Introduction to Catalysis - Lecture 5

University of Dortmund – Chemical Engineering Department – Chair for Reaction Engineering

University of Dortmund

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Introductionto Catalysis

David W. Agar

Short Course

26thJune – 4thJuly 2003

Chemical Engineering Department

IISc Bangalore




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University of DortmundFor those who were distracted from the engineering woodsby the chemical trees, a brief summary of yesterday‘s talk:

1. Transition metal – ligand complexes

and now back to more familiar engineering territory..

Hydroformylation – homogeneous catalysis for useful intermediates(RCH=CH2→RCH2CH2CHO)

2. Complex catalyst composition: - Co→Rh metallic active centre- Triphenylphosphine ligands- Triphenylphosphinesulfonate ligands

5. Two-phase catalysis – extraction of water soluble catalyst

3. Well-defined cyclic mechanism (Heck-Breslow)

4. Mild conditions (100°C, 10 bar), high selectivities (95% n-Aldehydes)

6. Reaction engineering: immobilisation, enhanced mass transfer



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Course schedule:

26.06.03 Principles of catalysis

27.06.03 Ammonia synthesis catalysis

30.06.03 Automotive exhaust catalysis

01.07.03 Hydroformylation catalysis

02.07.03 Catalytic partial oxidation of propene

03.06.03 Catalysis in polymerisation

04.06.03 Enzymatic glucose isomerisation



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Propene production

[1992 mio.t]

USA 10.3Western Europe 9.7Japan 4.5Germany 2.0GUS 1.3

world capacity 40



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Propene production and consumption [1984]

(1) acetone, acrolein, acrylic acid, allyl chloride, carbon disulfide, chlorinated solvents, cresols, dichloropentadiene, epichlorhydrin, ethylene-propylene rubber, 4-methyl 1-pentene, oxalic acid, polymethyl methacrylate, paramins...

(2) Steam cracking and catalytic cracking. In 1986 the worldwide production capacity of propylene was 28.3·106 t/a with the following distribution:

geographic areas Western Europe United States Japan WorldUses (% product)

acrylonitrile 17 18 20 18cumene 9 9 5 9isopropanol 6 6 3 5oxo alcohols 13 8 10 11polypropylene 34 35 47 36propylene oxide 10 11 6 9oligomers 7 1 4micellaneous(1) 6 8 8Total 100 100 100 100

Sources (% product)steam cracking 86 53 89 75catalytic caracking 14 47 11 25Total 100 100 100 100

Production (106 t/a) 7.2 7.0 3.0 22.5Capacity (106 t/a)(2) 8.7 9.9 3.0 28.5Consumption (106 t/a) 7.1 6.8 2.9 21.5

11

United States 9.7 Western Europe 8.2 Middle East 0.2Canada 0.7 Eastern Europe 3.3 Japan 3.0Latin America 1.3 Africa 0.1 Asia and Far East 1.8



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polymerisation47%+NH3/O2

12%

+CO/H2

10%+O2

10%

+benzene7%

+H2O4%

+O2

+Cl2

polypropylene

acrylonitrile

butanal

propylene oxide

cumene

isopropanol

acrylic acid

allyl chloride

isohexenedimerisation

propene

polyacrylonitrileacrylamideadiponitrile

acetone

polyacrylic acid,acrylatesepichlorhydrin

isoprene

phenolacetone

propylenglycololigomer

n-butanol2-ethylhexanol



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Catalytic partial oxidation of propenesubstrates catalysts products

propene +oxygen

acrolein

acrylicacid

acetone

propyleneoxide

acetic acid

1,5-hexa-diene

benzene



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Acrolein derivatives

acrolein

allyl alcohol+H2 (cat.)

acrylic acid+O2 (cat.)

methionine3 stages

pyridine+NH3

CHO



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Acrylic acid derivatives

acrylic acid

acrylates (n-Bu,Et,Me,2-EH)+alcohol

polyacrylic acidpolymeris.

copolymers+ alkenesacrylamide

salts+NH3,

NaOH

COOH



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Acrylic acid production

Acrylic acid derivativesPolyacrylic acid &saltsn-Butylacrylate

Ethylacrylate

Methyl & 2-Ethylhexylacrylatesspecialty acrylates

miscellaneous

Worldwide production: 1.2 (2.0) Mio.tpa

End uses [%]: USA Europe Japan

Surface coatings 42 35 34Textiles 23 18 16Acrylic fibres 6 7 14Adhesives 5 15 20Others* 24 25 16*superabsorber, detergents, water treatment, dispersants

Regional production of acrylic acid & acrylates (1982)

USA

Europe

JapanOther



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Life cycles of acrylic acid syntheses

propene oxidation

new processesReppe process

Cyanohydrin acrylonitrile & propiolactoneprocess



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Historical acrylic acid synthesesReppe process

• low efficiency of acetylene manufacture• toxic & polluting nature of Ni carbonyls

2

140 ,200

2 2 2 2 .bar C

NiBr CuBrC H CO H O CH CHCOOH

°

−+ + → =

2 4H SOaq.soln.

2 2 2 2 255-60°C 175°C

4 4

.

CH CH HCN HOCH CH CN CH CHCOOH

O NH HSO

− + → → =

+

Cyanohydrin process

• toxicity of HCN & ethene cyanohydrin• waste salt by-product

Ketene process

• toxicity of β-propiolactone• multistep synthesis

2 2 4

3 2 2 2750

2

.

H O H SOHCHO

CCH COOH CH C O CH C O CH CHCOOH

CH O

− +

°→ = = → − = → =

−



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Production of acrylic acid from acetylene

(BASF process -Reppe synthesis)



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Acrylic acid synthesis via propene oxidation

3 2 2 2 2

3 2 2 2 2

2 2 2

3 . 594.92

. 340.8

1. . 254.12

R

R

R

CH CH CH O CH CHCOOH H O H kJ

CH CH CH O CH CHCHO H O H kJ

CH CHCHO O CH CHCOOH H kJ

= + → = + ∆ = −

= + → = + ∆ = −

= + → = ∆ = −

Net Reaction:

Two step process:

• limited yield of single step process (50-60%)

• rapid deactivation of single step catalyst (TeO2)

• 1st step: Bi/Mo-Oxide - 300-400°C - >85% yield

• 2nd step: Mo/V-Oxide – 250-350°C - >96% yield

• gas composition: 10% propene, 50% air, 40% steam

• pressure 1.3-2 bar - residence time: 1-3 s.



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Acrolein

Degussa process

mixed aldol-condensation (obsolete)

CH3CHO + HCHO CH2=CHCHO + H2ONa-silicates/SiO2

300-320°C

Direct oxidation of propene

mechanism: via allyl-radicals

catalyst: bismuth molybdate

phosphorus molybdate

selectivity: ~80% (by-products: acrylic acid, acetic acid, acetaldehyde)

CH2=CHCH3 + O2 CH2=CHCHO + H2O ∆RH = -368 kJ/molCat.

350-450°C

+Fe-, Co-, Ti- oxides



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Development of acrolein catalyst

>77310-330Bi, Mo, Fe, P, Ni, Co, W, Si, K1970

60-70330-350Bi, Mo, Fe, P, Ni, Co1964

50-60350-400Bi, Mo, Fe1959

30-40450-500Bi, Mo1957

Acroleinyield [%]

Reactiontemp.[°C]

Catalystcomposition

Year

• catalyst lifetime: 3 years

• transition metals lower operating temperature

• Fe, Co, Ni regulate gas-lattice oxygen exchange

• P & K maintain selectivity



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Catalyic partial oxidation of propene

propene

acrolein

Propene oxidation on bismuth molybdates

hydrogenabstraction allyl radical

adsorption

Desorption

reoxid-ation

(high pO2)eqm.oxid.step



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Acrylic acid catalystproduction

Precipitation

Drying

Activation

activephase

microporosity

Mechanicalproperties of the catalyst pellet

Thermalproperties

porestructure

porestructure

active phase

catalystproperties

Precipitation

Drying

Calcination

Grinding

Adding of formu-lation excipients

Precompression

Shaping

Drying

Sieving

Activation/ Annealing

solution A solution B solution C(NH4)6Mo7O24·4H2OH3PO4

Ni(NO3)2 · 6H2OFe(NO3)2 · 6H2OSiO2

Bi(NO3)3 · 5H2O



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Development of acrolein oxidation catalyst

• catalyst lifetime: >7 years

• by-products: CO2, acetic acid, furfural, oligomers

• two stage structured temperature profile

Catalyst composition Reaction Acrolein Acrylic acid(O-free) temp.[°C] conversion [%] yield [%]

Mo12V1.9Al1.0Cu2.2 (Al-sponge) 300 100 97.5Mo12V3W1.2 (SiO2) 240 98.0 87.0Mo12V3W1.2Mn3 255 99.0 93.0Mo12V2W2Fe3 230 99.0 91.0Mo12V3W1.2Cu1Sb6 272 99.0 91.0Mo12V4.6Cu2.2W2.4Cr0.6 (Al2O3) 220 100 98.0Mo12V2(Li2SO4)2 300 99.8 92.4Mo12V4.8Cu2.2W2.4Sr0.5 (Al2O3) 255 100 97.5Mo12V2.4Cu0.24 (SiC) 290 99.5 94.8Mo12V3W1.2Ce3 288 100 96.1Mo12V4.7W1.1Cu6.3 360 99.0 96.0



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Acrylic acid synthesis via propene oxidation

Two-stageprocess



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Recuperative Heat ExchangeMulti-tubular reactor for partial oxidation reactions

Acrylic acid synthesis h salt bath = 2000 W/m2 • Kh gas = 150 ~ 200 W/m2 • K



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Temperature (& Concentration) profiles inreactors may be manipulated using:

B. Recuperationspatial segregation between reactionmedium & material/heat-sink/sourcee.g. multitubular reactor

T

z

A. Convectionaddition or withdrawal of sidestreams

e.g. cold-shot reactor

T

z



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... or, less frequently:

C. Regenerationchronological segregation betweenreaction medium & material/heat -sink/sourcee.g. reverse flow reactor

T

z

D. Reactiondirect coupling of main reactionwith thermally/materially compatible supplementary reactione.g. oxydehydrogenation

T

z

C → DA → B



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What are Hot-spots?Unwanted temperature maxima arising in tubular reactorswith exothermic reactions due to heat transfer limitations

Heat exchange area (A)~ 100 m²/m³Heat transfer coefficient (h)~ 100 W/m²K

Hot-spot adversely effects:- conversion- selectivity- safety- catalyst lifetime- choice of reactor materials

≤ 30K

≤ 100K

25 mm

≤ 25 000 tubes

Reactor feed

Coolant



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Characteristic dimensions of heterogeneouslycatalysed gas phase reactions in tubular reactors

• Chemical activity (Nanostructure)internal surface area ~ 100m²/g⇒ pore diameter ~ 10 nm

• Pore diffusion (Microstructure)Weisz-Modulus < 0,3 ⇒ ‘pellet diameter’ ~ 1mm

• Heat transport (Macrostructure)∆T < 50K ⇒ ‘tube diameter’ ~ 1cm



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Mechanisms of heat transfer in a fixed-bed

1a

1b2a,b

3a

3b

Heatflow

Fluid flow

1a

1b

2a

2b

3a 3b

1

2

3

Heat transfer in fixed-bed reactors



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Classification of the continuum modelsfor steady-state fixed bed reactors

sw

gw

err

eax

h,h,,D,:Parameterdispersion radialHT2:HT3

,:Parameterdispersion radial PH1:PH3

0

ldimensiona two

,,:Parametergradients internal HT1:HT2

,:Parameterboundary phase the at gradients reactor,flow plug Ideal:HT1

,:Parameterdispersion axial PH1:PH2

:Parameterreactorflow plug Ideal:PH1

0

ldimensiona one

ousheterogene shomogeneoupseudo

λλλ

+λ

+

≠∂∂

λ

+

λ+

=∂∂

≠≠==−

esr

grgrw

eg

g

ax

sgsgsgsg

D,h,,,kD,h

Dh,kU,

hkU,

DU,

U

TT;ccTT;cc

r

r



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Mean conversions & axial temperature profilein multitubular partial oxidation reactor

z (m)

x Am

xB

m

x Cm

xAm

xBm

xCm

T'm

T'm



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Multitubular partialoxidation reactor

Radial temperature profiles at various bed depths

T’=T

-To

Rr

Hot-spot scale-upproblems:

• catalyst activation

• thermoelement dis-tortions

• coolant side non-uni-formities



I t CUniversity of Dortmund - Chemical Engineering Department - Institute for Reaction Engineering

How can hot-spots be eliminated?Improved co-ordination between the rates

of heat generation & heat removal in reactor

Cooling

Reaction

Hydrodynamics

Microreactor

Benchmark:Multitubular

reactor

Debottlenecking Strategies:

- diminish catalyst activity• catalyst dilution

- enlarge heat exchange surface (A)• Linde-reactor

- raise heat transfer coefficient (h)• Fluidised bed

- increase both A & h• Microreactor• Millireactor

Fluidised bed



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Improved co-ordination of reaction &cooling using a fluidised bed reactor

Increased heat transfer coefficients due to efficient convective-regenerative particle transport mechanism

+ excellent isothermal behaviour (h~600 W/m²K)

+ higher degree of catalyst utilisation

+ facile catalyst regeneration

– very mechanically resilient catalyst needed

– limited hydrodynamic loading range

– undesirable backmixing

– scale-up?e.g. Ammonoxidation of propene to

acrylonitrile (Sohio process)



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Limitations of catalytic activity

• the chemical processes occurring at the active sites impose an upperlimit on reaction rate (~1µmol/g.cat.s*) and heat generation (~500kW/m³*)

• for chemically limited kinetics, a specific heat exchange surface of ~1,000 m²/m³ is usually adequate to ensure rapid heat removal.

⇒ Microreactors offer excessiveheat exchange surface

⇒ reactor dimensions of 1-5mm,i.e. ‘Millireactors’

* for typical industrial synthesis reactions W.Gerhardt, DECHEMA-GVC, Wernigerode, 06.04.00

0,01

0,1

1

10

100

0,001 0,01 0,1 1 10 100 1000

k [1/s]

d [m

m]

Da =2NTU =100Nu = 3,7a =10-5 m²/s



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Developments

• extension to similar reaction systems e.g. methacrylic acid

• use of propane instead of propene

• operation in rich & even ‚ex‘- composition regionExplosion limits: 2.0 LEL-11.1(15.3) UEL % C3H6

• use of microreactors to improve heat removal

• cut losses due to unwanted polymerisation



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Methyl MethacrylatePreviously

Propylene

Cumene

Acetone

Cyanhydrine

Methacryl Amide

Methyl Methacrylate

Benzene

Hydrocyanic Acid

Sulfuric Acid

Methanol

Now

Ethylene

Propionaldehyde

Methacrolein

Methacrylic Acid

Methyl Methacrylate

CO/H2

Formaldehyde

Air

Methanol



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Acrylate via acrylic acid esterification

Yield: 95% based on acrylic acid



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University of DortmundCourse schedule:

26.06.03 Principles of catalysis

27.06.03 Ammonia synthesis catalysis

30.06.03 Automotive exhaust catalysis

01.07.03 Hydroformylation catalysis

02.07.03 Catalytic partial oxidation of propene

03.06.03 Catalysis in polymerisation

04.06.03 Enzymatic glucose isomerisation

Disposable cats?

Documents

Introduction to Catalysis - Lecture 5