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Chapter 6 - Multiple Reactions

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Mohammad Tazli Azizan

on 17 February 2017

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Transcript of Chapter 6 - Multiple Reactions

Why yield & selectivity is important in multiple reactions?
Case Evaluation:
Key Factor:
Concentration & Proper Choice of Reactor
YIELD & SELECTIVITY
Parallel reaction: Maximising desired product for one reactant
Consider the following reaction:

Rate of disappearance of A:

Case 1:
a
1>
a
2
Keep high concentration of key reactant A
Gas phase: Carry out reaction without inert & at high P
Liquid phase: Keep the use of inert to minimum
Use batch or PFR since in this two reactors, the initial concentration of reactant is high and drops progressively through reaction
Under no circumstances CSTR is recommended since in CSTR, the reactant concentration drops to low value immediately after entering the reactor

FREE
Tuesday, 10 March 2015
Fogler, 4th Ed
Learning Outcome:
CHAPTER 6: MULTIPLE REACTIONS
DOES TEMPERATURE AFFECT THE MULTIPLE REACTIONS?
After completing Chapter 6 the student will be able to:
Choose the appropriate reactor scheme to maximise the production of desired product for any multiple reaction systems
Developed equations of concentration profile in any reactor for complex reactions

Selectivity is related to activation energy, E, through:





If ED>EU: rD increases rapidly with increasing temperature wrt rU, thus the reaction should be carried out
at highest temperature possible
If Eu>ED: rU increases rapidly with increasing temperature wrt rD, thus the reaction
should be carried out at lowest temperature possible
, but not too low such that the desired reaction does not proceed to any significant effect

THE KINETICS & REACTOR DESIGN TABLOID
Types of multiple reactions
Parallel reaction: Maximising desired product for one reactant
Consider these reactions:
Instantaneous Selectivity:
Overall Selectivity:
Instanenous Yield:
Overall Yield:
Write the overall selectivity & yield for batch reactor
A is the key reactant
Corresponding rate laws:

Selectivity:
Case 2:
a
1<
a
2
Keep low concentration of key reactant A
Either dilute feed with inert or run reaction at low concentration of A
Use:
CSTR: Concentration is kept to low value
Recycle reactor with product stream acting as feed diluent
Example 6-2:

Reactant A decomposes by three simultaneous reactions to form three products; the desired product, B, and two undesired product, X and Y.

These gas phase reactions together with their corresponding rate laws are given below.

How and under what conditions (reactor type, pressure, temperature etc) should the reaction be carried out to minimize the concentration of unwanted X and Y?


LET'S DO SOME EVALUATIONS
KRD
KRD
The best|The latest| Factual
Deduce the selectivity formula for

A ---> D

A ---> U1

A---> U2
CHAPTER 6: MULTIPLE REACTIONS - Part 2
DIFFERENT TYPES OF REACTORS - WHAT AND WHY?
MAXIMISING SELECTIVITY IN SERIES REACTION
What is the most important factor in obtaining the maximum selectivity of the desired product?
Let's evaluate this:
(1) Mole Balance
(2) Net Rates of Reaction
Suppose we have these reactions:
(2) Cont..
Net rate of reactions:
(3) Stoichiometry: Relative rate of reaction

Relative rate of reaction
(4) Stoichiometry: Concentration terms
Highly recommended link
http://www.umich.edu/~elements/course/lectures/six/
Full transcript