Steam Reforming - Carbon Formation

transcript

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

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

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

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

0.1 550 600 650 700 750 800

1100 1200 1300 1400 (°F)

Temperature (°C)

High Methane Concentrations

Increasing Potential for Carbon Deposition

0.1 550 600 650 700 750 800

1100 1200 1300 1400 (°F)

Temperature (°C)

Increasing Rate of Carbon Deposition

0.1 550 600 650 700 750 800

Temperature (°C)

Carbon Deposition Zone

1200 1300 1400 (°F)

Deposition possible but rate low

Deposition not favored

0.1 550 600 650 700 750 800

Temperature (°C)

1200 1300 1400 (°F)

Composition - temperature profile along reformer tube

No carbon deposition

0.1 550 600 650 700 750 800

Temperature (°C)

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

0.1 550 600 650 700 750 800

Temperature (°C)

Faster rate of carbon removal shrinks CDZ

No carbon deposition

1300 1400 (°F)

0.1 550 600 650 700 750 800

Temperature (°C)

1200 1300 1400 (°F) CDZ

0.1 550 600 650 700 750 800

Temperature (°C)

1200 1300 1400 (°F) CDZ

0.1 550 600 650 700 750 800

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

Temperature (°C)

400 500 600 700 800 900 1000 1100

Carbon Free Region

Carbon Forming Region

Steam Reforming - Carbon Formation

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