Gas Purification

Arthur L Kohl, Richard Nielsen

• Publisher: Gulf Professional Publishing
• Year: 1997
• ISBN:
  9780080507200
• Topic: Mathematics
• Subtopic: Mathematics General
• Edition: 5

This massively updated and expanded fifth edition is the most complete, authoritative engineering treatment of the dehydration and gas purification processes used in industry today. Of great value to design and operations engineers, it gives practical process and equipment design descriptions, basic data, plant performance results, and other detailed information on gas purification processes and hardware. This latest edition incorporates all significant advances in the field since 1985.You will find major new chapters on the rapidly expanding technologies of nitrogen oxide control, with discussions of regulatory requirements and available processes; absorption in physical solvents, covering single component and mixed solvent systems; and membrane permeation, with emphasis on the gas purification applications of membrane units. In addition, new sections cover areas of strong current interest, particularly liquid hydrocarbon treating, Claus plant tail gas treating, thermal oxidation of volatile organic compounds, and sulfur scavenging processes.This volume brings you expanded coverage of alkanolamines for hydrogen sulfide and carbon dioxide removal, the removal and use of ammonia in gas purification, the use of alkaline salt solutions for acid gas removal, and the use of water to absorb gas impurities. The basic technologies and all significant advances in the following areas are thoroughly described: sulfur dioxide removal and recovery processes, processes for converting hydrogen sulfide to sulfur, liquid phase oxidation processes for hydrogen sulfide removal, the absorption of water vapor by dehydrating solutions, gas dehydration and purification by adsorption, and the catalytic and thermal conversion of gas impurities.

Chapter 1 Introduction

DEFINITIONS

PROCESS SELECTION

PRINCIPLES OF ABSORPTION

Introduction

Contactor Selection

Design Approach

Material and Energy Balance

Column Height

Column Diameter

General Design Considerations,

REFERENCES

DEFINITIONS

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:

1. Absorption into a liquid

2. Adsorption on a solid

3. Permeation through a membrane

4. Chemical conversion to another compound

5. Condensation

Absorption 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.

Adsorption, 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.

Membrane permeation 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.

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 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.

Condensation 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.

PROCESS SELECTION

The principal gas phase impurities that must be removed by gas purification processes are listed in Table 1-1.

Table 1-1 Principal Gas Phase Impurities

1. Hydrogen sulfide 2. Carbon dioxide 3. Water vapor 4. Sulfur dioxide 5. Nitrogen oxides 6. Volatile organic compounds (VOCs) 7. Volatile chlorine compounds (e.g., HCl, Cl2) 8. Volatile fluorine compounds (e.g., HF, SiF4) 9. Basic nitrogen compounds 10. Carbon monoxide 11. Carbonyl sulfide 12. Carbon disulfide 13. Organic sulfur compounds 14. Hydrogen cyanide

Selecting the optimum process for removing any one or combination of the listed impurities is not easy. In many cas...