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"Pounds per square inch absolute.

"Pounds per square inch absolute.

8.2.2 Alternative Control Structures

Three alternative control structures are studied. In the first two, pressure is controlled by manipulating the flowrate of the vapor distillate stream from the reflux drum. This is the conventional configuration that is recommended in most papers and books. In the third control structure, pressure is controlled by condenser heat removal.

Fundamentals Conventional single-end control is used in all configurations since dual-composition control is rarely used with this propane/butane separation. The temperature on a tray in the rectifying section of the column, where the temperature changes from tray to tray are large, is controlled by manipulating reboiler heat input. The steady-state design shows a temperature on stage 8 of 128°F in the large-distillate case and 155°F in the small-distillate case. Figure 8.4 gives the temperature profiles for the two design cases.

The tray temperature controllers are tuned by inserting a 1-min deadtime in the loop and using the relay-feedback test to determine the ultimate gain and ultimate frequency. Then the Tyreus-Luyben settings are used. Table 8.2 gives the tuning constants.

Note that the tuning constants listed in Table 8.2 are different for the two design cases because of the different vapor distillate flowrates. Note also that the tuning constants are different for some of the different control structures because the effect of reboiler heat input (or the equivalent vapor flowrate to the condenser) on pressure varies depending on what manipulated variable is used to control pressure. Proportional level controllers are used with gains of 2. The default pressure controller tuning constants from Aspen Dynamics are used.

Control Structure CS1 Figure 8.5a shows the control structure that is probably most commonly used for distillation columns with partial condensers. The main features of this

Figure 8.4 Temperature profiles.

structure are pressure controlled by manipulating vapor distillate flowrate and reflux drum level controlled by manipulating condenser heat removal. Reflux flowrate is fixed or ratioed to feed.

The interaction between the level and pressure loops is present because any disturbance that affects either loop will propagate to the other loop. For example, suppose that the feed composition changes and more ethane enters the column. The temperature in the reflux drum will drop and the rate of heat transfer in the condenser will decrease for a fixed flow-rate of cooling water. The rate of condensation will decrease. Pressure will increase, so the pressure controller will increase the distillate flowrate. The drop in condensation will also decrease the reflux drum level. When the level controller increases cooling water flowrate to increase the level, pressure will decrease. This interaction can cause the pressure controller and the level controller to start fighting each other.

One fundamental concept in distillation control is to prevent rapid changes in pressure. If pressure increases too quickly, vapor rates through the trays decrease, which can cause weeping and dumping. If pressure decreases too rapidly, vapor rates increase, which can

TABLE 8.2 Temperature Controller Tuning Constants

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