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 21 Web Lecture 21 Class Lecture 17 Tuesday 3192013 ID: 266183
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Slide1
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 21Slide2
Web Lecture 21Class Lecture 17 – Tuesday 3/19/2013
Gas Phase Reactions
Trends and Optimums
2Slide3
3
User Friendly Equations relate T, X, or F
i
Review Last Lecture
1. Adiabatic
CSTR
,
PFR, Batch, PBR achieve this:Slide4
4
User Friendly Equations relate T, X, or F
i
2.
CSTR
with
heat exchanger
, UA(T
a-T) and a large coolant flow rate:
T
T
aSlide5
5
User Friendly Equations relate T, X, or F
i
3.
PFR
/
PBR
with
heat exchange:
F
A0
T
0
Coolant
T
a
3A.
In terms of conversion, XSlide6
6
User Friendly Equations relate T, X, or F
i
3B. In terms of molar flow rates, F
i
4. For multiple reactions
5.
Co-
Current
BalanceSlide7
7Reversible Reactions
endothermic
reaction
exothermic
reaction
K
P
T
endothermic
reaction
exothermic
reaction
X
e
TSlide8
Heat Exchange8
Example
: Elementary liquid phase reaction carried out in a
PFR
F
A0
F
I
T
a
Heat Exchange Fluid
The
feed
consists
of
both
inerts I and Species A with the
ratio
of inerts to the species A
being
2 to 1.Slide9
Heat Exchange9
Adiabatic
.
Plot X,
X
e
, T and the
rate
of disappearance as a function of V up to V = 40 dm3.
Constant
Ta. Plot X,
Xe, T, T
a and rate
of disappearance of A when there is a heat loss to the coolant and the coolant temperature is constant at 300 K for V = 40 dm3
. How do these curves differ from the adiabatic case. Slide10
Heat Exchange10
Variable T
a
Co-Current
. Plot X,
X
e
, T, T
a and rate of disappearance of A when there is a heat loss to the coolant and the coolant temperature varies along the length of the reactor for V = 40 dm3
. The coolant enters at 300 K. How do these curves differ from those in the adiabatic case and part (a) and (b)?
Variable T
a Countercurrent
. Plot X, Xe
, T, Ta and
rate of disappearance of A when there is a heat loss to the coolant and the coolant temperature varies along the length of the reactor for V = 20 dm3
. The coolant enters at 300 K. How do these curves differ from those in the adiabatic case and part (a) and (b)?Slide11
Heat Exchange11
Example:
PBR
A
↔ B
5) Parameters
For adiabatic:
Constant Ta:
Co-current: Equations as is
Counter-current: Slide12
Reversible Reactions12
1) Mole
BalancesSlide13
Reversible Reactions13
2) Rate
LawsSlide14
Reversible Reactions14
3) Stoichiometry
Note: Nomenclature change for 5th edition
p
≡
ySlide15
Reversible Reactions15
ParametersSlide16
3) Stoichiometry:
Gas Phase
16
Example
:
PBR
A
↔ B
Reversible Reactions
Gas Phase Heat EffectsSlide17
17
Reversible Reactions
Gas Phase
Heat
Effects
Example
:
PBR
A ↔ BSlide18
18
Exothermic Case:
X
e
T
K
C
T
K
C
T
T
X
e
~1
Endothermic Case:
Example
:
PBR
A
↔ B
Reversible Reactions
Gas Phase
Heat
EffectsSlide19
19
Case 1:
Adiabatic and ΔC
P
=0
Additional
Parameters (17A) & (17B)
Reversible Reactions
Gas Phase
Heat
EffectsSlide20
Heat effects:
20
Case 2:
Heat
Exchange
– Constant T
a
Reversible Reactions
Gas Phase
Heat
EffectsSlide21
Case 3.
Variable T
a
Co-Current
Case 4.
Variable T
a
Countercurrent
Guess
T
a
at V = 0 to match T
a0 = Ta0 at
exit, i.e., V = V
f21
Reversible Reactions
Gas Phase
Heat
EffectsSlide22
22Slide23
23Slide24
24Slide25
25Slide26
26Slide27
27Slide28
Conversion on temperature
Exothermic
ΔH is negative
Adiabatic Equilibrium temperature (T
adia
) and conversion (Xe
adia
)
X
X
e
adia
T
adia
T
28
Adiabatic EquilibriumSlide29
X
2
F
A0
F
A1
F
A2
F
A3
T
0
X
1
X
3
T
0
T
0
Q
1
Q
2
29Slide30
X
T
X
3
X
2
X
1
T
0
X
e
30Slide31
31Slide32
T
X
Adiabatic
T and
X
e
T
0
exothermic
T
X
T
0
endothermic
Trends:
Adiabatic
Gas Flow
Heat
Effects
32Slide33
Effects of
Inerts
in the Feed
33Slide34
Endothermic
34
As inert flow
increases
the
conversion
will
increase
.
However
as inerts
increase, reactant
concentration decreases
, slowing down the
reaction
.
Therefore
there
is an optimal inert flow rate to
maximize
X.
First Order IrreversibleSlide35
Adiabatic:
35
As T
0
decreases the conversion X will increase, however the reaction will progress slower to equilibrium conversion and may not make it in the volume of reactor that you have.
Therefore, for
exothermic
reactions there is an optimum inlet temperature, where X reaches
X
eq right at the end of V. However, for
endothermic reactions there is no temperature maximum and the X will continue to increase as T increases.
X
T
X
e
T
0
X
T
X
T
Gas Phase
Heat
EffectsSlide36
Adiabatic:
36
Effect of adding
i
nerts
X
T
V
1
V
2
X
T
T
0
X
e
X
Gas Phase
Heat
EffectsSlide37
Exothermic Adiabatic37
As
θ
I
increase
, T
decrease
and
k
θ
ISlide38
38Slide39
39Slide40
Endothermic
Exothermic
40
AdiabaticSlide41
Heat Exchange
Endothermic
Exothermic
41Slide42
End of Web Lecture 21End of Class Lecture 17
42