Page 282 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

284 Chapter 11 Distillation

Configuration options for continuous distillation systems
In case of continuous distillation which has to split a feed into two streams, a distillate and a product,
there is not much to decide on the configuration. However, when a feed has to be split into three or more
streams, there can be several competing alternative options. Fig. 11.2 shows the common options for three
product streams using two columns in sequence as well as using a single column with side stream draw.

(A) A (B) A (C)
A

B
F F B
F
B
C
C C

(D) (E)
A B A

F F

C C B

FIGURE 11.2
Product fractionation schemes e (A) side stream draw, (B) side stream draw with stripper,
(C) prefractionation, (D) direct sequencing and (E) indirect sequencing.

A configuration option could also incorporate a flash stage or a prefractionator column before the
main fractionator (Fig. 11.2C). This reduces the loading of the main distillation column. Also if much
of the lighter components are removed in a prefractionator, the vapour volume to be handled in the
fractionator comes down along with lowering of its operating pressure. The optimum is struck between
the additional costs due to the prefractionation column and lowered costs of the main column. Use of
prefractionator column is common when the feed has a large fraction of volatiles in large-scale pro-
cesses like crude oil distillation.
Among the plausible configuration alternatives synthesised during the conceptual design, the final
configuration is selected based on heuristics and economic considerations, wherever possible. A set of
heuristics rules stated below helps in synthesising and short-listing the design alternatives:
• Small capacity usually calls for batch distillation.
• Batch distillation is preferred when the product quality needs frequent changes.