Wow! what a simple question !
I will try to answer the question's first part, in repsect to capacity enhancement.
you need to get vapor+liquid flows on each trays, downcomer backup, flooding margin, velocity under donwcomer, increased pressure drop and adequate margins for foaming tendencies.What kind of trays you have in column, if antispin valve trays are there.... This is besides the reboiler and condenser duties.
You can best refer to Distillation "Design" and "operation" by Henri Kister.
