Lecture 10
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 – Tuesday 2/8/2011
 Instantaneous SD/U = rD/rU
 Overall = FD/FU
2
D
A  B 
D
rD  k D C A C B
(Desired)
A  B  U
rU  k U C A C
(Undesired
k
2
kU
Selectivity
Yield
Instantaneous SD/U = rD/rU
ŜD/U = FD/FU
Overall
S D /U 
3
rD
rU
2

k D C AC B
k u C AC
2
B

Keep CA high and CB low.
2
B
kDC A
kU C B
Y D  rD /  r A
YˆD  F D /( F A 0  F A )
)
 Semi 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
B, v0
m
A
Initial V
Liquid level and volume increase
5
Mass Balance:
dm
 m
dt
m   0  0
dm
dt
dV
dt
 0
and
dV
dt
  0 0
 0
t  0 V  V0
6
V  V0   0t
m  V 0
1) Mole balances:
Species A:
[in] – [out] + [gen] = [acc]
0  0  r AV 
dN
A

d [ C AV ]
dt
dV
dt
dt
dt
dt
 0
dC A
7
V
 rA 
dC A
 0C A
V
dN
A
dt
 CA
dV
dt
1) Mole balances:
Species B:
F B 0  0  rB V 
dN
B
dt
V
dC B
dt
 CB
dN
dV
dt
F B 0  C B 0 0
dC B
dt
8
 rB 
C B 0  C B  0
V
dt
B
9
 0C A
1 
dC
2 
dC B
3 
dC C
4 
dC D
5 
V  V0   0t
A
dt
dt
dt
dt
 rA 
 rB 
 rC 
 rD 
V
 0 (C B 0  C B )
V
 0C C
V
 0C D
V
6 
 rA
r A  kC A C B

1
1

rC

rB  r A
8 
rC   rA
9 
rD   r A
X 
rD
1
7 
10 
10
 rB
1
N A0  N A
N A0
11 
N A 0  C A 0V 0
12 
N A  C AV
C A0 , V0 ,  0 , k , C B 0
11
12
Consider the following reaction:


A  B 
 C  D
Everything is the same as for the irreversible case, except for the
rate law:

13

CCC D 
 rA  k A  C A C B 

K
C


Where:
C A
C B
N A 0 1  X
V
 FB 0 t  N A 0 X 
V
C C CD 
At equilibrium,
K C
14
C Ce C De
C Ae C Be

 rA  0
N Ce N De
N Ae N Be
Xe changes with time.

N A0 X
V
then
2

N A0 X e
1  X e  F B 0 t  N A 0 X e 
P6-6B
Soldium Bicarbonate+Ethylene ChrolohydrinEthylen Glycol+NaCl+CO2
NaCHO3 + CH2OHCH2Cl  (CH2OH)2 + NaCl + CO2 
A + B  C + D + CO2 
Semibatch Balance in Terms of Moles
A + B  C + D + CO2
A
B
C
D
(1)
(2)
(3)
(4)
dN
a
dt
dN b
dt
dN c
 r AV
 F B 0  rB V
 rC V
dt
ND  NC
0   FCO 2  rCO 2 V
CO 2
(5 )
FCO 2  rCO 2 V
 r A   rB  rC  r D  rCO 2
16
(6)
dV
  0   CO 2
dt
(7 )
2
MW  44
(9 )
RHO  1000
(10 )
Ca  N A V
(11 )
CB  N B V
(12 )
r A   kC A C B
N a0  N a
X 
N a0
N a 0  V0C a 0
(14 )
2
RHO
(8 )
(13 )
17
 CO 
FCO 2 MWCO
Rest of the Polymath Statements
Similar to Concentration Program
P6-6 Semibatch: Moles, Na, Nb, etc.
19
20
21
P6-6 Semibatch: Concentrations CA, CB, CC
23
24
Three Forms of the Mole Balance applied to Semi Batch Reactors:
1. Molar Basis
dN
A
 r AV
B
 F B 0  rB V
dt
dN
dt
2. Concentration
Basis
dC A
 rA  C A
dt
dC B
dt
3. Conversion
25
dX
dt

0
dN
V
dt
 rB  C B 0  C B 
 r AV
N A0
0
dN
V
dt
A
 r AV
B
 F B 0  rB V
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
dX
dt

k 1  X
Concentration
 N Bi
 FB 0 t  N A 0 X
V0   0t

dC A
dt
dC B
26
dt
 rA  C A
No. of Moles
0
dN
V
dt
 r A  C B 0  C B 
0
dN
V
dt
A
 r AV
B
 F A 0  rB V
27
End of Lecture 10
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