Page 289 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.4 Fractionator 291

cold reflux stream that may lead to corrosion in presence of H 2 S and chloride ion. Only the
warmer hydrocarbon liquid is refluxed back. Two sets of exchangers are provided in series. After
the first set, part of the hotter liquid is refluxed to the column and the rest is subcooled in the
second set of exchanger.
• Condenser with hot (vapour) bypass: This is used in some naphtha debutaniser columns with
flooded condenser. The column pressure is manipulated to change the submergence of the
condenser tubes. Opening of the hot bypass valve varies the differential pressure between the
column top and the reflux drum. One may note that the reflux drum pressure is slightly lower than
the column pressure.

Air cooled
Column condenser Reflux drum with
water boot
Non-condensable
gases
Hot reflux
#1

Condensate
Water cooled (water)
exchanger
Top product

FIGURE 11.6
Schematic showing hot refluxing in a crude distillation column.

Column bottom section
Supply of vapour reflux stream below the bottom tray is essential for fractionation to take place in
the stripping section. The reboiler generates this vapour stream from the liquid falling from the bottom
tray. The reboiler is essentially a heat exchanger where a hot fluid supplies the latent heat of vapor-
isation. The reboilers shown in Figs. 11.1D and 11.3 are ‘kettle type’. Liquid from the column flows to
the reboiler by gravity and the vapour is returned below the bottom tray and above the column bottom
liquid level. The hot fluid for heating the reboiler may be condensing steam or some other hot stream
available in the plant. Utilisation of such hot stream adds to the plant economy by saving both hot and
cold utility. Use of small fired heaters (furnaces) as reboiler is also possible, though not very common.
In some configurations, a pump circulates the column bottom liquid through the reboiler and the hot
liquid flashes, releasing reboiled vapour inside the column on its return. Such forced circulation
reboilers are needed for viscous bottom product.
For cases where the bottom product is unstable, i.e., it has a tendency to crack at temperatures
slightly above the bottom temperature, reboilers cannot be used as it exposes the material to higher
temperature for a long time. Crude distillation column (Fig. 11.5) is an example of such a fractionator
without a reboiler. Bottom product ‘Reduced Crude Oil (RCO)’ from the column starts cracking at