6.Multiple_reactions.ppt

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Scope:

• Limited to Qualitative treatment

• Design aspects – not included

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What is a multiple reaction ?

• Require rate w.r.t. more than one species

• Require more than one stoichiometric equation and equilibrium expression

Classification: • Series (consecutive) reactions

A R S• Parallel (competing) reactions

A R A S

• Series – Parallel reactions 3

Criteria for selecting the reactor:• Good product distribution • Size of the reactor

Good product distribution could be the dominant criteria in many cases.

The above two criteria are contradictory to each other. Good design w.r.t one requirement may be poor w.r.t. the other.

An economic analysis will give the best compromise.

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Selectivity: A R (desired reaction)A S (undesired reaction)

Selectivity = rdesired/rundesired

The selectivity should be maximum for the chosen reactor.

Yield = rdesired /-rA

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Parallel reactions

Series reactions

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Series-Parallel reactions

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Parallel reactions

A R (desired reaction)A S (undesired reaction)

A

RR Ck

dt

dCr 1

AS

S Ckdt

dCr 2

k1, k2, , are all constant for a specific system at a given temperature.

How to maximize S ??

AS

R Ck

k

dC

dCS

2

1

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AS

R Ck

k

dC

dCS

2

1

How to decide the concentration level??

Case – 1: > , ndesired > nundesired

High concentration is desirable as it maximizes the selectivity.

Case – 2: < , ndesired < nundesired

Low concentration is desirable as it maximizes the selectivity.

Case – 3: = , ndesired = nundesired

Concentration level does not affect the selectivity.

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Concentration can be maintained low:• Using a CSTR• Designing for high conversions• Increasing inerts in the feed• Decreasing pressure in gas phase

reactions

Concentration can be maintained high:• Using a PFR• Designing for low conversions• Decreasing inerts in the feed• Increasing pressure in gas phase reactions

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How to decide the temperature level??

Case – 1: ED > EU

High temperature is desirable as it maximizes the selectivity.

Case – 2: ED < EU

Low temperature is desirable as it maximizes the selectivity.

Case – 3: ED = EU

Temperature level does not affect the selectivity.

S is maximized at higher k1/k2

AS

R Ck

k

dC

dCS

2

1

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Example: A + B R (desired reaction)A + B S (undesired reaction)

2211

2

1 BAS

R CCk

k

dC

dCS

Case – 1: 1 > 1 and 2 > 2 Maintain both CA and CB high.

Case – 2: 1 < 1 and 2 < 2 Maintain both CA and CB low.

Case – 3: 1 = 1 and 2 = 2 CA and CB levels will not affect S.

Temperature levels can be decided based on the activation energies 12

Contacting patterns for batch operation:

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Contacting patterns for continuous operation:

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•For parallel reactions, the concentration level of the reactants is the key to control product distribution.

•High reactant concentration favors the higher order reaction

•Low reactant concentration favors the lower order reaction

•Similarly high temperature favors the reaction with high activation energy

•Low temperature favors the reaction with low activation energy

Conclusions:

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Series reactions:A R S (I order at constant density)

meanopt kkk

kkt

log

12

12 /1)/ln( )(

2

1

0

max,12

2kk

k

A

R

k

k

C

C

Batch Reactor:

meanopt kkk

kk

log

12

12 /1)/ln(

)(

2

1

0

max,12

2kk

k

A

R

k

k

C

C

Plug Flow Reactor:

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CSTR:

A

AA

A

AA

Ck

CC

r

CC

1

00

10

1 kC

C

A

A

Material balance for species R: vCR0 = vCR + (-rR)V

0 = vCR + (-k1CA+ k2CR)V 0 = CR + (-k1CA + k2CR) CR(1 + k2) = k1CA

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)1)(1(1 21

01

2

1

kk

Ck

k

CkC AAR

For maximum concentration of R: dCR/d = 0

2120

max,

1/

1

kkC

C

A

R

21

1

kkopt

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Consecutive I-order reactionsCB exhibits a maximum at opt

PFR

CSTR

CB,max (PFR) > CB,max (CSTR)19

•For series reactions, the time of reaction / space time is the key for obtaining the maximum concentration of the desired product.

•For higher order reactions and series-parallel reactions, such analytical derivations for optimum space time would be tedious.

•In such cases, optimization methods can be used to find the optimum space time and the corresponding maximum concentration of the desired product.

Conclusions:

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Assignment:1. Find expressions for optimum space time

and maximum concentration of R for the following series reaction, if k1 = k2 in both CSTR and PFRA R S (I order at constant density)

2. Find expressions for optimum space time and maximum concentration of R for the following series reaction, if k1 = 2k2 in both CSTR and PFRA R S (I order at constant density)

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ANY CLARIFICATIONS ?

Berrill, N.J.A great teacher is not simply one who imparts knowledge to his

students but is one who awakens their interest in the subject and makes them eager to pursue it for themselves. An outstanding

teacher is a spark plug, not a fuel line.

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