3 replies to this topic

[Estiqaatsi]

Dear all,

I am facing a problem within a dynamic simulation in Aspen Plus. I need to operate a controller based on the temperature measured in the catalyst bed. Apparently the only property that I can get from the reactor is the outlet pressure. But at the same time I can easily know and plot that temperature value. This doesn't make sense to me, and maybe I am missing something.

The only option that I see, if that property is not available, is to split the reactor into different parts, in order to make a stream appear in between, being then able to use all the stream properties (including of course temperature).

Has anyone experience with this kind of problem?

Thanks in advance

[Pilesar]

Splitting the reactor into two parts seems the easy way and should not harm the fidelity of the model. Why not just do that? Have you investigated using a calculator value as a control process variable? (My dynamic simulation experience is with other programs and I am not familiar with the features you have available.)

[Estiqaatsi]

Thank you Pilesar for your answer.

Actually the reactor is already split (to simulate catalyst and inert), so my point was: is it possible to avoid splitting this simple unit op? What if I had different temperature transmitters installed in different points? Ok it is easy, but looks overcomplicated for a simple task. I am not aware of a calculator (I think you are referring to something like the Spreadsheet on Hysys), I should check.

[MojoZeJojo]

You can do this: connect a control signal on the PV of your controller. The simulation will become underspecified, but don't worry about that. Then go to the Flowsheet folder in the Simulation Explorer, right click Edit, and add this code:

 

TC.PV = Blocks("B1").T(10);

 

For example if TC is the name of the PIDincr controller, and B1 is the name of the reactor block, and you want for example the temperature in node 10. You need to look in the AllVariables table of the reactor to find out the "catalyst" temperature. Finally, right click, Compile the flowsheet constraints. As you compile, you'll see the simulation is square again (because this equation says "make the PV of TC equal to the temperature T(10) of the reactor block).

 

The flowsheet constraints have access to all variables, and you can also add your own equations. It's a bit like the "calculator" or "spreadsheet" but using Aspen Custom Modeler syntax. 

 

The only drawback is that the PV will not have the right units (it will have no units), and the "initialize" button on PIDincr block won't work for the PV/SP. You can enter manually the SP and range for PV/SP.