In this Assignment file i try to easily describe the Deactivation mechanism of any catalysis reaction .Furthermore i will describe some Regeneration and prevention method of deactivated catalysts. and in the last part of this assignment i will show very easily the heterogeneous reaction kinetics.

1. 1
Bangabandhu Sheikh Mujibur Rahman Science and
Technology University, Gopalganj-8100
An assignment on
“Deactivation and Regeneration of Catalysts and
Heterogeneous Reaction Kinetics.”
Course Code: ACCE 513
Course Title: Chemical Reaction Engineering and Reactor Design
Submitted by Submitted to
Name: Bapi Mondal
ID No: 20151207052
Year: M.Sc. (Engg.)
Semester: 1st
Dept. of Applied Chemistry &
Chemical Engineering
BSMRSTU
M. Mehedi Hasan Babu
Assistant Professor,
Dept. of Applied Chemistry &
Chemical Engineering
BSMRSTU
Date of submission: 10-07-2021

2. TABLE OF CONTENT
2
1. Catalyst ..........................................................................................................................................................3
1.1 Catalytic Process or Catalysis ...............................................................................................................3
1.1.1 Classification of Catalytic process or Catalysis............................................................................3
2. Catalyst Deactivation.....................................................................................................................................4
3. Types of Catalyst Deactivation......................................................................................................................4
3.1 Catalyst Deactivation by Poisoning ......................................................................................................5
3.1.1 Common Poisons..........................................................................................................................5
3.1.2 Poisoning Effects..........................................................................................................................5
3.1.3 Types of Poisoning.......................................................................................................................6
3.1.4 Important poisoning parameters ...................................................................................................7
3.1.5 Poisons for selected catalysts .......................................................................................................7
3.1.6 Advantages of catalyst poisoning.................................................................................................7
3.1.7 Disadvantages of Catalyst poisoning............................................................................................7
3.2 Catalyst deactivation by Fouling or Coking..........................................................................................7
3.2.1 Coke formation on a supported metal catalyst .............................................................................8
3.2.2 Coke formation on metal oxide and sulphide catalysts. ...............................................................9
3.3 Catalyst deactivation by Sintering.......................................................................................................10
3.3.1 Factors Affecting Metal Particle Growth during sintering .........................................................10
3.4 Catalyst Deactivation by Phase Transition..........................................................................................11
3.4.1 Reactions of gas/vapor with solid to produce volatile compounds.............................................11
3.4.2 Reactions of gas/vapor with solid to produce inactive phases....................................................11
3.5 Deactivation by Mechanical degradation............................................................................................12
4. Deactivation affects catalyst performance ...................................................................................................12
5. Catalyst Regeneration ..................................................................................................................................13
5.1 Regeneration of Poisoned Catalysts....................................................................................................13
5.2 Regeneration of Catalyst Deactivated by Coke or Carbon..................................................................13
5.3 Regeneration of Sintered Catalyst.......................................................................................................14
6. Prevention of Catalyst deactivation .............................................................................................................14
6.1 Prevention of poisoning ......................................................................................................................14
6.2 Prevention of coking ...........................................................................................................................15
6.3 Prevention of sintering ........................................................................................................................15
6.4 Prevention of mechanical degradation ................................................................................................15
7. Catalyst Characterization.............................................................................................................................15
8. Heterogeneous Reaction ..............................................................................................................................17
9. Heterogeneous reaction Kinetics .................................................................................................................18
10. Overall rate expression for Heterogeneous reaction................................................................................18
11. Contacting Patterns for two phase system ...............................................................................................20
12. Kinetics for heterogeneous reactions system...........................................................................................21
12.1 Kinetics for Heterogeneous reaction ...................................................................................................22

3. SECTION -1
3
Deactivation and Regeneration of Catalysts
1. Catalyst
A catalyst is a substance that affects the rate of a reaction but emerges from the process
unchanged. A catalyst usually changes a reaction rate by developing a different molecular path
or mechanism for the reaction.
The catalyst used for mainly to speed up the chemical reaction rate or decreasing the reaction
rate. Positive catalysts increase the chemical reaction rate and negative catalysts decrease the
chemical reaction rate. Several reactions can be affected by using various types of catalysts.
For Example, gaseous hydrogen (H2) and oxygen (O2) are virtually inert at room temperature
but when exposed to platinum (Pt) they react rapidly.
Figure 1 Difference of Reaction Rate and reaction path variation by using the catalyst and Without catalyst.
1.1 Catalytic Process or Catalysis
Catalysis is the method of changing the rate of a chemical reaction by adding a substance
known as a catalyst. Catalysts are not consumed in the reaction and remain unchanged after it.
Often only very small amounts of catalyst are necessary to modify the chemical reaction rate.
Catalysis is the study, and use of catalysts and catalytic processes. The term catalysis was
introduced as early as 1836 by Berzelius.
For a chemical reaction to take place, it requires a certain minimum amount of energy, called
its activation energy (Ea). If a substance can lower this activation energy without itself being
changed or consumed during the reaction, it is called a catalyst or catalytic agent. The action
of a catalyst is called catalysis.
1.1.1 Classification of Catalytic process or Catalysis
Catalysts are primarily categorized into four types according to their phase involved. They
are (1) Homogeneous, (2) Heterogeneous (solid), (3) Heterogenized homogeneous catalyst,
and (4) Biocatalysts.

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2. Catalyst Deactivation
Catalyst deactivation means Loss in catalytic activity due to chemical, mechanical or
thermal processes. catalyst deactivation is a temporal or permanent loss of active sites, caused
by chemical and physical reasons. The main reasons for catalyst deactivation include the
formation of inactive phase, catalyst poisoning, carbon deposition, and metal sintering.
Catalytic deactivation adds another level of complexity to the analysis of the reaction rate law
parameters and pathways.
The activity of an industrial catalyst with time can be described employing several basic types
can be represented by following figure 2
Figure 2 Deactivation behaviors of various types of catalysts.
3. Types of Catalyst Deactivation
We know that various types of catalysts have been used for various types of reactions.
According to the catalyst, phase involved catalyst are various types such as homogenous
catalyst, heterogeneous catalyst, Heterogenized homogeneous catalyst, and Biocatalyst.
Among these types of catalysts, Heterogeneous catalysts are more prone to deactivation.
Catalyst Deactivation may be categorized according to Loss in catalytic activity due to
chemical, mechanical or thermal processes. And also these three processes are subdivided into
various types which are represented by the following figure 3.
Figure 3 Types of Catalyst Deactivation Process

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The many deactivation mechanisms reported for heterogeneous catalysts can be broadly
classified into eight distinct types. This is listed below.
1. Poisoning (strong chemisorption of species on catalytic sites)
2. Fouling (physical deposition of species on the catalyst surface and in its pores)
3. Sintering or Thermal degradation (thermally induced loss of active phase or support
surface area, support area)
4. Vapor compound formation or leaching accompanied by transport (reaction of fluid
with catalyst phase to produce volatile or soluble compounds)
5. Inactive phase formation
6. Attrition/crushing
7. Photo degradation (loss of activity due to exposure to light)
8. Electro degradation (changes in catalyst structure due to applied voltage)
3.1 Catalyst Deactivation by Poisoning
Poisoning is caused by the chemisorption of compounds in the process stream and these
compounds block or modifies active sites on the catalyst. The poison may cause changes in the
surface morphology and surface structure of the catalyst either by surface reconstruction or
surface relaxation or may modify the bond between the metal catalyst and the support.
Deactivation by the Poisoning mechanism occurs when the poisoning molecules become
irreversibly chemisorbed to active sites, thereby reducing the number of sites available for the
main reaction. Poisoning is the strong chemisorption of reactants, products, or impurities on
the active sites of catalyst and that is available for catalysis. Poisoning refers specifically to
chemical deactivation, rather than other mechanisms of catalyst degradation such as thermal
decomposition or physical damage.
3.1.1 Common Poisons
Poisons or poisonous chemicals mainly include various types of chemical species or
compounds and these poisonous chemical are mainly responsible for catalyst deactivation.
These poisonous chemicals or poisons are mainly classified according to their chemical nature
as well as structure.
 Groups VA and VIA elements in the periodic table such as N, P, As, Sb, O, S, Se, Te,
etc.
 Group VIIA elements in the periodic table such as F, Cl, Br, I, etc.
 Toxic heavy metals and ions (Pb, Hg, Bi, Sn, Zn, Cd, Cu, Fe)
 Molecules, which adsorb with multiple bonds (CO, NO, HCN, benzene)
 Molecules with reactive heteroatoms. Example sulfur.
3.1.2 Poisoning Effects
 Geometric effect: Blocking an active site due to poisoning, therefore, change the shape
and surface morphology of the catalyst and also catalyst support.
 Electronic effect: Electronic effects alter the adsorptivity of other species.
 Chemical effect: Chemical effects alter the chemical nature of the active site and for
this reason catalyst active site loss its initial startup properties.
 Reconstruction: Formation of new compounds during the reaction process and these
compounds may be unwanted and this may affect the selectivity of the reaction.

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Figure 4 Poisoning Effect in Catalyst Deactivation mechanism
3.1.3 Types of Poisoning
Mainly five types of poisoning are found in catalyst deactivation that is discussed below
 Selective poisoning: Selective poisoning involves preferential adsorption of the poison
on the most active sites at low concentrations. An example of selective poisoning is the
deactivation of platinum(Pt) by Carbon monoxide for the para-H2 conversion.
 Anti-selective: If the catalyst active sites of lesser activity are blocked initially then this
type of poisoning is called anti-selective. Lead (Pb) poisoning of CO oxidation on
platinum is anti-selective.
 Non-Selective: If the catalyst activity loss is proportional to the concentration of
adsorbed poison, the poisoning is non-selective. Arsenic (As) poisoning of
cyclopropane hydrogenation on Pt is non-selective.
 Reversible
 Non- reversible
A plot of activity that is the reaction rate normalized to initial rate versus normalized poison
concentration represents the poisoning selectivity. Poisoning selectivity is illustrated in Figure
5.
Figure 5 Poisoning selectivity ( Normalized activity vs normalized concentration)

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3.1.4 Important poisoning parameters
 Activity: Reaction rate at time t relative to that the initial time that is t = 0.
 Toxicity: Susceptibility of a given catalyst for a poison relative to that for another
poison.
 Resistance: Inverse to the deactivation rate and this property which determines how
rapidly a catalyst deactivates.
 Tolerance: Activity of the catalyst at saturation coverage. Many catalysts may have
negligible activity at saturation coverage.
3.1.5 Poisons for selected catalysts
Catalysts Reaction Type Poisons
Silica–alumina, zeolites Cracking Organic bases,
hydrocarbons,
Nickel, platinum,
palladium
Hydrogenation/dehydrogenation Compounds of S, P, As,
Zn, Hg,
Nickel Steam reforming of methane
and naptha
H2S, As
Iron, ruthenium Ammonia synthesis O2, H2O, CO, S, C2H2,
Cobalt, iron Fischer–Tropsch synthesis H2S, As, NH3, metal
Carbonyls
Noble metals on zeolite Hydrocracking reactions Ammonia, sulfur,
phosphorus, Selenium,
and Tellurium.
3.1.6 Advantages of catalyst poisoning
Usually, catalyst poisoning is undesirable as it leads to the wasting of expensive metals
or their complexes. However, the poisoning of catalysts can be used to improve the selectivity
of reactions. Poisoning can allow for selective intermediates to be isolated and desirable final
products to be produced.
 Pt-containing naphtha reforming catalysts are often pre-sulfided to minimize unwanted
cracking reactions.
 S and Cu added to Ni catalyst in steam reforming to minimize coking.
 V2O5 is added to platinum (Pt) to suppress SO2 oxidation to SO3 in diesel emissions
control catalysts.
3.1.7 Disadvantages of Catalyst poisoning
 Reduce the catalyst active sites.
 Change the catalyst surface structure and morphology and its geometric shape.
 Alter the electronic effects of catalyst.
 Change in the structure of catalyst pellets and their pores.
3.2 Catalyst deactivation by Fouling or Coking
Fouling occurs when materials present in the reactor are deposited upon the surface of
the catalyst and blocking active sites of the catalyst. The most common form of fouling is by
carbonaceous species which is known as coking. Coke or carbonaceous matter may be
deposited in several forms including graphite, high-molecular-weight polycyclic aromatics
(tars), and metal carbides. When catalytic reactions involve hydrocarbon or carbon monoxide
then there is the possibility of carbon deposition on the catalyst surface and thereby physically

8. 8
blocks the active sites of the catalyst. Especially catalysts with acidic or hydrogenating/
dehydrogenating properties are responsible for coking.
Important examples include mechanical deposits of carbon and coke in porous catalysts,
although carbon- and coke-forming processes also involve chemisorption of different kinds of
carbons or condensed hydrocarbons that may act as catalysts poisons.
Carbon is generally a product of carbon monoxide (CO) disproportionation while coke is
formed by decomposition or condensation of hydrocarbons on catalyst surfaces and typically
consists of polymerized heavy hydrocarbons.
The form of the coke depends upon the catalyst, the temperature, and the partial pressure of the
carbonaceous compound. The chemical nature of cokes or carbons formed in catalytic
processes varies with reaction type, catalyst type, and reaction conditions.
Figure 6 Conceptual Model of Fouling
Coking can be classified under two headings
1. Coke formation on supported metal catalysts
2. Coke formation on metal oxide and sulphide catalysts.
3.2.1 Coke formation on a supported metal catalyst
Processes of carbon deposition and coke formation on metal catalysts from carbon
monoxide and hydrocarbons, including methane during steam methane reforming (SMR) for
hydrogen production represented by the following reaction in figure 7. Different kinds of
carbon and coke that vary in configuration and reactivity are formed in these reactions. Carbon
may chemisorb strongly as a monolayer and physically as a multilayer on the catalyst surface
and reduce the catalytic active surface sites. And then totally encapsulate the metal particle and
deactivate the particle also blocking the micro and macropores of the catalyst.
For example, CO dissociates on metals and formed an adsorbed atomic carbon (Cα), Cα can
react to Cβ which is a polymeric carbon film. Reactive and amorphous carbon is formed at low
temperatures which are then converted at high temperatures over some time to less reactive,
graphitic forms.

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CO(a) → C(α) + O (a)
Figure 7 Formation, transformation, and gasification of carbon on nickel
Here in the above reactions a, g, and s represent adsorbed, gaseous and solid states respectively.
The formation and transformation of coke on metal surfaces can be represented by the
following reactions.
Figure 8 Formation and transformation of coke on a metal surface
Here (Cβ) Polymeric, amorphous films or filaments (Cv) Vermicular filaments or fibers, (Cγ)
Nickel carbide (bulk), Cc Graphitic (crystalline) platelets or films.
3.2.2 Coke formation on metal oxide and sulphide catalysts.
In hydrocarbon involving reactions, coke may be formed in the gas phase and on both
non-catalytic and catalytic surfaces. The formation of coke on oxides and sulfides is generally
a result of cracking reactions involving coke precursors which are typically olefins or aromatics
compounds are catalyzed by acid sites.
Dehydrogenation and cyclization reactions of carbocation intermediates formed on acid sites
lead to an aromatic compound that reacts further to form higher molecular weight polynuclear
aromatics that condense as coke. Due to the maximum stability of the polynuclear carbocation,
they can continue to grow on the metal oxide catalyst surface for a relatively long time. The
formation of coke on metal oxide and sulphide can be easily demonstrated by the following
diagram.

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3.3 Catalyst deactivation by Sintering
Sintering is caused by the growth or agglomeration of small crystals which make up the
catalyst or its support. The structural rearrangement observed during sintering leads to a
decrease in surface area of the catalyst consequently an irreversible reduction in catalyst sites.
At high temperatures, the materials gathered together and form a cluster.
Sintering processes generally take place at high reaction temperatures that are greater than
500˚C and are generally accelerated by the presence of water vapor. Sintering generally occurs
if the local temperature of the catalyst exceeds approximately one-third to one-half of its
melting temperature. Two factors mainly affect the sintering process that is Temperature and
reaction atmosphere.
Figure 9 Metal Crystallite due to sintering
The processes of crystallite and atomic migration are illustrated in above figure 9. Crystallite
migration involves the migration of entire crystallites over the catalyst support surface followed
by collision and coalescence.
3.3.1 Factors Affecting Metal Particle Growth during sintering
Temperature, environment, metal type, promoters/impurities, support surface area,
texture, and porosity are the principal parameters that influence the rates of sintering.
Variables Effect of Variables during sintering
Temperature Sintering rates are exponentially dependent on Temperature. Sintering rates
increase exponentially with temperature.
Atmosphere Sintering rates are much higher for noble metals in O2 than in H2 and higher
for noble and base metals in H2 relative to N2. Sintering rates are decreases
for supported Platinum catalyst in the following order NO, O2, H2, N2.

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Metal Metals sinter relatively rapidly in oxygen and relatively slowly in hydrogen,
although depending upon the support
Support Metal–support interactions are weak and bond strengths vary from 5–15
kJ/mol. Thermal stability for a given metal decreases with support in the
following order: Al2O3 > SiO2 > carbon
Promoters Some additives decrease atom mobility. Examples are C, O, CaO, BaO
others increase atom mobility. Examples are Pb, Bi, Cl, F, or S.
Pore size Sintering rates are lower for porous materials and higher for nonporous
supports.
3.4 Catalyst Deactivation by Phase Transition
In addition to poisoning, several chemical routes are leading to catalyst deactivation.
1. Reactions of gas/vapor with solid to produce volatile compounds.
2. Reactions of gas/vapor with solid to produce inactive phases.
3.4.1 Reactions of gas/vapor with solid to produce volatile compounds
Loss of metal through direct vaporization is generally an insignificant pathway to
catalyst deactivation. Metal loss through the formation of volatile compounds. Examples are
metal carbonyls, oxides, sulfides, and halides. Carbonyls are formed at relatively low
temperatures. Halides can be formed at relatively low temperatures and low concentrations of
halogens. Metal loss via the formation of volatile metal compounds can occur at moderate
temperature even at room temperature.
Types and Examples of Volatile Compounds Formed in Catalytic Reactions are discussed
below.
Gaseous Environment Types of Compound formation Example of
Compounds
Carbon monoxide (CO), Oxide
of Nitrogen (NOx)
Carbonyls, nitrosyl carbonyls, Ni(CO)4
Oxygen Oxides PbO
Hydrogen Sulphide (H2S) Sulphides MoS2
Halogens Halides CuCl2, PdBr2
3.4.2 Reactions of gas/vapor with solid to produce inactive phases.
Dispersed metals, metal oxides, metal sulfides, and metal carbides are typical catalytic
phases, the surfaces of which are similar in composition to the bulk phases. If one of these
metal catalysts is oxidized, sulfided, or carbides, it will lose essentially all of its activity. While
these chemical modifications are closely related to poisoning. The difference here is that rather
than losing activity owing to the presence of an adsorbed species, the loss of activity is due to
the formation of a new phase altogether.
Examples of Reactions of Gases/Vapors with Catalytic Solids to Produce Inactive Phases are
listed below.
Catalytic Process Gas-vapour
composition
Catalysts Formation of
inactive phases
Ammonia synthesis and
regeneration
H2, N2 Fe/K/Al2O3 FeO

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Catalytic Cracking HCs, H2, H2O La-Y Zeolite H2O induced Al
migration from
zeolite causing
zeolite destruction
Fischer- tropsch CO, H2, H2O,
CO2, HCs
Co/ SiO2 CoO.SiO2 and
collapse of SiO2, by-
product water
Steam reforming and
Regeneration in H2O
CH4, H2O, CO,
H2, CO2
Ni/ Al2O3 Ni2Al2O4
Automobile emission
and control
N2, O2, HCs, CO,
NO, H2O, SO2
Pt-Rh/ Al2O3 RhAl2O4
CO oxidation N2, O2 Pt/Al2O3 Al2(SO4)3
which blocks
catalyst
pores
3.5 Deactivation by Mechanical degradation
Mechanical failure of catalysts is noticed in several different forms which are described below
 Catalyst is deactivated by the influence of applied pressure or load which causes catalyst
crushing or attrition. Thus, attrition and crushing are physical as well as mechanical
processes.
 The size reduction of catalyst granules or pellets to form fines pores especially in fluid or
slurry beds sometimes causes degradation. Erosion or sometimes it is called corrosion of
catalyst particles or monolith coatings at high fluid velocities causes catalyst degradation.
 Collision of particles with each other or with reactor walls degrades catalyst surface and
morphology. While gravitational stress at the bottom of a large catalytic bed cause erosion
of a catalyst.
4. Deactivation affects catalyst performance
Catalyst deactivation may affect the performance of a reactor in several ways.
 A reduction in the number of catalyst sites can reduce catalytic activity and decrease
fractional conversion. After all, some reactions depend solely on the presence of metal
while others depend extremely on the configuration of the metal.
 Catalyst performance is affected depends upon the chemical reaction to be catalyzed and
how the catalyst has been deactivated. For example, deposition/chemisorption of sulfur,
nitrogen, or carbon on the catalyst generally influences hydrogenation reactions more than
exchange reactions.
 If parallel reactions are to be catalyzed deactivation may cause a shift in selectivity to favor
non-hydrogenated products.
 Heavy metals such as Ni, Fe present in the feed stream of catalytic crackers can deposit on
the catalyst and subsequently catalyze dehydrogenation reactions.
 Catalyst deactivation may affect performance is by blocking catalyst pores and active sites.
This is particularly prevalent during fouling, when large aggregates of materials may be
deposited upon the catalyst surface.

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5. Catalyst Regeneration
Regeneration is a method to restore the catalytic activity of a spent catalyst. It generally
involves thermal treatment to remove surface coatings and absorbed species. it is possible to
build up temporarily or completely the activity of a catalyst through chemical treatment. The
regeneration method may be slow, either because of thermodynamic limitations or diffusional
limitations rising from the blockage of catalyst pores. Although the rate of desorption generally
increases at high temperatures prolonged exposure of the catalyst to a high-temperature gas
stream can lead to sintering and irreversible loss of activity.
By increasing the useful life of the spent catalyst, regeneration eliminates the need for catalyst
disposal. Some catalyst is too damaged to be reused and must be disposed of then it can be
ultrasonically cleaned to remove all poisons.
Again we know that catalyst loss its catalytic activity by a different mechanism such as
poisoning, sintering, coking or fouling, phase transformation, etc. By controlling these
deactivation mechanisms, we can easily protect the catalysts from deactivation, and also by
preventing these mechanism steps we can regenerate the catalyst catalytic activity.
When the activity has rejected to a critical level, a choice must be built among four alternatives:
(1) Restore the activity of the catalyst, (2) Use it for another application, (3) Reclaim and
recycle the important and/or expensive catalytic components, or (4) discard the catalyst.
Among these, the first steps are almost always preferred.
5.1 Regeneration of Poisoned Catalysts
Much of the previous literature has focused on the regeneration of sulfur-poisoned
catalysts used in hydrogenations and steam reforming. regeneration of sulfur-poisoned Ni, Cu,
Pt, and Mo with oxygen/air, steam, and hydrogen. 80% removal of surface sulfur from Mg-
and Ca-promoted Ni, steam reforming catalysts occurs at 700 °C in steam. Sulfur poisoned Ni
catalyst can be regenerated by the following reaction mechanism.
Ni-S+ H2O→ NiO+H2S
H2S+ 2 H2O → SO2+ 3 H2
This treatment is partially successful in the case of low-surface-area steam reforming catalysts.
But at high temperatures, these reactions would cause sintering of most high-surface-area
nickel catalysts.
Regeneration of sulfur-poisoned noble metals in the air is more easily accomplished than with
steam. Regeneration of sulfur-poisoned nickel catalysts using hydrogen is impractical because
(1) adsorption of sulfur is reversible only at high temperatures at which sintering rates are also
high and (2) rates of removal of sulfur in H2 as H2S are slow even at high temperature.
5.2 Regeneration of Catalyst Deactivated by Coke or Carbon
Carbonaceous compounds which deposits on the catalyst surface can be removed by
gasification with O2, H2O, CO2, and H2. The temperature necessary to gasify these deposits at
a reasonable rate varies with the type of gas, the structure and reactivity of the carbon or coke,
and the activity of the catalyst.
Coked catalyst regeneration may be accomplished by gasification with oxygen, steam,
hydrogen, or carbon dioxide.
C+ O2 → CO2

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C+H2O → CO+ H2
C+ 2H2 → CH4
C+ CO2 → 2CO
The first reaction is strongly exothermic and may lead to high local temperatures within the
catalyst. Thus, the temperature must be carefully controlled to avoid sintering.
Furthermore, Promoters can be added to increase the gasification rate such as K or Mg in Ni
for steam reforming. Gasification rates of coke or carbon are greatly accelerated by the same
metal or metal oxide catalysts upon which carbon or coke deposits.
5.3 Regeneration of Sintered Catalyst
The regeneration process for regenerating the original metal dispersion of a sintered
catalyst or even improved dispersion is known as re-dispersion which has been utilized for
recovery of catalysts containing precious metals such as Pt, Au, and Ag.
The regeneration procedure for the sintered catalyst is the re-dispersion of the platinum phase
by a high-temperature treatment in oxygen and chlorine which is generally known as
oxychlorination. A mechanism for platinum (Pt) re-dispersion by oxygen and chlorine is shown
in Figure 10.
Figure 10 mechanism for redispersion by oxychlorination of alumina-supported platinum
But the main problem of regeneration of sintered catalyst is very hard to reverse. Redispersion
of alumina-supported platinum catalyst is also possible in a chlorine-free oxygen atmosphere
if chlorine is presented on the catalyst.
6. Prevention of Catalyst deactivation
6.1 Prevention of poisoning
 Since poisoning is caused by strong adsorption of feed impurities and since poisoned
catalysts are generally difficult or impossible to regenerate, it is best prevented by removal
of impurities from the feed site and that will enable the catalyst to operate at its optimum
lifetime.
 Adding some extra additives compounds which selectively adsorb poisonous chemicals or
poisons.

15. 15
 By careful choice of reaction conditions that lower the strength of poison adsorption and
that simultaneously prevent poisoning.
6.2 Prevention of coking
 Avoid using coke precursors.
 By adding gasifying agents such as H2, H2O we can reduce the formation of coke.
 The formation and growth of carbon or coke species on metal surfaces are minimized by
choosing reaction conditions.
 Coke deposition on oxide or sulfide catalysts surface occurs mainly on strongly acidic sites.
For this reason, decrease the acidity of oxide or sulphide.
 Catalyst additives such as MgO, K2O facilitate H2O or CO2 adsorption and dissociation to
oxygen atoms which promote gasify coke precursors.
 Use shape-selective molecular sieves.
 Control the reaction temperature because excess high temperature increases the possibility
of sintering.
6.3 Prevention of sintering
 First of all, lower the reaction temperature because sintering is caused by high temperature.
 Catalysts may be doped with stabilizers which may have a high melting point to prevent
formation clusters of small crystals. For example, chromium, alumina, and magnesia which
have high melting points are often added as stabilizers of finely divided metal catalysts.
 Adding small quantities of chlorinated compounds to the gas stream may prevent sintering.
Examples Prevention of sintering for Platinum catalyst. In this case, chlorine increases the
activation energy for the sintering process thus, reduces the sintering rate.
 The addition of Ba, Zn, La, Si, and Mn oxide promoters improves the thermal stability of
alumina and these additives can affect product selectivity and extend the productive life of
the catalysts.
 Must avoid water vapor formation and other substances that facilitate metal migration.
6.4 Prevention of mechanical degradation
 Design of processes and catalysts in preventing or minimizing mechanical degradation
that is increasing strength by advanced preparation methods.
 Use binders to facilitate plastic deformation and thus protect against brittle fracture.
 Choose supporters, support additives, and coatings that have high fracture toughness.
 Chemical or thermal tempering of agglomerates to introduce compressive stresses, which
increase strength and attrition resistance.
7. Catalyst Characterization
Mainly four types of catalyst characterization techniques implemented to analyze the
catalyst properties as well as catalyst morphologies, structure, thermal behaviors. These
techniques are described below
 BET analysis: Braunauer-Emmet-Teller theory is a popular theory for determining solid
catalyst surface area. This equation can represent multilayer adsorption equilibrium for
many systems. This equation is extremely useful for determining surface area as well as
pore volume and pore size distribution. Example: The deactivation of Cu/ZnO/Al2O3
catalyst used in a methanol synthesis because of sintering. After the reaction, the overall
surface area of a catalyst and a metal area of Cu decreases with time.

16. 16
Catalyst Fresh Cu/ZnO/Al2O3 catalyst Spent Cu/ZnO/Al2O3 catalyst
BET surface area
(m2
/g)
96.0 41.5
Cu surface area
(m2
/g)
25.4 11.1
 SEM (Scanning Electron Microscopy): SEM technique gives the information of external
morphology text, the surface topography of the solid catalyst.
 TEM (Transmission Electron Microscopy): TEM technique provides information on the
structure, texture, shape, and size of the sample. This technique is also valuable for
determining either the catalyst is amorphous or crystalline.
 TPO (Temperature programmed oxidation): TPO analysis method gives us information
about the percent of coking in a given catalyst. Here Gas mixture such as Oxygen diluted
in Helium is used to perform analysis. Dynamic TPO with on-line mass spectrometry is
incorporate to monitor oxygen consumption and which confirms percent coking that
occurred in a catalyst.

17. SECTION -2
17
Heterogeneous Reaction Kinetics
8. Heterogeneous Reaction
A heterogeneous reaction is a chemical reaction where the reactants are in different phases
from each other.
Examples: The reaction between acid and metal is heterogeneous. A reaction between a gas
and a liquid as between air and seawater is heterogeneous. A reaction at the catalyst surface is
heterogeneous. Heterogeneous reaction examples are broadly explained by the following
reaction.
 The Burning of a carbon particle in the air
When carbon particle reacts with environmental air then there are two possible products will
be formed that is carbon dioxide (CO2) and carbon monoxide(CO). And if we assume that here
only one type of product will be formed that is carbon dioxide and we ignore the possible
formation of carbon monoxide. The reaction is given below
C+O→CO2
From the above reaction we see that there are two steps in series are involved in this reaction
(1) mass transfer of oxygen to the surface and (2) reaction at the surface of the particle.
Figure 11 Burning of Carbon particle in Air
 Aerobic fermentation
When air bubbles through a tank of liquid that contains dispersed microbes and is taken up by
the microbes to produce product materials and there are up to seven possible resistance steps.
And these seven steps are similar to the catalytic reactions steps. But in this reaction one step
involves.
Figure 12 Aerobic fermentation reaction all possible steps

18. 18
9. Heterogeneous reaction Kinetics
Since more than one phase is present, the movement of material from phase to phase must
be considered in the rate equation. Thus the rate expression, in general, will incorporate mass
transfer terms in addition to the usual chemical kinetics term. These mass transfer terms are
different in type and numbers in the different kinds of a heterogeneous system, no single rate
expression has general application.
10. Overall rate expression for Heterogeneous reaction
If we want to get the overall rate expression for a heterogeneous reaction, then we must
write the individual rate steps on the same basis. And these individual rate steps may be unit
surface, unit volume of the reactor, unit volume of cells, etc.).
Let consider a general reaction and calculate the overall rate expression
A B
Here A is the reactant and B is the Product in the above reactions.
We know that the rate of a reaction is the moles of A reacted per unit time per volume. And
with time the reactant A reduces in amount and product B increase in amount. That is A loss is
a quantity with time and B increases with time. The rate expression for reactant A is
−𝑟𝐴= −
1
𝑉
𝑑𝑁𝐴
𝑑𝑡
=
𝑚𝑜𝑙𝑒𝑠 𝐴 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑜𝑟 × 𝑡𝑖𝑚𝑒
Here minus sign (-) indicates that the reactant is reduced with time.
But we know that maximum time in a heterogeneous reactions catalyst is involved. But the
most time this catalyst is solid or in other words, the catalyst is in the solid phase. And this
must be considered when we calculate an overall rate expression because when we consider a
catalytic reaction and calculate overall rate expression, the weight and surface area of the
catalyst must be considered. The catalyst weight and surface area are of significant importance
in the heterogeneous catalytic reaction.
Rate expression when the weight of the solid substance is involved
−𝑟′𝐴= −
1
𝑊
𝑑𝑁𝐴
𝑑𝑡
=
𝑚𝑜𝑙𝑒𝑠 𝐴 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑 × 𝑡𝑖𝑚𝑒
Rate expression when the surface area of a solid substance is involved
−𝑟′′𝐴= −
1
𝑆
𝑑𝑁𝐴
𝑑𝑡
=
𝑚𝑜𝑙𝑒𝑠 𝐴 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑖𝑎𝑙 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 × 𝑡𝑖𝑚𝑒
Put all the mass transfer and reaction steps into the same rate form and then combine all these
rate steps we get an overall rate equation
−𝑟𝐴= −
1
𝑉
𝑑𝑁𝐴
𝑑𝑡
=
𝑚𝑜𝑙𝑒𝑠 𝐴 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑜𝑟 × 𝑡𝑖𝑚𝑒

19. 19
or, (−𝑟𝐴) × 𝑉 = −
𝑑𝑁𝐴
𝑑𝑡
-------------------(1)
similarly, we get
(−𝑟′𝐴) × 𝑊 = −
𝑑𝑁𝐴
𝑑𝑡
-------------------(2)
And,
(−𝑟′′𝐴) × 𝑆 = −
𝑑𝑁𝐴
𝑑𝑡
-------------------(3)
Now combine the no 1,2 and 3 equations we get an overall rate term that is
𝑚𝑜𝑙𝑒𝑠 𝐴 𝑟𝑒𝑎𝑐𝑡𝑒𝑑
𝑡𝑖𝑚𝑒
= (−𝑟𝐴) × 𝑉 = (−𝑟′𝐴) × 𝑊 = (−𝑟′′𝐴) × 𝑆-----------(4)
Here the minus sign indicates that the rate of disappearance of reactant. If we neglect the minus
sign from equation no 4 we can easily write the following equation,
(𝑟𝐴) × 𝑉 = (𝑟′𝐴) × 𝑊 = (𝑟′′𝐴) × 𝑆------------------(5)
By rearranging equation 5 we can easily write the following equation,
(𝑟𝐴) × 𝑉 = (𝑟′𝐴) × 𝑊
or, 𝑟𝐴 =
𝑊
𝑉
(𝑟′
𝐴)
Again we can write, (𝑟𝐴) × 𝑉 = (𝑟′′𝐴) × 𝑆
or, 𝑟′′𝐴 =
𝑉
𝑆
(𝑟′
𝐴)
and finally, we can write, (𝑟′𝐴) × 𝑊 = (𝑟′′𝐴) × 𝑆
or, 𝑟′𝐴 =
𝑆
𝑊
(𝑟′
′𝐴)
Here, the rate of reaction of reactant A (𝑟𝐴) with respect to volume (V), (𝑟′𝐴) is the rate of
reaction with respect to solid catalyst weight (W) and (𝑟′′𝐴) is the reaction rate with respect to
surface area (S).
If the reaction steps are in series that means the following reactant form an intermediate
substance which is decomposed to the final desired product, we can re-write the following
overall rate expression,
𝒓𝒐𝒗𝒆𝒓𝒂𝒍𝒍 = 𝒓𝟏 = 𝒓𝟐 = 𝒓𝟑
And if the reaction is in parallel that means a reactant from the desired product as well as an
undesired product. So for the parallel reaction, we can write the overall rate expression which
is represented below,
𝒓𝒐𝒗𝒆𝒓𝒂𝒍𝒍 = 𝒓𝟏 + 𝒓𝟐
In the general case, if all the steps are linear in concentration, then it is easy to combine them.
However, if any of the steps are nonlinear, then you will get a messy overall expression.

20. 20
11. Contacting Patterns for two phase system
A phase is defined as one of the states of the matter. It can be a solid, a liquid, or a gas.
Multiphase flow is the simultaneous flow of several phases. The simplest case of multiphase
flow is a two-phase flow. Two-phase flow can be solid-liquid flow, liquid-liquid flow, gas-
solid flow, and gas-liquid flow.
There are many ways that two phases can be contacted, and for each, the design equation will
be unique. Contacting patterns for two-phase systems generally mean the mixing of two
different phases from a different direction and different patterns.
Ideal contacting patterns for two flowing fluids can be broadly classified into five different
types. And these flow patterns are
 Co current flow pattern
 Countercurrent flow pattern
 Crosscurrent flow pattern
 One laminar flow and one mixed flow
 The mixed flow of two-phase.
Co-Current Flow: Co-current flow of a two-phase system means that the flow of two-phase is
in the same direction. If the two-phase is A and B the according to the co-current flow patterns
phase A and phase B mixing is in the same direction or in other words they enter the reactor in
the same direction. And this type of flow may also know as parallel flow.
Figure 13 Co current flow of two phase
Counter-Current Flow: Counter-current flow of two-phase system means that the flow of two-
phase is in the opposite direction. If the two-phase is A and B the according to the counter-
current flow patterns phase A and phase B mixing are in the opposite direction or in other
words they enter the reactor in the opposite direction.
Figure 14 Countercurrent flow of two phase
Cross-Current Flow: Crosscurrent flow of two-phase system means that the flow of two-phase
is in the perpendicular direction. That means phases flow in the horizontal axis and phase B
flow in the vertical axis. If the two-phase is A and B the according to the cross-current flow
patterns phase A and phase B mixing is in the perpendicular direction.

21. 21
Figure 15 Crosscurrent flow of two-phase
One laminar flow and one mixed-flow: In this type of contacting pattern one phase is
introduced into the reactor by maintaining the laminar flow and another phase is the mixed-
phase that maintenance turbulent flow. The mixed-phase maybe Micro or macro. And these
two types of phases are introduced into the reactor according to crosscurrent flow.
Figure 16 Plug A and Mixed B Flow pattern
The mixed flow of two-phase: when two-phase is introduced into the reactor by maintaining
cross-current flow pattern but the two-phase are mixed-phase then these flow properties is
known as the mixed flow of two-phase. Here the two phases are micro-macro or micro-micro.
And here the flow of two phases is turbulent.
Figure 17 Mixed A and Mixed B flow pattern
12. Kinetics for heterogeneous reactions system
A heterogeneous reaction is a chemical reaction where the reactants are in different phases
from each other. For describing the kinetics of heterogeneous reactions systems we generally
consider the heterogeneous catalysis reaction. Because in heterogeneous catalysis reaction
more than one phase is present.

22. 22
For most cases, heterogeneous catalytic reactions can be divided into seven steps and these
steps are crucial for catalytic reactions. The steps are listed below
1. Mass transfer of the reactants from the bulk fluid to the external surface of the catalyst
pellet.
2. Diffusion of the reactant from the pore mouth through the catalyst pores.
3. Adsorption of reactant onto the catalyst surface.
4. Reaction on the surface of the catalyst.
5. Desorption of the products from the surface.
6. Diffusion of the products from the interior of the pellet to the pore mouth at the external
surface.
7. Mass transfer of the products from the external pellet surface to the bulk fluid.
Among these seven steps 3,4 and 5, no steps are most important because these three steps
controlling the reaction rates.
12.1 Kinetics for Heterogeneous reaction
Now we consider a porous catalyst particle bathed by reactant A.
Figure 18 Porous Catalyst Bathed by Reactant A
The rate of reaction of A for the particle as a whole may depend on the following steps.
1. Surface kinetics
2. Pore diffusion resistance
3. Particle temperature gradients within the particle
4. Film temperature gradients between the outer surface of the particle and the main gas
stream
5. Film diffusion resistance
Surface kinetics: Surface kinetics means all possible steps involved in the surface of the
particle. On the surface, there are three important steps are involved. These steps are adsorption
of reactant A onto the surface, reaction on the surface, or desorption of product back into the
gas stream. For initiating a chemical reaction in the heterogeneous system the reactant particle
first transport from the bulk fluid concentration to the catalyst pellets surface. And then this
particle diffuses through the particle external surface to the internal surface and then completes
the reaction and desorbed from the catalytic surface.

23. 23
Pore diffusion resistance: Pore diffusion resistance acts as resistance or barrier for the
movement of reactant to diffuse through the internal part of the catalyst particle. which may
cause the interior of the particle to be starved for reactants.
Particle temperature gradients within the particle: During the surface reaction, large heat
release or absorption is mainly responsible for the temperature gradients within the particle.
Film temperature gradients between the outer surface of the particle and the main gas
stream: all the particles are uniform in temperature throughout the reactor and here the
temperature is hotter than the surrounding gas. And this is responsible for temperature
gradients.
Film diffusion resistance: Concentration gradient across the bulk gas film surrounding the
catalyst particles. And these concentration gradients determine the diffusional flow.
For gas and porous catalyst systems slow reactions are influenced by 1 alone, in faster reactions
2 intrudes to slow the rate, then 3 and/or 4 enter the picture, 5 unlikely limits the overall rate.
Diffusion: Spontaneous mixing of atoms or molecules by random thermal motion.
External diffusion: Diffusion of the reactants or products between bulk fluid and the external
surface of the catalyst is known as external diffusion.
Internal diffusion: Diffusion of the reactants or products from the external pellet surface or to
the pore mouth to the interior of the pellet is known as internal diffusion.
When the reactants diffuse into the pores within the catalyst pellet, the concentration at the
pore mouth will be higher than that inside the pore and the whole catalytic surface is not
accessible to the same concentration. Reaction rate for the external diffusion as well as internal
diffusion is mainly depend on the thickness of the boundary layer.
Figure 19 External and internal diffusion of reactant to the catalyst particle
In the above figure, (CAb) is the concentration of reactant A in the bulk gas phase and (CAs) is
the concentration of reactant A in the catalyst surface.

24. 24
REFERENCES
REFERENCES OF SECTION-1
1. https://getrevising.co.uk/grids/catalysts
2. https://www.inkbottlepress.com/interesting/faq-what-is-a-catalyst-in-literature.html
3. https://en.wikipedia.org/wiki/Catalysis
4. https://www.legaladvantage.net/blog/overview-different-types-catalysts/
5. https://www.sciencedirect.com/topics/chemistry/catalyst-deactivation
6. Elements of Chemical Reaction Engineering H. SCOTT FOGLER Fifth Edition
7. Chemical Reaction Engineering by Octave Levenspiel Third Edition.
8. Introduction to chemical reaction engineering and kinetics by Ronald W. Missen, Charles A Mims
and Bradley A. Saville
9. Bartholomew, C. H. (2001). Mechanisms of catalyst deactivation. Applied Catalysis A: General,
212(1–2), 17–60. https://doi.org/10.1016/S0926-860X(00)00843-7
10. Bartholomew, C. H., & Farrauto, R. J. (2010). Catalyst Deactivation: Causes, Mechanisms, and
Treatment. Fundamentals of Industrial Catalytic Processes, 260–336.
https://doi.org/10.1002/9780471730071.ch5
11. https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527619474.ch7
12. https://www.peertechzpublications.com/articles/IJNNN-4-123.php
13. Industrial catalysis a practical approach by Jens Hagen second edition.
14. https://books.google.com.bd/books?id=nZhxBgAAQBAJ&redir_esc=y
15. https://www.catalystseurope.org/index.php/safety-and-regulation/catalyst-regeneration
REFERENCES OF SECTION-2
1. https://www.thoughtco.com/definition-of-heterogeneous-reaction-605207
2. Chemical Reaction Engineering by Octave Levenspiel Third Edition.
3. Elements of Chemical Reaction Engineering H. SCOTT FOGLER Fifth Edition
4. Introduction to chemical reaction engineering and kinetics by Ronald W. Missen, Charles
A. Mims and Bradley A. Saville
5. https://en.wikipedia.org/wiki/Countercurrent_exchange
6. https://www.youth4work.com/Talent/Chemical-Engineering/Forum/115149-what-is-the-
diff-between-counter-and-co-current-flow?yFast=On
7. https://www.intechopen.com/books/an-overview-of-heat-transfer-phenomena/two-phase-
flow