Chemical Reaction Engineering

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Chemical Reaction Engineering
Lecture 9
Lecturer ???
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This course focuses on catalysis and catalytic
reactors.
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Catalysts

• Early example of catalysts
• wine, cheese and bread production
• Berzelius (1835)
• initially suggested that small amount of a
  foreign source could affect the course of
  chemical reactions.
• This force is called catalytic.
• Ostwald (1894)
• Catalysts are substances that accelerate the rate
  of chemical reactions without being consumed.

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Catalyst

• A catalyst affects the rate of a reaction but the
  process unchanged.
• A catalyst usually changes a reaction rate by
  promoting a different molecular path/mechanism
  for the reaction.
• A catalyst makes it possible to obtain an end
  product by a different pathway, it can affect
  both the yield and the selectivity.
• A catalyst changes only the rate of a reaction
  it does not affect the equilibrium.

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Homogeneous and Heterogeneous

• Homogeneous catalysis concerns processes in which
  a catalyst is in solution with at least one of
  the reactants.
• A heterogeneous catalytic processes involves more
  than one phase usually the catalyst is a solid
  and the reactants and products are in liquid or
  gaseous form.
• Heterogeneous catalysis is the more common type
  and has a major advantage
• Simple and complete separation of the fluid
  product mixture from the solid catalyst (Many
  catalysts are quite valuable)
• Only heterogeneous catalysts will be studied here.

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Catalyst types

• Porous
• Molecular sieves
• Monolithic
• Supported
• Unsupported

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Catalyst
catalyst reaction
fluid-solid interface
reaction rate
interfical area
Many catalysts are provided by porous structures
(called porous catalyst).
A typical silica-alumina cracking catalyst has a
pore volume of 0.6 cm3/g and an average pore
radius of 4 nm. The corresponding surface area is
300 m2/g.
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• Sometimes pores are so small that they will admit
  small molecules but prevent large ones from
  entering. Materials with this type of pore are
  called moleular sieves.
• Example zeolite
• They are quite selective catalysts.
• The pores can control the residence time of
  various molecules near the catalytically active
  surface.
• May allow only the desired molecules to react.

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• Some catalysts are sufficiently active that the
  effort to create a porous catalyst would be
  wasted. Monolithic catalysts are encountered in
  processes where pressure drop and heat removal
  are major considerations.
• Example platinum gauze reactor used in the
  ammonia oxidation portion of nitric acid
  manufacture
• Example catalytic converters used to oxidize
  pollutants in automobile exhaust

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Supported and unsupported catalysts

• A catalyst consist of minute active particles
  (frequenly a pure metal or metal alloy) dispersed
  over a less active substance (support), such a
  catalyst is called a supported catalyst.
• Unsupported catalysts have active ingredients as
  the major part. Other substances are promoters,
  which increase the activity.

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The deactivation of catalysts

• Deactivation the decline in a catalysts
  activity as time progresses.
• By aging phenomenon
• gradual change in surface crystal structure
• By poisoning phenomenon
• deposit of a foreign material on active portions
  of the catalyst surface
• example coking in the catalytic cracking of
  petroleum naphthas

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Micoscopic view of catalytic rxns
Focus on gas-phase reactions catalyzed by solid
surfaces
adsorption
Adsorption takes place by two different
processes physical adsorption and chemisortion
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Adsorption

• Physical adsorption is similar to condensation
• exothermic (low heat of adsorption 1 15
  kcal/g mol)
• weak attraction forces van der Waals forces
• amount of adsorption decreases with increasing
  temperature
• Chemisorption affects the rate of a chemcial
  reaction
• strong attaching force valence forces (same
  type as the force between bonded atoms in
  molecules)
• exothermic (heat of adsorption is as high as a
  chemical reaction 10 100 kcal/g mol)
• the reaction must be carried out within the
  temperature range where chemisorption of the
  reactants is appreciable.

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• Active site is a point on the catalyst surface
  that can form strong chemical bonds with an
  adsorbed atom or molecule.
• Turnover frequency is used to quantify the
  activity of a catalyst and defined as the number
  of molecules reacting per active site per second
  at the conditions of the experiment (i.e.
  molecules/sites)
• Dispersion is fraction of the catalytic atoms
  deposited that are on the surface (i.e. active
  sites)

Fisher - Tropsch Sythesis
Catalyst 0.5 wt Ru on ?-Al2O3.
The catalyst dispersion percentage of atoms
exposed, determined from hydrogen chemisorption,
was found to be 49. At a pressure of 988 kPa and
a temperature of 475 K, a turnover frequency of
0.044 s-1 was reported for methane. What is the
rate of formation of methane in mol/s.g of
catalyst ?
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Catalysts used in different classes of reactions

• Alkylation reactions
• alkylation is the addition of an alkyl group to
  an organic compound, example
• frequently used catalysts
• Friedel-Crafts catalyst - AlCl3 trace HCl
• Pd

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• Dealkylation reactions
• cracking of petroleum products
• frequently used catalysts
• Silica-alumina, Silica-magnesia
• Clay (montmorillonite)
• AlCl3
• Pd
• Isomerization reactions
• conversion of normal hydrocarbon chains to
  branched chains, which have higher octane number
• frequently used catalysts
• Acid-promoted Al2O3, Pt/Al2O3
• AlCl3
• Zeolites

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• Hydrogenation and Dehydrogen reactions
• Low temperatures for hydrogenation and high
  temperatures (at least 600 C) for
  dehydrogenation.
• The bonding strength between hydrogen and metal
  surface increases with an increase in vacant
  d-orbitals.
• Maximum in catalytic activity occurs when there
  is approximately one vacant d-orbital per atom
• frequently used metal catalysts
• Co, Ni, Rh, Ru, Os, Pd, Ir, and Pt.
• MoO2 and Cr2O3
• Too strong bonding (a large number of vacant
  d-orbitals)
• relatively inactive, because of the strong
  adsorption for the reactants or the products or
  both
• V, Cr, Mo, Ta, and W

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• Oxidation reactions
• Frequently used catalysts
• Transition group element (group VIII) and
  subgroup I
• Ag, Cu, Pt, Fe, Ni, and their oxides
• V2O5 and MnO2
• principal types
• Oxygen addition
• Oxygenolysis of carbon-hydrogen bonds
• Oxygenation of nitro-hydrogen bonds
• Complete combustion

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• Hydration and dehydration reactions
• Catalysts need to have strong affinity for water
• frequently used catalysts
• Al2O3 - dehydration of alcohols to form olefins
• MgO
• Silica-alumina gels
• Phosphoric acid
• Phosphoric acid salts on inert carriers
• Clay (montmorillonite)
• Example

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• Halogenation and dehalogenation reactions
• Catalysts may not needed
• However, catalysts can increase the product
  selectivity and decrease the operating
  temperature
• frequently used catalysts
• CuCl2
• AgCl
• Pd

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Steps in catalytic reaction
Page 598

• Mass transfer (External diffusion)
• reactants external surface of the
  catalyst pellet
• Internal diffusion
• reactant from the pore mouth through the catalyst
  pores to the immediate vicinity of the internal
  catalytic surface
• Adsorption
• reactant attaches onto the catalyst surface
• Reaction
• Desorption
• product leaves from the catalyst surface

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The overall rate of reaction is equal to the rate
of the slowest step in the mechanism.
In this course, we assume that the diffusion
steps are very fast, such that the overall
reaction rate is not affected by mass transfer in
any fashion. We will focus on the adsorption,
surface reaction and desorption steps.
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Development of rate laws in heterogeneous
catalysis

• Seldom follow power law models
• Algorithm for the development
• postulate catalytic mechanisms
• (typically three steps) adsorption, surface
  reaction, desorption
• one of which is usually rate-limiting
• derive rate laws for the various mechanisms

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Adsorption
S represents an active site (vacant site , with
no atom, molecule, or complex adsorbed on it)
B
A
When species A is adsorbed on the site S
Site balance equation
Total molar concentration of active sites per
unit mass of catalyst
Molar concentration of vacant sites per unit mass
of catalyst
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Adsorption isotherms

• Adsorption data are frequently reported in the
  form of adsorption isotherms.
• Isotherms portray the amount of a gas adsorbed on
  a solid at different pressures, but at one
  temperature.

Postulate model(s)
Experimental data
Fit ?
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Postulated model
For carbon monoxide on metal, two models may be
proposed
(1) molecular or nondissociated adsorption
(2) Dissociated adsorption
Which one is the correct model?
Ans depends on the surface
C
O
O
C
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Carbon monoxide molecular adsorption
The carbon monoxide does not react further after
being absorded
It is treated as an elementary reaction and
Proportional to the number of collisions between
molecules and the surface per unit time
The net rate of adsorption
adsorption equilibrium constant
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Molecular adsorption equilibrium constant
Virtually independent of temperature
Increase exponentially with increasing temperature
When CO is the only material adsorbed on the
catalyst
At equilibrium, rAD 0
CCOS f (PCO), an equation of adsorption
isotherm Langmuir isotherm
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Langmuir isotherm
CCOS
PCO
Check if Langmuir isotherm ?
PCO/CCOS
linear ?
PCO
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Carbon monoxide dissociative adsorption
The carbon monoxide does not react further after
being absorded
Adsorption (1) Two adjacent vacant active sites
are required rather than the single site needed
when a substance adsorbs in its molecular
form. (2) The probability of two vacant sites
occuring adjacent to one another is proportional
to the square of the concentration of vacant site.
Desorption (1) Two occupied sites must be
adjacent
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The net rate of adsorption
Dissociated adsorption equilibrium constant
Increase exponentially with increasing temperature
Increase exponentially with increasing temperature
decrease with increasing temperature
When CO is the only material adsorbed on the
catalyst
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At equilibrium, rAD 0
for
PCO1/2/COS
linear ?
PCO1/2
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More than one substance...

• Same principle as the single substance
• When the adsorption/desorption of both A and B
  are first order processes, and both A and B are
  adsorbed as molecules

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Surface reaction

• Once a reactant has been adsorbed onto the
  surface, it is capable of reacting in a number of
  ways
• Single - site
• only the site on which the reactant is adsorbed
  is involved in the reaction
• for example isomerize, decompose
• rate law

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• dual - site
• the adsorbed reactant interacts with another site
• 1st mechanism react with a vacant site
• rate law
• 2nd mechanism reaction between two absorbed
  species
• rate law
• 3rd mechanism reaction of two species adsorbed
  on different types of sites S and S
• rate law

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Reactions involving either single - site or dual
- site mechanisms are referred to as following
Langmuir - Hinshelwood kinetics

• single - site and non-adsorbed molecule
• the adsorbed molecule interacts with a molecule
  in the gas phase
• rate law

Reactions involving single - site and non -
adsorbed molecule is referred to as following
Eley - Rideal mechanism
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Desorption
The products of the surface reaction adsorbed on
the surface are subsequently desorbed into the
gas phase.
C
When a species CS is desorbed from a site
Desorption rate law
Desorption step for C is the reverse of the
adsorption step for C
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Rate-limiting step

• At steady state
• However, one particular step in the series
  (adsorption - surface reaction - desorption) is
  usually found to be rate-limiting or
  rate-controlling
• Langmuir - Hinshelwood approach is usually
  applied to determine catalytic and heterogeneous
  mechanisms.

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Langmuir - Hinshelwood approach example
The synthesis of ammonia from hydrogen and
nitrogen,
rapid
mechanisms
rate - limiting (dissociative adsorption)
rapid
rapid
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Two noxious automobile exhaust product react to
form environmentally acceptable products
mechanisms
rapid
rate - limiting (surface reaction)
rapid
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Development of rate laws for catalytic reactions
that are not diffusion-limited
Example the decomposition of cumene
mechanisms
Benzene adsorption equilibrium constant
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Whats next?
Assume one of the steps to be rate - limiting
Formulate the reaction rate law in terms of the
partial pressure of the species present
Determine the variation of the initial reaction
rate with the initial total pressure
Compare with the experimental observation
No
Yes
END
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The adsorption of cumene rate - limiting
cannot be measured and must be replaced!
Steady state
Set as 0
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Initial rate (PP PB 0)
-rC0
linear ?
PC0
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The surface reacting rate - limiting
cannot be measured and must be replaced!
Steady state
Set as 0
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(No Transcript)
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Initial rate (PP PB 0)
-rC0
low initial partial pressure of cemene
PC0
high initial partial pressure of cemene
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The desorption of benzene rate - limiting
cannot be measured and must be replaced!
Steady state
Set as 0
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(No Transcript)
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Initial rate (PP PB 0)
-rC0
PC0
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Final decision...
From experimental observation
Therefore, cumene decomposition is
surface-reaction-limited.
In fact, more than 75 of all heterogeneous
reactions that are not diffusion-limited are
surface-reaction-limited.
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Adding of inert?
What would be affected?
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Another example isomerize straight-chain
hydrocarbon molecules to increase the octane
number (i.e. the reforming process)
We will study this part.
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Isomerization of n-pentene to i-pentene
Select rate-limiting step
Elimination
Propose mechanisms
This does not fit the experimental
observation. The results obtained from the
selection of the other steps as the rate-limiting
steps are not fit, either.
Propose other reaction mechanisms !!
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Isomerization of n-pentene to i-pentene
Select rate-limiting step
Elimination
Propose other mechanisms
Fit the experimental observation !