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CHAPTER 12

Absorption of Gases
12.1. INTRODUCTION
The removal of one or more selected components from a mixture of gases by absorption
into a suitable liquid is the second major operation of chemical engineering that is based
on interphase mass transfer controlled largely by rates of diffusion. Thus, acetone can be
recovered from an acetone–air mixture by passing the gas stream into waterin which
the acetone dissolves while the air passes out. Similarly, ammonia may be removed from
an ammonia–air mixture by absorption in water. In each of these examples the process
of absorption of the gas in the liquid may be treated as a physical process, the chemical
reaction having no appreciable effect. When oxides of nitrogen are absorbed in water to
give nitric acid, however, or whencarbon dioxide is absorbed in a solution of sodium
hydroxide, a chemical reaction occurs, the nature of which influences the actual rate
of absorption. Absorption processes are therefore conveniently divided into two groups,
those in which the process is solely physical and those where a chemical reaction is
occurring. In considering the design of equipment to achieve gas absorption, the mainrequirement is that the gas should be brought into intimate contact with the liquid, and
the effectiveness of the equipment will largely be determined by the success with which
it promotes contact between the two phases. The general form of equipment is similar to
that described for distillation in Chapter 11, and packed and plate towers are generally
used for large installations. The methodof operation, as will be seen later, is not the
same. In absorption, the feed is a gas introduced at the bottom of the column, and the
solvent is fed to the top, as a liquid; the absorbed gas and solvent leave at the bottom, and
the unabsorbed components leave as gas from the top. The essential difference between
distillation and absorption is that in the former the vapour has to be produced ineach
stage by partial vaporisation of the liquid which is therefore at its boiling point, whereas
in absorption the liquid is well below its boiling point. In distillation there is a diffusion
of molecules in both directions, so that for an ideal system equimolecular counterdiffusion takes place, though in absorption gas molecules are diffusing into the liquid, with
negligible transfer in thereverse direction, as discussed in Volume 1, Chapter 10. In
general, the ratio of the liquid to the gas flowrate is considerably greater in absorption
than in distillation with the result that layout of the trays is different in the two cases.
Furthermore, with the higher liquid rates in absorption, packed columns are much more
commonly used.

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ABSORPTION OF GASES

12.2.CONDITIONS OF EQUILIBRIUM BETWEEN
LIQUID AND GAS
When two phases are brought into contact they eventually reach equilibrium. Thus, water
in contact with air evaporates until the air is saturated with water vapour, and the air is
absorbed by the water until it becomes saturated with the individual gases. In any mixture
of gases, the degree to which each gas is absorbed is determined by its partialpressure.
At a given temperature and concentration, each dissolved gas exerts a definite partial
pressure. Three types of gases may be considered from this aspect — a very soluble one,
such as ammonia, a moderately soluble one, such as sulphur dioxide, and a slightly soluble
one, such as oxygen. The values in Table 12.1 show the concentrations in kilograms per
1000 kg of water that arerequired to develop a partial pressure of 1.3, 6.7, 13.3, 26.7,
and 66.7 kN/m2 at 303 K. It may be seen that a slightly soluble gas requires a much
higher partial pressure of the gas in contact with the liquid to give a solution of a given
concentration. Conversely, with a very soluble gas a given concentration in the liquid
phase is obtained with a lower partial pressure in the vapour phase. At...