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CH7: Reaction Mechanisms and Pathways
RE7
Chemical Engineering Guy
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Chemical Reaction Engineering Methodology
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CH3: Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)
Chemical Reaction Engineering Methodology
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CH3: Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)
Introduction to Block RE7
• This is just an OVERVIEW
• This is typical Master or Special-Topic– This is typically studied by CHEMISTS not Engineers!
• There is a part on BioEngineering– This is also another subject for Master
– This is typical study for Biochemical Engineers
• Relax! Just know that this exists and read a little bit so you look like you know =)
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Content
• Section 1 Reaction Mechanisms– Active Intermediates
– Non-Elementary Rate Laws
– Mechanisms Proposal
– Chain Reactions
• Section 2 Reaction Pathways– Pathways
• Section 3 BioReactors and Enzymes– NOT INCLUDED
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Section 1
Reaction Mechanisms
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Revisiting Elementary Rate Law
• From Chapter 3 we got:
– Typical Elementary Rate Law
n·A Products
-rA = K·Can
• In which we studied 0, 1 and 2 typically
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Non-Elementary Rate Laws
• There are many reactions that does NOT follow the Elementary Rate Law!
• Examples…
• These generally involve a series of steps of Elementary Rate Laws!
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Active Intermediate
• A high-energy molecule that reacts virtually fast as it is formed:
A + M A* + M
• Where A* is an active intermediate, an unstable molecule
• Free radicals are an example of Active Intermediates!
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Pseudo-Steady-State Hypothesis (PSSH)
• By definition, the rate of reaction of an active intermediate is zero!
-rA*= 0
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PSSH in Azomethane
• We need to explain this second order with the reaction mechanism!
In theory… Applying Elementary Rate Law:
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PSSH in Azomethane
• We need to explain this second order with the reaction mechanism!
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PSSH in Azomethane
• Lets see the mechanism
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PSSH in Azomethane
• Applying
• Solve for Cazo*
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PSSH in Azomethane
• Substitute in Product
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PSSH in Azomethane
• Lets analyze the denominator…
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Apparent Order of Reactions
• In the last example
– At high concentrations the reaction seems to be of first order (with respect to CAZO)
– At low concentrations the reaction seems to be of second order with respect to CAZO
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Reaction Mechanism Steps
• Activation:
• Deactivation:
• Decomposition:
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Reaction Mechanism Proposal
• Typical concept
– Add a unit in the denominator
• We will explain a general methodology to propose a reaction mechanism
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Reaction Mechanism Methodology
1. Species having the concentration in the denominator are probably colliding with the active intermediate
A +A * collision product
2. If a constant appears in the denominator, one of the reactions steps is a decomposition of an intermediate
A* Decomposition product
3. Species having Concentration in the numerator, probably are the product of the active intermediate
Reactant A* + Products
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Reaction Mechanism Methodology
CH7: Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)
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Exercises
• Go to the website if you need more Study Material such as: problems, examples, exercises and more theory!
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Courses
Reactor Engineering
CH7: Mechanism and Pathways
Remember that this is only an OVERVIEW
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End of Topic
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Chain Reactions
• Chain Reactions consist of:1. Initiation
2. Propagation
3. Termination
• As the name implies, is a reaction that starts with one reaction (initiation) follows series of reaction (propagation) and then finishes (termination)
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Chain Reactions
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Chain Reactions
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Chain Reactions: Initiation
• Formation of the Active Intermediate
• Example:
– Ethane converts to 2 free radials
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Chain Reactions: Propagation
• Propagation: AKA chain transfer, is the interaction ofan active intermediate with the reactant or productto produce another active intermediate
• Example: the free radicals interact with other ethanemolecules
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Chain Reactions: Termination
• Termination: deactivation of the active intermediate to form products
• Example:
– The radical of 2C2H5 reacts with other molecule of the same kind to form butane
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Chain Reactions: Conclusion
• 2·C2H6 C4H10
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Exercises
• Go to the website if you need more Study Material such as: problems, examples, exercises and more theory!
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Courses
Reactor Engineering
CH7: Mechanism and Pathways
Remember that this is only an OVERVIEW
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Section 2
Reaction Pathways
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Reaction Pathways
• Typical for multiple reactions of different species
• Visual Map in which all reactions possible are displayed
• They are used to find the Key Reaction
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Reaction Pathways
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Reaction Pathways
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Example: Smog Formation
• The formation of Smog is very important due to its pollutant properties. Smog is composed mainly by exhaust gases…
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Example: Smog Formation
• A Free radical gets contact with NO
• NO2 is formed and decomposed by UV
• That Oxygen will react with O2 and form Ozone
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Example: Smog Formation
• Ozone may react with other olefins… which react to form Aldehyde and Free Radicals!
• These Aldehydes may react further to Free radicals (irritants to the eye) and
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Smog Pathway
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Smog Pathway: Conclusion
• Many byproducts are formed with only one molecule!
• NO2 is “regenerated” and can continue reacting much more
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End of Block RE7
• You know what is a reaction mechanism and why it is important to study it
• You know how to propose basic reaction mechanisms
• You can define an Active Intermediate
• You know the concept of the PSSH
– Pseudo Steady State Hypothesis
• You know what a Pathways
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Questions and Problems
• Go to the website if you need more Study Material such as: problems, examples, exercises and more theory!
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• Remember that this is only an OVERVIEW
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More Information…
• Get extra information here!
– Directly on the WebPage:
• www.ChemicalEngineeringGuy.com/courses
– FB page:
• www.facebook.com/Chemical.Engineering.Guy
– Contact me by e-mail:
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Text Book & Reference
Essentials of Chemical Reaction EngineeringH. Scott Fogler (1st Edition)
Chemical Reactor Analysis and Design FundamentalsJ.B. Rawlings and J.G.
Ekerdt (1st Edition)
Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)
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Bibliography
Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)
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We’ve seen CH7