Terry L. Roberts Potash & Phosphate Institute Potash & Phosphate Institute of Canada
The Benefits and Disadvantages of Potash in Steam Reforming
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Transcript of The Benefits and Disadvantages of Potash in Steam Reforming
The Benefits and Disadvantages of Potash In Steam Reforming
Gerard B. Hawkins Managing Director
www.GBHEnterprises.com
Introduction
Why do we include potash ? What are the benefits ? What are the disadvantages ?
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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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Why do we include Potash ? Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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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Why do we include Potash ? Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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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Why do we include Potash ? Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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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Why do we include Potash ? Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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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Why do we include Potash ? Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
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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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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Why do we include Potash ? Catalyst Deactivation
Carbon Deposition : Impact of Potash
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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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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Why do we include Potash ? Definition of carbon formation
margin
pH22
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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Why do we include Potash ?
600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Tem
pera
ture
(°C
)
Inside TWTCarbon Equilibrium
Margin to Carbon Formation
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Why do we include Potash ?
600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Tem
pera
ture
(°C
)
Base Case Inside TWT
Base Case CarbonForming Equilibrium
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Why do we include Potash ?
600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Tem
pera
ture
(°C
)
Base Case Inside TWT
Base Case CarbonForming EquilibriumPotash CarbonFormation
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Carbon Formation Margin
-50
0
50
100
150
200
250
0 2 4 6
Distance Down Tube (m)
Tem
pera
ture
(°C
)
Base Case Margin
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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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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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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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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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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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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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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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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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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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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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What are the Disadvantages ? Effect on Catalyst
As potash is removed then strength of catalyst is reduced
Causes excessive breakage
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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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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 Tim
e w
ithou
t ste
am (m
ins)
46-3 46-3Q
VSG-Z101 survives where Comp fails www.GBHEnterprises.com
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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