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Chemical Reaction Engineering Lecture 6: Complex Reactions
Jayant M. Modak Department of Chemical Engineering Indian Institute of Science, Bangalore
Topic 3: Complex systems
! Analysis of “Simple complex” systems ! Kinetics of complex systems
" Chain reaction " Catalysis " Polyermization
! Lumping analysis
Complex systems - Examples
! Large number of reactions and reactants
3 8 3 6 2
3 8 2 4 4
3 8 2 4 2 6 3 6
Thermal cracking of alkanesC H C H HC H C H CHC H C H C H C H
Cracking of crude to petrol
Metabolic network insidecell
! +! ++ ! +
Complex systems - Examples
! Chain reactions
3 4
2
( )
Thermal decompositionCH CHO CH CO
Auto oxidationR H O ROOH
Polymerizationstyrene poly styrene
! +
"" + !
!
Complex systems - Examples
! Catalytic reactions
Thermal decompositionC12 H22O11 + H2O
acid! "!! C6 H12O6 + C6 H12O6
Ammonia synthesis12
N2 +32
H2Fe! "! NH3
Yield – conversion diagram
Polymer weight distribution
Catalytic reaction kinetics
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.0
0.5
1.0
1.5
2.0
2.5
3.0
0.061 0.132 0.263 0.526 0.789
rate
pEthylene
pO2
Complex reactor behavior
Cracking of ethane to ethylene
! New questions " Are all products useful? " How to monitor the reaction? " Is conversion of ethane the only criteria for design?
26 24 2CHCHH!+
26 38 42CHCHCH!+
24 222CHCH!+
Parallel reactions
2 6 2 4 2
2 6 38 42CHCHHCHCHCH! +!+
A1 !A3
"A2
Series reactions
2 6 2 4 2
2 4 22 2CH CHHCH CH! +!+ A1 ! A2 ! A3
Complex (Series-parallel) reactions
2 6 2 4 2
2 6 3 8 4
3 8 2 4 4 8 4
2CH CHHCH CHCHCHCH CHCH
! +! ++ ! +
A1 !A3
"A2
A2 + A3 ! A4
Independent Reactions
2 6 2 4 2
3 8 3 6 2
CH CHHCH CHH! +! +
A1 ! A2
A3 ! A4
Desired and Undesired Reactions
A1 ! A2
A1 ! A3
A1 ! A2 ! A3
26 24 2CHCHH!+
26 38 42CHCHCH!+
Yield
Desired reaction A1 ! A2 r1
Undesired reaction A1 ! A3 r2
OverallYield Y2 =
Exit molar flowrateof desired productInlet molar flowrateof reactant
Selectivity
Desired reaction A1 ! A2 r1
Undesired reaction A1 ! A3 r2
InstantenousSelectivity s2 =
r1
r1 + r2
OverallSelectivity !S2 =
Exit molar flowrateof desired productExit molar flowrateof all products
Selection of reactor type and conditions
CSTRC2
C10 ! C1
=r1
r1 + r2
PFR
C2
C10 ! C1
=1
C10 ! C1
r1
r1 + r2C1
C10
" dC1
Desired reaction A1 ! A2 r1
Undesired reaction A1 ! A3 r2
Complex systems - selectivity
1 0 -1
se
lect
ivity
(C1)
C10-C1
q2-q1
Complex systems – series reactions
0 10
0.0
0.2
0.4
0.6
0.8
1.0
!=0.1 A1
A2
A3
Concentration
Time
A1 ! A2 r1 = k1C1
A2 ! A3 r1 = k2C2 " = k2 / k1
Concept of yield-conversion diagram
A1 ! A2 r1 = k1C1
A2 ! A3 r1 = k2C2 " = k2 / k1
Concept of rate determining step
0 10
0.0
0.2
0.4
0.6
0.8
1.0
!=0.1 A1
A2
A3
Concentration
Time
A1 ! A2 r1 = k1C1
A2 ! A3 r1 = k2C2 " = k2 / k1
Concept of rate determining step
0 10
0.0
0.2
0.4
0.6
0.8
1.0
!=5
A1
A2
A3
Concentration
Time
A1 ! A2 r1 = k1C1
A2 ! A3 r1 = k2C2 " = k2 / k1
Concept of quasi-equilibrium approximation
0 1 2 3 4 50.0
0.2
0.4
0.6
0.8
1.0 k1=1, k-1=0.5, k2=k-2=1
C1
C2
C3
concentration
time
A1! A2 r1 = k1C1 ! k!1C2
A2 ! A3 r1 = k2C2 ! k!2C3
Concept of quasi-equilibrium approximation
0 1 2 3 4 50.0
0.2
0.4
0.6
0.8
1.0 C1
C2
C3
conntration
time
k1=1, k-1=0.5, k2=k-2=10
A1! A2 r1 = k1C1 ! k!1C2
A2 ! A3 r1 = k2C2 ! k!2C3
Concept of quasi-equilibrium approximation
0 1 2 3 4 50.0
0.2
0.4
0.6
0.8
1.0 C1
C2
C3
C1 (e) C2 (e)
conntration
time
k1=1, k-1=0.5, k2=k-2=10
A1! A2 r1 = k1C1 ! k!1C2
A2 ! A3 r1 = k2C2 ! k!2C3
Concept of quasi-steady state approximation
0 10
0.0
0.2
0.4
0.6
0.8
1.0
!=5
A1
A2
A3
Concentration
Time
A1 ! A2 r1 = k1C1
A2 ! A3 r1 = k2C2 " = k2 / k1
0 1 2 3 4 50.0
0.2
0.4
0.6
0.8
1.0
1 10 50 !
C 3/C
10
k1t
k2
Concept of quasi-steady state approximation
A1 ! A2 r1 = k1C1
A2 ! A3 r1 = k2C2 " = k2 / k1
Chemical Reaction Engineering Lecture 6: Complex Reactions
Jayant M. Modak Department of Chemical Engineering Indian Institute of Science, Bangalore
Chain reactions
! Combustion reactions ! Decomposition reactions ! Autooxidation!! Polymerization!
Chain reactions – decomposition of acetaldehyde CH 3CHO! CH 4 + CO
CH 3CHO! CH 3• + CHO•
CH 3• + CH 3CHO! CH 3CO
• + CH 4
CH 3CO• ! CH 3
• + CO2CH 3
• ! C2H6
r = kCCH3CHO3/2
Polymerization
! Chain polymerization of " Ethylene (X=H), vinyl chloride (X=Cl) " Styrene (X=C6H5) etc
! Initiator I (!-!)!
CH2 = CHX (RX )
Initiation C6 H5COO ! OOCC6 H5 " 2C6 H5COO •
# • +RX "#! RX •Propogation #! RX • +RX "#! (R) ! RX •
#! (R) j!1 ! RX • +RX "
#! (R) j ! RX •
Termination#! (R) j!1 ! RX • + • XR ! (R)i!1 ! #"
#! (R) j!1 ! RX ! XR ! (R)i!1 ! #
Polymerization
Initiation I k0! "! 2# k0 = 10$4 $10$6
# + M ki! "! R1
Propogation R1 + Mkp! "! R2 kp = 102 $104
Rj$1 + Mkp! "! Rj
Termination Rj + Rika! "! Pi+ j ka = 106 $108
Polymerization
Species Appearance disappearanceI k0 I
! r0 = 2 fk0 I ri = ki!M
R1 ri kp MR1 + ka R1 Rj"Rj kP MRj kp MRj + ka Rj Ri"Pj
ka
2Rj# i Ri"
Polymerization
Initiation rate ri = 2 fk0 I
Total radicals !Rj = "0 =ri
ka
#
$%&
'(
1/ 2
Monomer consumption rM = kp M"0
Radical concn Rj =ri
kP M
#
$%&
'(1
1+ ri / rM
#
$%&
'(
j
polymer generation rPj= Rj j )1( ) ka
2ri
kP M
#
$%&
'(
Polymerization
0 500 1000 1500 2000 2500 3000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
wei
ght f
ract
ion
number of monomers
0.04 0.02 0.08
Initiator
0 500 1000 1500 2000 2500 3000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
wei
ght f
ract
ion
number of monomers
360 180 60
time (min)
Polymer weight distribution
Chemical Reaction Engineering Catalytic reactions
Jayant M. Modak Department of Chemical Engineering Indian Institute of Science, Bangalore
Catalytic reactions
Solid
Gas
(2nd Liquid)
Liquid
Catalyst
Heterogeneous catalysis
Catalytic reactions
H2O2 soluti
on 25°C
stable over months
>320°C uncontrolled,
thermal decomposition in
seconds
25°C controlled, catalytic
or enzymatic
decomposition in seconds
Example: Hydrogen peroxide decomposition
2H2O2 2H2O + O2
Catalytic reactions Efficiency of Phthalic Acid Anhydride Production
Non-catalytic Oxidation of naphthalene in fluid phase
with MnO2+HCl (1872), Chromic acid (1881),
Oleum (1891)
Catalytic Oxidation of o-Xylene in the gas phase
on V2O5-catalyst
Yield: 5-15%
Yield: 75-87%
Catalytic reactions Efficiency of nitrogen fixation
catalytic processes
Catalytic reactions Product spectrum from partial oxidation of propene
substrates catalysts products
propene + oxygen
acrolein
acrylic acid
acetone
propylene oxide
acetic acid
1,5-hexa- diene
benzene
Catalytic reactions
Steps during the course of the reaction
External diffusion Internal diffusion Adsorption on the active sites Surface reaction forming the products Desorption of the products Internal diffusion External diffusion
!
" # $
% &
'
reaction: substrate A product P
fixed bed reactor
Boundary layer
Lumping analysis
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0 A1
A2
A3
Concentrations
Time
A10=1, A20=0, A30=0
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0A10=1, A20=0, A30=0
A1+A2
A3
Â1
Â2
Concentrations
Time
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0A10=0.5, A20=0.5, A30=0
Concentrations
Time
B
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6
0.8
1.0A10=0.5, A20=0.5, A30=0
A1+A2
A3
Â1
Â2
Concentration
Time
Lumping analysis
0.0 0.1 0.2 0.3 0.4 0.5
0.6
0.8
1.0Â1=A1+A3, Â2=A2
1,0,0 0,0,1
Concentration
Time
Initial A1,A2,A3
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