Figure 11.28 Control structure with 95% BP control.
428 GJ/h). The 95BPt controller has a deadtime of 3 min and is tuned in the same way as the temperature controller (KC = 0.821 and t7 = 26 min) with a boiling point temperature transmitter range of 150-250oC and a maximum reflux flowrate of 70,000 kg/h.
The effectiveness of this control structure is demonstrated in Figure 11.29a for a 20% increase and a 20% decrease in the flowrates of both crude feeds and steam to the base of the column. The peak dynamic deviations in the 95% boiling point of the naphtha are about 128C. The new steady state for an increase in feed has higher flowrates of reflux, lights, and bottoms (CDU Feed in Fig. 11.29a) but only a small increase in the flowrate of the naphtha. The new steady state for a decrease in feed has lower flowrates of reflux, naphtha, and bottoms (CDU Feed), but the reduction in lights is smaller than is the increase for the positive change in feed. Increasing the load on the column reduces the yield of naphtha at the expense of lights for the same naphtha 95% boiling point of 191 oC (3750F).
Figure 11.29b gives results when there is a change in the ratio of the two crude oils. The solid lines are for a decrease in Crude1 from 280 to 224 T/h, while Crude2 is increased from 275 to 329 T/h. Since Crude2 is lighter than Crude1, there is an increase in the flow-rate of the "lights" and a decrease in the flowrate of the bottoms. Unexpectedly, the naphtha decreases slightly. The dashed lines represent an increase in Crude1 from 280 to 336 T/h while Crude2 decreases from 275 to 221 T/h. Responses are almost the mirror image of the reverse disturbance.
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