Page 226 - Separation process principles 2

Part 2

~eparations by Phase

Addition or Creation

Among the most widely used industrial separation The continuous distillation of binary mixtures in
operations are absorption, stripping, various types of multiple-stage, trayed or packed columns is covered
distillation, and liquid-liquid extraction, all of which in Chapter 7, with emphasis on the classical McCabe-
involve separations by selective mass transfer of com- Thiele graphical, equilibrium-stage model. Typically,
ponents from one fluid phase to another. The other this separation operation utilizes energy to achieve the
phase is created by thermal energy input (energy- separation and two sections (rectifying and stripping),
separating agent) or by addition (mass-separating which make possible, with non-azeotropic-forming
agent). In most cases, these operations are based on the mixtures, the separation of a binary mixture into two •"i "'
use of countercurrent cascades of multiple stages. nearly pure products. Equipment-sizing methods of
Detailed descriptions of, and design and analysis Chapter 6 generally apply to distillation in Chapter 7.
calculations for, these vapor-liquid and liquid-liquid When the separation of a liquid binary mixture by
operations are presented in Chapters 6 through 13. Two distillation is infeasible or too expensive, liquid-liquid
types of mathematical models are considered: ( 1) stages extraction using a selective solvent is considered, as
that attain thermodynamic phase equilibrium and presented in Chapter 8. Although many equipment con-
(2) stages that do not reach phase equilibrium but are figurations are available, columns or vessels with me-
governed by rates of mass transfer. The less-complex chanically assisted agitation are the most useful when
equilibrium-stage models are more widely used, with a multiple stages are needed to achieve the desired recov-
stage efficiency, but the availability of fast and inex- ery. This chapter emphasizes graphical, equilibrium-
pensive digital computations is encouraging an increase stage methods using triangular diagrams for treating
in the application of more tedious, but more accurate, ternary systems.
mass-transfer models. Equilibrium-stage models and calculations for
Absorption (vapor-phase feed) and stripping (liquid- multicomponent mixtures are considerably more com-
phase feed) are covered in Chapter 6. These two plex than those for binary mixtures, as in distillation, and
operations usually rely on the addition of a mass- for ternary mixtures, as in absorption, stripping, and
separating agent (liquid absorbent and vapor-stripping liquid-liquid extraction. Approximate algebraic methods
agent, respectively), but may also use heat transfer to are presented in Chapter 9, while rigorous algebraic
produce the other phase. In general, these operations methods are developed in Chapter 10. These methods
are not used to make a sharp separation, but can achieve are implemented in process simulators and widely used.
a high recovery of a key component in the feed by its Chapter 11 considers equilibrium-stage calculation
transfer to the other phase. Absorption and stripping methods for so-called enhanced distillation of mixtures
equipment most often consists of columns containing that are difficult to separate by conventional distillation
trays or packing for contacting the two phases, with con- or liquid-liquid extraction. An important aspect of
tinuous flow of the fluid phases; calculation methods, enhanced distillation is the determination of feasible
graphical and algebraic, are presented for both types products, which uses residue-curve maps. Extractive,
of contacting. Methods for estimating tray efficiency, azeotropic, and salt distillation use mass-addition as
column height, and diameter are also presented. well as thermal energy input to achieve separation.

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