulating the feedstream F1 using a temperature controller labeled "TC126." This controller has already been tuned in the single-end control structure. The temperature on stage 11 is controlled by manipulating the reflux to feed ratio using a temperature controller labeled "TC111." This controller was tuned with the TC126 controller on automatic, using a relay-feedback test. The resulting controller tuning constants were KC = 6.1 and t/ = 28 min. The resulting response was found to be too underdamped, so the gain was cut to 3.

For the high-pressure column C2, the SVD results suggest the use of stages 10 and 24. The temperature on stage 24 is controlled by manipulating the reboiler heat input using a temperature controller labeled "TC224." This controller has already been tuned in the single-end control structure. The temperature on stage 10 is controlled by manipulating the reflux to feed ratio using a temperature controller labeled "TC210." This controller is tuned with the TC224 controller on automatic, using a relay-feedback test. The resulting controller tuning constants are KC = 5.1 and t/ = 15 min.

Figure 8.23 gives the complete dual-temperature control structure. The two-column heat-integrated process has a fairly complex control structure. There are four temperature controllers and three ratios, in addition to the conventional four level controllers, two pressure controllers, and a total feedflow controller.

The performances of this structure for feed composition disturbances ranging from 60 to 70 mol% methanol and from 60 to 50 mol% methanol are shown in Figure 8.24. A comparison with the single-temperature control results given in Figure 8.21c shows a very significant improvement. Product purities are maintained much closer to their specification levels.

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