Page 230 - Separation process principles 2

6.0 Instructional Objectives 195

the temperature and/or increasing the pressure. In- (a strong base) is used as the absorbent to dissolve an acid gas,
creasing the pressure also serves to reduce the diameter of the absorption is accompanied by a rapid and irreversible neutral-
equipment for a given gas throughput. However, temperature ization reaction in the liquid phase and the process is referred
adjustment by feed-gas refrigeration and/or absorbent refrig- to as chemical absorption or reactive absorption. More
eration, and/or adjustment of the feed-gas pressure by gas complex examples of chemical absorption are processes
compression can be expensive. For these reasons, the ab- for absorbing COz and H2S with aqueous solutions of
sorber in Figure 6.1 operates at near-ambient conditions. monoethanolamine (MEA) and diethanolamine (DEA),
For a stripper, the stripping factor, S = 1/A = KV/L, is where a reversible chemical reaction takes place in the liquid
crucial. To reduce the required flow rate of stripping agent, phase. Chemical reactions can increase the rate of absorption,
operation of the stripper at a high temperature and/or a low increase the absorption capacity of the solvent, increase selec-
pressure is desirable, with an optimum stripping factor in the tivity to preferentially dissolve only certain components of the
vicinity of 1.4. gas, and convert a hazardous chemical to a safe compound.
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In this chapter, trayed and packed-column equipment for
Absorption and stripping are technically mature separa- conducting absorption and stripping operations is discussed
tion operations. Design procedures are well developed and and fundamental equilibrium-based and rate-based (mass-
conlmercial processes are common. Table 6.1 lists represen- transfer) models and calculation procedures, both graphical
tative, commercial absorption applications. In most cases, and algebraic, are presented for physical absorption and strip-
the solutes are contained in gaseous effluents from chemical ping of mainly dilute mixtures. The methods also apply to
reactors. Passage of strict environmental standards with re- reactive absorption with irreversible and complete chemical
spect to pollution by emission of noxious gases has greatly reactions of the solute in the liquid phase. Calculations for
increased the use of gas absorbers in the past decade. concentrated mixtures and reactive absorption with reversible
When water and hydrocarbon oils are used as absorbents, chemical reactions are best handled by computer-aided calcu-
no significant chemical reactions occur between the absorbent lations, which are discussed in Chapters 10 and 11. An intro-
and the solute, and the process is commonly referred to duction to calculations for concentrated mixtures in packed
as physical absorption. When aqueous sodium hydroxide columns is given in the last section of this chapter.

Table 6.1 Representative, Comnlercial Applications of Absorption

Solute Absorbent Type of Absorption
Acetone Water Physical
Acryloiiitrile Water Physical
Ammonia Water Physical
Ethanol Water Physical
Formaldehyde Water Physical
Hydrochloric acid Water Physical
Hydrofluoric acid Water Physical
Sulfur dioxide Water Physical
Sulfur trioxide Water Physical
Benzene and toluene Hydrocarbon oil Physical
Butadiene Hydrocarbon oil Physical
Butanes and propane Hydrocarbon oil Physical
Naphthalene Hydrocarbon oil Physical
Carbon dioxide Aq. NaOH Irreversible chemical
Hydrochloric acid Aq. NaOH Irreversible chemical
Hydrocyanic acid Aq. NaOH Irreversible chemical
Hydrofluoric acid Aq. NaOH Irreversible chemical
Hydrogen sulfide Aq. NaOH Irreversible chemical
Chlorine Water Reversible chemical
Carbon monoxide Aq. cuprous ammonium salts Reversible chemical
C02 and H2S Aq. monoethanolamine (MEA) or Reversible chemical
diethanolamine (DEA)
C02 and H2S Diethyleneglycol (DEG) or Reversible chemical
triethyleneglycol (TEG)
Nitrogen oxides Water Reversible chemical