CHEE 321: Chemical Reaction Engineering Module 5: Multiple Reactions (Chapter 6, Fogler)

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CHEE 321 Chemical Reaction EngineeringModule
5 Multiple Reactions (Chapter 6, Fogler)
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Topics to be covered in this Module

• Types of multiple reactions
• Introduction to selectivity and yield
• Qualitative Analyses (Parallel and Series
  Reactions)
• Maximizing the reactor operation for single
  reactant systems
• Maximizing the reactor operation for two reactant
  systems
• Algorithm for Reactor Design of Complex Reactions
• Mole Balance
• Net Rates of Reactions
• Stoichiometry
• Additional Topic Rate Law of Reversible Reaction

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Multiple Reactions

• Types of Multiple Reactions
• Series Reactions
• Parallel Reactions
• Complex Reactions Series and Parallel
• Independent

Use molar flow rates and concentrations DO NOT
use conversion!
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Selectivity and Yield
Instantaneous Global
Desired Reaction Undesired Reaction
Selectivity Yield

• What should be the criterion for designing the
  reactor ?
• Is it necessary that reactor operates such that
  minimum amount of undesired products are formed ?

Economics
NO
Cost
Reactant Conversion
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Maximizing Selectivity for Series Reactions
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Series Reactions
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Series Reactions
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Series Reactions
We can calculate, the concentration of
undesired species C when B is maximum
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Maximizing Selectivity for Parallel Reactions
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Selectivity for Single Reactant Systems
Example (parallel reaction) Desired
Reaction Undesired Reaction
BTW, what is the net rate of reaction of A ??
Let us examine some reactor operating scenarios
to maximize selectivity.
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Selectivity for Single Reactant Systems
Case 1 ?D-?U gt0
High CA favors D
How can we accomplish this?

• For gas phase reactions, maintain high pressures
• For liquid-phase reactions, keep the diluent to
  a minimum
• Batch or Plug Flow Reactors should be used
• CSTR should NOT be chosen

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Selectivity for Single Reactant Systems
Case 2 ?D-?U lt 0
Low CA favors D
How can we accomplish this?

• For gas phase reactions, operate at low
  pressures
• For liquid-phase reactions, dilute the feed
• CSTR is preferred

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Selectivity for Single Reactant Systems
Case 3 ?D-?U 0
Concentration cannot be used operating parameter
for selectivity maximization
What now?

• Operate reactor at highest possible temperature

• Operate reactor at lowest possible temperature

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Maximizing Selectivity for Parallel Reactions
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Selectivity for Two Reactant Systems
Example Desired Reaction Undesired Reaction
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Selectivity for Two Reactant Systems
Case 1 ?1gt?2 ?1 gt ?2
Let, a ?1-?2 b ?1 - ?2
For high SDU, maintain both A B as high as
possible
How can we accomplish this?

• Use Batch reactor
• Use Plug Flow reactor

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Selectivity for Two Reactant Systems
Case 2 ?1gt?2 ?1 lt ?2
Let, a ?1-?2 b ?2 - ?1
For high SDU, maintain concentration of A high
and of B low
How can we accomplish this?

• Use semi-batch reactor where B is fed slowly
• Use Tubular reactor with side streams of B being
  fed continuously
• Use series of small CSTR with A fed only to
  first and B to each reactor

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Selectivity for Two Reactant Systems
Case 3 ?1lt?2 ?1 lt ?2
Let, a ?2-?1 b ?2 - ?1
For high SDU, maintain both concentration of A
and B low
How can we accomplish this?

• Use CSTR
• Use Tubular reactor with large recycle ratio
• Use diluted feed

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Selectivity for Two Reactant Systems
Case 4 ?1lt?2 ?1 gt ?2
Let, a ?2-?1 b ?1 - ?2
For high SDU, maintain concentration of B high
and of A low
How can we accomplish this?

• Use semi-batch reactor where A is fed slowly
• Use Tubular reactor with side streams of A being
  fed continuously
• Use series of small CSTR with B fed only to
  first and A to each reactor

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Multiple Reactions System Implementation of
Numerical Method
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Modification to the CRE Algorithm for Multiple
Reactions

• Mole balance on every species
• Rate Law Net Rate of reaction for each species,
  e.g., rA S riA
• Stoichiometry
• a) Liquid Phase use CA
• b) Gas Phase use
• Combine - MATLAB will combine for you

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Design Equation for Reactors
Gas-Phase Liquid Phase
Batch Semi-Batch CSTR PFR PBR
NOTE the design equations are EXACTLY as it were
for single reaction
Reaction rates are NET rates of reaction
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Net Rate of Reaction

• For N reactions, the net rate of formation of
  species A is

For a given reaction i ai A bi B ? ci C di D
NOTE You can use stoichiometric coefficients to
relate relative rates of reaction of species for
a specific reaction only
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Net Rate of Reaction

• The following reactions follow elementary rate
  law

Write net rates of formation of A, B and C
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Multiple Reactions - Gas Phase
Recall,
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Gas Phase Multiple Reactions - Algorithm
1. Mole Balance
A
Remember, unlike single-reactions, for multiple
reactions mole balance for each species must be
written
B
C
D
rA, rB, rC, rD are all NET rates of reactions
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Gas Phase Multiple Reactions - Algorithm
Species A
Species B
Species C
Species D
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Gas Phase Multiple Reactions - Algorithm
3. Stoichiometry
4. Combine