Sensitivity Runs in Aspen

A sensitivity run in Aspen allows you to vary one element of a system, such as the temperature of a reactor, and then view the effects this variance has on other aspects of the system, such as the mole composition of the reactor effluent. As part of the example of creating a flash tank model in Aspen, we will use a sensitivity run to show how the fraction vapor changes with the flash temperature for fixed feed and pressure.

Since we already have a flash file created, we will use it to create the sensitivity run. There are many steps to follow, but some that are shown here may not be required. If you find a better or simpler way to do this, let us know so we can correct our notes. Here are the steps we followed:

  • First create a sensitivity tool window by:
  • Second, give the sensitivity run a name or accept the default by clicking "OK" in the window.  In most cases, we will simply click on the "Next" button to go on to further steps.
  •  Third, create a defined variable to be sampled. We created the variable VFLOW by pressing the "New" button:
  •  VFLOW is in the "Streams" category, of type "Stream-Var", in the stream "VAPOR", and it holds the "MOLE-FLOW". The substream is mixed, but that is the default value.
  •  Using the "Next" button allows us to bring up a window in which we can set the Variable Numer as "1" by clicking on "New" and then specify that:
    • it is the "TEMP
    • of type "Block-Var"
    • in the block "FLASH".
    • We specify the range as 105C to 125C with 10 points used in that range.

    Note that the window does not show the units for temperature, you must deduce this from the fact that we are using the "METCBAR" units.

  •  We will tabulate "VFLOW" in the first column of out results.
  •  We see (using the "Next" button again) that we have finished setting conditions for the sensitivity run and can click "OK" to execute the program.

  •  Note that in the "Control Panel" you can see that 11 flash operations were performed.
  •  Here is the first result window that is displayed. If you click on the "<<" button you will find the sensitivity results.
  • They list the vapor flow rate for each flash temperature. Now let's plot those results:
  •  If you a) click on the head of the "TEMP" column and then b) use the Plot menu, you can make a plot with those temperatures as the X-Axis Variable.
  • If you a) click on the head of the "VFLOW" column and then b) use the Plot menu, you can make a plot with the flows as the Y-Axis Variable. Then you can use the "PLOT" menu to display the plot as seen below.

 

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