Post on 30-Jan-2021
transcript
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
1
Dr.-Eng. Zayed Al-Hamamre
Advance Chemical Reaction Engineering
Catalysis and Catalytic Reactions
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
2
Content
Catalysts
Catalyst Structure and Properties
Adsorption to the Catalyst Surface
Catalyst Active Site
Steps in a Catalytic Reaction
Concentration Profiles
Adsorption Isotherms
Adsorption rate
Surface Reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
3
The first observed uses of catalysts were in the making of wine, cheese, and bread.
Catalysis is a term coined to describe the property of substances that affects the rate of a chemical reactions without being consumed in them.
Catalysts provide an alternate path for the reaction to occur
Catalysts
Catalyst can provide a lower barrier for the desired reaction, leaving the undesired reaction rate unchanged
The catalyst provides a lower energy barrier path
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
4
Catalysts can strongly regulate reactions because they are not consumed as the reaction proceeds.
A catalyst changes only the rate of a reaction; it does not affect the equilibrium.
Very small amounts of catalysts can have a profound effect on rates and selectivity's.
However, catalysts can undergo changes in activity and selectivity as the process proceeds.
They are subject to deactivation, which refers to the decline in a catalyst’s activity as time progresses.
Catalyst deactivation may be caused by
An aging phenomenon, such as a gradual change in surface crystal structure, or by
The deposit of a foreign material on active portions of the catalyst surface (poisoning or fouling of the catalyst).
Catalysts
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
5
Homogeneous catalysis concerns processes in which a catalyst is in solution with at least one of the reactants
Catalysts can be
Catalysts
A heterogeneous catalytic process involves more than one phase, usually the catalyst is a solid and the reactants and products are in liquid or gaseous form
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
6
Catalyst Structure and Properties Most solid catalysts are supplied as cylindrical
pellets with lengths and diameters in the range of 2–10 mm.
More complex shapes and monoliths can be used when it is important to minimize pressure drop.
The catalyst is micro-porous with pores ranging in diameter from a few angstroms to a few microns.
The pores may have a bimodal distribution of sizes
The internal surface area, accessible through the pores, is enormous, up to 2000m2 per gram of catalyst
A catalyst may consist of minute particles of an active material dispersed over a less active substance called a support
The active material is frequently a pure metal or metal alloy.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
7
Adsorption to the Catalyst Surface For gas-phase reactions catalyzed by solid surfaces, reactants must become attached
(adsorbed) to the surface.
This adsorption takes place by physical adsorption or chemisorption.
Physical adsorption is similar to condensation.
The process is exothermic,
the heat of adsorption is relatively small, being on the order of 1 to 15 kcal/g mol.
The forces of attraction between the gas molecules and the solid surface are weak.
The amount of gas physically adsorbed decreases rapidly with increasing temperature,
Above its critical temperature only very small amounts of a substance are physically adsorbed.
Physical adsorption
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
8
This type of adsorption affects the rate of a chemical reaction.
The adsorbed atoms or molecules are held to the surface by valence forces of the same type as those that occur between bonded atoms in molecules.
As a result the electronic structure of the chemisorbed molecule is perturbed significantly, causing it to be extremely reactive
Chemisorption
Chemisorption is an exothermic process,
The heats of adsorption are generally of the same magnitude as the heat of a chemical reaction (i.e., 10 to 100 kcal/g mol).
If a catalytic reaction involves chemisorption, it must be carried out within the temperature range where chemisorption of the reactants is appreciable
Interaction with the catalyst causes bonds of the adsorbed reactant to be stretched making them easier to break
Adsorption to the Catalyst Surface
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
9
Adsorption to the Catalyst Surface
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
10
A reaction is not catalyzed over the entire solid surface but only at certain active sites or centers.
Catalyst Active Site
An active site as a point on the catalyst surface that can form strong chemical bonds with an adsorbed atom or molecule
One parameter used to quantify the activity of a catalyst is the turnover frequency, N.
It is the number of molecules reacting per active site per second at the conditions of the experiment.
When a metal catalyst such as platinum is deposited on a support, the metal atoms are considered active sites. The dispersion, D, of the catalyst is the fraction of the metal atoms deposited that are on the surface.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
11
Catalyst Active Site
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
12
Catalyst Active Site
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
13
Catalyst Active Site
Turnover frequency f (TOR) an expression used to quantify the activity of a catalyst.
It is the number of molecules reacting per active site per second at the conditions of the experiment
When a metal catalyst such as platinum is deposited on a support, the metal atoms are considered active sites.
The dispersion, D, of the catalyst is the fraction of the metal atoms deposited that are on the surface.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
14
Example
The number of molecules reacting per active site per second
The dispersion, D, i.e., the fraction of the metal atoms deposited that are on the surface.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
15
Example Cont.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
16
Range of turnover frequencies as a function for different reactions and temperatures.
Catalyst Active Site
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
17
For H2 chemisorption on Pt
Dihydrogen is perhaps the most common probe molecule to measure the fraction of exposed metal atoms
Catalyst Active Site
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
18
Catalyst Active Site The dispersion of Pt, or the fraction of exposed metal atoms,
As a rule of thumb for spherical particles the Pt particle diameter is
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
19
Length scales in the reactor
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
20
Possible pore structures
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
21
Various forms of catalyst particles
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
22
Steps in a Catalytic Reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
23
Steps in a Catalytic Reaction
The overall rate of reaction is equal to the rate of the slowest step in the mechanism.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
24
Steps in a Catalytic Reaction
Concentration profile of a reacting species in the vicinity of a porous catalyst particle. Distances are not to scale.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
25
Steps in a Catalytic Reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
26
Steps in a Catalytic Reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
27
Reactant concentration profiles in x-direction perpendicular to the flow direction z expected for flow over porous catalyst pellets. External mass transfer and pore diffusion produce the reactant concentration profiles shown
Concentration Profiles
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
28
Concentration Profiles
Reactant concentration profiles around and within a porous catalyst pellet for the cases of reaction control, external mass transfer control, and pore diffusion control. Each of these situations leads to different reaction rate expressions.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
29
Diffusion from the Bulk to the External Transport
: The diffusion coefficient
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
30
The external resistance decreases as
The velocity across the pellet is Increased,
The particle size is decreased.
The boundary layer becomes smaller and the mass transfer coefficient (mass transfer rate) increases,
Diffusion from the Bulk to the External Transport
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
31
Reactant Concentration Profiles
krkr
kr
Reactant concentration profiles around a catalyst pellet for reaction control and for external mass transfer control
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
32
Internal Diffusion
For large pellets, it takes a long time for the reactant A to diffuse into interior compared to the time it takes for the reaction to occur on the interior surface
The reactant is only consumed n the exterior surface of the pellet and the catalyst near the center of the pellet wasted catalyst
For very small pellets it takes very little time lo diffuse into and out of the pellet interior and, as a result, internal (fusion no Longer limits the rate of reaction.
The rare of reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
33
Reactant concentration profiles within a porous catalyst pellet for situations where surface reaction controls and where pore diffusion controls the reactions.
Reactant Concentration Profiles
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
34
Adsorption Isotherms
The adsorption of A on a site S is represented by
The total molar concentration of active sites per unit mass of catalyst is equal to the number of active sites per unit mass divided by Avogadro's number
The molar concentration of vacant sites, is the number of vacant sites per unit mass of catalyst divided by Avogadro's number
In the absence of catalyst deactivation, we assume that the total concentration of active sites remains constant
Define
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
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
35
Adsorption Isotherms
The site balance
Adsorption of carbon monoxide
As molecules
Example
molecular or nondissociated adsorption
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
36
As oxygen and carbon atoms
Example Cont.
dissociative adsorption
Adsorption rate
i. Adsorption of carbon monoxide as molecules
Considered as an elementary reaction
The rate of attachment of to the active site on the surface is proportional to the number of collisions that these molecules make with a surface active site per sec.
The collision rate is, in turn, directly-proportional to the carbon monoxide partial pressure
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
37
Adsorption Rate
The net rate of adsorption
or
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
38
At equilibrium, the net rate of adsorption equals zero
Adsorption Rate
But the site balance
Then
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
39
Adsorption Rate
Langmuir isotherm for adsorption of molecular carbon monoxide
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
40
Adsorption Rate
ii. The isotherm for carbon monoxide adsorbing as atoms is derived
where
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
41
Adsorption Rate
the site balance
Then
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
42
Adsorption Rate
The adsorption isotherm of A in the presence of adsorbate B is given by
Show this
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
43
Surface Reaction1. The surface reaction may be a single-site mechanism in which1 only the site on which the
reactant is adsorbed is involved in the reaction.
Because in each step the reaction mechanism is elementary
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
44
2. The surface reaction may be a dual-site mechanism in which the adsorbed reactant interacts with another site (either unoccupied or occupied) to form the product
Surface Reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
45
3. Eley-Rideal: the reaction take place between an adsorbed molecule and a molecule in the gas phase
Surface Reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
46
Desorption
For the desorption of a species
The desorption step for C is just the reverse of the adsorption step for C
And
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
47
The Rate-Limiting Step
At steady state,
i.e. the rates of each of the three reaction steps in series (adsorption, surface reaction, and desorption) are equal.
The slowest step in the reaction mechanism represents the rate limiting or rate-controlling step.
The approach in determining catalytic and heterogeneous mechanisms is usually termed the Lungmuir-Hinshelwood approach
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
48
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step The decomposition of cumene to form benzene and propylene.
Sequence of steps in reaction-limited catalytic reaction
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
49
The decomposition of cumene is represented by a series of elementary reactions
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
1.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
50
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
Where
2.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
51
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
Propylene is not adsorbed on the surface
3.
But
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
52
At steady state, there is no accumulation of reacting species on the surface the rates of each step in the sequence are all equal:
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
If the Adsorption of Cumene Rate-Limiting
The reaction rate constant of this step (in this case kA) is small with respect to the specific rates of the other steps (in this case kS and kD), i.e.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
53
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
But
But
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
54
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
At equilibrium
Overall partial pressure equilibrium constant
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
55
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
And
: The change in the Gibbs free energy
The concentration of vacant sites, CV can be eliminated
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
56
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
Initially
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
57
Synthesizing a Rate Law, Mechanism, and Rate-Limiting StepIf the Surface Reaction Rate-Limiting
The rate of surface reaction is
kA is large by comparison when surface reaction is controlling
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
58
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
The concentration of vacant sites, CV can be eliminated
The initial rate is
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
59
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
At low partial pressures of curnene
At high partial pressures
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
60
If the Desorption of Benzene Rate-Limiting
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
Initially
the initial rate would be independent of the initial partial pressure of cumene
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
61
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step The experimental observations
The rate law derived by assuming that the surface reaction is rate-limiting agrees with the data
Actual initial rate as a function of partial pressure of cumene
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
62
If the feed contains inert, then it would not participate in the reaction but would occupy sites on the catalyst surface
Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step
The site balance becomes
Since the adsorption of the inert is at equilibrium, the concentration of sites occupied by the inert is
Then, the rate of reaction is
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
63
Temperature Dependence of the Rate Law
The specific reaction rate, k, will usually follow an Arrhenius temperature dependence and increase exponentially with temperature
The adsorption of all species on the surface is exothermic.
o The higher the temperature, the smaller the adsorption equilibrium constant. o As the temperature increases, KA and KB decrease resulting in less coverage of the surface
by A and B
o At high temperatures
For a surface-reaction-limited irreversible isomerization
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
64
Temperature Dependence of the Rate Law
Initial rare versus total pressure for various rate controlling steps
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
65
Design of Reactors for Gas-Solid Reactions For an ideal batch reactor, the differential form of the design equation for a heteriogeneous
reaction is
For a packed-bed reactor, the differential form of the design equation for a heterogeneous reaction is
The design equation for a perfectly mixed "fluidized" catalytic reactor can be replaced by that of a CSTR
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
66
Heterogeneous Data Analysis for Reactor Design
Data obtained form differential reactor for the Hydrogen and toluene reaction over a solid mineral catalyst containing clinoptilolite (a crystalline silica-alumina)
To design the PBR, the rate law must be determined
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
67
Dependence on the product methane
Heterogeneous Data Analysis for Reactor Design
If the methane were adsorbed on the surface, the partial pressure of methane would appear in the denominator of the rate expression and the rate would vary inversely with methane concentration:
From runs 1 and 2
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
68
Heterogeneous Data Analysis for Reactor Design
Dependence on the product benzene
In runs 3 and 4,
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
69
The rate decreases with increasing concentration of benzene. A rate expression in which the benzene partial pressure appears in the denominator could explain this dependency:
Heterogeneous Data Analysis for Reactor Design
Dependence on toluene
Runs 10 and 11 and runs 14 and 15
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
70
Heterogeneous Data Analysis for Reactor Design At low concentrations of toluene (runs 10 and 1 l), the rate increases with increasing partial
pressure of toluene, while at high toluene concentrations (runs 14 and 15), the rate is essentially independent of the toluene partial pressure. A form of the rate expression that would describe this behavior is
The rate law may be of the form
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
71
Dependence on hydrogen
Heterogeneous Data Analysis for Reactor Design
Runs 7, 8, and 9
The rate increases linearly with increasing hydrogen concentration and we conclude that the reaction is first-order in H2
hydrogen is either not adsorbed on the surface or
it’s coverage of the surface is extremely low
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
72
Heterogeneous Data Analysis for Reactor Design
Combining
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
73
Evaluation of the Rate Law Parameters
The multiple regression techniques can be used to evaluate the rate parameters.
One can use the linearized least squares analysis to obtain initial estimates of the parameters k, KT, KB , in order to obtain convergence in nonlinear regression.
Selecting specific experiments to evaluate the rate parameters.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
74
Example
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
75
Example Cont.
Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888
76
Example Cont.