steam reforming - the basics of reforming

C2PT Catalyst Process Technology

By Gerard B Hawkins Managing Director

Steam Reforming Catalysis : ◦ Chemical reactions

◦ Catalyst shape design

◦ Catalyst chemistry

◦ Carbon formation and removal

The conversion of hydrocarbons to a mixture of CO, CO2 and H2 Two reactions: Reforming and Shift

Steam Reforming (very endothermic)

CH4 + H2O CO + 3H2

CnH2n+2 + nH2O nCO + (2n + 1)H2

Water gas shift (slightly exothermic)

CO + H2O CO2 + H2

Overall the reaction is highly endothermic

Both reforming and shift reactions are reversible

Rate of shift is fast compared to reforming

Methane conversion favored by: – low pressure – high temperature – high steam to carbon ratio

Steam

SecondaryReformer

Steam

Steam + Gas

SteamReformer

Air / Oxygen500°C

780°C

450°C

1200°C

950°C

10% CH4 0.5% CH4

The primary reformer is a heat exchanger Its function is to heat up process gas Catalyst and reaction in the tubes Combustion on the shell side Dominant heat transfer by radiation

0 0.2 0.4 0.6 0.8 1200

300

400

500

600

700

800

900

fraction down tube

tem

pera

ture

(°C

)

gas temp

Eq tempATE

Nickel on a ceramic support Three key factors in catalyst design:

– geometric surface area – heat transfer from tube to gas – pressure drop

Also of concern: – packing in the tube – breakage characteristics

Top Fired Reformer

0 0.2 0.4 0.6 0.8 1660680700720740760780800820840860

fraction down tube

tube

wal

l tem

pera

ture

(°C

)

base case

base case with twice GSA

base case with twice heat transfer

Outside tube wall temperature 830°C

Bulk ProcessGas Temp.715°C

1200°CFluegas

Inside tube wall temperature 775°C

Gas film

Tube Wall

Need to minimize thickness of gas film at tube wall

Smaller catalyst particles improve heat transfer from wall to bulk gas and reduce tube temperatures

Smaller particles increase pressure drop

Catalyst shape should be optimized for high heat transfer with low pressure drop

The traditional catalyst shape is a ring

Smaller rings give high activity and heat transfer but higher pressure drop

Optimized catalysts offer high surface area and heat transfer with low PD

Important that shape also provides good packing and breakage characteristics

Relative Pressure DropRelative HTC

Voidage

1 0.9 0.9 0.8

1 2 3 4

1 1.3 1.1 1.0 0.49 0.6 0.58 0.59

1 2 3 4

Design of catalyst shape is a complex optimization of:

– Higher surface area (needed for activity -

diffusion control) – Higher heat transfer (needed for cooler

reformer tubes) – Lower pressure drop (efficiency consideration)

Need also to consider breakage characteristics and loading

pattern inside the reformer tube

Catalyst loading can be improved using various dense loading techniques

Carbon formation is totally unwanted

Causes catalyst breakage and deactivation

Leads to overheating of the tubes

In extreme cases carbon formation causes a pressure drop increase

Carbon Formation and Prevention

Giraffe Necking

Hot Tube Hot Band

Reformer tube appearance - Carbon laydown

Cracking – CH4 C + 2H2 – C2H6 2C + 3H2 etc

Boudouard – C + CO2 2CO

Gasification

– C + H2O CO + H2

Under normal conditions carbon gasification by steam and CO2 is favored (gasification rate > C formation rate)

Problems of carbon formation occur when: – steam to carbon ratio is too low – catalyst is not active enough – higher hydrocarbons are present – tube walls are too hot – catalyst has poor heat transfer characteristics

Use of a potash doped catalyst reduces probability of carbon formation

Methods of preventing carbon formation:

– Use more active catalyst – Use better heat transfer catalyst – Reduce level of higher hydrocarbons – Increase the steam ratio – Use VSG-Z102 (3-7) -hole tailored catalysts

catalyst (potash-promoted)

Alkali greatly accelerates carbon removal

Addition of potash to the catalyst support reduces carbon formation in two ways: a increases the basicity of the support

b promotes carbon gasification

Potash is mobile on the catalyst surface

Potash doped catalyst is only needed in the top half of the reformer tube

C + H2O CO + H2 OH -

Increasing the content of alkali (potash)

– Higher heat flux possible for light feeds – Heavier hydrocarbons can be steam reformed – Lower steam to carbon ratios – Faster carbon removal during steaming

Fraction Down TubeTop Bottom

Non-AlkalisedCatalyst

Ring Catalyst

Optimised Shape(4-hole Catalyst)

Inside Tube WallTemperature

920 C(1688 F)

820 C(1508 F)

720 C(1328 F)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

AlkalisedCatalyst

Carbon FormingRegion

O

OO

OO

O

For light feeds and LPG etc using lightly alkalised catalyst VSG-Z101 – Potash is chemically locked into catalyst

support – Potash required only in the top 30-50% of the

reformer tube

– Catalyst life influenced by Poisoning Ni Sintering Process upsets etc

VSG-Z101

VSG-Z102

0

0.5

1

1.5

2

2.5

3

1.2m 3m 5m 6m 9mCatalyst samples at various depths down

reformer tube

Fresh1 year2 years4 years6 years

wt% of potash

VSG-Z102

VSG-Z102

Requirements : ◦ High and stable activity ◦ Low pressure drop ◦ Good heat transfer ◦ High resistance to carbon ◦ High strength ◦ Robust formulation/simple operation

Best achieved with VSG-Z101 (3-7) -hole tailored catalysts