Dynamic Control

The flowsheet shown in Figure 10.20 does not show the plumbing required to run a realistic dynamic simulation. The key feature is that the pressure in the rectifier must be less than that in the main column so that vapor can flow from the main column into the rectifier. Therefore, in the simulation, a control valve is placed on the vapor line and a pump and a control valve are placed in the liquid recycle line from the bottom of the rectifier back to the main column. All this plumbing is shown in Figure 10.21. In a real physical setup it may be possible to use elevation differences to provide the necessary differential pressure driving force to get the liquid to flow from the rectifier into the main column at a higher pressure and avoid the use of a pump.

The usual sizing calculations are performed for both the main column and the rectifier (10-min liquid holdups in column bases and reflux drum). The flowsheet is pressure-checked, and the file is exported to Aspen Dynamics.

The development of a control structure for this complex system turned out to be more difficult that for the stripper flowsheet. The initial control scheme evaluated is shown in Figure 10.21. This is a logical extension of the control structure used for the vapor sidestream column in which the vapor sidestream is ratioed to the reboiler heat input. As we will demonstrate below, this structure worked well for some disturbances, but it could not handle decreases in the composition of methanol in the feed, resulting in a shutdown of the unit.

The various loops of the initial control structure are described below. The key issues are how to manipulate the sidestream and how to maintain the compositions of the three products:

1. Feed is flow controlled.

2. With a reflux ratio of 0.5, the reflux drum level is controlled by manipulating distillate flowrate.

3. The temperature on stage 4 of the main column is controlled by manipulating reflux flowrate. Stage 12 is tested as an alternative later.

4. Base levels are controlled by manipulating bottoms flowrates.

5. Pressure in the main column is controlled by manipulating condenser heat removal in the main-column condenser.

6. Pressure in the rectifier is controlled by manipulating condenser heat removal in the rectifier condenser.

7. The temperature on stage 51 in the main column is controlled by manipulating reboiler heat input.

8. The temperature on stage 9 in the rectifier is controlled by manipulating reflux flowrate in the rectifier.

9. The sidestream flowrate is ratioed to heat input to the reboiler.

The locations of the temperature control trays are selected by looking at the temperature profiles in the two columns, shown in Figure 10.22. The three temperature controllers (TC4 and TC51 in the main column and TC9 in the rectifier) have 1-min deadtimes and 100 K temperature transmitter spans. They are tuned individually by running a

VI2 I

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