It looks like the vent from the buffer tank goes below tray T3 when the drawoff comes from above T3. There is a pressure difference (equal to the pressure drop on the tray) that may hamper liquid flow. A dip leg in the buffer tank would help.
What about organizational and project engineering skills required in such huge and time-lasting projects as the building of cathedrals, bridges, pyramids...
How did ancient "engineers" coordinate workers, mobilize resources, etc. ?
I would be interested if anyone had references about the history...
To get this kind of information you usually have to subscribe to a trade periodical.
You can get some data for petrochemicals in graphical form at http://www.platts.com