Absorption And Stripping Flashcards
COUPLING OF ABSORBER & STRIPPER
Absorbers are frequently coupled with strippers to
permit regeneration and recycling of the absorbent
Because stripping is not perfect, recycled
absorbent contains solutes (absorbates)
Absorption Factor &
Stripping Factor
The larger the value of A, the fewer
the number of stages required for
absorption
The required absorbent flow rate low
the K-value of solute low
temperature low and/or pressure high
• Stripping factor, S = 1/A = KV/L/
Equipment for absorption and stripping. Give 5 equipment.
Trayed tower
Packed column
Spray tower
Bubble column
Centrifugal contractor
Contacting Tray in a Trayed Tower
Ideal operation
: phase equilibrium at
each tray between
the vapor and liquid
streams leaving the
tray
→ Each tray is treated
as an equilibrium
stage
Tray Openings for Vapor Passage
Perforation
Valve cap
Bubble cap
Tray with valve cap
Trayed Tower & Packed Column
Additional separation capacity can be achieved by replacing all or
some of the trays with sections of random or structured packing
3 types of packing
Structured packing
Packing support
Random packing
More about stripping
❑Counter current flow. Gas
containing the solute enters
from the bottom and flows out
at the top. Solvent liquid, enters
at the top and flows out the
bottom.
❑ Solvent is chosen such that the
solute gas is preferentially
soluble in the solvent rather
than the gas mixture.
/
❑ Two film theory can be used to
explain diffusion from gas to
liquid phase.
Selection of Solvent
❑Gas Solubility - high solubility of a gas in the solvent
is preferred and it should not dissolve carrier gas. If
chemical reaction takes place between solute and
solvent, rate of absorption is extremely high. But the
reaction should be reversible to recover solvent
during desorption.
❑ Volatility - Low volatility or low vapor pressure of the
solvent enhances the adsorption operation as
solvent loss with carrier gas is very small
❑ Viscosity - Greater amount of power is required for
high viscous solvent and flooding is also caused at
lower liquid and gas flow rates. I
What is the goal of absorption or stripping
operations?
✓ Maximum extraction with 100% efficiency
✓ High throughput (low residence time)
✓ Smallest and simplest possible system (low
capital)
How do we accomplish this?
✓ Choose the right absorbing (liquid) or
stripping (gas) phase (equilibrium &
immiscibility)
✓ Optimize column diameter and height
✓ Maximize contact area/volume within
column.
Absorption Systems - Physical
❑ Physical absorption - solubility of a particular gas
in a liquid.
❑ This solubility is often quite low; consequently, a
relatively large amount of liquid solvent is needed
to obtain the required separation – alternatively
chemical absorption.
❑ This liquid solvent containing the solute is
typically regenerated by heating or stripping to
drive the solute back out.
I
Absorption Systems – Chemical
Chemical absorption relies on reaction of a particular
gas with a reagent in a liquid. Examples:
✓CO2 / H2S and aqueous ethanolamines
✓CO2 / H2S and aqueous hydroxides
✓CO and aqueous Cu ammonium salt
✓SO2 and aqueous dimethyl aniline
✓HCN and aqueous NaOH
✓HCl / HF and aqueos NaOH
❑ This absorption can often be quite high; consequently,
a smaller amount of liquid solvent/reagent is needed
to obtain the required separation.
❑ However, the reagent may be relatively expensive, and
it is often desirable to regenerate when possible.
Assumptions for Absorption & Stripping
✓ The carrier gas is insoluble (very low
solubility), e.g, N2 or Ar in water.
✓ The solvent is nonvolatile (or it has a low
vapor pressure), e.g., water in air at low
temperatures.
✓ The system is isothermal. e.g., the effects
of heat of solution or reaction are low or
there is a cooling or heating system in the
column.
✓ The system is isobaric.
✓ The concentration of the solute is low, say
<10%
Application of absorption?
✓ Removal of gases like H2S from hydrocarbons
using amines as solvent.
✓ Washing of ammonia from ammonia-air mixture
using water as solvent.
✓ Gas purification by removal of CO2
.
