In the alternative control structure, the throughput is set by the flowrate of the methanol fresh feed instead of the C4 fresh feed. Now the methanol feed is flow controlled, and the C4 feed is ratioed to it, as shown in Figure 15.9. The ratio is reset by a composition controller that maintains the isobutene composition at 6.98 mol% on stage 10. As shown in Figure 15.2, this is the location of the peak in the isobutene composition profile.
The rest of the control loops are identical to those used in the previous structure. Note that flow controller FCC4 is on cascade with its setpoint coming from the ratio element whose other input is the output signal of composition controller CC. Note also that this output signal is the C4-to-methanol ratio, which is 3.58 at base case conditions. Controller faceplates are given in Figure 15.10. The new tuning parameters for the composition controller are provided in Table 15.1.
Disturbances in both throughput and feed composition were used to evaluate the effectiveness of this alternative control structure. Figure 15.11 shows the responses to 20% step changes in the setpoint of the methanol feed flow controller. Solid lines are increases, and dashed lines are decreases. The increase is handled quite well. It is surprising that the decrease is not handled well. There is a significant transient drop in MTBE purity in the bottoms product (middle left graph, Fig. 15.11), which lasts for several hours. In addition and of more importance, there is a very large increase in the isobutene lost in the distillate.
Even though the methanol feed is decreased, the C4 feed actually ends up at a new steady state that is at a higher flowrate. The distillate isobutene composition climbs to about 14mol%.
Figure 15.12 gives the responses for changes in the composition of the C4 feedstream. An increase in isobutene concentration produces a large transient drop in MTBE purity.
20% Methanol Feed Flowrate Disturbances 1000r u?
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