Control Structure with Flow Control of Ethanol Feed

The second control structure switches from flow controlling the C4 feedstream to flow controlling the ethanol fresh feed. Thus, the production rate handle is ethanol, not C4. In addition, the composition of isobutene is controlled on a tray inside the column instead of controlling the composition of ethanol.

Figure 15.21 shows this control structure. The FFetoh stream is flow controlled. Then the C4 feed is ratioed to it, with the ratio being reset by a composition controller that holds the isobutene composition on stage 20 at 22.5 mol%. This stage is selected by looking at the composition profiles shown in Figure 15.14. The peak in the isobutene composition occurs at stage 20.

The temperature controller is the same as in the previous control structure. The temperature on stage 22 is controlled at 383 K by manipulating the reboiler heat input. It required no retuning. However, the composition controller was retuned for the new structure. Sequential tuning was used (the temperature controller was on automatic when tuning the composition controller). The composition controller was tuned by running relay-feedback tests and using Tyreus-Luyben tuning. Table 15.2 gives the parameter values. Faceplates are shown in Figure 15.22. Note that in this structure the FCc4 flow controller is on cascade. In addition, note that the output signal of composition controller CCic4 is the ratio of the flowrates of the C4 feed and the ethanol feed (1767/707 = 2.5).

Figure 15.21 Control structure for ETBE with flow control of ethanol stream.

The results for disturbances in throughput are provided in Figure 15.23. At 0.2 h, step changes in the setpoint of the C4 feedflow controller are made. The solid lines are for +20% steps; the dashed lines are for -20% steps. The control structure handles these disturbances quite well with ETBE purity held very close to its desired value. A comparison of Figure 15.23 with Figure 15.18 shows that the two control structures handle throughput disturbances almost identically.

In Figure 15.24 the disturbances are large 10% increases and decreases in the concentration of isobutene in the C4 fresh feedstream. The solid lines correspond to a change from 40 to 50mol% isobutene, with a corresponding decrease in the n-butene

Figure 15.22 Faceplates for ETBE with flow control of ethanol stream.

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