Working on Dynamics Models

Steady State versus Dynamics

The same modeling environment is used for steady state and dynamics. It is possible to adapt an existing steady state case and use it in dynamics mode. In many cases the lined out dynamics model will provide the same results as the steady state model.

However, as demonstrated in the Key Concepts section, there are several fundamental differences between steady state and dynamics models. Due to static head considerations and specifications that require simultaneous solution, switching a complex dynamic model, especially one that has been built or extended in dynamics mode, to steady state, will require modification to solve successfully.

Once a case is in dynamics mode, the general practice is to keep it in dynamics mode and not switch back to steady state again. After all, the dynamics solver can also provide a lined out steady state solution in dynamics mode.

If a steady state model is to be transitioned to dynamics mode, the following should be kept in mind:

• Make sure the model is fully solved in steady state. This will provide a solution from which the dynamics mode integrator can continue.
• Consider where extra equipment such as valves or pumps may be required.
• Consider the size of equipment such as vessels and tray sections. In steady state the size of a vessel is often not relevant, but it can be a key factor in dynamics.
• Consider setting elevations where static head contributions will be relevant.
• Consider the pressures in the model. Steady state models often disregard pressure drops or have the same pressure throughout the model. Rectify this.
• Depending on the model complexity and specifications you choose, you may also need a control strategy. A simple model will likely not need a control strategy, but will possibly also be less flexible because of that.

You can use the dynamics assistant tool (described under Tools) to examine your model and get an idea of what changes may be required.

Building

The best way to develop a large model is by doing so in incremental steps. Start with a smaller model that runs fine. Then add onto it, and run the integrator to make sure it behaves as expected. Then add another section and so on.

Do note that you do not need to use recycle unit operations in dynamics mode. In dynamics mode, when a specification is changed or deleted, the case will also not forget (erase) values. When the integrator is started the model will always continue from the previous values, using them as a starting point. There are a few exceptions, such as the stream which will erase some values if you delete a specification, such that another specification can be made.

A Simple Model Step by Step

As an example and to illustrate some concepts, here is a simple model, to be constructed next:

Follow these steps:

• Create a new case. Since this is a level control exercise, we shall need vapour and liquid. Click File, New, Case. Create a component list and add N2 and n-Octane to it. Exit to the build environment. Note that clicking on the new case icon on the button bar will open a standard starting case which includes many component. You can make use of that case, intended for steady state refinery modeling, if you wish, though in general, for speed, you want to eliminate components that are not needed.
• Click on the dynamics mode button to switch to dynamics mode. This will ensure that dynamics mode specific pages are visible in the unit operation views.
• Now create a stream in the PFD. Hit F4 to open the palette if need be, and using the left or right mouse button drag a blue stream to the palette. Or double click on the stream icon in the palette to create a stream.
• Double click on the stream in the PFD to open it. Specify a pressure of 101.325 kPa, a temperature of 25 C, a flow rate of 1 kgmol/hr and set the composition to 0.5 mole N2 and 0.5 mole n-Octane. The stream will flash and solve.
• Create a vessel (Separator) and connect the feed to it. Also attach vapour and liquid streams to the vessel. Note that, since the integrator is not running, the vessel does not solve. But when you connect new streams to it, it initializes those steams from the feed stream. This will provide a starting point for the integrator.
• Go to the vessel rating page and specify a volume of 1 m³.
• Attach a valve to the liquid outlet stream. Note how the valve shows red, indicating that it needs to be sized.
• Open the Valve view on the Dynamics, Specs page, enter a C_v value of 0.5.
• Now the static head contribution from the liquid level should provide a driving force for the liquid, but open the Valve outlet stream. On the Dynamics, Specs page, check Active for pressure, and change the value to 91 kPa. Go to the vessel top outlet and also set the pressure specification for on. For the vessel feed stream, disable the pressure specification. There can only be one pressure specification among all streams directly connected to the vessel.
• Now at this point the model can be run, but the vessel will likely drain empty or overflow (either of which is fine).
• So add a controller from the palette. Open its view. On the Connections tab, set its output to be the liquid valve. Set the controller's process variable to be the vessel's liquid percent level. On the Parameters tag, set the PV minimum and maximum values to 0 and 100. Set the tuning values K_c to 1.2 and T_i to 0.4 (just as initial rough guesses, feel free to try and improve upon these later on). Finally set the controller mode to auto. Set the controller action to Direct.
• Now it would be nice to have a strip chart, so that we can monitor the controller. Open the vessel view and on the Dynamics, Strip Chart page, select "Small Dynamic" from the drop down. Then delete all the variables that show up, except Liquid Percent Level. Click Create Stripchart. On the controller, Parameters tab, use the right mouse and drag the OP value and drop it on the strip chart. You can access the stripchart settings in the Databook (press Control-D).
• Now run the integrator by click Go on the button bar, or press Control-I to open the Integrator view and click on the Start or Continue button.

Note how the vessel level is zero initially (since it is set by default to do a dry startup). The controller initially closes the valve which lets the vessel level rise. Once it start getting around 50%, the controller will start to open the valve.

• Later on, similarly, stop the integrator again. You can not add operations or delete them or change connections while the integrator is running.
• Now, lets add a pipe unit operation. Create one from the PFD and connect it up as shown. Attach an energy stream to the pipe and specify a value of 0 for it. On the Valve outlet stream, on the Dynamics, Specs page, uncheck Active for the pressure. On the pipe outlet stream, check the Pressure specification as Active, and set it value to 81 kPa. Now open the Pipe view. On the Rating, Sizing page, add one segment. Set it length to 10 m and the outside diameter to 8 mm and the inside diameter to 7 mm.
• Now start the integrator.

Note that the simulation will run more slowly because of the pipe unit operation. In many cases pipe sections can be ignored, or can be modeled more efficiently (but using a much simpler model) via the pipe option in the valve unit operation. It can be accessed on the Dynamics, Pipe page of the valve view.

The dynamics assist may come up during this exercise, depending on your preferences. It is discussed in the |Tools section. For now just dismiss its dialog boxes or choose to proceed in any event.

< Sample Cases | Index | Tools >