Rewind I Cancel

Rewind I Cancel

Figure 7.21 Rewinding to Snapshot.

in Chapter 6, there are several ways to approach this problem. These include looking at the shape of the temperature profile in the column, calculating steady-state gains, and using SVD analysis.

7.5.1 Tray Temperature Control

Let us first discuss using a temperature controller to maintain a tray temperature in the column. Looking at the temperature profile in Aspen Plus, we see that stage 9 displays a fairly steep slope. Its temperature is 337.36 K.

A controller is installed on the flowsheet in the normal way, except that instead of using a PID controller, we select a PIDIncr controller. The important difference between these two is that the PIDIncr controller has a built-in relay-feedback test capability, which make this dynamic test a breeze.

The PV is selected to be the temperature on stage 9. The OP is selected to be the reboiler heat input QRebR. Figure 7.22a shows the controller faceplate and the Tuning page tab after the Initialize Values button has been clicked. The normal controller output is 27,166,000 W. The controller action should be set at Reverse because if the tray temperature is going up, the reboiler heat input should be decreased. It is convenient to change the range of the temperature transmitter from the default 273-401 K to a more convenient and narrower range of 320-370 K, as shown on the Ranges page tab in Figure 7.22b.

The program is run to make sure that everything works okay without a lag or a deadtime in the loop. Now we back up and insert a deadtime element on the flowsheet between the column and the TC9 temperature controller. The reason for installing the controller initially without the deadtime element is to avoid initialization problems that sometimes crop up if you attempt to install the deadtime and the controller all in one shot.

Before we proceed, it might be wise to save some of the work. Since a fair amount of time has been spent in setting up the faceplates and arranging them on the screen, we can avoid having to do this again by clicking on Tools in the toolbar at the top of the screen and selecting Capture screen layout. The window shown in Figure 7.23 opens, on which we enter an appropriate name. When the program is restarted, the screen layout can be reinstalled by going to the Exploring window, clicking Flowsheet, and double-clicking on the icon in the lower Flowsheet Contents window with the name you provided.

Now let us install the deadtime element. The line from stage 9 temperature is selected. Right-clicking, selecting Reconnect Destination, and placing the icon on the arrow pointing to the deadtime icon, we connect the input to the deadtime. A new control signal is inserted between the deadtime and the controller. The deadtime icon then is selected. Right-clicking, selecting Forms from the dropdown list, and selecting All Variables, we open the window shown in Figure 7.24a. The DeadTime value is initially 0 min. Note that the Input and Output values are set at a default number, not the actual 337.36 K value. A deadtime of 1 min is entered, and performing an Initialization run fills in the correct values, as shown in Figure 7.24b. The final flowsheet and controller faceplates are shown in Figure 7.25.

Everything is ready for the relay-feedback test. Clicking the Tune button opens the window shown in Figure 7.26a. We specify a Closed loop ATV as the Test method. The default value of the Relay output amplitude is 5%, which is usually good. For a very nonlinear column, the amplitude may have to be reduced.

Figure 7.22 (a) Initial installation of temperature controller; (b) Ranges page tab.
Figure 7.23 Saving screen layout.

To start the test, click the Run button at the top of the screen and click the Start test button on the Tune window. To be able to see the dynamic responses, click the Plot button at the top of the controller faceplate. After several (4-6) cycles have occurred, click the Finish test button. Figure 7.26b gives the results. The predicted ultimate gain is 5.9, and the ultimate period is 4.5 min.

The timescale in Figure 7.26b is fairly small. To get good-looking plots, the plot time interval must be reduced from the default value of 0.01 h. Aspen Dynamics calls this parameter "communication time," and it can be accessed by going to the top toolbar in the Aspen Dynamics window and selecting Run and Run Options. A window opens on which Communication Time can be set. A value of 0.0005 h was used to get the plots shown in Figure 7.26b. This parameter does not affect the results of a dynamic simulation, except for slowing it down somewhat. It only affects the appearance of plots.

Finally the Tuning parameters page tab is clicked, the Tyreus-Luyben Tuning rule (Fig. 7.26c) is selected, and the Calculate button is pushed. The resulting controller settings are gain KC = 1.84 and integral time t = 9.9 min. The Tyreus-Luyben tuning

Q deadl.AllVariables Table

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