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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’


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


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


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


Porous catalytic walls

Flow pattern


Flow patternFlow pattern


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


Turbine Reactor Screw Impeller Stirred Reactor


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





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:


Chapters 3 and 8

Kinetics Reactor theory Experimental aspects

– Interpretation– Reactors– Interfering phenomena

• Mass transfer• Diffusion • Dispersion• Criteria

Problems/questions


Structure

Reactor engineering Catalysis

Reaction modelsKinetics

Behaviour singleparticle

Ideal reactorsBatchCSTRPlug flow

Non-ideal reactors

Catalytic Reactor

Transportphenomena

Heat & Mass


Kinetics of Catalysed Reactions

Why Reaction Kinetics Derivation rate expressions Simplifications

– Rate determining step– Initial reaction rate

Limiting cases– Temperature dependency– Pressure dependency

Examples


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%


31%

Other1%


26%


8%Process

Development30%


37%

Other4%


27%


2%

Process Development

15%Process

Optimisation17%

Other0%


56% Mechanistic Research

12%

Process Development

56%


28%


1%


15%

(a) Chemical Companies (b) Oil Companies

(c) Catalyst Companies (d) Engineering Companies

Other0%


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


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 = ??


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


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


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


Quasi-equilibrium / rate determining step

r1

r2

r3

r-1

r-2

r-3

r‘quasi-equilibrium’

rate determining

r = r2 - r-2


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:


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:


‘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/

/


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’


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


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


Multicomponent adsorption / inhibition

Langmuir adsorption

AA

A i i

K pK p K p

1

11

Inhibitors


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


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


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


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+)


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


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


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


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)


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


Terminology

Heterogeneous catalysis BiocatalysisReactants SubstratesMolecules

Reactor performance

CSTR, autoclaveResidence time, space time

Flow rate

Chemostat, fermentor

Dilution rate


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


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


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


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


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#-



2. Weak adsorption r k N K pT A A 2


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



3. Strong adsorption Br k N K p

K pT A A

B B

2


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


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


n-Alkanes cracking

Energy diagram

Initial state Transition state

Adsorbed state

A + *

HadsEa2

Ea,obs

A*B*,C*


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


Dual site reaction, contd.

*3333 CBAT kkNsrrr

Number of neighbouring sites (here: 6)


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


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


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


Examples kinetics


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??


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


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


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



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


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


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


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


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


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


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


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


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?

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