Why do we include potash ?
What are the benefits ?
What are the disadvantages ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
Carbon formation margin
Reaction chemistry (Tube inlet)
Hydrocarbons undergo cracking reactions on hot surfaces at the tube inlet
Products of catalytic cracking reactions can form polymeric carbon
"Subclassing and Composition – A Pythonic Tour of Trade-Offs", Hynek Schlawack
The Benefits and Disadvantages of Potash in Steam Reforming
1. The Benefits and Disadvantages of
Potash In Steam Reforming
Gerard B. Hawkins
Managing Director
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2. Introduction
Why do we include potash ?
What are the benefits ?
What are the disadvantages ?
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3. Why do we include Potash ?
Two reasons
• Prevents carbon formation within
The catalyst
On the inside wall of the tube
• If carbon is laid down, helps gasification
of carbon
Potash has to be mobile so that it gets to
the hottest point that the process gas sees
• The inside tube wall
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4. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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
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5. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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
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6. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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
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7. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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
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8. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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
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9. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Prevention
• If carbon deposition occurs by :
CH4 C + 2 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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10. Why do we include Potash ?
Catalyst Deactivation
Carbon Deposition : Impact of Potash
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)
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11. Why do we include Potash ?
In terms of modelling we consider the
margin to carbon formation
This is defined as the difference between
the
• Equilibrium temperature
• Process gas temperature
Assumes GOM natural gas
Care with gases that are heavier than this
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12. Why do we include Potash ?
Definition of carbon formation
margin
pH2
2
pCH4
10
1.0
0.1
550 600 650 700 750 800
1100
100
Temperature (°C)
CDZ
1200 1300 1400 (°F)
No carbon
deposition
Margin to Carbon
Formation
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13. Why do we include Potash ?
600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Temperature(°C)
Inside TWT
Carbon Equilibrium
Margin to Carbon
Formation
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14. Why do we include Potash ?
600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Temperature(°C)
Base Case Inside TWT
Base Case Carbon
Forming Equilibrium
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15. Why do we include Potash ?
600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Temperature(°C)
Base Case Inside TWT
Base Case Carbon
Forming Equilibrium
Potash Carbon
Formation
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17. Naphtha Steam Reforming
Reaction chemistry (Tube inlet)
• Hydrocarbons undergo cracking reactions on hot
surfaces at the tube inlet
• Products of catalytic cracking reactions can form
polymeric carbon
• High strength catalysts required
• Carbon resistant catalysts required
CxHy Cx + y/2H2
CxHy CH4 + H2 + Cx-1H2x-2
Thermal
Catalytic
Polymers
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18. Steam Reforming - Reaction Chemistry
Thermal cracking &
carbon formation
Catalytic cracking and
olefin polymerisation
Steam reforming
reactions
Water gas shift reaction
Heavy
Naphtha
Light
Naphtha
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19. Steam Reforming - Catalyst Design
High strength catalysts
to tolerate carbon
deposition
Carbon resistant
catalysts
High activity catalysts
High activity catalysts
Heavy
Naphtha
Light
Naphtha
or
Mixed
Feeds
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20. So What are the Benefits ?
Prevents carbon formation in applications
were carbon formation is an issue
For example
• Highly stressed reformers
High heat fluxes
High throughput
• High levels of C2+
Even then can be insufficient
• Low steam to carbon ratios
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21. What are the Disadvantages ?
It is assumed that the potash will be
captured on the catalyst in the bottom of the
tubes
This is not always the case
In a number of situations there have been
problems
Usually linked to potash loss
• In some cases fouls the WHB
• Sometimes reaches the HTS
• Can cause SCC of downstream heat
exchangers
• Loss of catalyst strength
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22. What are the Disadvantages ?
Waste Heat Boiler Fouling
For NG feeds main issue has been;
• H2 plants are more vulnerable as process
gas temperatures are higher
• Rate of evolution of potash from catalyst
is higher
Customer X WHB exit temperature increased
at 2°F per day
• Fouling in WHB was 23% potash
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23. What are the Disadvantages ?
Stress corrosion cracking
On train at South American Methanol Plant
• 2nd BFW heater (3 in total) suffered from
SCC
• Process gas on shell side
Design flaw – should have been on tube
side as it is the dirty duty
• Material specified as SS due to supply
issue
Design flaw – should have been CS
• Huge amount of cracking – due to SCC
Exchanger irreparable
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24. What are the Disadvantages ?
Effect on WHB
For Naphtha feeds – a variety of problems on plants
• Again with fouling of WHB
• Older plants have large fouling margin
• Modern plants have had this cut and so have
insufficient surface area
• Requires regular WHB cleaning
• Sometimes bypass valve becomes coated and does
not operate correctly
• Sometimes potash deposited in cold end of WHB
and needs digging out
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25. What are the Disadvantages ?
Effect on HTS
Most WHB have internal bypass for exit
temperature control – automatic control
If tubes are fouled, tube exit temperature rises
Bypass therefore closes to keep WHB exit
temperature constant
Once bypass fully closes, exit WHB T rises
Therefore inlet HTS temperature rises
So CO slip rises
If HTS catalyst or vessel temperature limit is
reached then plant rate has to be reduced
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26. What are the Disadvantages ?
Effect on HTS
Potash can also coat the surface of the HTS bed
Deactivates leading edge of the bed
Increases CO slip
Increases DP
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27. What are the Disadvantages ?
Effect on Catalyst
As potash is removed then strength of catalyst is
reduced
Causes excessive breakage
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28. What are the Disadvantages ?
Effect on Catalyst
0
20
40
60
80
100
120
140
160
Z203 Z202 Z201 Z101
Catalyst type
kg
Min
Avg
Max
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29. What are the Disadvantages ?
Effect on Catalyst
Carbon Laydown
Test:
Feed
cyclohexane +
steam at 500 oC
and S/C = 3.5
isolate steam
form carbon
remove/inspect
sample after set
times
2
9
15
Timewithoutsteam(mins)
46-3 46-3Q
VSG-Z101 survives where Comp fails
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30. Conclusions
Potash has many more advantages than
disadvantages
We have options for the majority of plants
to address carbon formation
Even the very worst cases
On some plants there are problems were the
WHB is very sensitive to fouling
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