Figure 8.24 Feed composition disturbance with dual temperature.
The modeling of the two heat-integrated column discussed above assumes that the pressure controller in the high-pressure column can freely manipulate the heat removal rate in the condenser/reboiler heat exchanger. This is rigorously not true because the heat transfer area is fixed, and the heat transfer rate really depends on the temperature difference between the reflux drum of the high-pressure column and the base of the low-pressure column.
A more rigorous dynamic simulation of this system could be fairly easily put together by using Flowsheet Equations in Aspen Dynamics. The heat-transfer rate could be calculated using the area A^x, overall heat transfer coefficient U, and the differential temperature (temperature on stage 1 in the high-pressure column minus the temperature on stage 32 in the low-pressure column). This would fix QC2 and QR1, which must be equal but of opposite sign. No pressure controller would be used on the high-pressure column. Its pressure would float. The low-pressure column would have the normal pressure controller manipulating condenser heat removal.
The heat-integrated process provides an excellent example of the power and usefulness of dynamic simulation of distillation column systems. Alternative control structures can be easily and quickly evaluated.
8.4 CONTROL OF AZEOTROPIC COLUMNS/DECANTER SYSTEM
The dehydration of ethanol using benzene as a light entrainer was studied in Chapter 5. The process consisted of two distillation columns, one decanter, and two recycle streams. One of the recycle streams was successfully closed, but the second would not converge using steady-state Aspen Plus.
In this section we demonstrate how this second recycle loop can be successfully converged in Aspen Dynamics. A plantwide control structure is developed and its effectiveness evaluated. A very counterintuitive level control loop is shown to be required for stable operation.
8.4.1 Converting to Dynamics and Closing Recycle Loop
The usual base and reflux drum sizing procedure gives column base diameters of 1.76 and 0.855 m, respectively, in columns C1 and C2. Assuming as aspect ratio (L/D) of 2 gives the lengths. There is no reflux drum in the first column. The decanter is sized to provide 20 min of holdup based on the total liquid entering in the two liquid phases (aqueous and organic). The resulting decanter has a diameter of 2 m and a length of 4 m. Horizontal orientation is specified to aid in the phase separation. With the equipment sized and the flowsheet pressure checked, it is exported into Aspen Dynamics. Figure 8.25a shows the initial flowsheet that opens. Pressure controllers are installed on both columns. In column C1, pressure is held by manipulating the position of valve V12 in the overhead vapor line upstream of the condenser. Note that the Reflux recycle loop is not closed.
As discussed in Chapter 5, the column C1 is very sensitive to the amount of organic reflux. If too much is fed, the benzene comes out the bottom with the ethanol product.
If too little is fed, the water is not entrained overhead and comes out the bottom with the ethanol. Therefore, the organic reflux is selected to control the temperature on stage 28 in column C1, where there is a sharp change in the temperature profile. We know that the temperature control will not be very tight because of the liquid hydraulic lag between a change in reflux and a tray temperature 27 trays down the column. This controller is added before the reflux recycle loop is closed, as shown in Figure 8.25b. An Initialization run is made to make sure that everything is running okay.
Now we are ready to close the recycle loop. The block "V1" and the streams "REFLUX2" and "ORGREC" are deleted from the flowsheet. The "REFLUX" stream is selected and right-clicked. Selecting Reconnect Source, this stream is attached to the mixer "M1." The little red light appears at the bottom of the window. Double-clicking opens the window shown in Figure 8.26a, telling us that the simulation is overspecified by two variables. Double-clicking the Analyze button opens Figure 8.26b, where changing temperature of the reflux stream from Fixed to Free is suggested. This makes sense because the reflux stream is now coming from the organic liquid phase in the decanter. The other suggestion is to make a change in the temperature controller, which is not reasonable. What is reasonable is to change the pressure of the reflux from Fixed to Free. This is done selecting the stream "REFLUX," right-clicking, selecting Forms and then All Variables. Scroll down to P and change to Free, as shown in Figure 8.26c. The green light appears at the bottom, indicating that the simulation is now "square" (as many variables as equations, i.e., 0 degrees of freedom). An Initialization run and a Dynamics run are made to check that the integrator is working okay with both recycle streams connected.
The other controllers are now added in the usual way. Base levels are held by bottoms flowrate. Reflux drum level in column C2 is held by the distillate flowrate (the RECYCLE stream back to column C1). The tray with the sharpest temperature change in column C2 is stage 20 (365.14 K at steady state). The temperature controller TC2 manipulates reboiler heat input. A third temperature controller is added on the heat exchanger before the decanter to control the temperature of the stream entering the decanter.
The control of the two liquid inventories in the decanter is critical. Since only a very small amount of benzene is lost, the organic level basically floats up and down as changes occur in the reflux flowrate. An organic phase level controller adjusts the benzene makeup stream, but it is so small that the level changes are significant. This does not hurt anything as long as the decanter does not overfill or the organic level is lost.
The control of the aqueous level would appear to be straightforward. A level controller would manipulate the valve "VD1." Conventional wisdom says that this controller is Direct-acting. If the level goes up, more is fed to column C2. Very surprisingly, this setup was found to not work. The system shut down. However, making the controller Reverse-acting produced a stable control structure. We demonstrate this in the next section.
Figure 8.27 shows the final control structure. Features not mentioned include ratioing both the reboiler heat and the reflux flowrate input in column C1 to the feed flowrate. The temperature controller TC1 in column C1 changes the reflux to feed ratio.
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