Absorption of Gases

1
Absorption of Gases
Packed tower absorption (with and without
reaction)
Pure water in
Clean air out
Liquid film
packing
solid
Gas stream
Polluted air in
Contaminated water out
2
Phase Equilibrium (Henrys Law)
y (or) P (or) Ca
x (or) Cw
y m x (or) P mx (or) P KH Cw (or) Ca Hc
Cw
3
Film Theory of Mass Transfer
Case I Evaporation from Water to Air.
R3
R4
R2
and
with
4
Other Definitions for Flux
Based on individual Phase concentrations (molar)
Local mass transfer Coefficients
Based on individual Phase concentrations (mole
fractions)
Overall mass transfer Coefficients
5
Other Cases
Case II Absorption from Air into Water
6
More Definitions for Absorption Flux
Note that all mass transfer coefficients are
related to one another.
Examples
7
Examples of Mass Transfer in Environmental Systems

• Dissolution of gases into water from the air.
• Reaeration of natural streams.
• Flux of dissolved organics from wastewater
  impoundments and polluted streams.
• Evaporation of pure chemicals to the atmosphere.
• Wet and dry deposition of pollutants from the
  atmosphere.

8
Evaluation of mass transfer coefficients from
correlations
L
v
Example binary mixtures.
D
Recall for heat transfer Nu Nu (Re,Pr,L/D)
For mass transfer Sh Sh (Re, Sc, L/D)
Sh kxD/cDAB
1 Find Sh from empirical correlation 2 Find
kx 3 - Find kGkx/(PB)ln or kLkx/(cB)ln 4
Find NA kG (PA,bulk PAo) kL (CAo
CA,bulk) 5 - Find WA