Re: Column overhead temp as an mv vesus reflux as an mv

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: As Tod mentioned in his post, you usually like to nail down one side of the column.  However, there are also cases where you do not tie down either end at the regulatory level.

: The question I would ask is how well the temperature loop you plan to close (or open) controls.  Is it sluggish or very slow?  How does it handle setpoint changes and disturbances?

: If you currently run with this temperature loop closed, open it up and see how the tower responds to feed rate changes, feed composition changes and other disturbances.  If you have to make frequent compensating moves to the reflux to keep the tower operating within specifications, then you will need to keep the temperature loop closed.

: I recently worked on a depropanizer where we left the  top  and bottom temperature loops open.  The top temperature responded very slow and as in most depropanizers, the overhead composition (or tray temperature) acts very similar to a ramp (or integrator).  In otherwords, making moves in reboiler or reflux cause the temperature to keep moving until the move is returned to original value.  After the independent is returned to its original value, the measured temperature “lines-out” instead of returning to its original value as in a “steady-state” variable.  This characteristic is not uncommon in high purity columns. This temperature was put in the controller as a pseudo-ramp type variable.


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