Mechanisms and Pathways RE7

CH7: Reaction Mechanisms and Pathways

RE7

Chemical Engineering Guy

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Chemical Reaction Engineering Methodology


CH3: Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)

Chemical Reaction Engineering Methodology



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 =)


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


Section 1

Reaction Mechanisms


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


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!


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!


Pseudo-Steady-State Hypothesis (PSSH)

• By definition, the rate of reaction of an active intermediate is zero!

-rA*= 0


PSSH in Azomethane

• We need to explain this second order with the reaction mechanism!

In theory… Applying Elementary Rate Law:


PSSH in Azomethane

• We need to explain this second order with the reaction mechanism!


PSSH in Azomethane

• Lets see the mechanism


PSSH in Azomethane

• Applying

• Solve for Cazo*


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PSSH in Azomethane

• Substitute in Product


PSSH in Azomethane

• Lets analyze the denominator…


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


Reaction Mechanism Steps

• Activation:

• Deactivation:

• Decomposition:


Reaction Mechanism Proposal

• Typical concept

– Add a unit in the denominator

• We will explain a general methodology to propose a reaction mechanism


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


Reaction Mechanism Methodology



Exercises

• Go to the website if you need more Study Material such as: problems, examples, exercises and more theory!

www.ChemicalEngineeringGuy.com

Courses

Reactor Engineering

CH7: Mechanism and Pathways

Remember that this is only an OVERVIEW


http://www.ChemicalEngineeringGuy.com

End of Topic


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)


Chain Reactions


Chain Reactions


Chain Reactions: Initiation

• Formation of the Active Intermediate

• Example:

– Ethane converts to 2 free radials


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


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


Chain Reactions: Conclusion

• 2·C2H6 C4H10


Exercises



Courses

Reactor Engineering

CH7: Mechanism and Pathways

Remember that this is only an OVERVIEW



Section 2

Reaction Pathways


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


Reaction Pathways


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Reaction Pathways


Example: Smog Formation

• The formation of Smog is very important due to its pollutant properties. Smog is composed mainly by exhaust gases…



• A Free radical gets contact with NO

• NO2 is formed and decomposed by UV

• That Oxygen will react with O2 and form Ozone



• 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


Smog Pathway


Smog Pathway: Conclusion

• Many byproducts are formed with only one molecule!

• NO2 is “regenerated” and can continue reacting much more


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


Questions and Problems



• Remember that this is only an OVERVIEW



More Information…

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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)


Bibliography

Elements of Chemical Reaction EngineeringH. Scott Fogler (4th Edition)


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