ERT 313
BIOSEPARATION ENGINEERING
ADSORPTION
Prepared by:
Pn. Hairul Nazirah Abdul Halim
Adsorption ≠ Absorption !
• Absorption – a fluid phase is transferred from one medium
to another
• Adsorption – certain components of a fluid (liquid or gas)
phase are transferred to and held at the surface of a solid
(e.g. small particles binding to a carbon bed to improve
water quality)
• Adsorbent – the adsorbing phase (carbon, silica gel, zeolite)
• Adsorbate – the material adsorbed at the surface of
adsorbent
Application of Adsorption
• Used in many industrial processes:
– Adsorbing the desired product from fermentation broths
– Isolation of proteins
– Dehumidification
– odour/colour/taste removal
– gas pollutant removal (H2S)
– water softening and deionisation
– hydrocarbon fractionation
– pharmaceutical purification
Nature of Adsorbent
• Porous material - Large surface area per unit mass
- internal surface area greater than the
external surface area
- often 500 to 1000 m2/g.
• Granular (50μm - 12 mm diameter), small pellets or beads
• Suitable for packed bed use
• Activated carbon, silica gel, alumina, zeolites, clay
minerals, ion exchange resins
• Separation occurs because differences in molecular
weight, shape or polarity of components
• Rate of mass transfer is dependent on the void fraction
within the pores
Silica structure
Zeolite structure
Types of Adsorption
1. Ion exchange
– Electrostatic attachment of ionic species to site
of the opposite charge at the surface of an
adsorbent
Types of Adsorption
2. Physical Adsorption
– result of intermolecular forces causing
preferential binding of certain substances to
certain adsorbents
– Van der Waal forces, London dispersion force
– reversible by addition of heat (via steam, hot
inert gas, oven)
– Attachment to the outer layer of adsorbent
material
3. Chemisorption
– result of chemical interaction
– Irreversible, mainly found in catalysis
– change in the chemical form of adsorbate
Adsorption Equipment
• Fixed-bed adsorbers
• Gas-drying equipment
• Pressure-swing adsorption
Fixed-bed Adsorber
• Adsorbent particles: 0.3 – 1.2 m deep supported on
a perforated plate
• Feed gas passes down through the bed
• Downflow is preffered because upflow at high rates
may fluidize the particles, causing attrition and loss
of fines.
• The feed gas is switched to the other bed when the
conc. Of solute in exit gas reaches a certain value.
• The bed is regenerate by steam / hot inert gas.
Regeneration
• To remove unwanted particles from the adsorbent
surface after the adsorption process
• using steam/hot inert gas
• Steam condenses in the bed, raising the temp. of
the solid, provide energy for desorption
• The solvent is condensed, separated from water.
• Then the bed is cooled and dried with inert gas
Adsorption from liquid
• Use of activated carbon to remove pollutants from
aqueous wastes
• Use carbon beds up to 10 m tall, several ft in
diameter, several bed operating in parallel.
• Tall beds are needed to ensure adequate treatment
Adsorption Isotherm
• Adsorption isotherm – equilibrium relationship
between the concentration in the fluid phase and the
concentration in the adsorbent particles.
• For gas – concentration in mole % or partial
pressure
• For liquid – concentration in mg/L (ppm) or μg/L
(ppb)
• Concentration of adsorbate on the solid = mass
adsorbed (g) per unit mass of original adsorbent (g).
Types of Isotherms
•
4 types of Adsorption Isotherms
1. Linear Isotherms
- Adsorption amount is proportional to the
concentration in the fluid
2. Irreversible – independent of concentration
3. Langmuir Isotherm
4. Freundlich Isotherm
LANGMUIR ISOTHERM
•
Often been used to correlate equilibrium adsorption
data for protein.
•
Isotherms that convex upward are called favorable.
 Kc 
W  Wmax 

 1  Kc 
•
Where:
W = adsorbate loading (g absorbed/g solid)
c = the concentration in the fluid (mg/L)
K = the adsorption constant
K >> 1 : the isotherm is strongly favorable.
•
Wmax and K are constants determined experimentally
by plotting 1/W against 1/c
FREUNDLICH ISOTHERM
– strongly favourable
– Describe the adsorption of variety of antibiotics,
steroids and hormones.
– high adsorption at low fluid concentration
W  bc
m
where b and m are constant
- Linearize the equation: Log W = b + m log c
- Constant determined from experimental data by
plotting log W versus log c
- Slope = m, intercept = b
FIGURE 25.3 Adsorption isotherms for water in air at 20 to 50 0C.
Principles of Adsorption
• In fixed bed adsorption, the concentrations in the
fluid phase and the solid phase change with:
a) time
b) as well as the position in the bed.
• At first, most of the mass transfer takes place near
the inlet of the bed.
• The fluid contacts the adsorbent.
• After a few minutes, the solid near the inlet is nearly
saturated.
• Most of the mass transfer takes place farther from
the inlet.
• The concentration gradient become S-shaped.
• Concentration profile in fixed beds
Figure 25.6(a)
Breakthrough Curves
•
•
•
•
•
tb – time when the concentration reaches break
point
The feed is switched to a fresh adsorbent bed
Break point – relative concentration c/co of 0.05 or
0.10
Adsorption beyond the break point would rise
rapidly to about 0.50
Then, slowly approach 1.0 (concentration liq in =
liq out)
• t* is the ideal adsorption time for a vertical
breakthrough curve
• t* is also the time when c/co reaches 0.50
• Amount of adsorbed is proportional to the
rectangular area to the left of the dashed line at t*
• Solute feed rate (FA) = superficial velocity (uo) X concentration
(co)
Where:
Wo = initial adsorbate loading
Wsat = adsorbate at equilibrium with the fluid (saturation)
L = length of the bed
ρb = bulk density of the bed
Length of Unused Bed (LUB)
•To calculate LUB, determine the total solute adsorbed up
to the break point by integration

c


1

0  co dt
t
•The break point time, tb is calculated from the ideal time
and the fraction of bed utilized: