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Date post: | 02-Nov-2014 |
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Gerard B. Hawkins Managing Director
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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
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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
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Weld
Hot BandsWWW.GBHENTERPRISES.COM
Three main types 1 Carbon cracking 2 Boudouard carbon formation 3 CO reduction
Main focus on number 1 since it is most common
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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
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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
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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
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Hollow carbon fibre
Nickel crystallite0.0001mm (1/250 thou)
Pellet surface
Carbon
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pH22
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
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pH22
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
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pH22
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
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pH22
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
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pH22
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
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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
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pH22
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)
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pH22
pCH4
10
1.0
0.1 550 600 650 700 750 800
1100
100
Temperature (°C)
1200 1300 1400 (°F) CDZ
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pH22
pCH4
10
1.0
0.1 550 600 650 700 750 800
1100
100
Temperature (°C)
1200 1300 1400 (°F) CDZ
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pH22
pCH4
10
1.0
0.1 550 600 650 700 750 800
1100
100
Temperature (°C)
1200 1300 1400 (°F)
Margin
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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
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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
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Temperature
Kp Gas
Equilibrium Carbon forming
region
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Boudouard Carbon - Equilibrium Carbon Operation
Temperature
Kp Gas
Equilibrium Carbon forming
region
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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 =
P C
O2 /
(P C
O2)
bar -
1
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