| Select Chapter >> | TOC | Preface | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | Appendices |
Chapter 10: Catalysis and Catalytic Reactors
Topics
- Octane Rating
- Steps in Catalytical Reaction
- Rate Limiting Step
- Regulation for Automotive Exhaust Emissions
- Chemical Vapor Deposition
- Types of Catalyst Deactivation
- Temperature-Time Trajectories
- Moving Bed Reactors & Straight Through Transport Reactors
| Octane Rating | top |
A Typical Engine Piston
|
|
Determine the compression ratio, CR, The more compact molecules are (for a given |
 
|
to further explanation on octane rating
English |
|
| Espanol | |
| Svenska |
Molecular Adsorption
|
At equilibrium: |
|
Langmuir Isotherms
|
Dissociative Adsorption
|
At equilibrium: |
|
|
| Steps in a Catalytic Reaction | top |
|
|||
Adsorption on Surface |
|
||
Surface Reaction |
Single Site |
|
|
Dual Site |
|
||
Desorption from Surface |
|
||
|
||
Adsorption on Surface |
|
|
|
||
Surface Reaction |
Dual Site |
|
|
||
Eley-Rideal |
|
|
Desorption from Surface |
|
|
|
||
Example: Catalytic Reaction to Improve the Octane Number of Gasoline
|
||
Rationale: |
n-pentane: Octane No. = 62 |
|
The difference in octane ratings provides |
||
Steps in this reaction: |
||
|
||
Focusing on the second reaction: |
||
|
||
| Rate Limiting Steps | top |
Adsorption |
|
|
Surface Reaction |
|
|
Desorption |
|
|
Assume surface reaction is rate limiting |
|
|
If the surface reaction is limiting then: |
||
|
see also stirctly speaking link |
 |
see also strictly speaking link |
|
 |
||
Site balance: |
|
|
Substituting for CN-S, CI-S, and CV into CT = CV (1 + KN PN + KI PI) : |
||
|
||
where KP is the thermodynamic equilibrium constant for the reactor. |
||
Linearizing the Initial Rate: |
||
 
|
on setting rad/ka=0
| Single site | |||
| A) | 
|
|
 |
| Dual Site | |||
| B) | 
|
|
|
| C) | 
|
|
|
| Eley-Rideal | |||
| D) | 
|
|
|
 |
| Regulations for Automotive Exhaust Emissions | top |
|
|
|
|
| Principle Reactions: | |
|
|
|
|
|
|
|
|
| Surface reaction limiting: | |
|
|
|
|
|
|
|
|
|
|
Example
| Let's see what fraction of sites are covered by CO at the optimum: | |
|
|
| Multiplying by CV: | |
|
|
| (A) | 
|
| (B) | 
|
|
|
Dimethyl Ether Examples
Example Exam Questions
| Chemical Vapor Deposition | top |
Manufacturing of a Silicon Layer
 |
We see that a number of the key steps in the microelectronic
fabrication involve CVD, we shall consider the CVD of silicon.
| I Mechanism | ||
(1) 
|
 |
|
(2) 
|
 |
|
(3) 
|
 |
|
|
||
| II Rate Limiting Step (Reaction 3) | ||
| rdep=rs=ksfSiH2 | ||
| III Expressing fSiH2 in Terms of Partial Pressures | ||
|
||
|
||
| IV Site / Surface Area Balance: | ||
|
||
 |
 |
|
| For the homogeneous reaction: | ||
| then | 
|
|
| where: | 
|
|
|
||
| Types of Catalyst Deactivation | top |
| Separable Kinetics: |  |
Types of Decay
| 1.) Sintering |  |
|
| 2.) Coking |  |
|
| 3.) Poisoning
|
 |
|
| 4.) Slow Decay | Temperature-Time Trajectories | |
| 5.) Moderate Decay | Moving Bed | |
| 6.) Rapid Decay | STTR |
| Temperature-Time Trajectories | top |
The catalyst decay rate is a function of temperature, so you can vary the temperature with time to keep the rate of decay as constant as possible.
 |
|
| Then: | |
 |
|
 |
|
or solving for
|
|
 |
| Moving Bed Reactors & Straight Through Transport Reactors | top |
Catalyst Decay Example
The gas-phase, irreversible reaction
is elementary
with first order decay. The reaction is carried out at constant temperature and
pressure.
| Batch Reactor | Moving Bed Reactor | Straight Through Transport Reactor |
|
|---|---|---|---|
| Mole Balance: |  |
 |
 |
| Rate Law: |  |
 |
 |
| Decay Law: |  |
 |
 |
 |
 |
 |
|
 |
 |
||
 |
 |
||
| Stoichiometry: | gas phase, but
 , T = T0, and P = P0 |
||
 |
 |
 |
|
| Combine: |  |
 |
 |
 |
 |
 |
|
 |
 |
||
 |
 |
 |
|
 |
 |
 |
|
Another Catalyst Decay Example
The second-order, irreversible reaction
is carried out
in a moving bed reactor. The catalyst loading rate is 1 kg/s to a reactor
containg 10 kg of catalyst. The rate of decay is second order in activity and
first order in concentration for the product, B, which poisons the catalyst.
Plot the conversion and activity as a function of catalyst weight down the
reactor.
Additional information:
 |
 |
 |
 |
Solution:
| Polymath | ||
|---|---|---|
| Mole Balance: |  |
|
| Rate Law: |  |
|
| Decay Law: |  |
|
 |
||
 |
||
| Stoichiometry: |  |
|
 |
||
| Combine: |  |
|
 |
||
 |
||
 |
||
| Conversion vs. Catalyst Weight |
 |
| Catalyst Activity vs. Catalyst Weight |
 |
Example 10-7: Strictly Speaking
| When there is a change in the velocity due to a change in the number of moles up through the STTR, one cannot directly substitute t = z/U in the coking activity equation: | ||
 |
(1) | |
| Instead, one must add another equation to the Polymath program. We know that at any location, the gas velocity up the column is: | ||
 |
(2) | |
| Then: | ||
 |
(3) | |
| where t = 0 at z = 0. You can use either Polymath or MatLab to solve this equation and substitute it for t in the activity equation: |
||
|
|
||
| Along with: | ||
 |
||
|
|
||
 |
||
| etc. (same as the program in Table E10-7.1) |
||
Finding the rate law and mechanism for
Finding the rate law and mechanism for A+B<=>C+D
CO + NO; -r'CO = f(PCO , PNO , PN2 , PCO2)
Calculating Fractional Coverage
Decaying Catalyst in a Batch Reactor
Moving Bed Reactor
Catalyst Decay in a Packed Bed
Object Assessment of Chapter 10 * All chapter references are for the 4th Edition of the text Elements of Chemical Reaction Engineering .