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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.
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Semibatch reactors
A + B → C + D
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A
B, v0
Initial V
m
Liquid level and volume increase
Semibatch reactors
Semibatch reactors
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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
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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
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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
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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
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Semibatch
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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:
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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
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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.
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20
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P6-6 Semibatch: Concentrations CA, CB, CC
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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:
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End of Lecture 10
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