Page 285 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.4 Fractionator 287

also includes the set of isometric drawings showing exact location and orientation of the equipment
and piping.
We discuss the design of a fractionator in the next section, followed by design of flash and batch
distillation in Sections 11.6 and 11.8, respectively. Details of the column and its internals are covered
in Chapter 14. Associated piping design features are covered in Chapter 16. Column shell and fittings
are detailed in Chapter 17.

11.4 Fractionator
The fractionating column/fractionator (Fig. 11.1D) with plates/trays and/or packed section(s) offer
multiple stages of vapoureliquid contact. Feed is
introduced at an intermediate point in the column at
the feed tray or the feed zone in case of a packed
Fractionator configuration and components
column. Contacting sections above and below the
feed entry are the enrichment and the stripping
sections of the column. Reboiled stripper
(Fig. 11.1E) is a special configuration, where feed enters the top stage. Fig. 11.3 shows a fractionator
with trays. P&ID of a fractionator with a side stream draw is shown in Fig. 11.4.The figure depicts
the following:

• column with its feed entry arrangement, enriching section and stripping section with trays/
packing.
• overhead system comprising of the overhead vapour line, condenser and the reflux drum
(accumulator vessel) and arrangements for overhead product draw, reflux flow to top tray and
venting of noncondensable vapour.
• side stream draw.
• column bottom system, with the reboiler providing the vapour reflux to the column and
arrangement for bottom product draw.
Thus, the simplest fractionator has a feed stream, a distillate and a bottom product stream. More
complex configurations may include multiple feed, one or more side stream draw and external
refluxing arrangement. Fig. 11.5 shows the configuration of an atmospheric distillation column for
crude oil with prefractionator column, side stream (kerosene, light gas oil and heavy gas oil) draws and
steam stripping columns for the side streams from the main column. The figure also shows three
circulating refluxes for the main column. These are provided in addition to the overhead reflux on the
top tray. This column does not include a bottom reboiler but uses stripping steam supplied at the
bottom for the same purpose. Such design configurations are decided based on technical requirements,
economics and requirements of operational flexibility.