Page 12 - Gas Purification 5E

operation of gas purification processes and is used in a large fraction of the systems described
in subsequent chapters. Because of its importance, a section on absorption and basic absorber
design techniques is included in this introductory chapter.

A&o@o~, as applied to gas purification, is the selective concentration of one or more
components of a gas at the surface of a microporous solid. The mixture of adsorbed cow
nents is call4 the adsorbate, and the microporous solid is the admrbent. The attractive forces
holding the adsorbate on the adsorbent are weaker than those of chemical bonds, and the
adsorbate can generally be released (desorbed) by raising the temperature or reducing the
partial pressure of the component in the gas phase in a manner analogous to the stripping of
an absorbed component from solution. When an adsorbed component reacts chemically with
the solid, the operation is called chemisorption and desorption is generally not possible.
Adsorption processes are described in detail in Chapter 12, which also includes brief discus-
sions of design techniques and references to more compdmsive texts in the field.

Membrane permealion is a relatively new technology in the field of gas purification. In
this process, polymeric membranes separate gases by selective permeation of one or more
gaseous components from one side of a membrane barrier to the other side. The components
dissolve in the polymer at one surface and ~IE transported across the membrane as the result of
a concentratioIl gradient. The concentration gradient is maintained by a bigh partial pressure of
the key components in the gas on one side of the membrane barrier and a low partial pressure
on the other side. Although membrane permeation is still a minm factor in the field of gas
purification, it is rapidly finding new applications. Chapter 15 is devoted entirely to membrane
permeation processes and includes a brief discussion of design techniques.

Chemical conversion is the principal operation in a wide variety of processes, including
catalytic and noncatalytic gas phase reactions and the reaction of gas phase components with
solids. The reaction of gaseous Species with liquids and with solid particles suspended in liq-
uids is considezed to be a special case of absorption and is discussed under that subject. A
generalized treatment of chemical reactor design broad enough to cover all gas purification
applications is beyond the scope of this book; however, specific design parameters, such as
space velocity and required time at tempera-, are given, when available, for chemical con-
version processes described in subsequent chapters.

Condensation as a means of gas purification is of interest primarily for the removal of
volatile organic compounds (VOCs) from exhaust gases. Tk process consists of simply
cooling the gas stream to a temperatme at which the Organic compound has a suitably low
vapor pressure and collecting the condensate. Details of the process are given in Chapter 16.

PROCESS SELECTION

The principal gas phase impurities that must be removed by gas purification processes are
listed in Table 1-1.
Selecting the optimum process for removing any one or combination of the listed impuri-
ties is not easy. In many cases, the desired gas purification can be accomplished by several
different processes. Determining which is best for a particular set of conditions ultimately
requires a detailed cost and performance analysis. However, a preliminary screening can be