Figure 15.16 shows the control structure in which the FFC4 stream is flow controlled. Then the ethanol feed is ratioed to it, with the ratio being reset by a composition controller that holds the ethanol composition on stage 17 at 11.58 mol%. This stage is selected by looking at the composition profiles shown in Figure 15.14. The ethanol composition is changing rapidly at this location. Above stage 17 in the upper part of the reactive zone, the ethanol composition profile is flat at about 25 mol%. The isobutene compositions are low in the upper part of the reactive zone, but they begin to increase at around stage 17.
The temperature controller holds stage 22 at 383 K by manipulating the reboiler heat input. Note that the temperature profile in the ETBE system (Fig. 15.15) is much sharper than in the MTBE system (Fig. 15.3). Therefore, a larger temperature transmitter span was used.
The rest of the control structure is the same as that used for the MTBE system, as described in Section 15.1.2. The composition controller and the temperature
controller are tuned by running relay-feedback tests and using Tyreus-Luyben tuning. Table 15.2 lists the parameter values. The temperature controller was tuned first, with the composition controller on manual. Then the composition controller was tuned with the temperature still on automatic (sequential tuning). The level controllers are proportional only with gains of 2. Faceplates are shown in Figure 15.17. Note that in this
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