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Chemical Reaction Engineering (CRE) is thefield that studies the rates and mechanisms of
chemical reactions and the design of the reactors inwhich they take place.
Lecture 5d
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Lecture 1 Tuesday 2 12 2 13
2
Block 1: Mole Balances
Block 2: Rate Laws
Block 3: Stoichiometry
Block 4: Combine
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Selectivity in Multiple Reactions
3
Selectivity Yield
Instantaneous SD/U= rD/rU
Overall D/U= FD/FU )/( 0 AADD FFFY =
ADD rrY = /
Keep CA high and CB low.
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Semibatch Reactors Semibatch 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
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Semibatch reactors
A + B C + D
5
Liquid level and volume increase
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6
1) Mass Balance: mdt
dm=
andm00
= 0
Vm =
0=dt
dV
00 VVt ==
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1) Mole Balance on Species A:
dt
dNVr
A
A
=+ 00
0=
dtdV
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[in] [out] + [gen] = [acc]
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8
1) Mole Balance on Species B:
Semibatch Reactors
( )
V
CCr
dt
dCBB
B
B 00
+=
000
BB
CF =
dt
dNVrF
B
BB =+ 00
0
=
dt
dV
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( )
( )
( )
( )
( ) tVVV
Cr
dt
dC
V
Cr
dt
dC
V
CCr
dt
dCV
Cr
dt
dC
D
D
D
C
C
C
BB
B
B
A
A
A
00
0
0
00
0
5
4
3
)(2
1
+=
=
=
+=
=
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Semibatch Reactors
1) Mass and Mole Balance Summary
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2) Rate Laws3) Stoichiometry
1111
DCBA rrrr
==
=
( ) AC rr =8
( ) AD rr =9
( )0
010
A
AA
N
NNX
=
( ) VCNAA
=12
4) Parameters0000 ,,,, BA CkVC 1
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12
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Everything is the same as for the irreversible case,
except for the rate law:
Consider the following reaction:
13
Equilibrium Conversion in Semibatch
Reactors with Reversible Reactions
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14
Equilibrium Conversion in Semibatch
Reactors with Reversible Reactions
Where:
( )
V
XNtFC
AB
B
00 =
( )V
XNC
A
A
=
10
At equilibrium, then0rA =
Xe changes with time.
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15
P6-6B - Semibatch Reactors
Sodium Bicarbonate + Ethylene Chlorohydrin Ethylene Glycol + NaCl + CO2
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P6-6B - Semibatch Reactors
Semibatch Reactors in terms ofMoles
2
22
22
)5(
0)4(
)3(
)2(
)1(
2
0
CODCBA
COCO
COCO
CD
C
c
BB
b
A
a
rrrrr
VrF
VrFCO
NND
Vrdt
dNC
VrFdt
dNB
Vrdt
dNA
====
=
+=
=
=
+=
=
Stoichiometry
Mole Balances
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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
CVN
N
NN
X
CkCr
VNC
VNC
RHO
MW
RHO
MWCOF
dt
dV
=
=
=
=
=
=
=
=
=
Rate Laws
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P6-6 Semibatch: Moles, Na, Nb, etc.
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2
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21
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P6-6 Semibatch: Concentrations CA, CB, CC
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Three Forms of the Mole Balances applied to Semibatch Reactors:
Vrdt
dN
A
A=
VrFdtdN
BB
B+=
0
1. Molar Basis
0A
A
N
Vr
dt
dX =3. Conversion
25
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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 numberof moles, conversion, and/or concentration:
Conversion Concentration No. of Moles
Thisimagecannotcurrently bedisplayed.
Vrdt
dNA
A
=
VrFdt
dN
BA
B+=
0
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Semibatch Reactors
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27
Polymath Equations
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End of Lecture 5d
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