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Catalytic Reaction Engineering (CRE)Kinetics Catalytic reactions
• Examples reactor systems
• Description ideal reactors• batch, CSTR and plug flow
• Catalytic kinetics
• Effects of catalyst properties• mass and heat transfer
• Labscale reactors - performance testing• purpose
• criteria
Catalytic conversion process selection and design
Reactor
?
Reactants
Desired productsUndesired productsUnconverted reactants
Economics
Environment & Safety
Processrequirements
Minimum cost of overall process
•Maximum selectivity•maximum conversion•ease of scale-up•high throughput•low pressure drop•…….
•Intrinsically safe•WRAP•…….
Energy
Ammonia oxidation reactorsInstallation
wire-mesh gauze
Ammonia oxidation 2NH3 + 2O2 NO + 3H2O Pt-Rh gauzes – various structures
Pd – gauze (Pt entrapment)
‘Bispin’ ‘Warp knitted’
Fixed bed reactors
Hydrotreatingadiabatic
Methanol synthesis Isothermal reactor(Linde type)
Aromatization – Amoco Ultrafining
Hexane Benzene + 3H2
Spherical reactorFixed bedPressure drop Endothermal
Reactors in seriesadiabatic
FTS – Multi-tubular reactor
• Maximum weight = 900 tonnes• Diameter = 6 m• Height = 20 m• 8000 tubes• Reactor productivity = 300 tonnes
MD/day • Cooling by steam generation: water
evaporation
G L
Fischer-Tropsch synthesis I - Secunda
Sasol Slurry Phase Distillate(waxes)
Bubble column slurry reactor
• Fine catalyst particles ~ 50m• 2500 bbl/day production• Diameter 5m, height 22 m• Cooling by steam generation:
water evaporation
nCO + 2nH2 -(CH2)n- + nH2O
Steam reforming -CH4 + H2O 3 H2 + CO
CO + H2O H2 + CO2
Catalytic cracking
Heavy feedstock Lighter Products (gasoline) + coke
Catalyst deactivation~50 m particles
Batch – CSTR reactor
agitators
cooling/heating tube/jacket
stirrer motorHand holes for reactor charging
LiquidGas-liquidGas-liquid-solid
Batch – CSTR reactors
Cryo-reactor
2200 l
100 l
Fermentation - biocatalysis
Beer brewing
Researchfacility
Bioreactors – waste water teatment
Aerobic reactorsCatalytic process Slow stirring
Environmental – Automotive TWC
gases
Monolith wall
poroussupport
activeactivecomponentcomponent
Low pressure dropNO, CO, HC removalPt-Rh/Al2O3 catalyst
Diesel - Johnson Matthey CRT
Pre-oxizider Wall-flow monolith
Gas distributor
Exhaustgas inlet
Exhaust gas outlet
NO +O2 NO2NO +O2 NO2
NO2 + C CO2 + NONO2 + C CO2 + NO
Solids, gasesSeparatorBifunctional catalysis
MicroreactorsExcellent heat removalHigher selectivities
Selective oxidation
CVD reactors - semiconductors
Multiwafer reactors
Hot-wall
Cold-wall
Home appliances
Matsumoto et al., 1993US 5266543
Tefal Azura
Reaction coupling – SMART reactor ABB
Ethylbenzene dehydrogenation
Elimination heat exchanger,hot piping & steam superheater
Higher conversion per pass (80%)Lower energy consumption
H 118 kJ/mol
Reactor supermarket(Krishna)
stirred tank
G
Cat
fluid bed
circulatingfluid bed
membrane reactor
G LCat
Cat
bunkerreactorslurry
reactor
multi-tubulartricklebed
GGpacked bed
G
Riser
methanol,i-butene,n-butene
methanol
MTBE
catalytic distillationreactor
GL
cyclone
G L
Catalytic reactors
• What to choose?• How to design?
Catalytic Reaction Engineering (CRE)
• Examples reactor systems
• Description ideal reactors• batch, cstr and plug flow
• Effects of catalyst properties• mass and heat transfer
• Labscale reactors• purpose
• criteria
• Tutorials – application/illustration
• Biocatalytic reactor engineering
Ideal reactor types
Continuous stirred tank reactor (CSTR)
Plug flow reactor(PFR)
Batch reactor
Continuous Discontinuous
cT
c(z)T(z)
c(t)T(t)
How to describe these?
Gas/Liquid/Solid Reactors
Mechanicallyagitated
Bubblecolumn
Slurry
Trickle-bed Monolith
BedFixed
The Chemical Engineer’s tool
Input Output
Input - Output + Production = Accumulation
units: mol/sunits: mol/s
2 kg/s ??Production ?
Accumulation ?
Steady state
3 mol A /s ??
Steady state = 0
Water-tap: Liquid volume in bucket
steady stateunsteady stateor transient
bucketliquid VV tV tapliquid
t=0
Rate definitions - units
In chemistry usually: mol A / m3 s
m3reactor ,m3
catalyst ???
mol / sm reactor3
mol / sm particle3
mol / skg catalyst
p
V
W
V
rrr
per m3reactor
kg catalyst
m3particle
mol As
In mass balance
Rate definitions - units
R rV A A V, rate expression
stoichiometric coefficient+ products- reactants
V r V r W rV p V wp
mol / sm reactor3
mol / sm particle3
mol / skg catalyst
In mass balance unit: mol A / s
Continuous (flow) stirred tank reactor(CSTR)
Isothermal
FA0 FA
rW,W
cA0
cA
Molar balance
00 WRFF WAA
0A A
WF X R
W
)1(0 AAA XFF
W
A
A RX
FW
0
‘space time’
In - Out + Production = Accumulation
Design equation CSTR
Introduce X = conversion
Graphical interpretation
0
1A
A W
W XF r
1
Wr
AX
CSTR
n > 0Area
CSTR operates here
zero order?negative order?
Order of reaction?
W
A
A RX
FW
0
‘space time’
2 CSTRs in series??
Continuous (flow) stirred tank reactor(CSTR) 1st order reaction
FA0 FA
rW,W
cA0
cA
W
A
A RX
FW
0
A .......
AAAW Xckckr 1011
01
0
1 w
AA k
cc
01
01
1
w
wA k
kX
A= -1
0
00
A
A
FcW
spacetime
Relationship between CA0 and CA??
Batch reactor type
rW,W,V
Molar balance
dt
dcVdt
cVddt
dNWR AAAAW
,00
VRW
dtdc AWA ,
)1(0 AAA Xcc
dXR
tVW
c
AX
AWA0 ,0
11
AWA
A RVW
cdtdX
,0
1
Design equation Batch reactor
tVW
B ‘batchspace-time’
In - Out + Production = Accumulation
Plug flow reactor (PFR)Isothermal
dWFA0
FA FA+dFA
rW
Molar balance
0, dWRdFFF AWAAA
dXFdFXFF
AA
AA
0
0 )1(
AWA
A RdWdXF ,0
AWA
A RFWd
dX,
0
‘space time’
cA0
cA
0 W
XA
In - Out + Production = Accumulation
dXRF
W AX
AWA0 ,0
1
(integral)
Reactor design equations
AW
AA
A
A
RXc
FcW
,0
0
00
AX
AWA
A
A
RdXc
FcW
0 ,0
0
00
AX
AWAB R
dXctVW
0 ,0Batch
Plug flow
CSTR
similarity !
simplicityFA0 FA
r,W
r,V,W
FA0 FAr(X,z)
,
0 0
W A A W
A V A
R rF c
Reactor characteristics: CSTR versus PFR
0AFW
Wr1
AX
CSTR
PFR
n > 0
Wr1
AX
CSTR
PFR
n < 0
Similar conditions:• W/F PFR < CSTR positive orders• W/F PFR > CSTR negative orders• CSTR operates at lowest reactant concentrations• PFR at maximum local concentrations
Area=
Which one is most efficient???
Series reaction - Profilesmost efficient: PFR or CSTR??
A Q Pkw1 kw2
2s-1 1s-1
Max. yield PFR>CSTR(n>0)
0.00 0.50 1.00 1.50 2.000.00
0.20
0.40
0.60
0.80
1.00
Ci
Plug flow/BatchCSTR
P
A
Q
0 / kgcat m-3 s-1
Maximum yields
0.0
0.2
0.4
0.6
0.8
1.0
k2/k110-6 10-5 10-4 10-3 10-1 100 101 102 10310-2
k2/k110-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
0.0
0.2
0.4
0.6
0.8
1.0Maximum yieldsMaximum yields
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
k2/k110-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103
k2/k110-6 10-5 10-4 10-3 10-1 100 101 102 10310-2
Series reaction - max. yields
kw2/kw1
YQ,maxPFRCSTR
2
1
2max, 1
w
wQ k
kY 21
2
1
2max,
ww
wkk
k
w
wQ k
kY
Tutorial 1
A second order reaction A R has been studied in a Berty-reactor, a CSTR suited for the investigation of solid catalysed reactions. The following data are available:
V = 1 l W = 3 g catalyst v = 1 l h-1
cA0 = 2.0 mol/lcA = 0.5 mol/l
a. Determine the value of the rate constant and give its dimensionb. How much catalyst is needed to obtain 80% conversion in a packed
bed reactor at a volume flow rate of 1000 l/h and an inlet concentration cA0 = 1 mol/l ?
Tutorial 2 - Batch conversion sucrose
At room temperature sucrose can be hydrolysed by the enzyme sucrase:
sucrose products
Starting with an initial sucrose concentration of 1.0 mmol/l and an enzyme concentration of 0.01 mmol/l the following data have been obtained in a batch reactor. Concentrations have been determined by using polarized light.
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
tt
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
t
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
t
Verify that the data can be representedwell by a kinetic expression of the Michaelis-Menten type:
Determine the parameters k and Ms
Es
cMcck
r
0
Catalysis Engineering: Questions
• Measure and compare activities of catalysts for reactions?• Compare catalyst selectivities? For what purpose?• How? What does your reactor look like?
• How do you define your catalyst activity ?• Perform kinetic studies?• How would you define reaction rate and how to determine it ?
• What do you think plays a role in your measurements?• Are you sure you get the information you want?
Do you ever:
Ideal reactor types
Continuous stirred tank reactor (CSTR)
Plug flow reactor(PFR)
Batch reactor
Continuous Discontinuous
cT
c(z)T(z)
c(t)T(t)
How to describe these?
Reactor design equations
AW
AA
A
A
RXc
FcW
,0
0
00
AX
AWA
A
A
RdXc
FcW
0 ,0
0
00
AX
AWAB R
dXctVW
0 ,0Batch
Plug flow
CSTR
similarity !
simplicityFA0 FA
r,W
r,V,W
FA0 FAr(X,z)
Phenomena in catalytic reactor (fluid-solid)
PLUG FLOW MIXINGDISPERSIONVELOCITY PROFILE
DIFFUSIONREACTIONTRANSPORT PHENOMENA
Reactor level
Particle level
Temperature and concentration profiles within catalyst particle
T
c
Exothermal
T
c
Endothermal
Rates different from rate at bulk conditionsHow to handle ?
How would they qualitatively be??
GasGas
CatalystCatalystLiquidLiquid
Gas concentrationprofile
Three-phase catalytic process
Catalytic reactor design equationplug-flow, steady state
Wii
i rFWd
dX 0
stoichiometric coefficient i
‘catalyst effectiveness’
intrinsic rate
conversion i
‘space time’
deactivation function
bb,Tc at raterate effective
Use: effective rate
External mass transfer - isothermal
cs
cb
film layer around particle
)()(
at raterate real
b
s
b crcr
c
How to determine cs ?
obsr
Isothermal - external mass transfer
mol/scs
cb
reaction ratein particle:
mass transfer rateto particle: =
sbfp cckA svp crV =
rate per particle volume(mol/s.m3
p)mass transfer flux
(mol/s.m2)
mol/s
In - Out + Production = Accumulation
L/S, G/S, L/L reaction systems
Isothermal first order - external mass transfer
cs
cbfilm layer
)(
'1
1bvebv
f
v
obsv crck
kak
r
sbfp
psv cck
VA
ck '
' 1' 1f
s b bvf v
f
k ac c ckk a kk a
p
p
VA
a '
or:
sbfp cckA svp crV =v vr k c
Limits?
Mass transfer control
Kinetic control
''
f v
f v
a k ka k k
bvf
obsv c
kakr
11
'1
rate determined by
physical resistance and by chemical resistance
Effective rate:
Isothermal - internal mass transport
Slab-type catalyst
Diffusion and reaction
Concentration profile
Reaction rate profile
Profiles?Effectiveness factor
Effective diffusivity porous media
Flux direction
tortuous pathlonger
only fraction openfor diffusion
gradient dc/dxdirection
component gradientin flux direction
combined to ‘tortuosity’
DDeff
dxdcDN eff
Isothermal - internal mass transport
Mass balance, steady state difusion & reaction
02
2
ckdx
cdD ve1st order irreversible:
)cosh(
)cosh(
Lx
cc sSolution:
Lk
Dv
eff
0
Slab
xx+dx
L
c/ci
x/L1.0 0.8 0.6 0.4 0.2 0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.1
1.0
2.0
10.0
‘Thielemodulus’
Limits?
0
Some mathematics - Hyperbolic functions
1)tanh(3
121)cosh()tanh()sinh(
3.0
)sinh()(cosh')cosh()(sinh'
)coth(1
)cosh()sinh()tanh(
2)cosh(
2)sinh(
2
xx
xxxxx
x
xxxx
xxxx
eex
eex
xx
xx
0.0 0.5 1.0 1.5 2.0
x
0.0
1.0
2.0
3.0
4.0
cosh sinh
tanh
Catalyst effectiveness
pssv
V
v
i VTcr
dVTcrp
),(
),(
conditions surface external at raterate observed 0
0 11
i
i
i tanh
Slab:
Limits:1st order irrev.
0.1 1 10
0.1
1
Effectiveness factor- experimental
0.1 1 100.1
1
2
3
Post et al. AIChE-J 35(1989)1107
Fischer Tropsch synthesis
n CO + m H2 CnH2(m-n) + n H2O
Co,Zr/SiO2 catalystH2/CO=221 bar473-513 Kdp= 0.38-2.6 mm spheres
rv=kvpH2 (zero order CO)
Classical catalyst particlesFoam structures
200 cpsi200 cpsi 400 cpsi400 cpsi 600 cpsi600 cpsi
1.80/0.27 mm1890 m2/m3
= 0.72
1.27/0.16 mm2740 m2/m3
= 0.76
1.04/0.11 mm3440 m2/m3
= 0.8
Monoliths - cell density Generalizations - isothermal - internal
Geometry
slabcylinder
sphere
0.1 1 10
0.1
1
LVA a
p
p
1'
0 11
i
i
Slab:
Cylinder:
Sphere:
L L
LR
LR
2
3
tanhUse:
v
eff
kLD
L???
12
1 ni
n
e
v cDkLnth order:
Kinetics
Controlling regimes
• Kinetic control
• Diffusion control(internal)
• Mass transfer control(external)
bfbveobs
v ckacrr ')(
)()( bv
bviobs
vcrcrr
)( bvobs
v crr
• How to determine in which regime?• What do we observe?• How to determine in which regime?• What do we observe?
What’s observed?extraparticle limitation, first order kinetics
',obs
v p f br a k c
' 1aL
Slab:
Cylinder:
Sphere:
L L
LR
LR
2
3
' 1p
ad
• dependent of u, dp• first order• no activation energy
External mass transfer increases at increasing linear velocity
0.6 0.7fk uFrom literature
What’s observed? intraparticle limitation
11 nsev
chemchemobs cDk
Lrrr
particle size dependent
reaction order (n+1)/2
activation energy: Eatrue/2
Post et al AIChE-J 35(1989)1107
1.90 1.95 2.00 2.05 2.100.001
0.01
0.1
dp/mm
0.38
1.42.4
1000/T
kvobs
wide pore silica sphereseffect dp
Limiting case: ‘Falsified kinetics’
Tutorial 8
Observed temperature behaviourBernardo & Trimm Carbon 17(1979)115
Catalysed steam gasification coke on Ni catalyst
C + H2O CO + H2
Ni
• p(H2O)=26 kPa• thermobalance• coked catalyst:Ni/Al2O3
0.9 1.0 1.1 1.2 1.3 1.4
1000/T
0.01
0.1
1
5
r(ob
s)
061
164
10.75
0.6
Ea(kJ/mol)
order n
Summary dependencies rv,obsstrong mass transport limitations
External mass transfer:
Internal mass transfer:
'n
obs f b bm
ur a k c cL
11 niev
chemobs cDk
Lr
r
depends on: 1/L, (n+1)/2 reaction order, Eaapp= ½Eatrue
depends on: L, flow rate, 1st reaction order, Eaapp= 0
Kinetics unknown effectiveness cannot be calculated
How to check if limitations are present ?
Isothermal - external mass transfer
n
b
s
pbbv
pssve c
cVTcrVTcr
),(),(
conditions fluid bulk at raterate observed
b
s
b
sb
bf
sbf
bf
obs
cc
ccc
ckaccka
ckarCa
1)(
')('
'
Criterion:
Ca < 0.05/n
05.01e
Catalyst effectiveness:
Observable quantity:
0.001 0.01 0.1 10.01
0.1
1
10
n =
-1
0.5
1
2
Ca
e
e Ca 1
enCa 1
Criteria - experimental verification
External transfer:ncka
rCa
bf
obsv 05.0'
,
Criterion: 05.01,
, chemv
obsv
rr
5%deviation
p
p
VA
a ' mass transfer coefficient
reaction order
observed rate
particle properties
Diffusion control?
Kinetics unknown effectiveness cannot be calculated
Weisz-Prater: 2
2 12
observed rate'diffusion rate'
obs ni
e s
L rD c
(nth order)slab
15.02
12,2
n
cDLr
ieff
obsvGi
0.1 1 100.1
1
i
i2
cylinder
sphere
Criterion:
Effect temperature rise
How much increases rate constant ?
( ) exp 1 exp( )
a bib
b b i b
E Tk T Tk T RT T T T
1.0
1.1
1.2
1.3
1.4
1.5
0 2 4 6 8 10
80
40
T / K
k(Ti)/k(Tb)
Tb=500 KEa(kJ/mol): 120
Criterion 0.05
A few degreesalready critical !
11 exp 11
ne b
e
CaCa
External transfer: 05.0Cabe
When temperature effects?
External transfer: 05.0Cabe
Internal transfer:
05.02
2
isi
External gradient criterion more severe than internal criterion
5%Criterion
i
e s
e s
D H cT
( )
0-0.3 (exothermal)
Prater numbers
i
e
10-104 gas-solid
10-4-0.1 liquid-solid
b
bfe hT
ckH )(
b
ab RT
E
10-20
Temperature and concentration profiles
Largest T-gradientin film layer
Largest c-gradientinside particle
T
c
Exothermal
T
c
Endothermal
Temperature gradient in catalyst bed 05.01
8111
,
2,
t
p
wb
wbeff
tobsvr
w
a
dd
Bib
TrrH
TRE
18
)1(particle gradient-T film
gradient-T bed 2
,
,
2
b
effb
effp
p
t srr
criterioncriterion
extintintextbed gradcgradTgradcgradTgradT ,
temperature gradient in bed always develops first !
Summary:
Tutorials
• Tutorials #6, 7 & 10
Laboratory performance testing catalystsKinetic studies
Optimization
Preparation
Screening
Reaction network
Stability tests
Scale-up
Combinatorialstage
Quantificationstage
Increasing:• time• money• reality
Kinetics
Catalyst testing & Kinetic studiessolid catalysts
Intrinsic reaction rate data Not obscured by parasitic phenomena
reactor characteristics mass and heat transport phenomena
particle – reactor scale user manipulations catalyst misbehaviour
deactivation/fouling
Information wanted
• Comparison activities and selectivities• Kinetic modeling
For
How ?
How to obtain intrinsic performance data?
Choose a well-defined reactor– Ideal type: CSTR, plug flow,..– Dimensions: L, dt, dp, shape– Hydrodynamics
• Flow distribution• Wetting, contact phases
Avoid undesired gradients– C and T gradients on a particle scale– C and T gradients on the reactor scale
Starting point for example 1 0.05observed
ideal
RateRate
Laboratory Reactors
– simple– yields conversion data, not rates– deactivation noted directly– small amounts of catalyst needed
– direct rate data from conversions– larger amounts of catalyst and flows needed– deactivation noted directly
– non-ideal behaviour– continuous handling of solids possible
– limited to weight changes– careful date interpretation needed– often mass-transfer limitations
– yields conversion and selectivity data quickly over large range– Easy to change feed– catalyst deactivation hard to detect
PFR
CSTR
FBR
TGA
Batch
Proper comparison - Selectivity
A Q Pk1 k2
2s-1 1s-1
k / s-1
0.00 0.50 1.00 1.50 2.000.00
0.20
0.40
0.60
0.80
1.00
Ci
Plug flow/Batch
P
A
Q
catalysts of different activitydifferent product yieldskinetic selectivity = 2
Compare selectivities at similar conversion levels !
Important checks
Particle criteria: External temperature rise CarberryInternal mass transfer Weisz-Prater
Bed citeria: Temperature rise MearsFlow velocity profilePlug flow - dilution
Particle level 5% criteria – ‘Observables’
• External (film) gradients– Concentration
– Temperature
• Internal (particle) gradients– Concentration (Weisz-Prater)
– Temperature
,( ) 0.05'
v obsa r f bb e
b b f b
rE H k cCaR T h T k a c
2,2 1 0.15
2v obs
ieff s
r L nD c
2,2
,
0.1r eff s v obsas i i
s p eff s eff s
H D c r LER T T D c
, 0.05'v obs
f b
rCa
a k c n
‘Ten commandments of catalyst testing’ - Dautzenberg
Diagnostic tests mass transport limitations
1. Particle size variation
2. Flow rate variation at constant space time!
X
FA0,1 FA0,2 FA0,3
XA,1 XA,2 XA,3
W3W2W1
FA0,1 FA0,2 FA0,3
particle size
observed rate egg-shell catalysts?
Packed bed reactor - assumptions
equal res,,TT
axialdispersion
velocityprofile
radialtemperature
gradient
res varies T varies
plug flowisothermal
real lifeideal
Impact on observed conversion levels10t
p
Dd
“dispersion” analogous to diffusionDax “Dispersion coefficient”
33t
p
D Xd
Criterion:
Catalyst bed size
Practical catalyst: often dp = ~1 - 3 mm
‘large’ reactor needed
xPe
ndL
pp
b
11ln8
X =0.8n =1
L > 25-75 mm Dt > 25 –75 mm
Moreover, velocity profiles
0.03t
p
D Xd
Axial dispersion
pp
ax
u dPe
D
Temperature rise in catalyst bed
Mears:
05.0
81112
wt
p
w
a
wer
btobs
v
Birr
RTE
TbrrH
Effective thermalbed conductivity~ 1 J/s.m2K
Reactionheat production vs. conduction
Activation energy
Wall effectheat transfer vs. conduction
Biot wall number~ 0.8-10
b = fraction inertdiluent
Generally most severe temperature criterion
What to do ?
Catalyst testing - Bed dilution
Bed dilution (e.g. SiC)• Hydrodynamics determined bysmall particles (wetting, velocity)
• Longer bed, larger L/dp• Testing of real catalyst particles• Better heat conduction• Larger heat transfer area• Less heat produced per volume
Dt/dp>~10-15 or Dt/dp<4 Lb/dp>~50
Real particle
Heat transferarea
Diluent
decoupling hydrodynamics and kinetics
Bed dilution - bypassing ?
inhomogeneous distributioncatalyst by-passing
Berger, Perez et al.App.Catal.227(2002)321Chem.Eng.Sci. 57(2002)4921Chem.Eng.J. 90(2002)173
Do not: • dilute too much• use too high conversion
0.051 2
pobs
bed
dxbb L
Criterion:
b = fraction inert diluent
(= deviation rate constant less than 5%)
Bed dilution: detrimental?
non-porousquartz
non-porousquartz
Catalyst Diluent
catalyst by-passing?
inhomogeneous distribution
Practical exampleEffect of Catalyst/Diluent Distribution in Decomposition of N2O
0
20
40
60
80
100
120
140
160
Eaap
p/
kJ m
ol-1
Range I Range II137 kJ mol-1
-11
-10
-9
-8
-7
-6
-5
1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50
ln( k
obs )
Range I
Range II
Achieve a homogeneous mixture of catalyst and diluent !
1-xN2O
11(Wcat /FN2O,0) pN2O,0
kobs = ln
10 T -1/ K-1-3
Heat effects in packed-bed reactor
Heat production/consumption
Cooling-heating:• Reaction coupling• Heat exchange
• through wall• no wall
• Evaporation
Poor heat transfer• in bed• to wall
Improvements:• foams (ceramic,metal)
• catalytic coatings
• forced flowradial axial
T-profiles
250 Whm K
4210 Wh
m K
G.Kolios et al. Chem.Eng.Sci. 57(2002)1505TCR, UOP
Coated wall reactor
Better heat removal
Exothermal reactionsoxidationhydrogenation
But:Velocity profile?Concentration gradients?
Monoliths, microreactors, kinetic studies
Coated wall – flow patterns
0.160.23 'CWRX
nPe
1.48
1.04 'CWRXnPe
Flow pattern
Concentrationprofile
Porous catalytic walls
Flow pattern
Concentrationprofile
Porous catalytic walls
Flow pattern
Concentrationprofile
Flow patternFlow pattern
Concentrationprofile
20
,
'A rad
u L RPeD L
Criteria
R.J. Berger & F. Kapteijn Ind. Eng. Chem. Res. 46 (2007) 3863Ind. Eng. Chem. Res. 46 (2007) 3871
Coated wall reactors
• Monoliths
• Microreactors
• Kinetic studiesRedlingshöfer et al.Ind.Eng.Chem.Res.41(2002)1445-1453
washcoatsupport
mm size
0.05-0.2 mm
5-15 mm
Berty-type Carberry-type
Internally mixed Externally mixed
Flat blade basket Pitched blade basket
CSTR – fixed bed
GasLiquid
Alternative reactors for multiphase kinetics measurements
Batch – Liquid phase systems – fixed bed
Robinson-Mahoney
Recirculation reactor300 ml (turbine)
SISR
Alternative reactors for multiphase kinetics measurements
Turbine Reactor Screw Impeller Stirred Reactor
Alternative reactors for multiphase kinetics measurements
F.Kapteijn and J.A.Moulijn, Laboratory testing of solid catalysts in: Handbook of Heterogeneous CatalysisWiley-VCH Verlag, Weinheim, 2008, p. 2019-2045
(Semi-) batch – G-L-S systems Swinging capillary reactor
+ =
Fixedpoint
BentrodCapillary
In/out
Heating
S.Tajik et al., Meas.Sci.Technol. 1(1990)815
Monolithic Stirrer Reactor
I.Hoek et al. Chem.Engng.Sci. 59 (2004) 4975-4981R.K.Edvinsson-Albers et al. AIChE J. 44 (1998) 2459-2464
Mn-oxide/Alumina H2O2 decomposition
Principles Catalyst Performance Testing
Down scale as far as possible– Lower cost equipment– Less material consumption– Lower utility demands– Safer– Less labour– Less synthesis effort– More options to test
Do not mimic industrial reactor• Output industrial reactor: $$$$ or €€€€• Output laboratory reactor: knowledge
No Dinky Toy / Matchbox approach!
Scaling down steps
N. van der Puil
N. van der Puil
Observations
• Mainly fixed bed and batch slurry systems applied
• Massive parallelization• Cost reduction• Used for
– Catalyst screening– Catalyst performance– Kinetic studies
1960 1970 1980 1990 20000.01
0.1
12
0.5
Year
Manhour per reactor hour
Pilot plant(non-automated)
Microflow(automated)
Bench scale(semi-automated)
Bench scale(non-automated)
Sie, AIChE-J. 1996
parallellization
‘Workhorse’ in catalyst testing
Six-flow equipment
Plug flow - parallelization
VENT
ANALYSIS
MFC
MFC
MFC
MFC
MFC
MFCMFC
MFC
MFC
SV
BPC
P
FEED CONTROL
BPC
REACTOR
Diesel sootFTSN2O, NOxHDS
Pérez et al. Catal. Today 60(2000)93
N2O/NOx decomposition set-up
•• FischerFischer--TropschTropsch•• Soot abatement Soot abatement •• CFC, AutomotiveCFC, Automotive•• SCRSCR
GCNDIRGCMS, NOx
Other systems:
x
Particle sizedp1 dp5
x5
x1
referencecatalyst
x
FA0,1
Flow rateFA0,5
x1
x5
(b)
x1 x2 x3 x4 x5
W1 W2 W3 W4 W5
oAFo
AFoAFo
AF oAF o
AF
(a)
x1 x2 x3 x4 x5
oAFo
AFoAFo
AF oAF o
AF
(c) dp1
x5x1
dp5increasing
particle size
oAF o
AFoAF
(d)
x1 x5
W1 W5
increasingflow rate
constantoAii FW /
oAF o
AF 1oAF 5
Commercial developments
Nelleke van der Puil, dec. 2008
Commercial developments
Nelleke van der Puil, dec. 2008
Commercial developments
Nelleke van der Puil, dec. 2008
Kinetics
Procuring rate data laborious task
conversion vs. space time W/Ftemperaturepartial pressures / concentrations
Improve speed:• PC controlled equipment• Six-flow set-up (parallel reactors)• Temperature scanning• Sequential experimental design
Don’t forget: stable catalyst, blank runs, duplicates, criteria
Kinetics of catalysed reactions
Catalysis Engineering: Questions
Measure and compare activities of catalysts for reactions? Compare catalyst selectivities? For what purpose? How? What does your reactor look like?
How do you define your catalyst activity ? Perform kinetic studies? How would you define reaction rate and how to determine it ?
What do you think plays a role in your measurements? Are you sure you get the information you want?
Do you ever:
Kinetics of catalysed reactions
Chapters 3 and 8
Kinetics Reactor theory Experimental aspects
– Interpretation– Reactors– Interfering phenomena
• Mass transfer• Diffusion • Dispersion• Criteria
Problems/questions
Kinetics of catalysed reactions
Structure
Reactor engineering Catalysis
Reaction modelsKinetics
Behaviour singleparticle
Ideal reactorsBatchCSTRPlug flow
Non-ideal reactors
Catalytic Reactor
Transportphenomena
Heat & Mass
Kinetics of catalysed reactions
Kinetics of Catalysed Reactions
Why Reaction Kinetics Derivation rate expressions Simplifications
– Rate determining step– Initial reaction rate
Limiting cases– Temperature dependency– Pressure dependency
Examples
Kinetics of catalysed reactions
Process Development
34%
Process Optimisation
30%
Other1%
Catalyst Development
29%
Mechanistic Research
6%
Utilization of kinetic data in industryQuestionnaire 1997
Bos et al. Appl.Catal. A160 (1997) 185-190
www.eurokin.tudelft.nl Kinetics of catalysed reactions
Utilization of kinetic data for different chemical industry sectors
Process Development
34%
Process Optimisation
31%
Other1%
Catalyst Development
26%
Mechanistic Research
8%Process
Development30%
Process Optimisation
37%
Other4%
Catalyst Development
27%
Mechanistic Research
2%
Process Development
15%Process
Optimisation17%
Other0%
Catalyst Development
56% Mechanistic Research
12%
Process Development
56%
Process Optimisation
28%
Mechanistic Research
1%
Catalyst Development
15%
(a) Chemical Companies (b) Oil Companies
(c) Catalyst Companies (d) Engineering Companies
Other0%
Kinetics of catalysed reactions
Rate expressions
Rate expressions in principle crucial for– design– process start-up and control– process development and improvement– selection reaction model
General relationship
Often used– power rate models– models based on elementary processes
• extrapolation more reliable• intellectually process better understood
mj
ni ppkr
2
222
1/
BBAA
eqBABAT
pKpKKppKKksN
r
( ,....... , , ,........., ,......, )i T i i eqr f p T N k K K
Kinetics of catalysed reactions
Rate data, Examples A PBatch reactor
Rate equation??
CA
t
CA
t
CA
t
CA
t
CA
t
r = kcAr = k
r = kcn, n ~2
r = k(cA-cp/Keq)r = k
r = kcA
Does power rate equation fit?If so, n = ??
Kinetics of catalysed reactions
Role of catalyst?
Concentrating reactantsadsorption/complexation
Providing alternative reaction pathcatalyst selectivityother activation energy barrier
But:– other components adsorb, too
block ‘active sites’– fixed number of ‘active sites’
rate constant
affect rate
affect rate
affect rate
other form rate expression expectedKinetics of catalysed reactions
Rate expression – Catalysed reaction
CadsCB
adsA scksckr
forward ratebackwardSuccess frequency
amount of A adsorbed
chance of adjacent B adsorbed
Note:• cgas and cads differ• ratios components differ
Kinetics of catalysed reactions
Simple example: reversible reactionA B
A B
A* B*
‘Elementary processes’
‘Langmuir adsorption’
1r 1r
2r
2r
3r 3r
How many unknowns, when the overall rate is known?Kinetics of catalysed reactions
Elementary processes
Rate expression follows directly from rate equation
Eliminate unknown surface occupancies
1 1 1 * 1A T T Ar r r k p N k N
2 2 2 2T A T Br r r k N k N
3 3 3 3 *T B B Tr r r k N k p N
Kinetics of catalysed reactions
Site balance:
Steady state assumption:
Rate expression:
Elementary processes contd.
1 * A B
0
0
dtddt
d
B
A
321
321
:with(......)(......)(.....)
)/(
KKKK
ppKppkkkN
r
eq
BA
eqBAT
MicrokineticsMichaelis-Menten
Algebraic eqs.
Very simple case, nevertheless quite complex equation
1 2
2 3
r rr r
Kinetics of catalysed reactions
Quasi-equilibrium / rate determining step
r1
r2
r3
r-1
r-2
r-3
r‘quasi-equilibrium’
rate determining
r = r2 - r-2
Kinetics of catalysed reactions
Rate expression - r.d.s.
r r r k N k NT A T B 2 2 2 2
Rate determining step:
Eliminate unknown occupancies
Quasi-equilibrium:
r r k p N k NA T T A1 1 1 1 *
So:
A A
BB
K p K kk
pK
1 1
1
1
3
*
*
with:
Kinetics of catalysed reactions
Unknown still *
Rate expression, contd.
Substitution:
r r r k N K p k N p Kr k N K p k p k K K
T A T B
T A B
2 2 2 1 2 3
2 1 2 2 1 3
* *
*
//
K K K K ppeq
B
A eq
1 2 3
where:
Kinetics of catalysed reactions
‘lumped’
Rate expression, contd.
Site balance:
1 1 1 3 * * /A B A BK p p K
* /
1
1 1 3K p p KA B
Finally:
r
k N K p p KK p p K
T A B eq
A B
2 1
1 31/
/
Kinetics of catalysed reactions
Other rate determining steps
Adsorption r.d.s
Surface reaction r.d.s.
Desorption r.d.s.
r
k N K p p KK p p K
T A B eq
A B
2 1
1 31/
/
r
k N K K p p KK K p
T A B eq
A
3 1 2
2 11 1/
r
k N p p KK p K
T A B eq
B
1
2 31 1 1/
/ /
Rule of thumb: Generally surface reaction r.d.s.
‘lumped’
Kinetics of catalysed reactions
Thermodynamics
Equilibrium constantReaction entropy
Reaction enthalpy
ln ( ) ( )o o oeqRT K G T H T T S
)(, TGi
oifi
Adsorption constant
Adsorption enthalpy,<0(J/mol)
Adsorption entropy, <0(J/mol K)
atm-1
RTH
RSK
oo
A
ln
Data sources: Handbooks, API, JANAFChemsage, HSC, YAW’s Handbook Kinetics of catalysed reactions
Initial rate expressions
Forward rates Product terms negligible
r k p A '0 r k p
K pA
A A
'0
01 r k '
Adsorption Surface Desorption
r0
T1
T2
T3
pApA pA
T1
T2
T3
T1
T2
T3
low p high p
Kinetics of catalysed reactions
Langmuir adsorption
Uniform surface (no heterogeneity) Discrete number of sites No interaction between adsorbed species
A + * A*
AA
AAA pK
pK
1
KA /bar -1
0.1
1.0
10100
pA /bar
1.0
0.8
0.6
0.4
0
0.2
0 0.2 0.4 0.6 0.8 1.0
Irving Langmuir1881 - 1957
Nobel Prize 1932
Kinetics of catalysed reactions
Multicomponent adsorption / inhibition
Langmuir adsorption
AA
A i i
K pK p K p
1
11
Inhibitors
Kinetics of catalysed reactions
Surface occupancies
Empty sites
Occupied by A
Occupied by B
BB
A B
p KK p p K
//
3
1 31
AA
A B
K pK p p K
1
1 31 /
* /
1
1 1 3K p p KA B
BKK
3
1
Kinetics of catalysed reactions
Langmuir adsorption model
Generally used– Simplification (uniform, no interactions)– although nonlinear, mathematically simple– simple physical interpretation– rather broadly applicable
• multicomponent adsorption• non-uniform surfaces
– ‘compensation effect’– very weak and strong sites do not contribute much
to the rate• for microporous media (activated carbons) often
not satisfactory
RTH
RSK
oo
Aexp
Kinetics of catalysed reactions
N2O decomposition over ZSM-5 (Co,Cu,Fe)
2 N2O 2N2 + O2
Kapteijn et al. J.Catal.167(1997)256-265
Kinetic model
1. N2O + * N2 + O*2. N2O + O* N2 + O2 + *
Rate expression
r k N pk kT N O
1
1 2
2
1no oxygen inhibition1st order
Kinetics of catalysed reactions
Effect of CO on N2O decomposition
0.0 0.5 1.0 1.5 2.0
molar CO/N2O ratio
0.0
0.2
0.4
0.6
0.8
1.0
X(N
2O)
Co-ZSM-5 (693 K)
Cu-ZSM-5 (673 K)
Fe-ZSM-5 (673 K)
CO removes oxygen from surfaceso ‘enhances’ step 2, oxygen removal
now observed: rate of step 1 r1 = k1 NT pN2O
increase: ~2, >3, >100
CO + O* CO2 + *
CO + * CO* (Cu+)
Kinetics of catalysed reactions
Dissociative adsorption
O2 + 2* 2O*
2 2
2 2
0.5
0.51
O OO
O O
K p
K p
Two adjacent sites needed
Lower pressures:
Gerhard ErtlNobel laureateChemistry 2007
STM oxygen on Ru
Kinetics of catalysed reactions
Dissociative adsorption
H2 + 2* 2H*
HH H
H H
K p
K p
2 2
2 2
0.5
0.51
Two adjacent sites needed
Kinetics of catalysed reactions
Initial rates - CO hydrogenation over Rh
Koerts, Van Santen et al.
Kinetic model
1. CO + * CO*2. CO* + * C* + O* (r.d.s.)
r sN kT CO0 2 *
r sk N K p
K pT CO CO
CO CO0
221
Initial rate
0.20.4
0.60.8
1.0
Occupancy (-) 400450
500550
600
Temperature (K)
0
200
400
600
800
Rat
e
Kinetics of catalysed reactions
Langmuir-Hinshelwood/Hougen-Watson models (LHHW)
r kinetic factor driving forceadsorption term
( ) ( )
( )n
includes NT, k(rds)For: A+B C+D
pApB-pCpD/Keq
molecular: KApAdissociative: (KApA)0.5 = 0, 1, 2...
number species inand before r.d.s.
Cyril Norman Hinshelwood(1897 – 1967) Nobel Prize 1956
Irving Langmuir(1881 – 1957) Nobel Prize 1932
Leonor Michaelis(1875-1949)
Maud Menten(1879-1916)
Kinetics of catalysed reactions
adsorption constant
rmax
AA
AAT
pKpKNkr
1
Heterogeneous catalysisLangmuir-Hinshelwood 1916/20
Hougen-Watson
‘active site’
surfaceTNr
(s-1)
Terminology
Turnover numberTurnover frequencyNumber of turnovers
Kinetics
Rate expression
Linearization
Catalytic centre
Biocatalysis
Michaelis-Menten 1913
Lineweaver-Burke
enzyme
Michaelis constant
Vmax
AM
A
cKckEv
0
k (s-1)
number molecules converted/number complexesKinetics of catalysed reactions
Linearization rate expression
AM
A
cKckEv
0
00
111kEckE
Kv A
M
1/v
1/cA
Slope = KM/VmIntercept= 1/Vm
=-1/KM
=Vm
• Lineweaver-Burke• Hougen-Watson
Kinetics of catalysed reactions
Terminology
Heterogeneous catalysis BiocatalysisReactants SubstratesMolecules
Reactor performance
CSTR, autoclaveResidence time, space time
Flow rate
Chemostat, fermentor
Dilution rate
Kinetics of catalysed reactions
Enzyme Catalysis
1. Irreversible inhibition2. Competitive inhibition
3. Non-competitive inhibition
Biocatalysis Heterogeneous Catalysis
S + E ES E + PI + E EI
k2
'2 0'
S
M S
k E cvK c
S + E ES E + PI + E EI
k2’
k2 depends on intermediate concentration
1. Catalyst poisoning (irrev.)2. Competitive adsorption
or inhibition
3. Co-adsorbed intermediates change active sites(‘modifiers’)Affect activity and selectivity
Kinetics of catalysed reactions
What about observed: reaction orderactivation energy ?
Determination:
ii p
rnlnln
ln r
ln pi
slope = order ni
ln r
1/T
slope = -Eaobs/R
ln1
obsaE rR T
LHHW models ? r k N K p
K p K pT A A
A A B B
2
1
Svante Arrhenius(1859 – 1927)
Nobel Prize 1903
Kinetics of catalysed reactions
Reaction order - Activation energy
r k N K p
K p K pT A A
A A B B
2
1rate expression
Reaction order ? Activation energy ?
BB
AA
nn
1 BBAAaobsa HHEE 12
depend strongly on occupancy!vary during reaction
limiting cases? 1A AK p
2 Tr k N
2
1T A A
B B
k N K prK p
1A AK p
General:
Tutorial 13Try it yourself
Kinetics of catalysed reactions
Selective hydrogenation benzaldehyde
0
100
200
300
400
500
600
700
0 5000 10000 15000 20000 25000Time / s
Con
cent
ratio
n/ m
ol/m
3
Benzaldehyde Benzyl alcohol
Toluene
Kinetics of catalysed reactions
Limiting cases - forward rates
Surface reaction r.d.s. r k N K p
K p K pT A A
A A B B
2
1
1. Strong adsorption A r k NT 2
A*
B*
A#*
Ea2 H#Eaobs
kbarrierk#
+
k#-
Kinetics of catalysed reactions
Limiting cases - forward rates
2. Weak adsorption r k N K pT A A 2
Surface reaction r.d.s. r k N K p
K p K pT A A
A A B B
2
1
A* B(g)+*
A#*
Ea2
Eaobs =Ea2+ HA
HR
A(g)+*HA
B*
HB
Kinetics of catalysed reactions
Limiting cases - forward rates
3. Strong adsorption Br k N K p
K pT A A
B B
2
Surface reaction r.d.s. r k N K p
K p K pT A A
A A B B
2
1
A*
A#*
Ea2
Eaobs =Ea2+ HA HB
A(g)+ * +B(g)
A(g)+B*
HB
HA
Kinetics of catalysed reactions
Cracking of n-alkanes over ZSM-5J. Wei I&EC Res.33(1994)2467
Carbon number
AApKkr 20
Aaobsa HEE 2
negative!?Ea2
Eaobs
HA
kJ/mol
200
100
-200
-100
0
0 5 10 15 20
Kinetics of catalysed reactions
n-Alkanes cracking
Energy diagram
Initial state Transition state
Adsorbed state
A + *
HadsEa2
Ea,obs
A*B*,C*
Kinetics of catalysed reactions
Observed temperature behaviour
• T higher coverage lower• Step highest Ea increased most
Change in r.d.s.
1/T
ln robs
desorption r.d.s.
adsorption r.d.s.
Ea,observed depends on T, because of change r.d.s.Kinetics of catalysed reactions
Dual site reaction :A + B C
A + * A* B + * B* A* + B* C* + * C* C + *
(r.d.s.)
3 3 *T A B T Cr k N s k N s 4 unknowns, 4 equations
3 1 2
21 2 4
/
1 /T A B C eq
A B C
k s N K K p p p Kr
K p K p p K
Kinetics of catalysed reactions
Dual site reaction, contd.
*3333 CBAT kkNsrrr
Number of neighbouring sites (here: 6)
Kinetics of catalysed reactions
More than one reactant (no product inhibition)
• One-site models
• Two-site models
e.g. hydrogenation, oxidation
dual site reaction
single site reaction)1(1 BABAA
BAABATABT pKpK
ppKKkNkNr
2)1( BBAA
BBAATBAT pKpK
pKpKskNskNr
)1()1( BBAA
BBAATBAT pKpK
pKpKskNskNr
• Number of sites conditions dependent
21
210
1 AA
AATT pK
pKNN
different sites
optimal surface concentrationsoptimal adsorption strengths
Kinetics of catalysed reactions
Reaction kinetics, summary
Langmuir adsorption– uniform sites, no interaction adsorbed species,
finite number of sites, multicomponent Rate expressions derivation
– series of elementary steps– steady state assumption, site balance – quasi-equilibrium / rate determining step(s)– initial rates (model selection)
LHHW models– inhibition, variable reaction order, activation
energy
simpler
mechanism kinetics
Kinetics of catalysed reactions
Further kinetics
Concepts of Modern Catalysis and Kinetics. I. Chorkendorff, J.W. Niemantsverdriet2003WILEY-VCH VerlagGmbH & Co. KGaA, Weinheim
AppCatA342(2008)3–28 Microkinetics– Keep all elementary processes
• Estimate theoretically pre-exponentials (statistical physics) and activation energies (molecular modeling, DFT) or from experimental work (TPD)
• Active site concentration and limited number of constants estimated from experimental rate data
Single event modeling– Complex reaction schemes reduced to finite
number of single events– Detailed composition feed required– Further as microkinetics
Transient operation– Active site concentration and rate constant
decoupled Include lateral interactions, surface reconstruction,
dependency catalyst properties on exposed environment
Kinetics of catalysed reactions
Examples kinetics
Kinetics of catalysed reactions
Hydrodesulphurization kineticsSie, AIChE-J 42(1996)3498
• Apparent second order behaviourr = kcS
2
• H2S inhibits strongly
Example HDS vacuum gasoil
0.0 0.1 0.2 0.3 0.4 0.5 0.6
1/LHSV (h)
0.00.20.40.60.81.01.21.41.61.82.02.2
1/S
-1/S
0(1
/wt.%
)
GasoilCoMo-aluminatrickle flowL=0.2-0.4 m
0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
bed length
c
concentration
conversion
fast decrease followedby slow decrease
Second order silly, what is wrong??
Kinetics of catalysed reactions
Composition oil fractions
S
S
SR R
S
RS S
R
RS
R
0 5 10 15 20 25 30
Vacuum gasoil
Simulated distillation b.p.
Sulphur compounds
Thioethers
ThiopheneBenzthiophene
Dibenzthiophene
Substituteddibenzthiophene
complex mixturesdifferent reactivitieslumping
Kinetics of catalysed reactions
Simulated profiles - HDS reactivity lumping
0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
bed length
conc
entra
tion
Three lump model: first order reactionsSimulated model data:2nd orderk=10 m3/mol.sc0=2 mol/m3
Three lump model:1st ordersk1=36.1 s-1 c01=1.23k2=16.0 s-1 c02=0.59k3=7.5 s-1 c03=0.18
sum
1
23
Three lump model adequateInhibition through LHHW models Which groups lumped?
model studies
Kinetics of catalysed reactions
N2O decomposition
Kinetics of catalysed reactions
Effect of CO on N2O decomposition
0.0 0.5 1.0 1.5 2.0
molar CO/N2O ratio
0.0
0.2
0.4
0.6
0.8
1.0
X(N
2O)
Co-ZSM-5 (693 K)
Cu-ZSM-5 (673 K)
Fe-ZSM-5 (673 K)
CO removes oxygen from surfaceso ‘enhances’ step 2, oxygen removal
now observed: rate of step 1 r1 = k1 NT pN2O
increase: ~2, >3, >100
CO + O* CO2 + *
CO + * CO* (Cu+)
Kapteijn et al. J.Catal.167(1997)256-265
Kinetics of catalysed reactions
Effect of CO on N2O decomposition
rate without CO rate with CO
r k N pT N O 1 2
So k1/k2 = : 1 Co>2 Cu>100 Fe
ratio = 1 + k1/k2 and:21
21* 1 kk
kkO
O* 0.5>0.7>0.99
21
1
12
kkpNkr ONT
Kinetics of catalysed reactions
Apparent activation energies N2O decompositionCO/ N2O = 2
Cu r k N pk k K p
k N pK p
T N O
CO CO
T N O
CO CO
1
1 2
12 2
1
Co, Fe
r k N pT N O 1 2
E Eaobs
a 1
1obsa a COE E H
Apparent activation energies (kJ/mol)
only N2O CO/N2O=2
Co 110 115
Cu 138 187
Fe 165 78
Kinetics of catalysed reactions
Apparent activation energies N2O decompositionCO/ N2O = 0
Co,Cu
Fe
r k N pk kT N O
1
1 2
2
1
r k N pT N O 2 2 E Eaobs
a 2
E E Eaobs
a a mix( )1 2,
Apparent activation energies (kJ/mol)
only N2O CO/N2O=2
Co 110 115
Cu 138 187
Fe 165 78
Kinetics of catalysed reactions
N2O decomposition over ZSM-5 (Co,Cu,Fe)
Rate expression
r k N pk k K p
T N O
O
1
1 2 3
2
21
Oxygen inhibition model
1. N2O + * N2 + O*2. N2O + O* N2 + O2 + *3. O2 + * *O2
0 2 4 6 8 10
p(O2) / kPa
0.0
0.2
0.4
0.6
0.8
1.0
X(N
2O)
Fe-ZSM-5 Co-ZSM-5
Cu-ZSM-5
743 K
833 K
793 K
733 K688
773 K
Kapteijn et al. J.Catal. 167(1997)256
Kinetics of catalysed reactions
Catalysed N2O decomposition over oxidesWinter, Cimino
Rate expressions:
r k pp Kobs N O
O
2
21 30.5
r k pobs N O 2
r k p
pobsN O
O
2
2
0.5
1st order
strong O2 inhibition
moderate inhibition
Also: orders 0.5-1water inhibition
= Explain / derive =
Kapteijn et al. Appl.Catal.B: Env. 9 (1996) 25-64 Kinetics of catalysed reactions
N2O decomposition over Mn2O3
2 N2O 2N2 + O2
Rate expression
r k N K pK p p K
T N O
N O O
2 1
1 30.5
2
2 21
Kinetic model
1. N2O + * N2O*2. N2O* N2 + O*3. 2 O* 2* + O2
Yamashita & Vannice J.Catal.1996
Kinetics of catalysed reactions
N2O decomposition over Mn2O3Yamashita & Vannice J.Catal.1996
0.0 2.0 4.0 6.0 8.0 10.0
pO2 / kPa
0.0
0.1
0.2
0.3
0.4
r / 1
0-6m
ol.s
-1.g
-1
Oxygen inhibitionorder N2O ~0.78
Eaobs= 96 kJ/mol
648 K
638 K623 K608 K598 K
= Explain =
pN2O = 10 kPa
Kinetics of catalysed reactions
N2O decomposition over Mn2O3
Kinetic model
1. N2O + * N2O*2. N2O* N2 + O*3. 2 O* 2* + O2
Rate expression
5.031
12
22
2
1 KppKpKNkr
OON
ONT
Values
Ea2 130 kJ / mol
K J/mol 109SkJ/mol 92
3
3
H
K J/mol 38SkJ/mol 29
1
1
H
= Thermodynamically consistent =
Yamashita & Vannice J.Catal.1996
Kinetics of catalysed reactions
Effect reaction kinetics - batch operation
A + B C + D irreversible
21 DDCCBBAA
BABA
cKcKcKcKccKkKr
cA=cBKA=KBKD small
0 20 40 60 80 100 120 140 160 180 200
time
0.0
0.2
0.4
0.6
0.8
1.0
conv
ersi
on
KA=KC= 1
KA=KC= 0.1
KA=1KC=100
KA=10KC=1
Strong product inhibition
Kinetics of catalysed reactions
Kinetic coupling between catalytic cycles
Bifunctional catalysis: Reforming
Isomerization n-pentane: n-C5 -> i-C5
Pt-function: n-C5 -> n-C5=
surface diffusionAcid function: n-C5= -> i-C5=
surface diffusionPt-function: i-C5= -> i-C5
Coupled catalytic cycles on different sites
low concentrationclose proximity
See tutorial
NIOK course December 2009 Tutorial 1 A second order reaction A R has been studied in a Berty-reactor, a CSTR suited for the investigation of solid catalysed reactions. The following data are available: V = 1 l W = 3 g catalyst v = 1 l h-1
cA0 = 2.0 mol/l cA = 0.5 mol/l a. Determine the value of the rate constant and give its dimension b. How much catalyst is needed to obtain 80% in a packed bed reactor at a volume
flow rate of 1000 l/h and an inlet concentration cA0 = 1 mol/l ? Tutorial 2 At room temperature sucrose can be hydrolysed by the enzyme sucrase:
sucrose products Starting with an initial sucrose concentration of 1.0 mmol/l and an enzyme concentration of 0.01 mmol/l the following data have been obtained in a batch reactor. Concentrations have been determined by using polarized light. c mmol/l t (h) c mmol/l t (h) c mmol/l t (h) 0.84 1 0.27 5 0.018 9 0.68 2 0.16 6 0.006 10 0.53 3 0.09 7 0.0025 11 0.38 4 0.04 8 Verify that the data can be represented well by a kinetic expression of the Michaelis-Menten type:
Mc
cckr
S
ES
0 with M the Michaelis constant
Determine the parameter values in this rate expression. Tutorial 3 a. External mass transfer limitations can be verified by the Carberry number, Ca.
1. How would you calculate Ca 2. What are the limiting values of Ca, and why? 3. Give the physical interpretation of Ca
b. Pore diffusion limitations in porous catalysts can be verified by the Thiele modulus . 1. Give for a first order irreversible reaction and dimensions of the parameters 2. What is the physical meaning of 2 ? 3. Give the relation between the catalysts effectiveness and for the limits of
approaching 1 and approaching 0.
4. To be able to calculatate the kinetics of the reaction has to be known. If the kinetics are unknown give two ways to be able to check the presence or absence of pore diffusion limitations.
5. What is the effect on the observed reaction rate if one increases the dispersion of the active phase of a catalyst by a factor of two, while one operates in a strongly pore diffusion controlled regime? Motivate your answer.
Tutorial 4 For a first order catalysed gas-phase decomposition reaction under chemically controlled conditions the following data have been reported: rv = 10-6 mol s-1 (cm3
cat)-1 cA = 10-5 mol cm-3 @ 1 bar, 673 K De = 10-7 m2 s-1 Which maximum particle diameter of a spherical catalyst may still be used without diffusional disguise? Tutorial 5 A conversion rate of 8 mol s-1 is being observed for the isothermal gas phase decomposition of a component A in a catalyst bed of 0.5 m3 with a porosity b =0.4 at 600 K and at pA = 1 bar. The spherical catalyst particles have a diameter of 15 mm. In this case De = 2·10-6 m2 s-1. Are diffusion limitations present? Motivate your answer. Use the correct units. Tutorial 6 The data in the table below have been produced in a Berty reactor, a type of CSTR for heterogeneous catalysts with internal recirculation of the fluid. The isothermal reaction conditions were identical in all runs. What can you tell about transport limitations and catalyst porosity ?
Run no. Wcat dp FA0 Recycle rate rvobs
1 1 1 1 High 4 2 4 1 4 Very high 4 3 1 2 1 Very high 3 4 4 2 4 High 3
Tutorial 7 A first order catalysed decomposition has been studied in a labscale reactor. Use the data below to answer the following questions. a. Has external mass transfer been interfering ? b. Are diffusional disguises present ? c. Do temperature differences exist over the gasfilm or within the particle?
Data: Catalyst dp = 2.4 mm De = 1.4·10-8 m2 s-1 e = 0.45 J m-1 s-1 K-1 Gasfilm kf = 0.083 m s-1 h = 46 J m-2 s-1 K-1 Reaction kJ mol-1 cb = 20 mol m-3 (@ 1 bar, 609 K) rv
obs = 27 mol s-1 m-3cat
Tutorial 8 The Fischer-Tropsch reaction has been studied by Post et al. (AIChE-J. 35 (1989) 1107) using a wide-pore silica supported cobalt based catalysts (spherical particles). The
reaction can be described as a first order irreversible reaction in the hydrogen partial pressure. They calculated an observed first order rate constant at different temperatures and for different particle diameters, as indicated in the graph. A particle size dependency has been observed and the temperature dependency decreases with increasing particle size. Explain these phenomena and by first deriving an expression for the observed reaction rate under extreme diffusion limitations.
Tutorial 9 For the irreversible conversion of a component A into a product the following data are available: 1 g catalyst, kw = 10-3 m3 min gcat , cA0 = 3 mol m-3 and v= 10-3 m3 min-1. Calculate the (averaged) exit conversion for an ideal plug flow reactor for the cases a-c.
Do the same for a ten times lower catalyst activity. a. For an undiluted catalyst bed b. For the catalyst homogeneously diluted with the same volume
of inert particles c. Same as for b., but now the catalyst and inert particles form two
parallel beds in the reactor (see drawing)
1.90 1.95 2.00 2.05 2.100.001
0.01
0.1
dp/mm
0.38
1.42.4
1000/T
kvobs
1.90 1.95 2.00 2.05 2.100.001
0.01
0.1
1.90 1.95 2.00 2.05 2.100.001
0.01
0.1
dp/mm
0.38
1.42.4
1000/T
kvobs
Tutorial 10 In a thermobalance the catalysed oxidation of four char samples has been studied to investigate the effect of the catalyst precursor (copper salts) on the catalytic activity. a schematic diagram of the thermobalance used is given below, together with the observed reaction rate R (mg C per h and per mg C initially present). a. One observes at a certain temperature for each catalyst a strong increase in
reactivity and it becomes nearly constant at even higher temperatures. The authors explain this by a changing mode of catalytic action, ‘from a non-wetting to a wetting mode’. Give your explanation for this constant level.
b. Why is this level about the same for all samples ? c. Explain the increase in apparent activation energy with increasing temperature in
the intermediate temperature regime.
Tutorial 11 The first order irreversible decomposition of N2O into O2 and N2 has been studied in an internally recirculated reactor (Berty type). Under isothermal and kinetically controlled conditions (700 K) the observed conversion amounts to 0.7. The following additional data are available. Total flow rate 200 ml/min, amount of catalyst 1 gram, stainless steel reactor, internal reactor volume 100 ml, feed concentration N2O 40 * 10-6 mol/l. Furthermore, the reaction is not affected by other components that may be present. Design a packed bed reactor that has to convert 2000 ppm N2O (80 *10-6 mol/l) in a stack gas for 90% and a total flow rate of 24000 Nm3/h, i.e. calculate the weight of catalyst needed and the reactor volume needed for the following situation: similar temperature as the Berty reactor, isothermal operation, catalyst effectiveness 0.8 and 100 kg catalyst fits into 1 m3 reactor volume (monolithic catalyst). All volumetric dimensions given are identical in this problem. Hint: Use the design equations for the reactors.
Thermobalance
coolingwater
thermocouple
gas flow
sample inceramiccup
heatingcoil
10
1.0
0.11.40 1.45 1.50 1.55 1.60
RT (mg/h mgi)
1000/T (K-1)
Tutorial 12 Hosten and Froment studied the isomerization of n-pentane to i-pentane in the presence of hydrogen over a bifunctional Pt-Al2O3 catalyst. Globally first a dehydrogenation takes place over the metallic function, followed by an isomerization over the acidic alumina sites and finally a hydrogenation of the i-pentene takes place over Pt. The reaction sequences can be given as:
Dehydrogenation 1) A + * A* 2) A* + * M* + H2* 3) H2* H2 + * 4) M* M + *
Isomerization 5) M + # M# 6) M# N# 7) N# N + #
Hydrogenation 8) N + * N* 9) H2 + * H2* 10) N* + H2* B* + * 11) B* B + *
a. Derive a rate expression for this reaction where step 6. is rate determining. b. The overall reaction rate is found pressure independent. Is that in agreement with
your result? Tutorial 13 For the catalytic decomposition of alcohols into alkenes and water the following results have been obtained:
Alcohol Ea (kJ/mol) High pressure Low pressure Difference n-propanol 172 119 53 iso-propanol 163 109 54 n-butanol-1 184 117 67
Under all conditions water is adsorbed much stronger at the catalyst than the other two components. The apparent (observed) activation energy, obtained from an Arrhenius-plot of ln(r) versus 1/T , is significantly different for high and low pressure conditions. The backward reaction is negligible in all cases and a single-site kinetic model can be assumed for this reaction. 1. Demonstrate by means of a kinetic analysis what the physical meaning of the
constant difference of about 58 kJ/mol is. 2. Is it logical that this difference is about the same for all three alcohols?