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Tips & Tricks: Distillation Control Design

In this installment of tips and tricks we will look at one approach in which a candidate stage could be selected for a temperature control scheme in a distillation tower. The basic idea is that tray temperatures will often provide a good inference for composition in the tower. In the process we’ll highlight some useful features in VMGSim that could also be used in different applications namely the Process Calculator and Capture plot to static window. Begin with a steady state model of the tower which is set up to have the two manipulated variables specified.

Begin with a steady state model of the tower which is set up to have the two manipulated variables specified.

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In this case the manipulated variables are the reflux mole flow rate and the reboiler duty. This will represent the tower operating in open loop by adjusting the manipulated variables independently it is possible to see the effect that each has on the tower temperature profile. It is possible to streamline this process by using a process calculator to adjust both variables by a specified percentage away from the base values.

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For this example we will focus on the effect manipulating the reboiler duty has on the temperature profile. Navigate to the Profile tab in the distillation tower and then click the Plot button, and configure the plot to have only one y axis and select temperature.

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Click the Plot button, this will cause a plot of the tower temperature profile to appear.

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This is the base temperature profile. We would like to use this plot as the basis for comparison as we make changes to the reboiler duty in the selection of a candidate stage for a temperature control scheme. Right click on the profile plot and select Capture Pot to a New Static Window option. This will create a duplicate of the base temperature profile.

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 Make sure to leave both plot windows open. Now increase the reboiler duty by 5% using the process calculator.

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Navigate to the tower profile plot with the Print and Refresh button in the upper right hand corner of the plow window. Click the refresh button, you will notice the temperature profile will shift as the base case values contained in the plot are refreshed with the temperature profile with the increased reboiler duty. We would now like to overlay this new temperature profile on the static plot we created of the base reboiler duty temperature profile. We can do this by left clicking and holding down the mouse button on the icon on the tower profile plot and then dragging and dropping the pointer on to the same window. This will result in the static plot window having both the base case and increased reboiler duty temperature profile contained on it.

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We can repeat the process one more time to over lay the temperature profile in the tower when the reboiler duty is 5% less than the base case.

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The resulting static plot can be analyzed to find a stage in which there is a both a linear response and reasonable amount of gain. In this case Stage 18 looks like a reasonable candidate for a temperature control scheme using the reboiler duty as a manipulated variable. From this point, a dynamic model of the tower could be built to further evaluate the selection of this stage as a controlled variable.

The paper Improve Distillation-Column Control Design by Paul S. Fruehauf and Donald P. Mahoney, CEP March 1994 pg 75-83 provides a more detailed treatment of distillation control design.

Please contact your local VMG office for more information. 

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