Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of

chemical reactions and the design of the reactors in which they take place.

Lecture 10

Lecture 10 – Tuesday 2/8/2011Definition of Selectivity

Instantaneous SD/U = rD/rU

Overall = FD/FU

Semibatch Reactors

2

Selectivity in Multiple Reactions

)(Undesired

(Desired) 2

2

BAUUk

BADDk

CCkrUBA

CCkrDBAU

D

Selectivity YieldInstantaneous

SD/U = rD/rU

Overall ŜD/U = FD/FU

BU

AD

BAu

BAD

U

DUD Ck

Ck

CCk

CCk

r

rS

2

2

/

3

)/(ˆ0 AADD FFFY

ADD rrY /

Keep CA high and CB low.

Semibatch reactorsSemi Batch reactors can be very effective

in maximizing selectivity in liquid phase reactions.

The reactant that starts in the reactor is always the limiting reactant.

4

Semibatch reactors

A + B → C + D

5

A

B, v0

Initial V

m

Liquid level and volume increase

Semibatch reactors

Semibatch reactors

6

Mass Balance:m

dt

dm

andm 00 0Vm

000 dt

dV

dt

dm

0dt

dV

0 0 VVt

tVV 00

dt

dVC

dt

dCV

dt

VCd

dt

dNA

AAA ][

Semibatch

7

1)Mole balances:

dt

dNVr AA 00

Species A:

[in] – [out] + [gen] = [acc]

V

Cr

dt

dC AA

A 0

0dt

dV

V

CCr

dt

dC BBB

B 00

Semibatch

8

1)Mole balances:

000 BB CF

Species B:

dt

dNVrF BBB 00

dt

dVC

dt

dCV

dt

dNB

BB

tVVV

Cr

dt

dCV

Cr

dt

dCV

CCr

dt

dCV

Cr

dt

dC

DD

D

CC

C

BBB

B

AA

A

00

0

0

00

0

5

4

3

)( 2

1

Semibatch

9

2) Rate Law: BAA CkCr 6

3) Stoichiometry: 1111DCBA rrrr

AB rr 7

AC rr 8

AD rr 9

0

0 10A

AA

N

NNX

000 11 VCN AA

VCN AA 12

4) Parameters:0000 , , , , BA CkVC 10

Semibatch

11

Semibatch

12

Semibatch

Equilibrium Conversion in Semibatch Reactors with Reversible Reactions

C

DCBAAA K

CCCCkr

Everything is the same as for the irreversible case, except for the rate law:

Consider the following reaction:

13

A B C D

Where:

V

XNtFC ABB

00

V

XNCC ADC

0

V

XNC AA

10

At equilibrium, then

0rA

eABe

eA

BeAe

DeCe

BeAe

DeCeC XNtFX

XN

NN

NN

CC

CCK

00

20

1

14 Xe changes with time.

Equilibrium Conversion in Semibatch Reactors with Reversible Reactions

Soldium Bicarbonate+Ethylene ChrolohydrinEthylen Glycol+NaCl+CO2

NaCHO3 + CH2OHCH2Cl (CH2OH)2 + NaCl + CO2

A + B C + D + CO2

P6-6B

Semibatch

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Semibatch Balance in Terms of MolesA + B C + D + CO2

2

22

22

)5(

0

)4(

)3(

)2(

)1(

2

0

CODCBA

COCO

COCO

CD

Cc

BBb

Aa

rrrrr

VrF

VrFCO

NND

Vrdt

dNC

VrFdt

dNB

Vrdt

dNA

17

000

0

0

2

0

)14(

)13(

)12(

)11(

)10(

1000)9(

44)8(

)7(

)6(

2

2

2

aa

a

aa

BAA

BB

Aa

COCO

CO

CVNN

NNX

CkCr

VNC

VNC

RHO

MWRHO

MWCOFdt

dV

Rest of the Polymath StatementsSimilar to Concentration Program

P6-6 Semibatch: Moles, Na, Nb, etc.

19

20

21

P6-6 Semibatch: Concentrations CA, CB, CC

23

24

Semi Batch ReactorsThree Forms of the Mole Balance applied to Semi Batch Reactors:

Vrdt

dNA

A

VrFdt

dNBB

B 0

1.Molar Basis

VCr

dt

dCAA

A 0

V

CCrdt

dCBBB

B 00

Vrdt

dNA

A

VrFdt

dNBB

B 0

2.Concentration Basis

0A

A

N

Vr

dt

dX 3.Conversion

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Consider the following elementary reaction:A+B C+D-rA=kCACB

The combined Mole Balance, Rate Law, and Stoichiometry may be written in terms of number of moles, conversion, and/or concentration:

Conversion Concentration No. of Moles

tV

XNtFNXk

dt

dX ABBi

00

001

V

Crdt

dCAA

A 0

V

CCrdt

dCBBA

B 00

Vrdt

dNA

A

VrFdt

dNBA

B 0

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Polymath Equations:

27

End of Lecture 10

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