Lecture 23
Chemical Reaction Engineering (CRE) is the
field that studies the rates and mechanisms of
chemical reactions and the design of the reactors in
which they take place.
Today’s lecture
 Interstage Cooling
 Noble Prize 2007
 Catalytic Steps
2
Catalysts and Catalysis
 A Catalyst is a substance that affects the rate of chemical
reaction but emerges from the process unchanged.
 Catalysis is the occurrence, study, and use of catalysts and
catalytic processes.
Approximately 1/3 of the GNP of materials produced in the
U.S. involves a catalytic process.
3
Catalysts affect both Selectivity and Yield
Different reaction Paths
4
Different shapes and sizes of catalyst.
5
Catalytic packed-bed reactor, schematic.
6
Steps in a Catalytic Reaction
7
Active Site
 Reactions are not catalyzed over the entire surface but only at
certain active sites or centers that result from unsaturated
atoms in the surface.
 An active site is a point on the surface that can form strong
chemical bonds with an adsorbed atom or molecule.
8
The Adsorption Step
Vacant and occupied sites
For the system shown, the total concentration of sites is
Ct = Cv + CA.S + CB.S
NS 
rAD
9
N S

C NS 
 k A PN C V 

KN 

Adsorption
A  S  A S
rAD  k A PA CV  CAS / k A 
@ equilibriu m : rAD  0
rAD / k A  0
CT  CV  CAS  CV  k A PA CV
10
C AS  k A PA C V
C AS  k A PA C V
More Catalysis
CT
CV 
1  k A PA C V
C T k A PA
C AS 
1  k A PA
C AS C T k A PA

C T 1  k A PA
11
More Catalysis
C AS
CT
Increasing T
Slope=kA
Langmuir Adsorption
Isotherm
PA
12
The Surface Reaction Step
13
The Surface Reaction Step
14
The Surface Reaction Step
15
The Surface Reaction Step
16
The Surface Reaction Step
17
Desorption from surface
Vacant and occupied sites
BS  B  S
18
Steps in a Catalytic Reaction
AB
Adsorption on Surface
Surface Reaction
Single Site
A S  BS
Dual Site
A S  S  BS  S
Desorption from Surface
19
A  S  A S
BS  B  S
The Rate Limiting Step:
Which Step Has the Largest Resistance?
Electrical analog to heterogeneous reactions
20
Collecting information for catalytic reactor design
21
Octane Numbers
 Catalytic Reforming
 Normal Pentane Octane Number = 62
 Iso-Pentane Octane Number = 95
22
23
Steps in a Catalytic Reaction
AB
Adsorption on Surface
Surface Reaction
Single Site
A S  BS
Dual Site
A S  S  BS  S
Desorption from Surface
24
A  S  A S
BS  B  S
Steps in a Catalytic Reaction
A B  CD
A  S  A S
Adsorption on Surface
B  S  BS
Surface Reaction Dual Site
A S  BS  CS  D S
A  S  B  S'  C  S  D  S'
25
Steps in a Catalytic Reaction
Surface Reaction
Eley-Rideal
A  S  Bg  C  S  Dg
Desorption form Surface
26
CS  C  S
DS  D  S
End of Lecture 23
27