Catalysis and Catalytic reactors RE10

CH10: Catalysis and Catalyst
RE10
Chemical Engineering Guy
www. Chemical Engineering Guy .com

Chemical Reaction Engineering Methodology
CH3: Elements of Chemical Reaction Engineering
H. Scott Fogler (4th Edition)

Content
• Section 1: Catalysts
– Definitions
– Hetero-Homogeneous Catalysts
– Catalyst Properties
– Classification
• Section 2: Catalytic Reactions
– Common Industrial Applications
• Section 3: Steps In Catalytic Reactions
– Theory and Steps
– Example of Cumene Degradation

Section 1
Catalysis Basic Concepts

Catalysis Use and Importance
• Wine, cheese and bread  Previous batch was needed for
the next one
• Major users
– Petroleum refining
– Chemical Processes
– Automotive
• 1/3 of chemical processes will need eventually the use of a
catalyst
• The global demand on catalysts in 2010 was estimated at
approximately 29.5 billions USD.
• Automotive and Chemical industry overall
– the global catalyst market is expected to experience fast growth
in the next years.

Definition of Catalyst
• A substance that affects the rate of a reaction but
emerges from the process unchanged
• A catalyst usually changes a reaction rate by
promoting a different molecular path ("mechanism")
for the reaction

Definition of Catalyst
• A catalyst changes only the rate of a reaction;
it does nor affect the equilibrium.
– That is, no higher conversion is achieved

Catalysts
• Usually for “Faster” reaction design
• Less Activation Energy/Less free energy is required to reach
the transition state
• but the total free energy from reactants to products does not
change
– That is, the change in enthalpy or enthalpy of reaction

Catalysts Free Energy Diagram

Catalysts
• As a catalyst is regenerated in a reaction, often
only small amounts are needed to increase
the rate of the reaction.
• In practice, however, catalysts are sometimes
consumed in secondary processes.
• There are many type of catalyst recovery due
to the high prices of catalysts

Inhibitor
• The opposite of a catalyst, a substance that
reduces the rate of a reaction, is an inhibitor.
• This is typical for enzymes as well

Typical Mechanism
• Typical mechanism
X + C → XC (1)
Y + XC → XYC (2)
XYC → CZ (3)
CZ → C + Z (4)
• Overall reaction:
X + Y → Z

Typical Mechanism

Types of Catalysts
• Homogeneous
• Heterogeneous
• Enzymes and biocatalysts

Homogeneous Reaction
• The processes use the catalyst is in solution
• Types
– Acid Catalysis
– Oraganometallic Catalysis
– Enzymatic reactions

Enzymes
• Enzymes possess properties of both:
– Homogeneous
– heterogeneous catalysts.
• As such, they are usually regarded as a third, separate
category of catalyst.
• Typical for Biotechnological Processes

Heterogeneous Reaction
• Heterogeneous: involves more than one
phase: usually the catalyst is a solid and the
reactants and products are in liquid or
gaseous form
• A heterogeneous catalytic reaction occurs at
or very near the fluid-solid interface
• Reactions between gases-Iiquids are usually
mass-transfer limited

Heterogenous Reaction Examples
• From mighty Wikipedia

Catalyst Properties
• A large interaction area is almost always
essential in attaining a significant reaction rate
• This is provided by an inner porous structure
– i.e., i solid contains many tine pores, and the
surface of these pores supports the a needed for
the high rate of reaction
• These Cat. Are called poro-catalyst

Porous Catalysts

Chemisorption
• Chemisorption results in the sharing of
electrons between the adsorbate and the
adsorbent.

Chemisorption
• Two step process:
1. Molecular adsorption, where the adsorbate
remains intact.
• Example is alkene binding by platinum.
• In dissociation adsorption: one or more bonds break
concomitantly with adsorption.
2. The barrier to dissociation affects the rate of
adsorption.
• An example of this the binding of H2, where the H-H
bond is broken upon adsorption.

Surface Reactions
• Langmuir-Hinshelwood mechanism.
• Rideal-Eley mechanism.
• Precursor mechanism.

Langmuir-Hinshelwood mechanism.
• The two molecules A and B both adsorb to the
surface.
• While adsorbed to the surface, the A and B "meet,”
and bond
• The new molecule A-B desorbs.

Rideal-Eley mechanism.
• One of the two molecules, A, adsorbs to the surface.
• The second molecule, B, meets A on the surface,
having never adsorbed to the surface, and they react
and bind.
• Then the newly formed A-B desorbs.

Langmui vs. Rideal Models

Catalyst Examples
• Naturals:
– Clays
– Zeolite
• Synthetics
– Crystalline aluminosillicates

Clays and Zeolites

Zeolite use in Para-Xylene
• Benzene and Toluene enter the zeolite
• They both react to form a mix of ortho, para
metha xylene
• The size of the mouth only accepts p-xylene
going out

Zeolite use in Para-Xylene
• Many interior sites isomerize ortho and metha
xylene to para-xylene
• High selectivity of para-xylene

Synthetic Catalysts
• Aluminosillicates

Supported catalysts
• Finely, minute, pulverized catalyst (active material)
• It is dispersed on a less reactive material
– Support
• Promoters  small amounts of material that
increases the activity of the catalyst

Carbon-Supported Pt Catalyst

Catalysis+Support and Selectivity

Preparation of Catalyst+Support

Supported Catalysts: Examples
• Packed-Bed Catalytic converter of the auto
• Platinum-alumina for petroleum reformation
• Vanadium Pentoxide on silica for sulfuric acid
production

Deactivation
• Decline on catalyst’s
activity with time
– Aging phenomena
• gradual change in structure
– Poisoning
• irreversible deposition of
substances on the active sites
– Fouling/coking
• carbonous deposition on all
the entire surface

Deactivation by Sintering (Aging)
• Loss of catalytic activity due to a loss of active
surface area (due to high gas-phase
temperatures)
– Crystal agglomeration and growth of the metals
deposited on the support
– Loss of activity by narrowing or closing of the
pores inside the catalyst pellet.

Deactivation by Sintering (Aging)
• A change in the surface structure
– Recrystallization
– Formation or elimination of surface defects
• Sintering is usually negligible at temperatures below 40% of
the melting temperature of the solid

Deactivation by Coking or Fouling
• This mechanism of decay is common to
reactions involving hydrocarbon.
• It results from a carbonaceous (coke) material
being deposited on the surface of a catalyst.

Deactivation by Coking or Fouling
• When the catalyst is already Fouled or coked,
the material is normally called “spent catalyst”

Deactivation by Poisoning
• Poisoning molecules become irreversibly
chemisorbed to active sites
• This reduce the number of sites available for
the main reaction.
• Normally is done by impurities

• Petroleum feed stocks contain trace
impurities such as:
– sulfur, lead, and other components which are too
costly to remove.

Section 2
Catalytic Reactions

Basic Industrial Classification
• Alkylation and Dealkylation Reactions
• Isomerization Reactions
• Hydrogenation and Dehydrogenation
Reactions
• Oxidation Reactions
• Hydration and Dehydration Reactions.
• Halogenation and DehaIogenation Reactions.

Alkylation and Dealkylation Reactions
• Alkylation  addition of an alkyl group to an
organic compound
• Common catalyst: Friedel-Crafts AlCl3 + HCl

Alkylation and Dealkylation Reactions
• Dealkylation  Cracking of petrochemicals
• Common catalyst: silica-alumina; silica-
magnesia and even clays (montmorilonite)

Isomerization Reactions
• Change of Structure
• Hydrocarbon molecules are rearranged into a
more useful isomer
• The process is particularly useful in enhancing
the octane rating of petrol, as branched
alkanes burn more efficiently in a car engine
than straight-chain alkanes.

Hydrogenation and Dehydrogenation
Reactions
• Some metals used in Hydrogenation are:
– Co, Ni, Rh, Ru, Os, Pd, Ir, and Pt.
• Non-used metals:
– V, Cr, Nb, Mo, Ta, and W
– each of which has a large number of vacant d-
orbitals
– These are inactive as a result of the strong
adsorption for the reactants or the products or
both

Hydrogenation and Dehydrogenation
Reactions
• Hydrogenation reactions are favored at lower
temperatures (<200ºC)
• Dehydrogenation reactions are favored at high
temperatures (at least 200ºC)
• Example:
– Industrial butadiene (synthetic rubber production)
can be obtained by the dehydrogenation of
butenes

Hydrogenation and Dehydrogenation Reactions

Oxidation Reactions
• The transition group elements
(group VIII) and subgroup are used
extensively in oxidation reactions:
– Ag, Cu, Pt, Fe, Ni
• In addition, V2O5 and MnO2 are
frequently used for oxidation
reactions

Oxidation Reactions Types
• Oxygen Addition
• Oxygenolysis of carbon-hydrogen bonds

Oxidation Reactions Types
• Oxygenation of nitrogen-hydrogen bonds:
• Complete combustion

Hydration and Dehydration Reactions.
• Used to get rid of H2O molecules
• Hydration and dehydration catalysts have a strong
affinity for water
• One such catalyst is AI2O3, which is used in the
dehydration of alcohols to form olefins

Hydration and Dehydration Reactions.
• Other examples:
– Clays
– Phosphoric acid
– Phosphoric acid salts on inert carriers

HaIogenation and DehaIogenation
Reactions.
• Addition of Halogens Elements (group 7)
• Cl, F, Br, I, etc…
• Typical catalysts: CuCI2, AgCI. Pd, AlBr3, CCl4

HaIogenation and DehaIogenation
Reactions.

Catalyst Classification Summary

Section 3
Steps in a Catalytic Reaction

List of Steps in a Typical Heterogeneous
Catalytic Reaction
1. Mass transfer (diffusion) of the reactant(s) 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 to the immediate vicinity of the internal
catalytic surface
3. Adsorption of reactant A onto the catalyst surface
4. Reaction on the surface of the catalyst AB
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

List of Steps in a Typical
Heterogeneous Catalytic Reaction
1. External diffusion of reactant
2. Internal Diffusion of reactant
3. Adsorption of reactant A
4. Reaction on the surface of the catalyst AB
5. Desorption of the products from the surface
6. Internal diffusion of products
7. External diffusion of products

Overall Rate of Reaction
• Typically, is related to the rate of the slowest step in the
mechanism
• Classification of steps
– Mass Transfer related steps (1,2,6 and 7)
– Reaction Kinetic related steps (3,4 and 5)

Overall Rate of Reaction
• Then there are two cases
– Mass Transfer limitations
– Reaction Kinetic/Chemisorption limitations
• When the diffusion steps:
• If (1,2,6 and 7) are very fast vs. with the steps (3, 4
and 5)
– Transport or diffusion steps do not affect the overall rate
of the reaction.

Focus on Reactor Engineering
• We will focus in the actual Reaction
– Steps 3,4 and 5
– Adsorption, Surface Reaction, Desorption
• Mass Transfer phenomena limitations are more
commonly studied in other courses

Step 1: External Diffusion
• The reactant will diffuse to the “bulk” material
• The surface of the boundary layer is the one
with most resistance
• Lets call CAb to the concentration of reactant
A in the bulk

Step 1: External Diffusion
• Let Kc be the mass transfer coefficient
• Kc is function of Diffusion of A in B and the film length
• If diffusion of A in B is low and distance is large… you
have a slow coefficient and therefore a slow reaction
• At fast velocities  low length
• At low velocities  high length

Step 2: Internal diffusion
• Once the particle is “inside”, it must achieve
the activation site
• Suppose it diffuses to a Concentration of CAs
• Kr is dependent only of the particle diameter
Kr = 1/ Dp

Step 2: Internal diffusion
• The bigger the particle, the larger the path needed!

Step 3: Adsorption
• S  Active Site
• Two models of Adsorption
– Molecular/non-dissociated adsorption
– Dissociative adsorption

Step 3: Adsorption
• Rate of Attachment vs. Detachment
• Combining Both Rates:

Step 3: Adsorption
• If we apply the KA ratio (adsorption equilibrium)
• We will get:

Step 3: Adsorption
• Applying an Active site Balance:
• In equilibrium, the rate should be equal to 0
• Solving for CCO·S
• And rearranging…

Step 3: Adsorption
• This expression is generally called Langmuir
Isotherm

Step 4: Surface Reaction
• Recall that:
• After the reactant is absorbed, it may react in
the next ways:
– Single Site
– Dual Site
– Eley-Rideal

• Single Site
• Each step is elementary reaction
• The reaction occurs directly on-site
• The model is left as:
• Where Ks is the surface reaction constant
Ks = ks/k-s

• Dual-Site
• A reacts with B in the adjacent site
• This type of reactions are the so called
Langmuir-Hinshelwood kinetic model

• Eley-Rideal Mechanism Reaction
• Similar to Langmuir but only requires 1 site

Step 5: Desorption
• Let C be the product and S the Active Site
• Desorption to the gas phase… The rate of
reaction can be modeled with
• Let KDC be the equilibrium constant

Step 5: Desorption
• It is just the opposite (negative sign)
• For the Equilibrium Constant then:
• Therefore:

Step 6: Internal Diffusion of Products
• Similar to step 2
• The length of the particle is still a factor

Step 7: External Diffusion of Products
• Similar to step 1
• The diffusion factor of C in B is now the factor
– That is, now we are concerned with the product
rather than with the reactant

Summary of Rates
• Step 1
• Step 2
• Step 3
• Step 4
• Step 5
• Step 6
• Step 7
Mass Transfer Rates
Mass Transfer Phenomena
Adsorption Rates
 Chemisorption relevant
Rate of Reaction
 kinetic relevant

Application to Cummene Decomposition
• Not diffusion-limited
• Product: Benzene and Propylene
• Catalyst: Platinum bed
• Application of Langmuir Mechanism

Application to Cummene Decomposition
• Each step is treated as an elementary reaction
• Due to gas-phase
– We will use Partial Pressures
– Remember Concentration may be related to
Partial pressure: Pc = = Cc RT

Application to Cummene
Decomposition

Application to Cummene Decomposition:
Adsorption
• Adsorption of cummene in the Pt-bed

Visual Aid

Surface-Reaction
• The rate Law for the surface reaction step
producing adsorbed benzene and propylene in
the gas phase.
• Using the surface criterion equilibrium
• Propylene is not adsorbed on the surface.
Consequently, its concentration on the surface
is zero being

Surface-Reaction

Desorption

Rate-Limiting Step
• Typically, you would search for the rate-limiting step:
– Rate of Absorption
– Rate of Surface-Reaction
– Rate of Desorption

End of Block RE10
• By now you should know:
– Definition of a catalysis and a catalyst
– Importance of the Catalyst Industry
– What is an inhibitor
– Type of Catalytic Reactions (homo and
heterogeneous)
– The importance of chemisorption
– Basic Reaction Mechanisms such as: Langmuir
Models and Eley-Rideal Models

End of Block RE10
• You now know:
– The Importance of the Supported Catalysts
– Why deactivation occurs and its types (aging,
coking, poisoning)
– Common Industrial Processes and the type of
catalysts they use
– The basic steps of the Catalytic Reaction (7)
– What a limiting step is
– How to model a basic catalytic reaction
mechanism

Questions and Problems
• I included some extra problems and exercises
• All problems are solved in the next webpage
– www.ChemicalEngineeringGuy.com
• Courses
–Reactor Engineering
»Solved Problems Section
• CH10 – Catalysis and Catalytic Reactors

More Information…
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Text Book & Reference
Essentials of Chemical
Reaction Engineering
H. Scott Fogler (1st Edition)
Chemical Reactor
Analysis and Design
Fundamentals
J.B. Rawlings and J.G.
Ekerdt (1st Edition)
Elements of Chemical
Reaction Engineering

Bibliography
Elements of Chemical Reaction Engineering
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Catalysis and Catalytic reactors RE10

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