Steam Reforming - Carbon Formation

Gerard B. Hawkins
Managing Director
WWW.GBHENTERPRISES.COM

 The aim of this presentation is to
• Give an understanding of the reasons for carbon
formation
◦ Look at two main types
◦ Explain mechanisms
◦ Explain prevention of cracking

 Carbon formation
• Is side reaction
• Unwanted due to catalyst damage
◦ Breakage of catalyst
 Pressure drop increase
◦ Loss of activity and heat transfer
 Increased process gas temperature
◦ Hot bands
 Increased outside tube temperatures
 Reduction of tube life

Weld
Hot Bands

 Three main types
 1 Carbon cracking
 2 Boudouard carbon formation
 3 CO reduction
 Main focus on number 1 since it is most common

 Carbon is formed when
• Catalyst has low activity
◦ End of life
◦ Poisoned
◦ Wrong catalyst
• Steam to carbon to low
◦ Either during transient or normal operation
• Increased higher hydrocarbons
• Catalyst has poor heat transfer

 Natural gas feeds can produce carbon
• High temperatures can cause methane cracking
• Not likely at tube inlet
CH4 C + 2 H2
C2H6 2C + 3H2
C3H8 3C + 4H2
• High concentrations of H2 inhibit carbon formation
• Not likely at bottom of reformer tube
• Carbon formation zone at ~30% of tube length

 Methane cracking forms two types of carbon :
Whisker carbon :
• High concentrations of
carbon are dissolved within
the nickel metal crystallites
• Carbon precipitates as
tubular “whiskers” containing
nickel crystallites
• Filaments are robust and can
weaken catalyst pellets
Pyrolytic carbon :
• Carbon deposits across
catalyst surface
• Carbon covers active
surfaces
• Reduced catalyst activity

Hollow
carbon fibre
Nickel
crystallite0.0001mm (1/250 thou)
Pellet surface
Carbon

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100 1200 1300 1400 (°F)
100
Temperature (°C)
High Methane
Concentrations
Increasing Potential for
Carbon Deposition

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100 1200 1300 1400 (°F)
100
Temperature (°C)
Increasing Rate of
Carbon Deposition

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
Carbon Deposition Zone
1200 1300 1400 (°F)
Deposition
possible but
rate low
Deposition not favored

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
CDZ
1200 1300 1400 (°F)
Composition - temperature
profile along reformer tube
No carbon
deposition

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
CDZ
1200 1300 1400 (°F)
Zone of carbon deposition
30% of tube length

 If carbon deposition occurs by :
CH4 C + H2
 Then carbon deposition rate > carbon removal rate
 Deposition rate is difficult to modify
 Faster carbon removal is possible by leveraging an
additional removal reaction :
 C + H2O CO + H2
 Potash acts to increase the rate of this reaction

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
1200
Faster rate of carbon
removal shrinks CDZ
No carbon deposition
CDZ
1300 1400 (°F)

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
1200 1300 1400 (°F)
CDZ

pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
1200 1300 1400 (°F)
Margin

 Once carbon is formed then
• Activity of the catalyst is reduced
◦ Nickel sites blocked off - less reaction
◦ Higher process gas temperatures
• More resistance to flow - lower flow
◦ Higher process gas temperatures
• More heat transfer resistance
◦ Higher temperatures
• All cause temperature to be increased
◦ therefore an increased rate of carbon formation

 Reaction is
 2CO C + CO2
 Carbon is laid down between metal crystallites
 This induces stress at the micro level
 Grains the pop out
 Leads to classic pitting

Temperature
Kp Gas
Equilibrium
Carbon forming
region

Boudouard Carbon - Equilibrium
Carbon Operation
Temperature
Kp
Gas
Equilibrium
Carbon forming
region

2CO C + CO
10.0
1.0
0.1
0.01
Temperature (°C)
400 500 600 700 800 900 1000 1100
Carbon Free Region
Carbon Forming
Region
2
k
p=PCO2/(PCO2)bar-1

Steam Reforming - Carbon Formation

Steam Reforming - Carbon Formation

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