Page 167 - Design of Simple and Robust Process Plants

152 Chapter 5 Process Simplification and Intensification Techniques
5.3.2
Distillation

In this Section, the combination of functions within distillation is concentrated on
the separation of three or more product streams. In this respect, refineries have a
long tradition in the removal of more product grades from continuous distillation
units, such as crude stills. In the petroleum industry, these side streams traditionally
had a wide specification range, but today these ranges have been narrowed, this
being achieved by side strippers and improved process control. Batch distillations
are often also utilized for the separation of multiple components in different process
industries. For most applications of continuous distillations in the chemical indus-
try, the objective has been to produce high-purity materials, and therefore more
product streams (more than two) were rarely applied, surprisingly enough even
when pure streams were not a requirement.
The alternatives for a conventional distillation column for over two product
streams are:

. Side streams
. Divided wall column (two columns in one)
. Dephlegmator (rectification and cooling), also known as non-adiabatic rectifi-
cation.

5.3.2.1 Side streams
These are achieving increasingly wide applications (Figure 5.2). The drawbacks of
side streams are that the purity has a wide range, but quality control is not obvious.

Feed

Feed
Heat

Vapor
Feed
Distillation Vapor
with liquid C.W
side draws
Distillation with Feed
side stripper
Distillation with
vapor side draw
Distillation with side
draw and dephlegmator
Fig. 5.2. Distillation with side streams.