Introduction - Principles of Chemical Separations with Environmental Applications - page 3

more components into two or more products that differ in composition. These may either
remove a single component from a mixture or separate a solution into its almost pure
components. This is achieved by exploiting chemical and physical property differences
between the substances through the use of a separating agent (mass or energy).
Separation processes are used for three primary functions: purification, concentration,
and fractionation. Purification is the removal of undesired components in a feed mixture
from the desired species. For example, acid gases, such as sulfur dioxide and nitrogen ox-
ides, must be removed from power plant combustion gas effluents before being discharged
into the atmosphere. Concentration is performed to obtain a higher proportion of desired
components that are initially dilute in a feed stream. An example is the concentration of
metals present in an electroplating process by removal of water. This separation allows
one to recycle the metals back to the electroplating process rather than discharge them
to the environment. Lastly, in fractionation, a feed stream of two or more components is
segregated into product streams of different components, typically relatively pure streams
of each component. The separation of radioactive waste with short half-lives from that
having much longer half-lives facilitates proper handling and storage.
Analysis of separation processes can be placed into two fundamental categories:
equilibrium-based and rate-based processes. These separation categories are designated
using thermodynamic equilibrium relationships between phases and the rate of transfer of
a species from one phase into another, respectively. The choice of which analysis to apply
is governed by which is the limiting step. If mass transfer is rapid, such that equilibrium
is quickly approached, then the separation is equilibrium limited. On the other hand, if
mass transfer is slow, such that equilibrium is not quickly approached, the separation is
mass transfer limited. In some separations, the choice of analysis depends upon the type
of process equipment used.
Equilibrium processes are those in which cascades of individual units, called stages,
are operated with two streams typically flowing countercurrent to each other. The degree
of separation in each stage is governed by a thermodynamic equilibrium relationship
between the phases. One example is distillation, in which a different temperature at each
stage alters the vapor-phase equilibrium between a typically binary mixture. The driving
force for separation is the desire of a new equilibrium between the two phases at the
temperature of each stage. The end result is the separation of two liquids with dissimilar
boiling temperatures. Other equilibrium-based processes that will be covered in this text
include extraction and solid extraction, or leaching. Extraction is the removal of a species
from a liquid in which it is dissolved by means of another liquid for which it has a higher
affinity, and leaching is the removal of a species from a solid phase by means of a liquid
for which it has stronger affinity.
Rate-based processes are limited by the rate of mass transfer of individual components
from one phase into another under the influence of physical stimuli. Concentration gra-
dients are the most common stimuli, but temperature, pressure, or external force fields
can also cause mass transfer. One mass transfer based process is gas absorption, a process
by which a vapor is removed from its mixture with an inert gas by means of a liquid in