Control Structure with C4 Feedflow Controlled

The dynamic parameters in the Aspen Plus file are inserted. The volumes of the reflux drum (144 m3) and the column base (177 m3) are calculated so that the holdup times are 5 min when the levels are at 50%, based on the total liquid entering. Aspen Plus Tray Sizing gives a diameter of 7.4 m. Aspen Plus Packing Sizing gives a diameter of 5.4 m, and this

Block TI : Liquid Composition Praíles

Block TI : Liquid Composition Praíles

Stage

Figure 15.2 Composition profiles in MTBE reactive column.

Stage

Figure 15.2 Composition profiles in MTBE reactive column.

Stage

Figure 15.3 Temperature profile in MTBE reactive column.

Stage

Figure 15.3 Temperature profile in MTBE reactive column.

Mtbe Composition

Figure 15.4 The MTBE steady state from converged dynamic simulation.

Figure 15.4 The MTBE steady state from converged dynamic simulation.

is used in the dynamic simulation. To hold 800 kg of catalyst, the liquid height is about 0.044 m on the reactive trays. Pressure drops over the four valves in the system and pump heads are set to give adequate rangeability for all of the flows (about 4 atm pressure drop at design flowrates with control valves 50% open). The file is pressure checked and exported to Aspen Dynamics.

Structure. The control structure is shown in Figure 15.5. This structure is similar to that used in the ideal ternary system with inerts, which was studied in Chapter 12. It consists of the following loops:

1. The C4 feed is flow controlled.

2. The methanol feed is ratioed to the C4 feed with the ratio set by a composition controller that controls the methanol composition on stage 10 inside the column at 18 mol% methanol. Note that the methanol flow controller is on cascade with its remote setpoint coming from the output signal of the multiplier ratiomc4. The two inputs to the multiplier are the flowrate of the C4 stream and the controller output signal from composition controller CC. At steady state this ratio is 768/1976 = 0.389, so the output signal from the composition controller has this value, as shown on the faceplates given in Figure 15.6.

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Figure 15.5 Aspen Dynamics contxol structure with flow control of C4.

Figure 15.6 Controller faceplates MTBE with flow control of C4.

3. The temperature of stage 13 is controlled at 412.5 K by manipulating the reboiler heat input whose steady-state value is 8.15 x 106cal/s.

4. The reflux-drum level is controlled by manipulating the distillate flow.

5. The base level is controlled by manipulating the bottoms flow.

6. The reflux ratio is controlled by measuring the mass flowrate of the distillate, sending this signal through a multiplier RR (with a second input of the desired reflux ratio of 5.055) and setting the mass flowrate of the reflux.

7. Pressure is controlled by manipulating the condenser heat removal.

Controller Tuning. The composition controller and the temperature controller are tuned by running relay-feedback tests and using Tyreus-Luyben tuning. Table 15.1 provides 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.

TABLE 15.1 MTBE Column Tuning Parameters

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