Page 5 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN001H-01 May 7, 2001 16:18

2 Absorption (Chemical Engineering)

Operating line Line on the y–x diagram that represents Some common commercial applications of absorption are
the locus of all the points obeying the component listed in Table I.
material balance.
Rich gas Gas entering the absorber, containing both the
B. Choice of Solvent for Absorption
inerts and solutes.
Rich solvent Solvent leaving the absorber, which con- If the main purpose of absorption is to generate a specific
tains solute removed from the feed gas. solution, as in the manufacture of hydrochloric acid, the
Slope of equilibrium curve Ratio of the change of the solvent is specified by the nature of the product. For all
solute concentration in the gas to a given change in so- other purposes, there is some choice in selecting the ab-
lute concentration in the liquid when the solvent and sorption liquid. The main solvent selection criteria are as
solute are at equilibrium and when solute concentra- follows:
tions are expressed as mole fractions.
Solute(s) Component(s) absorbed from the gas by the 1. Gas solubility. Generally, the greater the solubility
liquid of the solute in the solvent, the easier it is to absorb the
Solvent Dissolving liquid used in an absorption process. gas, reducing the quantity of solvent and the equipment
Stripping (or desorption) Process in which the absorbed size needed for the separation. Often, a solvent that is
gas is removed from the solution. chemically similar to the solute or that reacts chemically
y–x diagram Plot in which the solute mole fraction in with the solute will provide high gas solubility.
the gas is plotted against the solute mole fraction in the 2. Solvent selectivity. A high selectivity of the sol-
liquid. vent to the desired solutes compared with its selectivity
to other components of the gas mixture lowers the quan-
tity of undesirable components dissolved. Application of
ABSORPTION is a unit operation in which a gas mixture a solvent of higher selectivity reduces the cost of down-
is contacted with a suitable liquid for the purpose of stream processing, which is often required to separate out
preferentially dissolving one or more of the constituents the undesirable components.
of the gas. These constituents are thus removed or par- 3. Volatility. The gas leaving the absorber is saturated
tially removed from the gas into the liquid. The dissolved with the solvent. The more volatile the solvent is, the
constituents may either form a physical solution with the greater are the solvent losses; alternatively, the more ex-
liquid or react chemically with the liquid. The dissolved pensive are the down-stream solvent separation facilities
constituents are termed solutes, while the dissolving liquid required to reduce the losses.
is termed the solvent. When the concentration of solute in 4. Effects on product and environment. For example,
the feed gas is low, the process is often called scrubbing. toxic solvents are unsuitable for food processing; noxious
The inverse operation, called stripping, desorption, or solvents are unsuitable when the gas leaving the absorber
regeneration, is employed when it is desirable to remove is vented to the atmosphere.
the solutes from the solvent in order to recover the solutes 5. Chemical stability. Unstable solvents may be diffi-
or the solvent or both. cult to regenerate or may lead to excessive losses due to
decomposition.
6. Cost and availability. The less expensive is the sol-
I. ABSORPTION IN PRACTICE vent, the lower is the cost of solvent losses. Water is the
least expensive and most plentiful solvent.
A. Commercial Application 7. Others.Noncorrosiveness,lowviscosity,nonflamm-
ability, and low freezing point are often desirable
Absorption is practiced for the following purposes: properties.

1. Gas purification, for example, removal of pollutants
from a gas stream. C. Absorption Processes
2. Production of solutions, for example, absorption of Absorptionisusuallycarriedoutinacountercurrenttower,
hydrogen chloride gas in water to form hydrochloric acid. through which liquid descends and gas ascends. The tower
3. Product recovery, for example, absorption of liqui- may be fitted with trays, filled with packing, or fitted with
fied petroleum gases (LPG) and gas olines from natural sprays or other internals. These internals provide the sur-
gas. face area required for gas–liquid contact.
4. Drying, for example, absorption of water vapor from A schematic flow diagram of the absorption–stripping
a natural gas mixture. process is shown in Fig. 1. Lean solvent enters at the top