Industrial Catalysis
LECTURE 17 – KETF20 CHEMICAL ENGINEERING PROCESSES 2017
Monolith extrusion
Image Frauenhofer Institute
Outline
• Introduction and recap
• Catalyst synthesis
• Reactor loading
• Catalyst activation
• Catalyst deactivation
2
Introduction and recap
• Structure
– An industrial catalyst consists of three (3) parts
• Carrier
• Support
• Active phase
– Each with its own purpose
3
Introduction and recap
• Carrier
– Provides structure to the reactor bed
– Determines of heat and mass transfer
– Determines bed pressure drop
– Ceramic or metallic
– Should be resistant to mechanical stress
4
Introduction and recap
Different catalyst carriers.
5
Introduction and recap
• Support
– Provides surface area for reaction
– May be the material of the carrier
– Common materials
• Alumina
• Silica
• Ceria
• Mixed oxides
• Active carbon
6
Introduction and recap
• Active phase
– Contain sites where reaction occur
– Metals or metal oxides
– Metals from transition metal group in periodic table
• Pt
• Pd
• Cu
• Co
• Ni
• Rh
7
Introduction and recap
The different elements to heterogeneous catalysis
Image from Rothenberg Catalysis concepts and
green applications
8
Catalyst manufacturing
• Catalyst manufacturing – black magic revealed
• Involves many different steps
– Support preparation
– Active phase preparation
– Post-treatment
– Forming
– Activation
9
Catalyst manufacturing
• Two main classes of catalysts
• Bulk catalysts
– Ammonia synthesis
– Hydrocracking
– Zeolites
• Impregnated catalysts
– Precious metal based
10
Catalyst manufacturing
Bulk
catalysts
Impregnated
Impregnated
catalysts
catalysts
Precipitation
(Silica/
Alumina)
Wet
impregnation
(Automotive)
Hydrothermal
synthesis
(Zeolites)
Incipient
wetness
(Pt/Sn/Al2O3)
Fusion/alloy
leaching
(Mixed oxides)
Ion exchange
(Acidic zeolite)
11
Catalyst manufacturing
• Precipitation
• Used for preparing
– The bulk catalyst
– The support material of impregnated catalysts
• Mixing of metal salts in water
• Adding reagent to shift pH
12
Catalyst manufacturing
Precipitation of carbonate salt
13
Catalyst manufacturing
Properties of aged precipitate
Images Hans T. Karlsson
14
Catalyst manufacturing
• Example
– Synthesizing NiAl2O4 for pre-reforming of higher
hydrocarbons
– Mix nitrate salts of Ni and Al in deionized water
– Use a ratio of 2:1 (Al:Ni)
– Heat solution to 60 C
– Add ammonium carbonate
15
Catalyst manufacturing
• Example
– Observe precipitation
– Age precipitate for 4h at 60 C
– Filter precipitate
– Wash filter cake with deionized water
– Dry at 120 C for 4 h
– Calcine dry precipitate at 1,200 C
– Verify phase by PXRD
– Then continue with pellet formation
16
Catalyst manufacturing
• Hydrothermal synthesis
– Heating of precipitate, gel or flocculate
– In aqueous solvent
– 100-300 C
– Changes in particles with respect to
• Texture
• Structure
• Crystal
• Particle size
17
Catalyst manufacturing
Hydrothermal synthesis
Image from Rothenberg Catalysis concepts and
green applications
18
Catalyst manufacturing
• Fusion/alloy leaching
– Starting with a metal material
– Melting
– Casting into ingots
– Cooling/annealing
– Crushing
19
Catalyst manufacturing
• Fusion/alloy leaching
– Screening (size sorting)
– Activation by leaching with caustic or acidic solution
– Raney nickel example
• NiAl alloy
• Al removed by leaching (70-100 C)
• Sponge Ni active catalyst
• Low cost alternative to PGM in saturating benzene
20
Catalyst manufacturing
• Wet impregnation
– Wetting entire catalyst with dissolved precursor
– Allowing precursor to adsorb onto catalyst
– Drying after allowing enough time for adsorption
– Determined by Darcy’s law
21
Catalyst manufacturing
Difference between wet impregnation (a)
and dry impregnation (b)
Image from de Jong 2009
Impregnation possibilities
Image from de Jong 2009
22
Catalyst manufacturing
• Wet impregnation
– Low precursor concentration - precursor-support
interaction dominating
– High precursor concentration - the species interacting
with support form seeds for crystallization upon drying
– Repeated many times (10)
– Two types of interactions
• Electrostatic interactions
• Grafting through ligand substitution
– Examples 3-way catalysts
23
Catalyst manufacturing
Electrostatic interactions(a)
Grafting via ligand exchange (b)
Image from de Jong 2009
24
Catalyst manufacturing
• Incipient wetness
– Measure the pore volume of the catalyst
– Decide the concentration of active phase
– Prepare a mixture with the right concentration
– Add enough liquid to fill the pore system
– Catalyst is still dry to the touch
– Example CuMnO/Al2O3 for VOC abatement
25
Catalyst manufacturing
• Ion exchange
– Used for changing cation (e.g. zeolites)
– Treating material with solution with desired cation
– Ion exchange between H+ and desired cation
– Can be mordenite substituted with Ce4+
– Improved selectivity for N2O decomposition
26
Catalyst manufacturing
• Summary
– Two main ways of preparing catalyst
• Bulk catalysts
• Impregnated catalysts
– Precipitation key step in forming the carrier/bulk
catalyst
– Active phase added in solution
• Wet impregnation
• Incipient wetness
27
Catalyst manufacturing
• Unit operations of catalyst manufacture
– Support preparation
– Active phase preparation on support
– Post treatment
– Forming
– Activation
– Storage
28
Precipitation
Support
preparation
Gelation
Active phase
preparation
Impregnation
Ion exchange
Filtration
Drying
Post-treatment
Calcination
Extrusion
Forming
Pelleting
Activation
Unit operations of catalyst manufacturing
29
Catalyst manufacturing
• Unit operations of catalyst manufacture
– Support preparation is usually produced via
precipitation or gelation
– Active phase added by impregnation
– Post treatment (washing, filtration, drying, calcination)
– Forming (pellet, monolith)
– Activation (reduction)
– Storage
30
Catalyst manufacturing
• Calcination
– High temperature process
– Inducing reaction of precursor
• Decomposition of nitrate
• Decomposition of acetate
• Removal of chlorine
– Assuring right crystal formation
– Atmosphere influence end-result
31
Catalyst manufacturing
Air gives more closed pore system
Air yields larger crystalites of Ni
J. Catal 260 (2008) 227
J. Catal 260 (2008) 227
32
Catalyst manufacturing
A typical calcination schedule for noble metal catalysts on alumina
33
Catalyst manufacturing
• Forming
– Dry pressing (pelletization of powder)
– Wet forming (making paste out of powder)
– Adding binders and additives
– May require second calcination step
34
Catalyst manufacturing
• Forming
– Required for nonfluidized catalysts
– Pre-densification
required
(kneading)
– Can be
accomplished in
washing/ion
exchange
– Combined with
addition of
tableting aids or
binders
35
Catalyst manufacturing
• Forming
– Tableting
– Extrusion
– Granulation (spherudizing)
• Inevitable lower activity compared to powder
• Introduces
– Diffusion limitations
– Aids for forming (graphite, silicates, MgOH/MgO)
36
Catalyst manufacturing
• For tableting the densified powder is dried and pulverized
• Powder is screened
– 6 mesh if 12-18 mm (1/2” to 3/4”)
– 10 or 12 mesh if 6-10 mm (1/4” to 3/8”)
• Pilling aid may also be introduced at this stage
• May require post-calcination
37
Catalyst manufacturing
38
Catalyst manufacturing
• Extrusion combined with densification
• Moist clay of catalyst powder + water + binder
• Pushed through a die to form catalyst
– Round
– Trilobe
– Quatrolobe
– Other shapes
• Limits in size 20 mm in diameter
• Expansion after die
• Dried, calcined, post-treated
40
Catalyst manufacturing
41
Catalyst manufacturing
42
Catalyst manufacturing
• Granulation
– Spraying slurry on tilted disk/pan
– Disk rotates and spheres build up
– Large particles are concentrated to top layer
– Falls out of pan at different size depending on tilt
angle
– Less uniform than tabletting
– Requires post treatment
43
Catalyst manufacturing
Catalyst manufacturing
• Activation
– Is performed on what is sometimes called a precatalyst
– Oxidation
– Reduction
– Be carful to avoid pyroforic situations (CuZnO/Al2O3)
– Initial and after deactivation (re-activation)
45
Catalyst manufacturing
• Industrial example of number of steps
1. Dissolve salts in water
2. Change pH to induce precipitation
3. Wait for secondary precipitation (aging)
4. Wash by deionized water
5. Filter
6. Wash filter cake
46
Catalyst manufacturing
• Industrial example of number of steps
7. Drying
8. Calcining
9. Reslurry in water (e.g. for ion exchange)
10. Filter
11. Wash
12. Dry
47
Catalyst manufacturing
• Industrial example of number of steps
13. Add solution of promoter and/or binder
14. Form paste via kneading
15. Extrude
16. Dry
17. Granulate
18. Mix with prilling lubricant or binder
48
Catalyst manufacturing
• Industrial example of number of steps
19. Prill
20. Calcine
21. Add active phase via impregnation
22. Calcine
23. Activate
24. Stabilize (inert environment etc.)
25. Store or use
49
Catalyst manufacturing
• Summary
– Several unit operations involved in catalyst synthesis
– Industrial schemes quite complicated
– Much washing of material
– Allowing for removal of catalyst poisons from
manufacture (Cl-)
– Generating quite a lot of wastewater
50
Reactor loading
• Important aspect of catalytic reactor design
• Ensure similar bed properties every time
– Heat transfer
– Pressure drop
– Temperature gradient
• Industrial scale
51
Reactor loading
52
Reactor loading
53
Reactor loading
54
Reactor loading
• Important to ensure correct packing of catalyst bed
• Make sure that catalyst mass fill entire reactor
• Evenly distributed and packed in the reactor
• Take care to have guard beds in place and exchange
material with set interval
• Make sure layering is correct
• Make sure the right dilution is used at the right position in
the reactor
55
Reactor loading
Catalyst bed with three concentrations of catalyst, e.g. Formox
56
Reactor loading
• Hot spot is to be avoided
• Using dilution of catalyst with inert
• Use catalyst with less active phase
• May contribute significantly to process economy and
operability
57
Catalyst deactivation
• Deactivation may have several causes
• Such as:
– Over-temperature
– Changes in inlet composition
– Process hygiene
• Concerns the deactivation that occur after the first initial
lowering of activity
• After 50 -100 h and onwards
58
Catalyst deactivation
59
Catalyst deactivation
• Poisoning
– Interaction with the active phase
– Usually with an electronegative specie
• Sulphur
• Chlorine
• Arsenic
• Phosphorous
• But also other compounds (e.g. CO, O2, NH3)
60
Catalyst deactivation
Most active
phases
Most catalyst
poisons
61
Catalyst deactivation
• Poisoning
– Selective towards the active phase
– One molecule may influence more than one active
site
– Electronic effects spill over to adjacent sites
– Competitive adsorption with reactants
– Reversible or irreversible
62
Catalyst deactivation
Example of catalyst poisoning of fuel cell catalyst
Image from Osakagas.co.jp
63
Catalyst deactivation
• Sulphur poisoning major problem in fuel processing
• Two mechanisms
– Surface coverage
– Formation of bulk sulphur
• The first process is reversible
• The second is irreversible
64
Catalyst deactivation
Ni-catalyst deactivation via sulphur addition
Image from Chrisgas special issue
65
Catalyst deactivation
• Clearly see two effects
• Initial lowering of activity due to surface adsorption
• Long term lowering of activity due to bulk sulphuide
formation
• Can be counteracted by higher temperatures
• Lower the stability of the surface sulphides
• Lower formation rate of bulk sulfide
66
Catalyst deactivation
• Fouling
– Covering of the active part of a catalyst
– Physical blocking of active sites
– Physical blocking of pores
– Renders part of the catalyst useless
– Can be formed or introduced
• Coke formation
• Ash coverage
67
Catalyst deactivation
• Three reactions to consider:
2 CO  C(s) + CO2 (exothermal)
CH4  C(s) + 2 H2 (endothermal)
CnHm  polymers  Coke + x H2 (endothermal, ir)
• Coke formation depends on the catalyst kinetics and
operation conditions
• Dent carbon appears at lower temperature in higher
amounts than predicted by equilibrium
68
Catalyst deactivation
• Intrinsic property for coking in
feedstock:
ethylene >> benzene, toluene > nheptane > n-hexane >
cyclohexane > trimethyl-butane ~
n-butane ~ carbon monoxide >
methane
• Inlet conditions important
Test rig for evaluating steam reforming and
pre-reforming catalysts
69
Catalyst deactivation
Coke formed on steam
reforming catalysts
70
Catalyst deactivation
• Prevention of coking in steam reforming
− Addition of alkali (not necessary for natural gas)
− Addition of PGM to Ni-phase
− Choice of carrier material
− Use of PGM catalyst
− Sufficient S:C ratio
− Inlet conditions controlled
− Product recycle
• Can be recovered in part by combined heat and steam
treatment
71
Catalyst deactivation
• Ash coverage
– Different from coking
– Not formed in reactor
– Introduced via gas stream
– Fouling and chemical transfer
72
Catalyst deactivation
10
Fresh catalyst
K2SO4 Exposed catalyst
Conversion (%)
8
6
4
2
600
620
640
660
680
Temperature (C)
Results from exposure of salt aerosols on steam reforming catalysts
From Hydrogen an energy carrier of the future
73
700
720
Catalyst deactivation
• Ash fouling
– Presence of ash covers active phase
– Multiple effects
• Physical coverage
• Chemical interactions
– Lowers performance drastically
74
Catalyst deactivation
• Sintering
– Two types
• Active phase
• Support
– Results in loss of metal surface directly or indirectly
75
Catalyst deactivation
• Sintering
– Oftentimes due to high temperatures
• Increased to maintain activity
• Excursions due to process upsets
– Can be rapid if excess temperatures are reached
– Initial deactivation often due to sintering phenomena
– Example(Catal. Today 214 (2013) 12)
• Pt catalysts for Fischer-Tropsch
• Initial metal dispersion 89%
• Reduced to 10%
76
Catalyst deactivation
• Sintering
– Example initial deactivation
– From Regale et al. Catal. Today 214 (2013) 12
• Pt catalysts for Fischer-Tropsch
• Initial metal dispersion 89%
• Reduced to 10% after 12 h on-stream
• Particle size increased from 3.7 nm to 10.9 nm
77
Catalyst deactivation
• Sintering of active phase
• Two mechanisms
– Atomic migration (THüttig=0.3 * Tmelt)
– Crystallite migration (TTamman=0.5 * Tmelt)
78
Catalyst deactivation
• Deactivation
– Important factor in catalyst design
– Avoid if possible
• Process hygiene
• Process control
– Design around if not
• Guard bed
• Egg yolk catalyst
Egg yolk catalyst
– Essential to include
in modelling
79
Concluding remarks
• Deactivation key to catalyst performance
– Poisoning
– Fouling
– Sintering
• Process hygiene and control essential
• Need to be taken into account in designing and modelling
• Will occur but deactivation rate should be
minimized/optimized
80
Concluding remarks
• Precipitation key step to catalyst synthesis
• Active phase added by impregnation
• Industrial schemes quite complicated
• Waste management should be taken into account
• Reactor loading important factor for repeatability
• Large scale industrially
81