reboiler return below the reboiler draw-out

[mahmmoud30]

Hi all
regarding the title, it is a naphtha stabilizer column which separate LPG from hydrotreated naphtha with a bottom temp. of 176 deg C at the reboiler draw-out and 190 deg C at the reboiler return.
the feed flow into this column is ranging from 24~35 ton/h
* my question is why the reboiler return is designed below the reboiler draw-out?
thanks in advance

 

[25362]

To mahmmoud30, for low-boiling applications, once-through thermosiphon reboilers vaporizing about 20-30% of the liquid descending down the column can be fed from a trapout pan attached to the bottom tray, therefore located above the reboiler return line.

In these cases, a valved start-up line is generally provided from the bottom of the column.

 

[mahmmoud30]

dear 25362,
thanks very much for ur valuable contribution, i just want more elaboration on the below,if you have a time,:
1- once-through thermosiphon reboilers
2- In these cases, a valved start-up line is generally provided from the bottom of the column.

thanks again

 

[25362]

Natural circulation or "thermo-siphon" reboilers may be horizontal (process fluid on the shell side) or vertical (process fluid inside the tubes).

In the horizontal units the process fluid velocity is lower although the vaporization % is higher than in vertical units, therefore they need more liquid head as by trapping the liquid off the bottom tray.

The start-up connection is needed to generate vapors (boil up) and bring the column to steady state conditions.
See also:

thread124-95533,

thread124-265704,

thread124-163883,

thread483-199678,

thread798-206595