We also want to control the 5% boiling point of the diesel. This is achieved by manipulating flowrate of the kerosene. To get this 5% boiling point, another Configure Sensor is used, looking at stream 25 of the flowsheet, which has the same composition as does the diesel. Note that this controller is reverse-acting. If the 5% boiling point of the diesel is too high, more light material needs to be dropped down into this sidestream. This means that the kerosene flow should be decreased.
Relay-feedback tests are performed on each loop individually with the other boiling point controllers on manual. Tuning results are given in Table 11.6.
All the controller faceplates are shown in Figure 11.51. There are 25 controllers. There should be three more flow controllers, one on each of the steam to the strippers. The final flowsheet is given in Figure 11.52. Not installed or shown are the steam flow controllers on the strippers because they add more congestion to an already cluttered picture.
The effectiveness of this control scheme is demonstrated in Figure 11.53. The disturbances are step changes in the setpoints of the two crude oil flow controllers at time equal to 0.2 h. The responses to both positive and negative 20% changes are shown. The maximum
deviations in the 95% boiling points of the light- and heavy-naphtha products are about 6°C. The deviations in the 5% and 95% boiling points of the diesel product are about 20°C for these quite large disturbances.
The 20% increase in feedflow resulted in the saturation of several control valves when the original design size was used (valves 50% open). The valve sizes were doubled to remove these limitations. The valves modified were V14 (LIGHTS from the preflash column), V25 (HNAPHTHA), and V22 (AGO). The steady-state positions of these three valves are now about 25% open, as can be seen on the faceplates of the "BChvynaph" 95% boiling point controller, the "PC1" pressure controller in the PREFLASH column, and the "FCwash" flow controller, which manipulates the AGO to hold constant the stage 19 liquid flowrate (see Fig. 11.51).
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