Plantwide Control Structure

In preparation for exporting the steady-state flowsheet into Aspen Dynamics, all equipment is sized. Column diameters are calculated by Aspen tray sizing. Reflux drums and column bases are sized to provide 5 min of holdup when 50% full, based on the total liquid entering the surge capacity. Pumps and control valves are specified to give adequate dynamic rangeability. Typical valve pressure drops are 2 atm.

When the flowsheet with a tubular reactor was exported into Aspen Dynamics, the program would not run. A liquid-filled plug-flow reactor will not run in version 12 of Aspen Dynamics. To work around this limitation, the tubular reactor was replaced by two continuous stirred tank reactors (CSTRs) in series. Operating temperatures in both reactors were set at 355 K and volume at 10 m3. This design gave the same reactor effluent as the tubular reactor.

The plantwide control structure is shown in Figure 9.14. The tray temperature is controlled in each column by manipulating reboiler heat input. The trays are selected by finding the location where the temperature profile is steep: stage 31 in column C1 (see Fig. 9.8), stage 7 in column C2, and stage 7 in column C3. In addition, an internal composition in column C1 is controlled by manipulating the flowrate of methanol to the column. Stage 18 is selected (see Fig. 9.8). The flowrate of methanol to the reactor is ratioed to the feed flowrate.

Mine Headframe Drawing Images
Figure 9.13 iC5/MeOH/TAME at 4 bar.

The flowrate of extraction water fed to the top of column C2 is ratioed to the feed to this column D1 by using a multiplier and a remotely set flow controller. The temperature of the extraction water is controlled by manipulating cooling water to the cooler. Base level is controlled by manipulating bottoms, and reflux drum level is controlled by manipulating distillate. The binary methanol/water mixture from the bottom of column C2 is fed to column C3. A constant reflux ratio is maintained in this column by adjusting reflux flowrate.

There are two key plantwide material balance loops associated with column C3. The level in the reflux drum provides a good indication of the inventory of methanol in the system. If this level is going down, more methanol is being consumed in the reaction than is being fed into the process. Therefore the control structure maintains the reflux drum level in C3 by manipulating the methanol fresh feed.

Note that the flowrate of the total methanol (D3 plus fresh methanol feed) is fixed by the two downstream flow controllers setting the flowrates to the reactor and to column C1. This means that there is an immediate effect of fresh feed flowrate on reflux drum level. The distillate flow D3 changes inversely with fresh feed flow because the downstream flowrate is fixed. Thus the reflux drum level sees the change in the methanol fresh feed instantaneously.

At the other end of the column, the base level provides a good indication of the inventory of water in the system. Ideally there should be no loss of water since it just circulates

Figure 9.14 Control structure for reactors and (a) C1 and (b) C2 and C3.

around between the extractive column and the recovery column. However, there is a small amount of water lost in the overhead from column C2. A water makeup stream is used to control the liquid level in the base of column C3. This makeup flow is very small compared to the water circulation, so the base of column C3 must be sized to provide enough surge capacity to ride through disturbances.

All temperature and composition controllers have one-minute deadtimes. The PI controllers are tuned by running a relay-feedback test and using the Tyreus-Luyben settings. All liquid levels are controlled by proportional controllers with gains of 2 for all level loops except the two reactors, which have gains of 10. Liquid levels in reflux drums are controlled by manipulating distillate flowrates. The reflux ratios in all columns are controlled by manipulating reflux. Column pressure controllers use default controller settings and manipulate condenser heat removal.

Figure 9.15a gives the responses of the process to 20% changes in feed flowrate. Figure 9.15b gives responses to changes in feed composition. Effective plantwide control is achieved. The control structure provides stable base-level regulatory control for large disturbances. The purity of the TAME product is held quite close to its specification.

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