kinetics & modeling of adsorption process.pptx

DR PRIY BRAT DWIVEDI, PHDSENIOR 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/q

e= C

e/S

m +1/K

LS

m (compare with y = mx + c, liner equation )

● Where Sm

and KL are the Langmuir constants.

● Graph plot between Ce

and Ce/q

e should give straight line with slope

value 1/Sm

and intercept 1/ KLS

m.


11

Freundlich Isotherm

● log qe = 1/n log C

e + log K

f (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 q

e 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/q

eLog q

eLog C

e

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 = q

e/C

e , where K

D is the distribution coefficient.

● ΔGo = – RT ln K

D

● ln KD = (ΔS

o/R) – (ΔH

o/RT)

● ΔGo is the Gibbs free energy, R is the universal gas constant, T is the

absolute temperature, ΔSo is the entropy change and ΔH

o 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 (ΔS

o) 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 q

e – k

1t/ 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.q

e2 + t/q

e

● 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, C

e, 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, ΔH

o, ΔS

o 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

Documents