Figure 7.12 Attaching control signal.
The action of the controller should be Direct (under Controller action in Fig. 7.15b) because if the level increases, the signal to the valve should increase (PV", OP ") to remove more bottoms. In some columns, base level is controlled by manipulating a valve in the feed to the column. In that control structure, the base level controller action should be Reverse.
Since we want proportional-only control, the controller gain is set equal to 2 and the integral time is set at a very large number (9999 min, as shown in Fig. 7.15b).
Next the Ranges page tab is clicked, which opens the window shown in Figure 7.16. The default value for the level transmitter range is 0-12.7 m. The default value of the controller output range is 0-100%. Both of these are what we want, so they require no changes. The Configure window is then closed.
The faceplate is located at some convenient spot in the window where we can keep an eye on what is going on with this level loop. Remember that there will eventually be five controller faceplates.
Let us look in detail at the faceplate. As shown in Figure 7.17, there are six buttons at the top. The first button on the left is the Auto button, the second is Manual, and the third is Cascade.
When the Auto button is pushed, the controller changes the OP signal automatically according to the current values of the setpoint SP and the PV. The value of the setpoint can be changed by double-clicking on the number in the box to the right of SP, typing in the desired number, and hitting Enter on the keyboard.
When the Manual button is pushed, you can manually set the OP signal. This is done by double-clicking on the number in the box to the right of OP, typing in the desired number, and hitting Enter on the keyboard. When the Cascade button is pushed, the controller receives its setpoint signal from some other control element. We will illustrate this later in this chapter.
Now would be a good time to check out the pressure controller, which was automatically set up when we started Aspen Dynamics. Figure 7.18a shows the faceplate (which appears after double-clicking the icon on the flowsheet) and the Configure page tab. The default controller tuning constants are a gain of 20 and an integral time of 12 min. These work pretty well in most column simulations. Note that the controller output is not a "% of scale" signal sent to a valve. It is a heat removal rate in the condenser. As a result, the controller is set up to be reverse-acting: when pressure goes up, the controller output signal goes down. The reason for this action becomes clear when we look at the Ranges page tab shown in Figure 7.18b. Note that the controller output ranges from a minimum of — 45,347,000 W to a maximum of 0W. This conforms with the Aspen convention that heat removal is negative.
In my experience the only modification that sometimes needs to be made to the pressure controller is to change to a more convenient pressure transmitter range. For example, in this column the operating pressure is 16.8 atm. We might change the pressure transmitter range to 14-19 atm from the very wide range used in the default setup. Of course, the gain should be correspondingly reduced.
The second-level controller LC12 for the reflux drum is installed and connected in the same way. The PV signal comes from the level on stage 1. The OP goes to valve V12. A direct-acting proportional-only controller is specified.
The final basic controller that we need to set up is a flow controller on the feed. A PID controller is placed on the flowsheet. Its PV signal is the molar flowrate of feedstream F1. Its OP signal goes to valve V1. After opening the Tuning page tab and clicking the Initialize Values button, we set the controller to be Reverse-acting, and use conventional flow controller tuning (KC = 0.5 and integral time = 0.3 min), as shown in Figure 7.19a. The most common error in setting up the flow controller is to forget to specify Reverse action. Since flow control is very fast and essentially algebraic, it seems to help the numerical integrator to use some filtering in a flow controller. The Filtering page tab is selected, the Enable filtering box is checked, and a small filter time constant (0.1 min) is typed in (see Fig. 7.19b).
The flowsheet now has four controller faceplates displayed, as shown in Figure 7.20. We have one more controller to add, a temperature controller that holds the temperature on a selected tray by adjusting the reboiler heat input.
It is important to clarify what is happening to the reflux flow. The column icon does not show the plumbing details of a reflux drum, pump, and reflux valve. As mentioned earlier, the default condition in Aspen Dynamics is that the mass flowrate of reflux is constant, unless otherwise adjusted. For example, if we wanted to control reflux drum level with reflux flowrate, the level controller OP signal would be connected to the column and the Reflux.FmR would be selected. The second common application would be if we wanted to ratio the reflux flowrate to the feed flowrate. We will illustrate these by examples in this and later chapters.
A word of caution is appropriate at this point. During the initialization of controllers, quirky things sometimes occur. There are some bugs in Aspen Dynamics that sometimes set the OP signal at the wrong initial value (e.g., at 100 instead of 50%) or the PV at a value
not equal to the steady-state value. To circumvent these problems, switch the controller to manual, type in the correct OP value, and run the simulation for a while. Then switch the controller to automatic and run out to a steady state.
While we are on the subject of running, once the simulation runs out in time and converges to a steady state, the file should be saved. It is convenient to save the file with time set equal to zero so that it can be used to establish initial conditions for new runs. To do this, make an Initialization run and then switch to Dynamic (but do not perform a run). Click the Rewind button, which is the fifth one from the right on the upper toolbar (see Fig. 7.20). A window opens (Fig. 7.21) on which you can select the Initialization Run as the Select rewind snapshot and then save the simulation file. Note that the "Sim Time" is 0. You can then "rewind" to these conditions whenever you want to start again at this steady state.
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