Page 7 - 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

4 Absorption (Chemical Engineering)

TABLE I (continued)
Type of plant Feed gas Solutes Solvent Commercial purpose Stripping practice

Ethylene oxide, Reactor effluent Ethylene oxide Water Ethylene oxide recovery Stripping is practiced
glycol to recover ethylene
oxide from the solution
Ketones from Hydrogen, ketones Ketones Water Ketone–hydrogen separation Stripping is practiced
alcohol to recover ketones
from the solution
Maleic anhydride Reactor effluent Maleic anhydride separation Water Maleic anhydride Stripping is practiced
from reactor gases to remove water from
the maleic acid formed
in the absorption
process, converting it
back to maleic anhydride
Isoprene Reactor effluent Isoprene, C 4 ’s, C 5 ’s Heavy oil Separation of C 4 ’s, C 5 ’s, Stripping is practiced
and isoprene from to recover the solute
light gases and regenerate the
oil for recycling to
the absorbent
Urea Reactor effluent CO 2 ,NH 3 Water Formation of ammoniumn Stripping not practiced
carbonate solution,
which is recycled
to the reactor

of the absorber and flows downward through the internals. solute-free lean gas, and bottom product is the rich sol-
Rich gas enters at the bottom of the absorber and flows vent, which contains the absorbed solute. The rich solvent
upward through the internals. The liquid and gas are con- then flows to the stripper where the solute is stripped from
tacted at the absorber internals, and the solute is absorbed the rich solvent, this operation being at a higher tempera-
by the solvent. Overhead product from the absorber is the ture and/or lower pressure than maintained in the absorber.
The solute leaves the stripper as the overhead product, and
the solute-free lean solvent leaves the stripper bottom and
is recycled to the absorber.

II. PRINCIPLES OF ABSORPTION

The important fundamental physical principles in absorp-
tion are solubility and mass transfer. When a chemical
reaction is involved, the principles of reaction equilibria
and reaction kinetics are also important.

A. Gas Solubility
At equilibrium, the fugacity of a component in the gas
is equal to the fugacity of the same component in the
liquid. This thermodynamic criterion defines the relation-
ship between the equilibrium concentration of a compo-
nent in the gas and its concentration in the liquid. The
quantity of gas dissolved in a given quantity of solvent
at equilibrium conditions is often referred to as the gas
solubility.
Gas solubility data are available from handbooks and
various compendia and often show solubility as a function
FIGURE 1 Typical schematic absorber–stripper flow diagram. of gas composition, temperature and pressure. A typical