This column needs an internal inspection.
There are several possible reasons why the bottom temperature might be too high:


1.        An excessive pressure drop across the column

  • Fouled/plugged rings
  • Deteriorated rings sticking together
  • Fouling at the feed pipe

2.        The bottom's composition is heavier than it used to be.

3.       The column top pressure is higher than it used to be.

4.       The steam is overly superheated.

A gamma scan might also expose the source of the problem.

It is "unusual" to control column temperature with reflux.  Column pressure, column compositions and steam temperature usually determine column temperature.