The easiest way to open the Example1.dynf file is to double-click on the filename in Windows Explorer. The screen that opens contains several windows (see Fig. 7.7).
The Process Flowsheet Window is where the control structure will be developed. The Simulation Messages window is where the progress of the simulation is shown and simulation time is displayed. The window in the upper left corner, Exploring, is where various types of controllers, control signals, and other elements can be found to "drop and drag" onto the flowsheet.
The very first thing to do with any newly imported file is to make an "initialization" run to make sure that everything is running. At the very top of the screen there is a little window that says Dynamic. Clicking the arrow to the right opens the dropdown menu shown in Figure 7.8a. Select Initialization and click the Run button, which is just to the upper right. If everything is set up correctly, the window shown in Figure 7.8b opens.
The next thing to do is to make sure that the integrator is working correctly. This is done by changing from Initialization back to Dynamic and clicking the Run button again. The Simulation Messages window at the bottom of the screen should start displaying advancing simulation time. An example is shown in Figure 7.8c. Note the green block at the bottom of the screen. If something is wrong, this block will turn red and you will not be able to run. The run is stopped by clicking the Pause button, which is the second button to the right of the Run button.
The initial flowsheet has some default controllers already installed. In this simple, single-column process there is only one default controller, the pressure controller. It is configured to measure condenser pressure and manipulate condenser heat removal. The action of the controller, the range of the pressure transmitter, the maximum heat removal rate, and the controller tuning constants are all set up at some nominal values. We will come back and look at these later. But this indicates the number of items that must be specified for each new controller that we add to the flowsheet. At a minimum, four additional controllers must be added to achieve effective operation of the column:
1. Reflux drum-level controller
2. Base-level controller
3. Feedflow controller
Which of the four remaining manipulated variables selected to control each of these four is the issue of control structure selection. A variety of control structures will be discussed here and in subsequent chapters.
We have not included reflux in the list above because the default configuration in Aspen Dynamics is to fix the mass flowrate of the reflux. We will discuss how this setup can be changed later in this chapter.
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