Page 219 - Design of Simple and Robust Process Plants

204 Chapter 5 Process Simplification and Intensification Techniques
Table 5.9. Distillation for one, two and three purified streams in order of increasing complexity.

Distillation No. of purified streams

Flash 0
Stripper 1
Absorber 1
Dephlegmator 1
Simple distillation 2
Distillation with side draw 2 pure, 1 impure
Distillation with purified side draw 3
(side stripper or dephlegmator/rectifier)
Divided wall column 3
Direct coupled distillation 3
Distillation with lights, top, side and bottom streams 4
Distillation with lights, top, purified side draw, bottom streams 4
Divided wall column lights, top, side and bottom streams 4
Divided wall column top, two side and bottom streams 4
Divided wall column and a distillation column 4
Two divided wall columns in series 4
Three distillation columns in series 4

available techniques, engineers still select the conventional approach of a series of
normal distillation columns. The only answer to this problem is conservatism.

5.7.3
Liquid±Liquid Extraction

Extraction is also a very old technique used to isolate a component from a homoge-
neous mixture. The differences in distribution coefficient of the components within
the solvent determines the separation. The solubility of the solvent in the raffinate is
preferably very low. Next to the distribution coefficients, the primary selection cri-
teria for the solvent include differences in density, tendency for emulsion formation,
and ease of separation of the extracted component. All types of process industries
utilize this technique, ranging from pharmaceuticals to food and fine chemicals. In
the pharmaceutical and fine chemical industries the quantities of material involved
are often relatively small, and this impacts upon the selection of equipment. In
these areas, batch extraction is quite often applied, using standard equipment such
as stirred tank reactors that are already being used for other steps in the production.
In this situation, settling, which is inherent to liquid-liquid extraction, is carried out
in the same vessel, as a batch-wise operation
The priority ranking for continuous operations is affected by the number of stages
required, as more stages require less solvent for processing. The selection of the
continuous phase is a choice to be made that can have a major impact on the perfor-