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 ?

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

 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

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

 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

 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)

 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

 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

600
650
700
750
800
850
0 2 4 6
Distance Down Tube (m)
Temperature(°C)
Inside TWT
Carbon Equilibrium
Margin to Carbon
Formation

600
650
700
750
800
850
0 2 4 6
Temperature(°C)
Base Case Inside TWT
Base Case Carbon
Forming Equilibrium

600
650
700
750
800
850
0 2 4 6
Temperature(°C)
Base Case Inside TWT
Base Case Carbon
Forming Equilibrium
Potash Carbon
Formation

Carbon Formation Margin
-50
0
50
100
150
200
250
0 2 4 6
Temperature(°C)
Base Case Margin

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

Steam Reforming - Reaction Chemistry
Thermal cracking &
carbon formation
Catalytic cracking and
olefin polymerisation
Steam reforming
reactions
Water gas shift reaction
Heavy
Naphtha
Light
Naphtha

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

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

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

 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

 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

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

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

Effect on HTS
 Potash can also coat the surface of the HTS bed
 Deactivates leading edge of the bed
 Increases CO slip
 Increases DP

Effect on Catalyst
 As potash is removed then strength of catalyst is
reduced
 Causes excessive breakage

Effect on Catalyst
0
20
40
60
80
100
120
140
160
Z203 Z202 Z201 Z101
Catalyst type
kg
Min
Avg
Max

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

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
A Business Unit of GBH Enterprises, Ltd.

The Benefits and Disadvantages of Potash in Steam Reforming

The Benefits and Disadvantages of Potash in Steam Reforming

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