This document discusses absorption, the process of transferring gas species to a liquid solvent, with applications such as separating gas mixtures and recovering chemicals. It covers various industrial equipment, design considerations, and examples of absorption calculations, emphasizing the importance of choosing an ideal absorbent and understanding phase equilibrium. Detailed case studies are provided to illustrate absorption scenarios and calculations required for desired separation efficiencies.

1. ABSORPTION

2. Basic Concepts
• In absorption, there is a transfer of one or more species from
the gas phase to a liquid solvent.
• The species transferred to the liquid phase are referred to as
solutes or absorbate.
• Absorption involves no change in the chemical species
present in the system.
• Absorption is used to separate gas mixtures, remove
impurities, or recover valuable chemicals.
• In stripping (desorption), a liquid mixture is contacted with a
gas to selectively remove components by mass transfer from
the liquid to the gas phase.

3. Industrial Equipment for Absorption and Stripping
a) Trayed tower
b) Packed column
c) Spray tower
d) Bubble column
a) b)
c) d)

4. GENERAL DESIGN CONSIDERATIONS
• Entering gas flow rate, composition, T and P
• Desired degree of recovery of one or more solutes
• Choice of absorbent, operating P and T, and allowable
gas pressure drop
• Type of absorption equipment
• Minimum and actual absorbent flow rate
• Number or equilibrium stages and stage efficiency
• Height of absorption equipment
• Diameter of absorption equipment

5. Ideal absorbent
• High solubility for the solute
• Low volatility to reduce loss
• Stability and inertness
• Low corrosiveness
• Low viscosity and high diffusivity
• Low foaming proclivities
• Low toxicity and flammability
• Availability
• Low cost
Most widely used: water, hydrocarbon oils, and aqueous
solutions of acids and bases.

6. Graphical Method for Trayed Towers
• Phase equilibrium is assumed between the vapor
and liquid leaving each tray.
Assume: Only
solute is
transferred from
one phase to the
other.
Assume: No
vaporization of
absorbent into
carrier gas or
absorption of
carrier gas by
liquid.

7. Equilibrium Curves
• K-value is:

8. Operating Lines

9. Operating Lines

10. Minimum Absorbent Flow Rate
Corresponds to a value of XN in equilibrium with YN+1
for n=N
Solving for XN and substituting into L’
For dilute solutes, If, the entering liquid, X00
A similar derivation of V’min:

11. Number of Equilibrium Stages

12. Ejemplo 1
El 95 % del vapor de acetona, contenida en una corriente
gaseosa con 85 % molar de aire, ha de absorberse por
contacto en contracorriente con agua pura en una columna
de platos. La columna operará esencialmente a 20ºC y 101
kPa. Los datos de equilibrio para acetona-agua en estas
condiciones se muestran en la tabla adjunta.
% mol de acetona en agua 3.30 7.20 11.7 17.1
Presión parcial de acetona en
aire, torr
30.0 62.8 85.4 103.0

13. Suponiendo comportamiento ideal, determinar:
a) El valor mínimo de L’/V’, la relación de moles de agua
por mol de aire.
b) El número de etapas de equilibrio que se requieren
utilizando un valor de L’/V’ 1.25 veces el valor mínimo.
c) La concentración de acetona en la corriente de agua a la
salida.

14. Example 2
A solute A is to be recovered from an inert carrier
gas B by absorption into a solvent. The gas
entering into the absorber flows at a rate of 500
kmol/h with yA=0.3 and leaving the absorbet with
yA=0.01. Solvent enters the absorber at the rate of
1500 kmol/h with xA= 0.001. The equilibrium
relationship is yA=2.8xA. The carrier gas may be
considered insoluble in the solvent and the solvent
may be considered nonvolatile. Construct the x-y
plot for the equilibrium and operating lines, and
determine the equilibrium stages.

15. Example 3
It is desired to absorb 95 % of acetone by water
from a mixture of acetone and nitrogen containing
1.5 % of the component in a countercurrent tray
tower. Total gas input is 30 kmol/h and water
enters the tower at a rate of 90 kmol/h. The tower
operates at 27 ºC and 1 atm. The equilibrium
relation is y=2.53x. Determine the number of ideal
stages necessary for the separation using a
graphical method.

16. Recovery of Alcohol
In a bioprocess, molasses is fermented to produce a liquor
containing ethyl alcohol. A CO2-rich vapor with a small amount of
ethyl alcohol is evolved. The alcohol is recovered by absorption
with water in a sieve-tray tower. Determine the number of
equilibrium stages required for countercurrent flow of liquid and
gas, assuming isothermal, isobaric conditions and that only
alcohol is absorbed.
Entering gas is 180 kmol/h; 98 % CO2; 2% ethyl alcohol; 30ºC,
110 kPa.
Entering liquid absorbent is 100 % water; 30ºC, 110 kPa.
Required recovery of ethyl alcohol is 97 %.
Use the following equilibrium equation Y = 0.57X / (1 + 0.43X).