Gas purification, as discussed in this text, involves the removal of vapor-phase impurities from gas streams. The processes which have been developed to accomplish gas purification vary from simple once-through wash operations to complex multiple-step recycle systems. In many cases, the process complexities arise from the need for recovery of the impurity or reuse of the material employed to remove it. The primary operation of gas purification processes generally falls into one of the following five categories:
Absorption into a liquid
Permeation through a membrane
Chemical conversion to another compound
refers to the transfer of a component of a gas phase to a liquid phase in which it is soluble. Stripping is exactly the reverse—the transfer of a component from a liquid phase in which it is dissolved to a gas phase. Absorption is undoubtedly the single most important
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.
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 components is called the adsorbate, and the microporous solid is the adsorbent. 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 discussions of design techniques and references to more comprehensive texts in the field.
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 are transported across the membrane as the result of a concentration gradient. The concentration gradient is maintained by a high 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 minor 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.
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 liquids is considered 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 temperature, are given, when available, for chemical conversion processes described in subsequent chapters.
as a means of gas purification is of interest primarily for the removal of volatile organic compounds (VOCs) from exhaust gases. The process consists of simply cooling the gas stream to a temperature at which the organic compound has a suitably low vapor pressure and collecting the condensate. Details of the process are given in Chapter 16