Page 290 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

292 Chapter 11 Distillation

about 400 C, whereas the bottom temperature is around 365 C. Using a reboiler would require its

heating tube surface temperature close to the cracking temperature and this would lead to coke
formation in the equipment and the associated pipelines. So instead of bottom reboiler, crude
distillation columns use superheated stripping steam injected below the bottom tray and above the
bottom liquid level to reduce the partial pressure and supply heat of vaporisation to strip out vapour
from RCO.
Feed zone
The feed enters the column through the feed nozzle on the column shell. This zone of the column is
also called ‘flash zone’ as part of the entering feed usually vaporises here. The pipeline carrying the
feed is called transfer line, particularly if it is carrying the feed heated in a furnace. Feed can be vapour,
liquid or a mixture of both. The temperature of the feed as it enters the column is not necessarily equal
to the temperature in the column at the feed tray location although such equality is desirable to increase
thermodynamic efficiency of the process. It is desirable to avoid a subcooled liquid or a superheated
vapour feed. Supply of a partially vaporised feed at the column temperature is desirable and is
commonly achieved by preheating the feed in a heat exchanger by the bottom product stream and/or
some other process stream available at a sufficiently high temperature. The spacing above the feed tray
is kept higher than that of other trays to reduce liquid entrainment with vapour. In case of packed
columns, the feed enters in between two consecutive sections of packing with arrangement for
distributing the liquid and the vapour across the column section. Feed entering the column from the
nozzle may be made to hit a splash baffle or a false downcomer for efficient vapoureliquid separation.
This is elaborated in Chapter 17. In large columns, the feed may have a tangential entry.
Side draws
Fractionators may be designed with one or more side stream draw with composition in between the
top and the bottom product stream. Such draws are from chimney trays. Chimney trays are not mass
transfer trays. Fig. 11.7 shows schematic of a chimney tray in a column. The vapour approaching the
chimney tray from the bottom passes through the chimney structures on the tray without coming in
contact with the liquid on the tray. The tray receives liquid from the upper section of the column and
routes it to the draw off pan. The side stream is drawn from the draw off pan through the (side stream)
draw off nozzle. The excess liquid overflows from the seal as internal reflux to the lower section of the
tower.
Circulating reflux streams, if provided to remove heat from a column, are drawn from an inter-
mediate tray in the column (usually between two side-stream draws or the overhead and the first side
stream draw). Arrangement for circulating reflux draw is similar to side stream draw. The stream is
cooled and returned two trays above its draw tray and the return tray arrangement is similar to the feed
tray section. Circulating reflux, common in crude distillation columns, increases the internal reflux
flow in the trays below draw-off trays and reduces vapour flow above the trays on which it is returned.
Trays involved in circulating reflux are ineffective as fractionating trays due to mixing of liquid in the
upper and the lower trays. The set of trays across which the reflux circulates is considered equivalent to
a single stage of contacting. The scheme shown in Fig. 11.5 includes three circulating refluxes for the
atmospheric distillation column.