Separations as unit operations - Principles of Chemical Separations with Environmental Applications - page 29

2.11 A unified view of separations

Having discussed the concept of unit operations in the context of separation, it is useful
at this point to reiterate what is gained by this approach. King [12] lists several major
gains in understanding, insight, capability, and efficiency that come from a unified view
of separations. The first such gain recognized historically is that methods of analyzing
the degrees of separation achieved in different separation processes are similar. Hence, a
comprehensive knowledge of separations enables one to transfer the uses of separations
to different scales of operation, ranging from analytical scale separations to large-process
separations. In addition, the interactions of mass transfer and phase equilibria and their
resultant effects are similar for related types of contacting equipment. The concepts used
for stage efficiencies (fraction of equilibrium attained) are common, as well.
Developments of powerful computational algorithms have provided immense gains in
computing capacity and the widespread use of personal computers has meant that much
less attention has to be given to methods of calculating degrees of separation. This allows
for much more emphasis on process selection, synthesis, and improvement. However,
while many simulators exist to model separation processes, it is critical that the designer
understands the fundamental principles prior to using them. A unified view makes it pos-
sible to identify and select among candidate separation processes for a given task on a
knowledgeable basis. An understanding of patterns of stage-to-stage changes in compo-
sition in countercurrent separations and the causes for particular patterns is useful across
the board for improvement of design and operating conditions to gain a greater degree of
separation and/or lesser flows and equipment cost. In addition, an understanding of the
factors governing energy consumption enables greater insight into reducing consumption
and/or achieving an optimal combination of equipment and operating costs.
A general understanding of separations facilitates developing entirely new methods of
separation. Insight into the capabilities of a variety of methods helps one to identify when
the ability to separate will pose a major process limit. An understanding of solution and
complexation chemistry makes it possible to identify and select among potential mass-
separating agents for different applications and to transfer the use of particular agents and
chemical functionalities among different types of separation processes.

2.12 Remember

The concept of a unit operation allows us to develop design methodologies for various
separation technologies that are the same for various sizes of equipment (scale-up), feed
composition, and separation requirements.
Separation processes use two primary mechanisms for performing the separation: par-
titioning between phases (equilibrium); or relative motion of various chemical species
(rate).