Kinetics & Modeling of Adsorption Process.pptx

DR PRIY BRAT DWIVEDI, PHD
SENIOR LECTURER, MIE DEPARTMENT,
CALEDONIAN COLLEGE OF ENGINEERING, OMAN
Validation, Kinetic Modeling &
Thermodynamics of Adsorption
Process Experiments

Discussion Plan
12/9/2015Priy Brat Dwivedi, PhD
2
● Adsorption Applications
● Adsorption vs Absorption
● Adsorption & Adsorbents
● Adsorption Isotherms
● Validation example
● Thermodynamics
● Kinetics

Technical Applications of Adsorption
● Separation processes of gaseous and liquid mixtures via molecular
sieves
● Heterogeneous catalysis
● Chemical analyses (various types of chromatography)
● Waste water treatment
● Gas mask filters
● Lubrication phenomena
● Optical coatings
● Corrosion-resistant coatings
3

Adsorption vs Absorption
4

What is Adsorption
● Adsorption is the adhesion of atoms, ions, or molecules from a gas,
liquid, or dissolved solid to a surface.
● This process creates a film of the adsorbate on the surface of
the adsorbent.
● This process differs from absorption, in which a solute is
dissolved by a liquid or solid (the absorbent).
● Adsorption is a surface-based process while absorption involves the
whole volume of the material.
5

Adsorption
● In a typical adsorption process, a gaseous, liquid or solid molecule (the
adsorptive) attached to a solid or liquid surface, the adsorbent, and
forms the adsorbate, a complex between adsorptive and adsorbent.
6

Adsorption
● Similar to surface tension, adsorption happen due to residual
surface energy.
● Inside matter, all the bonding requirements of atoms of the material
are filled by other atoms .
● However, atoms on the surface are not completely surrounded by
other atoms and therefore they can attract adsorbates.
7

Types of Adsorbents
● Oxygen based compounds – Are typically hydrophilic and polar,
including materials such as silica gel and zeolites.
● Carbon based compounds – Are typically hydrophobic and non-polar,
including materials such as activated carbon and graphite. Activated
carbon is used for adsorption of organic substances and non-polar
adsorbates and it is also usually used for waste gas (and waste water)
treatment.
● Polymer based compounds – Are polar or non-polar functional groups
in a porous polymer matrix.
8

Modeling Isotherms
● Adsorption is usually described through isotherms, that is, the amount
of adsorbate on the adsorbent as a function of its pressure (if gas) or
concentration (if liquid) at constant temperature. Adsorption
isotherms describe the interaction of adsorbate with adsorbents.
● These 3 isotherms are primary and most commonly used.
○ Freundlich Isotherm
○ Langmuir Isotherm
○ BET Isotherm
9

Modeling Isotherms
● The Langmuir adsorption model is based on the assumption that
maximum adsorption corresponds to a saturated monolayer of solute
molecules on the adsorbent surface, with no lateral interaction
between the adsorbed molecules.
● The Freundlich model can be applied to multilayer adsorption with
non-uniform distribution of adsorption heat and affinities over the
heterogeneous surface.
● BET isotherm is more advanced and involve more parameters.
10

Langmuir Isotherm
● The equilibrium adsorption of adsorbate, qe
(mg/g), can be related to
the equilibrium concentration of these metals, Ce
(mg/L), by the linear
form of the Langmuir isotherm model:
● Ce
/qe
= Ce
/Sm
+1/KL
Sm
(compare with y = mx + c, liner equation )
● Where Sm
and KL
are the Langmuir constants.
● Graph plot between Ce
and Ce
/qe
should give straight line with slope
value 1/Sm
and intercept 1/ KL
Sm
.
11

Freundlich Isotherm
● log qe
= 1/n log Ce
+ log Kf
(compare with y = mx + c)
● KF
is the Freundlich constant related to the adsorption capacity, where
1/n is related to the adsorption intensity.
● Graph plot between log Ce
and log qe
should give straight line with
slope 1/n and intercept log Kf
.
● Equilibrium adsorption qe
(mg/g) = adsorbent value / adsorbate
amount
● Equilibrium concentration Ce
(mg/L) = Starting value – adsorbent
value
12

Example: Pb adsorption on Saw Dust
● Pb (II) ions were adsorbed on Saw dust. Equilibrium concentration / %
removal of Pb (II) ions were estimated by AAS.
● If starting concentration of Pb (II) ions is 50 ppm and we use 2 g of saw
dust adsorbent.
● After 20 minutes of adsorption, if concentration of Pb (II) ions in
solution is 30 ppm, then
● qe
= (50-30)/2 = 20/2 = 10mg of ions /g of adsorbent
● Ce
= 30 ppm or mg/L
● These values are tabulated and log values are also calculated.
13

Example: Pb adsorption on Saw Dust
14
qe
Ce
Ce
/qe
Log qe
Log Ce
20 90 4.5 1.30 1.95
44 78 1.77 1.64 1.89
50 75 1.5 1.69 1.87
90 55 0.61 1.95 1.74
98 51 0.52 1.99 1.70
These data for contact time study for Pb (II) adsorption on Saw Dust

Langmuir Isotherm
Langmuir isotherm plot for the adsorption of Pb(II) ions
on sawdust for contact time study
15

Freundlich Isotherm
Freundlich isotherm plot for the adsorption of Pb (II)
ions on sawdust for contact time
16

Inference
● Line equation, slope, intercept is directly obtained from excel.
● R2
values > 0.99 is good agreement. It means selected model is fit with
data.
● Most of the trend lines are straight line.
● But few points deviate from line. It indicates small experimental
errors.
17

Adsorption Thermodynamics
● Calculate thermodynamic parameters with temperature variations.
● KD
= qe
/Ce
, where KD
is the distribution coefficient.
● ΔGo
= – RT ln KD
● ln KD
= (ΔSo
/R) – (ΔHo
/RT)
● ΔGo
is the Gibbs free energy, R is the universal gas constant, T is the
absolute temperature, ΔSo
is the entropy change and ΔHo
is the
enthalpy change.
18

Adsorption Thermodynamics
● The values of Gibbs free energy (ΔGo
) for various temperatures were
calculated from the experimental data.
● The values of enthalpy change (ΔHo
) and entropy change (ΔSo
) were
estimated from the slope and intercept of the plot of ln KD
Vs 1/T. (For
this calculation Temperature analysis is also required)
● The negative values of ΔGo
for the adsorption shows the spontaneous
nature of the adsorption process, negative values of ΔHo
shows
Endothermic process and positive values for entropy (ΔSo
) shows
increased randomness during process.
19

Adsorption Kinetics
● In order to investigate the mechanism of adsorption kinetic models
are generally used to test experimental data. Pseudo-first-order and
pseudo-second-order equations can be used assuming that the
measured concentrations are equal to surface concentrations.
● Integrated form of equation for first order reaction is
log (qe
-q) = log qe
– k1
t/ 2.303
● A straight line of log(qe
- q) versus t suggests that process followed
first order kinetics.
20

Adsorption Kinetics
● Integrated form of equation for second order reaction is
● t/q = 1/k2
.qe
2
+ t/qe
● The plot t/q versus t should give a straight line if second order kinetic
model is applicable
21

Important Points to Remember
● Record all data, with time.
● Do maximum variations like pH, contact time, adsorbate amount, shaking
speed, foreign ion interference, and most important Temperature
variation.
● Tabulate qe
, Ce
, q (adsorption value at any time ‘t’) values for each
variation. Then plot Langmuir and Freundlich isotherms and use trend
line, equation, R2
, slope and intercept values.
● Calculate ΔGo
, ΔHo
, ΔSo
values for each set to determine feasibility.
● Use first order & second order kinetics equation to decide the possible
mechanism of adsorption.
22

12/9/2015Priy Brat Dwivedi, PhD 23

Kinetics & Modeling of Adsorption Process.pptx

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