03 primary reformer - theory and operation
DESCRIPTION
johnson matthewTRANSCRIPT
Steam reforming catalyst theory
Asim Yadav
25 Mar 2015
Westin Hotel Gurgaon, India
Introduction
• Steam reforming catalysts – the chemistry
• Reactive metals
• Support materials
• Carbon formation and its prevention
• Steam reforming catalysts – the engineering
• Catalyst shape
• Catalyst strength and breakage
• Catalyst packing
• Heat transfer, pressure drop and surface area
Steam reforming – the basics
• CH4 + H2O ⇌ CO + 3H2 DH = +206 kJ/kmol
• CO + H2O ⇌ CO2 + H2 DH = -41 kJ/kmol
• Can combine with methane steam reaction above to give
• CH4 + 2H2O ⇌ CO2 + 4H2
• Reversible reactions, equilibrium limited • Overall endothermic – large heat requirement
• High temperature for high conversion
• Low pressure for high conversion but size and cost increases pressure
• C2H6 + 2H2O → 2CO + 5H2
• C3H8 + 3H2O → 3CO + 7H2
• ….. • Irreversible reactions can go to completion
• Reaction pathways more complex than shown
• Carbon forming intermediates such as ethylene
Steam reforming – more basics
• Steam reforming reactions are very fast
• Reaction occurs within the surface layer of the pellet
• Surface layer can react the gas faster than it arrives
• Diffusion of gas within the pellet is relatively slow
• Boundary layer
• Steam reforming pellets have a low porosity
• Need to be strong due to high temperature and thermal
movement
• Steam reforming catalyst activity is proportional to the
charged pellet area, not the charged weight
Steam reforming – active metals
• Many active metals promote carbon formation
• Highest activity metals are very expensive
• Nickel presents the optimum choice
Costly
Inactive Carbon formation
Optimum
Catalyst support – the basics
• 3 commercially available choices
• Alpha alumina
• Calcium aluminate
• Magnesium aluminate spinel
• Set of requirements placed on the support
• Chemically stable
• Physically stable
• Not detract from catalysis (if possible enhance the catalysis)
• Economic
• Pros and cons for each support type
• Other supports available for niche applications
• Stabilized zirconia for demanding applications (autothermal reforming)
Supports – nickel spinel formation
• Nickel can form spinel with alumina (NiAl2O4)
• Propensity depends on ionic radii of support atoms
• Ca2+ 0.99 Angstrom
• Ni2+ 0.68 Angstrom
• Mg2+ 0.65 Angstrom
• Al3+ 0.50 Angstrom
• Nickel and magnesium very similar size
• Relatively easy for nickel to migrate into magnesium aluminate spinel support at moderate temperatures
• Results in catalyst deactivation
• Nickel can only incorporate into alpha alumina or calcium aluminate at very high temperatures
• Typically seen in nipped tubes or after an over firing incident
Supports – catalyst reduction
• Nickel added to the catalyst as nickel oxide
• Must be reduced in situ to metallic nickel
• Nickel – support interaction controls ease of reduction
• Alpha alumina has +ve charged acidic sites
• Low interaction of +ve charged Ni ions with support
• Calcium aluminate has –ve charged basic sites
• Ratio of calcium : alumina = 0.5:1
• Dissimilar ionic sizes
• Moderate interaction of +ve charged Ni ions with support
• Magnesium aluminate has –ve charged basic sites
• Ratio of magnesium : alumina = 1:1
• Magnesium and nickel ions have similar spacing
• High interaction of + ve charged Ni ions with support
Increasing
difficulty of
reduction
Supports – reduction temperature
Mag
nesiu
m A
lum
ina
te
Calc
ium
Alu
min
ate
Alp
ha A
lum
ina
Temperature (°F)
800 1000 1200 1400 1600
Temperature (°C) 400 500 800 900 700 600
Supports - hydration
• Hydrolysis – reaction of oxides to hydroxides
• Occurs during start up and shut down
• Or during steaming for carbon / sulphur removal
• Alpha alumina is thermodynamically stable
• Calcium aluminate has a low Ca:alumina ratio
• Calcium oxide is locked into the support
• Not available for hydration
• Magnesium aluminate has a high Mg:alumina ratio
• Easier for free magnesia to be present
• Manufacturing process can not eliminate free magnesia
• MgO + H2O → Mg(OH)2
• Hydrolysis results in volume changes and weakening of the
support
Supports – carbon formation
• The support can have an impact on carbon formation
• +ve charged acidic sites enhance carbon formation rates
• Alpha alumina is an acidic support
• Calcium and magnesium aluminates are basic
• Test results
Support Highly acidic
support
Alpha
alumina
Magnesium
aluminate
Calcium
aluminate
Minimum
steam ratio
10.0 4.3 3.7 3.5
Carbon formation – gas feed
• Formed from hydrocarbon cracking
• CH4 ⇌ C(s) + 2H2
• C2H6 → 2C(s) + 3H2
• C3H8 → 3C(s) + 4H2
• …..
• Removed by steam
• C(s) + H2O ⇌ CO + H2
Carbon formation – hydrocarbon feed
• From cracking or polymerization of olefin intermediates
CxHy
Carbon formation – avoidance
• Carbon formation will always occur to some extent
• Balance of formation and removal reactions
• If rate of removal exceeds rate of formation then OK
• Need to slow the formation and speed up the removal
• Addition of potash (alkali) is the most common method
• This makes the support more basic and less prone to carbon
• The potash hydrolyses releasing volatile potassium
hydroxide
• Known carbon gasification promoter
• Johnson Matthey pioneered the use of alkali in the 1960s
and has been successfully using it since
Carbon formation – alkali
• Test results
• The potash is formed into the support as a mineral
• Aluminosilicates such as Kalsilite – KAlSiO4
• Slowly releases to maintain an active level
• Minimizes effect on activity
• Creates a long life before the potash is depleted
Support Alpha
alumina
Magnesium
aluminate
Calcium
aluminate
Alkalised
calcium
aluminate
Minimum
steam ratio
4.3 3.7 3.5 1.5
Carbon formation – effect of alkali
• Standard and alkalized catalysts tested
• Subject to carbon formation then removal conditions
• CO2 released during carbon removal measured
0
2
4
6
8
10
12
300 500 700 900
CO
2 in
exit
gas (
mo
l%)
Temperature (°C)
alkalized
Non alkalized
Carbon formation experiment
Carbon removal experiment
Catalyst shape – the basics
• Catalyst shape is a compromise between
• High activity (area) – small pellets with multiple holes
• High heat transfer – medium pellets with multiple large holes
– holes aligned in the radial direction
– good pellet to tube wall contact
• Low pressure drop – large pellets with multiple large holes
– holes aligned in the axial direction
– poor pellet to tube wall contact
• High strength – limited number of small or no holes
– thick ligaments / webs
• Good breakage – simple shape without stressed areas
Catalyst shape – more basics
• Can not develop a one size fits all catalyst
• Different sizes of the same optimized support shape
QUADRALOBE now available in 4 size options
• For use in different severity reformers
• For split charges in different parts of the reformer tube
• Lowest pressure drop with KATALCOJM 57-4XQ
Catalyst – range
• Range of sizes MQ Q GQ XQ
Catalyst shape – breakage
Poor shapes Good shape
Catalyst breakage – stress analysis
Load Load
Catalyst shape – packing
Poor shape Good shape
Catalyst shape – heat transfer
Poor shapes Good shape
Catalyst shape – heat transfer
Packing model Flow simulation
Tube wall temperature
Heat transfer
correlations
Conclusions
• The chemistry of steam reforming catalysts is complex
• The support can have a significant impact on the catalyst
performance
• The choice of support material is an optimisation between activity,
catalyst reduction, carbon resistance, strength and cost
• The engineering of steam reforming catalysts is complex
• The shape of the pellet is a key factor in the catalyst performance
• The choice of shape is an optimisation between activity, pressure
drop, heat transfer, strength and crushing characteristics
• Johnson Matthey has the best range of…
• catalyst and shapes for all reformer designs and feeds
• techniques for monitoring and optimising reformer performance
Thank you