Post on 23-Feb-2016
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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 18
Today’s lectureSolution to in-class problem
User friendly Energy Balance DerivationsAdiabaticHeat Exchange Constant TaHeat Exchange Variable Ta Co-currentHeat Exchange Variable Ta Counter
Current
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Adiabatic Operation CSTR
The feed consists of both - Inerts I and Species A with the ratio of inerts I to the species A being 2 to 1.
A B
FA0
FI
Elementary liquid phase reaction carried out in a CSTR
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Adiabatic Operation for CSTRa) Assuming the reaction is irreversible for CSTR, A
B, (KC = 0) what reactor volume is necessary to achieve 80% conversion?
b) If the exiting temperature to the reactor is 360K, what is the corresponding reactor volume?
c) Make a Levenspiel Plot and then determine the PFR reactor volume for 60% conversion and 95% conversion. Compare with the CSTR volumes at these conversions.
d) Now assume the reaction is reversible, make a plot of the equilibrium conversion as a function of temperature between 290K and 400K.
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CSTR Adiabatic Example
A B
T X
FA0 5 molmin
T0 300K
FI 10 molmin
H Rx 20,000cal mol A (exothermic)
Mole Balance:
exitA
0A
rXFV
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CSTR Adiabatic Example
T1
T1
RHexpKK
ekk
KCCkr
2
Rx1CC
T1
T1
RE
1
C
BAA
1
Rate Law:
XCC
X1CC
0AB
0AA
Stoichiometry:
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CSTR Adiabatic ExampleEnergy Balance - Adiabatic, ∆Cp=0:
X
36164000,20300X
182164000,20300T
CCXHT
CXHTT
IAi PIP
Rx0
Pi
Rx0
X 100300T
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CSTR Adiabatic ExampleIrreversible for Parts (a) through (c)
)K (i.e., X1kCr C0AA
(a) Given X = 0.8, find T and V
Given X Calc T Calc k Calc rACalc V
Calc KC(if reverible)
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CSTR Adiabatic Example
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A
0A
0A
0A
exitA
0A
dm 82.28.01281.3
8.05rXFV
81.3380
12981
989.1000,10exp1.0k
K3808.0100300T
X1kCXF
rXFV
Given X, Calculate T and V
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CSTR Adiabatic Example
(b) VrkTXGiven CalcA
CalcCalcCalc
CK Calc(if reverible)
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1
0AA
dm 05.24.0283.1
6.05V
min83.1k
6.0100
300TX
K360Tble)(Irreversi X1kCr
Given T, Calculate X and V
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CSTR Adiabatic Example(c) Levenspiel Plot
X1kCF
rF
0A
0A
A
0A
Choose X Calc T Calc k Calc rACalc
FA 0
rA
X100300T
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CSTR Adiabatic Example(c) Levenspiel Plot
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CSTR X = 0.95 T = 395
0
5
10
15
20
25
30
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
X
-Fa0/Ra
CSTR 60%
CSTR X = 0.6 T = 360
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CSTR Adiabatic Example
PFR X = 0.6
PFR X = 0.95
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CSTR Adiabatic Example
CSTR X = 0.6 T = 360 V = 2.05 dm3
PFR X = 0.6 Texit = 360 V = 5.28 dm3
CSTR X = 0.95 T = 395 V = 7.59 dm3
PFR X = 0.95 Texit = 395 V = 6.62 dm3
Summary
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CSTR Adiabatic Example
Choose T Calc KCCalc Xe , repeat
(d) At Equilibrium
e
e
e0A
e0A
Ae
BeC X1
XX1C
XCCCK
C
Ce K1
KX
T1
T1
RHexpKK
2
R2CC
Calculate Adiabatic Equilibrium Conversion and Temperature:
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(d) At Equilibrium
T1
2901
987.1000,20exp000,1KC
12.0X355.0X72.0X
937.0X999.0X
136.0K95.0K6.2K9.14K
000,1K
390T370T350T330T290T
e
e
e
e
e
C
C
C
C
C
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Calculate Adiabatic Equilibrium Conversion and Temperature:
300T 1.0X
300T000,20
200HTTCCX
Rx
0PIP IA
(e) Te = 358 Xe = 0.5918
Calculate Adiabatic Equilibrium Conversion and Temperature:
TTa
V+ΔVV
mc , HC
FA, Fi
In - Out + Heat Added = 0
PFR Heat Effects
D/4a
L4DV
LDA
turbine)(no 0W
2
S
TTAUQ a
V+ΔVVFi
T
Ta
Fi
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dVdFH
dVdHF
dVHFd
0TTUdV
HFd
0VTTUHFHF
ii
ii
ii
aaii
aaVViiVii
VTTUQ aa
PFR Heat Effects
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aiii
RPi0ii
rrdVdF
TTCHH
dVdTC
dVdH
Pii
RiiaiiPiiii HHrH
dVdTCF
dVHFd
PFR Heat Effects
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0TTUrHdVdTFC aaaRiPi
aaaRPii TTUrHdVdTCF
Pii
aaaR
CFTTUrH
dVdT
PFR Heat Effects
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Pii
rg
CFQQ
dVdT
Heat removed
Heat generated
XCCFCXFCF PiPii0APiii0APii
PFR Heat Effects
XCCFTTUrH
dVdT
PPii0A
aaaR
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User Friendly Equations Relate T and X or Fi
3. PBR in terms of molar flow rates
dTdW
rAHRx T
Uab
T Ta FiCPi
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4. For multiple reactions
€
dTdV
=
Uaρ B
Ta − T( ) + rijΔHRxij∑
FiCPi∑
5. Coolant Balance
€
dTA
dV=
Ua T − Ta( )˙ m cCPc
Heat Exchange ExampleElementary liquid phase reaction carried out in a PFR
The feed consists of both inerts I and Species A with the ratio of inerts to the species A being 2 to 1.25
FA0FI
Tacm Heat
Exchange Fluid
A BT
T1
T1
REexpkk )3(
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T1
T1
RHexpKK )4(
2
Rx2CC
FrdVdX )1( 0AA1) Mole
Balance:
C
BAA K
CCkr )2(2) Rate Law:
Heat Exchange Example
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0CP
6 XCC
5 X1CC
0AB
0AA
3) Stoichiometry:
9 CCC PIIPAPii
8 k1
kXC
Ceq
4) Heat Effects:
€
dTdV
=ΔHR( ) −ra( ) −Ua T − Ta( )
FA 0 θ iCPi∑ 7( )
Heat Exchange Example: Case 1- Constant Ta
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Parameters:
a
IPIPA0A
0Aaa2C1
21R
rrate
, ,C ,C ,C
,F ,T ,U ,k ,k
,T ,T ,R ,E ,H
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Heat Exchange Example: Case 1- Constant Ta
Pii
rg
CFQQ
dVdT
Heat removed
Heat generated
XCCFCXFCF PiPii0APiii0APii
PFR Heat Effects
€
dTdV
=ΔHR( ) ra( ) −Ua T − Ta( )
FA 0 θ iCPi + ΔCP X∑[ ]29
Energy Balance:Adiabtic and ΔCP=0Ua=0 )A16(
CXHTT
iPi
Rx0
Additional Parameters (17A) & (17B)
IAi PIPPi0 CCC ,T 30
Heat Exchange Example: Case 2 AdiabaticMole Balance:
€
dXdV
=−rA
FA 0
Adibatic PFR
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Find conversion, Xeq and T as a function of reactor volume
V
rate
V
T
V
XX
Xeq
Example: Adiabatic
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Heat Exchange:
iPi
aRxA
CFTTUaHr
dVdT
)B16( CF
TTUaHrdVdT
iPi0A
aRxA
33Need to determine Ta
A. Constant Ta (17B) Ta = 300K
Ua ,C ,TiPia
Additional Parameters (18B – (20B):
)C17( TT 0V , Cm
TTUadVdT
aoaP
aa
cool
B. Variable Ta Co-Current
C. Variable Ta Counter Current Guess ?T 0V
CmTTUa
dVdT
aP
aa
cool
Guess Ta at V = 0 to match Ta0 = Ta0 at exit, i.e., V = Vf
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User Friendly Equations
Coolant balance:In - Out + Heat Added = 0
0TTUdV
dHm
0TTVUHmHm
aaC
C
aaVVCCVCC
Variable Ta Co-current
dVdTC
dVdH
TTCHH
aPC
C
raPC0CC
All equations can be used from before except Ta parameter, use differential Ta instead, adding mC and CPC
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0aa
PCC
aaa TT 0V ,Cm
TTUdVdT
Heat Exchanger Energy Balance
In - Out + Heat Added = 0
All equations can be used from before except dTa/dV which must be changed to a negative. To arrive at the correct integration we must guess the Ta value at V=0, integrate and see if Ta0 matches; if not, re-guess the value for Ta at V=0
Variable Ta Counter-current
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PCC
aaaaa
CC
aaVCCVVCC
CmTTU
dVdT 0TTU
dVdHm
0TTVUHmHm
Heat Exchanger Energy Balance
Derive the User Friendly Energy Balance for a PBR
0HFHFdWTTUaii0i0ia
W
0 B
0dWdHFH
dWdF0TTUa i
iii
aB
Differentiating with respect to W:
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Aii
i rrdWdF
Mole Balance on species i:
T
TPiRii
R
dTCTHH
Enthalpy for species i:
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Derive the User Friendly Energy Balance for a PBR
Differentiating with respect to W:
dWdTC0
dWdH
Pii
0dWdTCFHrTTUa
PiiiiAaB
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Derive the User Friendly Energy Balance for a PBR
0dWdTCFHrTTUa
PiiiiAaB
THH Rii XFF ii0Ai
Final Form of the Differential Equations in Terms of Conversion:A:
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Derive the User Friendly Energy Balance for a PBR
Final Form of terms of Molar Flow Rate:
Pii
AaB
CF
HrTTUa
dWdT
B: T,Xg
Fr
dWdX
0A
A
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Derive the User Friendly Energy Balance for a PBR
Reversible Reactions
The rate law for this reaction will follow an elementary rate law.
DCBA
C
DCBAA K
CCCCkr
Where Ke is the concentration equilibrium constant. We know from Le Chaltlier’s law that if the reaction is exothermic, Ke will decrease as the temperature is increased and the reaction will be shifted back to the left. If the reaction is endothermic and the temperature is increased, Ke will increase and the reaction will shift to the right.
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Reversible Reactions
RTKK P
C
Van’t Hoff Equation:
2RPRR
2RP
RTTTCTH
RTTH
dTKlnd
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Reversible ReactionsFor the special case of ΔCP=0
Integrating the Van’t Hoff Equation gives:
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RR1P2P T
1T1
RTHexpTKTK
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Reversible Reactions
endothermic reaction
exothermic reaction
KP
T
endothermic reaction
exothermic reaction
Xe
T
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End of Lecture 18
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