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