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Column Bottom Arrangement
Bottom
Feed or Reboiler Return Inlet
Bottom feed and reboiler return inlets should never be submerged below the liquid level. Column of liquid above the submerged inlet can vary in height, and under certain conditions, slugs of liquid and vapour can be blown up the column and lift ("bump") trays off their support or packing support plates. Inlets below the liquid level can also be responsible for excessive entrainment and premature flooding.
The section of column wall directly opposite a bottom inlet nozzles often prone to corrosion and erosion attacks, especially for cases involving high inlet velocities, small column diameter and corrosive chemicals. These effects can be reduced by installing an impingement plate. The inlet should also not impinge on bottom seal pan, seal pan overflow or bottom downcomer.
3 arrangements are commonly used for column bottom:
Unbaffled arrangement - where both bottom product and reboiler liquid are withdrawn from a common bottom sump. See the Figure below (right).
Baffled arrangement - where the space at the bottom of the column is divided into a bottom draw-off sump and a reboiler feed sump by a preferential baffle. See the left Figure below.
Once-through reboiler arrangement - where the reboiler liquid is withdrawn from the bottom downcomer or from a chimney tray located above the bottom sump. See the Figure below.
Unbaffled arrangement has the advantage of simplicity
and low cost. They are preferred in small column (less than 3-ft diameter) where
baffles are difficult to inspect and maintain. They are used for kettle
reboilers because the bottom product is withdrawn from the reboiler surge
compartment, not from the column bottom sump. They are also used with forced-circulation
reboilers as the large circulation rate makes it difficult to achieve
a steady liquid overflow across the baffle.
Because it is difficult to deflect liquid raining from a packed bed above the reboiler sump into the reboiler side of the preferential baffle, unbaffled arrangement is normally used in packed column.
Baffled or once-through arrangements are usually preferred with thermosiphon reboilers in large columns (more than 3-ft diameter). They can supply a constant liquid head to the reboilers, and maximise bottom sump residence time when vapour disengagement is the main consideration.
Bottom
Outlet
The main consideration is to achieve the required
phase separation between vapour and liquid, and providing the required
surge capacity. Arrangements such as bottom sumps, chimney
trays, and surge drums are usually designed for avoiding the presence of
vapour in the liquid outlet. On the other hand, downcomer
trapouts are usually designed to allow for the presence of vapour.
Presence of vapour in liquid outlet lines can cause pump cavitation, erosion, column instability, etc. Some of the common causes are:
Insufficient residence time for vapour disengagement
from the liquid. Liquid arriving the pan or sump from which it is withdrawn
almost always contains entrained vapour bubbles.
Frothing
(waterfall pool effect) is caused by impact of falling liquid on liquid
surface in the sump or draw pan. Frothing is a far greater problem in tray column
sumps (where liquid fall resembles a waterfall) than in packed column sumps
(where liquid fall is more like rain).
Vortexing
occurs because of intensification of swirling motion as liquid converges towards
an outlet. Vortexing promotes entrainment of vapour into the draw-off line.
Vortex breakers can be installed to prevent vortexing.