4. Steam Reforming Reactions
The conversion of hydrocarbons to a mixture of CO,
CO and H2
Two reactions: Reforming and Shift
Water gas shift (slightly exothermic)
CO + H2O CO2 + H2
Steam Reforming (very endothermic)
CH4 + H2O CO + 3H2
CnH(2n+2) + nH2O nCO + (2n+1)H2
Overall the reaction is highly endothermic
5. Equilibrium Considerations
Both reforming and shift reactions are reversible
Rate of shift is fast compared to reforming
Methane conversion favoured by:
low pressure
high temperature
high steam to carbon ratio
CO conversion to CO2 favoured by:
low temperature
high steam to carbon ratio
GBHE kinetics developed using full size reformer
tube with whole pellets under industrial conditions
9. Primary Reformer
The Primary Reformer is a heat exchanger
Its function is to heat up process gas
Catalyst and reaction in the tubes
Combustion on the shell side
Dominant heat transfer by radiation
Multiple fuel supply points
10. Reformer Furnace
3 major types of reformer
Each tackles the duty in different ways
No clear best choice
Choice dictated by Contractor history
Terrace wall - Foster Wheeler
Side Fired - Topsoe, Selas, Chiyoda
Top Fired - H & G, Davy, Toyo, Howe Baker, Kellogg,
KTI etc
11. Many reformers now heat transfer limited
Catalyst not limiting reformer size or operation
Especially the case for hydrogen & methanol
plants
Important for design and uprating
Fluegas exit flow and temperature often limits
Heat exchange must not be ignored
Claims of +30% capacity treated with caution
Reformer Furnace
15. Heat Transfer - Top Fired
Nearly all heat transfer is by radiation
Radiation from the fluegas to the tubes
Little direct radiation from refractory to tube
Refractory acts as a reflector
Radiation from flame to tube at tube top
17. Heat Transfer - Side Fired / Terraced
Same for side fired and terrace walled
Nearly all heat transfer is by radiation
Radiation from the fluegas to the tubes
Major direct radiation from refractory to tube
Significant heat transferred from flame to wall
Carried out by convection
Radiation from flame to tube all down
19. Reformer Firing Balancing
Must have an even heat input to the furnace
Same reasons as for an even process gas flow
Exit temperature variations give high average
approach
Need to keep exit temps the same
Trim air and fuel flows to individual burners
Monitor tube wall temps and exit temps
Must be done as air ducting, tunnels etc may have
a systematic effect upon heat input
20. Reformer Firing Balancing
Usual problems for uneven heat input:-
Burner problems
Burner fouling with liquid fuels or offgas
Air leaks around burners or tube tops
Tunnel problems (mods or collapses)
Air ducting problems (internal refractory)
21. Typical Primary Reformer Catalyst
Loading
LoadedLength-12.98m
HalfLoadDip-7.38m
Tubes 352
Tubes id : 95 mm
Loaded Length : 12.98 m
Catalyst Types : 50% VSG-Z101
50% VSG-Z102
Loaded density : VSG-Z101 0.857 kg/l
VSG-Z102 0.828 kg/l
Total volume loaded 32.38m3
Full Tube Dip
0.89m
Catalyst
Support Grid
VSG-Z102
6.49m
VSG-Z101
6.49m
Ammonia Plant
22. Primary Reformer
Reforming involves heating the process gas
The position of equilibrium is constantly changing
The catalyst tries to react the gas to equilibrium
The catalyst is essentially chasing the heat input
Top of tube: slow reaction rate, high heat flux
Bottom of tube: high reaction rate, low heat flux
High approach to equilibrium at top of tube, low
approach at bottom of tube
Can never achieve a zero approach to equilibrium
25. Primary Reformer Catalyst
Requirements
High and Stable Activity
Low Pressure Drop
Good Heat Transfer
High resistance to Carbon Formation
High Strength
Robust Formulation / Simple Operation
26. High and Stable Activity
Low methane slip
Lower tube wall temperature
Reduced Fuel usage
27. Low Pressure Drop
Savings in Compression Power / Fuel
Possible Throughput increase
28. Improved Heat Transfer
Reduced tube wall temperatures
Increase firing /higher throughput
Smaller catalyst particles improve heat transfer
from wall to bulk gas
Smaller particles increase pressure drop
Catalyst shape should be optimised for high heat
transfer with low PD
29. Steam Reforming Catalysts
Nickel on a ceramic support
Three key factors in catalyst design:
i) geometric surface area
ii) heat transfer from tube to gas
iii) pressure drop
Also of concern:
i) packing in the tube
ii) breakage characteristics
30. Diffusion Limitation
The reforming reaction is very fast on the Ni sites
Reaction limited to catalyst surface (<0.1mm)
Reaction rate controlled by film diffusion
High geometric surface area gives high activity
31. Diffusion into and out of Catalyst
Bulk
Gas
Gas
Fil
m
Ni
Sites
CO
H O2
H2
CH4
Catalyst Support
32. Key Reaction Steps
1. Fast
Diffusion of the molecules in the bulk gas
phase
2. Slow
Diffusion of the molecules through the gas
film
3. Slow
Diffusion through catalyst pores
4. Fast
Absorption of the molecules onto the Ni
sites
5. Fast
Chemical reaction to produce CO2 and H2
33. Reaction Rate
Reaction rate controlled by film diffusion
- Most of the reaction takes place on
the catalyst surface (<0.1mm)
- Pore diffusion not limiting as film diffusion
controls the overall rate
Catalysts with higher geometric surface area
(GSA) per unit volume of catalyst will have a
higher activity.
Pore size/distribution is not significant for most
commercial grades of reforming catalyst
36. Outside Tube Wall T 830°C
Fluegas T 1200°C
Inside Tube Wall T 775°C
Gas film
Tube
Wall
Heat Transfer
Bulk Process
Gas T 715°C
37. Catalyst Heat Transfer
Reforming involves large heat flows into tubes
Absolute requirement to keep tubes cool
Major limitation is at the tube wall
Need to minimize thickness of stationary gas film
at tube wall
The catalyst acts as a heat transfer enhancer to
improve heat transfer from tube wall to gas
Promotes turbulence at the wall
Promotes gas mixing from walls to tube centre
Smaller catalyst particles improve heat transfer
from wall to bulk gas and hence reduce tube wall
temperatures
38. Catalyst Heat Transfer
Heat transfer to catalyst normally very good (high
GSA)
Minor limitation is radially in the catalyst
Catalyst also improves radial heat transfer
Smaller pellets improve wall transfer
Larger pellets improve radial transfer
Smaller usually better overall
BUT smaller particles increase pressure drop
Catalyst shape needs to be optimized for high heat
transfer with low PD
39. Catalyst Shape
The traditional catalyst shape is a ring
Smaller rings give higher activity and heat transfer
but higher pressure drop
Shape optimised catalysts offer high GSA and heat
transfer with low PD
Important that shape also provides good packing
and breakage characteristics
40. Tube Wall Temperature Profile
Top Fired Reformer
660
680
700
720
740
760
780
800
820
840
860
0 0.2 0.4 0.6 0.8 1
Fraction Down Tube
TubeWallTemperature(°C)
Base case with twice GSA
Base case with twice heat transfer
Base case
42. Catalyst Design Criteria
Conclusions
Design of catalyst shape is a complex optimization of:
- Higher GSA (Needed for activity - diffusion control)
- Higher HTC (Needed for cooler reformer tubes)
- Lower Pressure drop (Plant Efficiency / Capacity)
Need also to consider breakage characteristics
and loading pattern inside the reformer tube
44. Catalyst Breakage
Catalyst breaks up in service
Main mechanism due to startup / shutdown
The tube when cooling exerts massive forces
(several tonnes)
Forces exerted by carbon formation immeasurable
Pressure drop rises about 10% per year
Minimum Catalyst strength for handling & charging
approx. 10 kgf
The key is to ensure the catalyst does not fragment
into small bits/dust.
Careful charging essential
45. Breakage Characteristics
Contraction of tube
- some readjustment
- some breakage
Cold ColdHot
Initial
catalyst level
Expansion of tube
- some settling
All catalysts show breakage with time
No support can withstand tube forces
46. Pressure Drop due to Catalyst Breakage
Relativepd(%)
% Breakage
100
200
0 5 10 15 20
pd limit
Shape with good breakage
characteristics
Shape with poor breakage
characteristics Conventional rings
Breakage Characteristics is an
Important Consideration
49. Packing Characteristics
Uniform loading of catalyst
- Uniform tube pressure drops
- Uniform tube temperatures (no hot spots)
Long cylinders with hole(s) through
the centre give good uniform
packing
Short cylinders (tablets) with hole(s)
through the centre can stack resulting
in poor gas distribution down the
reformer tube
51. Catalyst Support
Three types commercially available
– Alpha Alumina
– Calcium Aluminate
– Magnesium Aluminate Spinel
52. Catalyst Support - Bulk Chemistry
Alpha
Alumina
Calcium
Aluminate
Magnesium
Aluminate
Spinel
Structure Corundum Spinel-like Spinel
Stability to
Sintering
Extremely
Stable
Relatively
Stable
Relatively
Stable
Chemical
Stability
(Hydrolysis)
Inert Stable ‘Free’ MgO
Hydration
under
Steaming
Conditions
53. Catalyst Support - Surface Chemistry
Alpha
Alumina
Calcium
Aluminate
Magnesium
Aluminate
Spinel
Surface Area Low Higher Higher
Basicity Inert Basic Sites Most Basic
Support
Surface
Interaction
with Ni / NiO
No Chemical
Interaction
Moderately
Reactive
Surface
Some
bonding of
Ni 2+
ions
Most
Reactive
Surface
Strongest
bonding of
Ni 2+
ions
54. Catalyst Support - Solid Solutions
Magnesium Aluminate Spinel
NiO / MgO
Solid solution
NiAl2O4
formed
NiO / Ni
Fresh catalyst
High surface area
Heat
In use - Low surface area
Difficult to reduce NiO
Important to consider in-service activity
and ease of catalyst reduction
55. Catalyst Support - Reduction
Temperatures
AlphaAlumina
CalciumAluminate
Temperature (°F)
Temperature (°C)
800 1000 1200 1400 1600
400 500 600 700 900
Magnesium aluminate
spinel material usually
supplied pre-reduced
MagnesiumAluminateSpinel
56. Tube Wall Temperature Stability
0 200 400
820
840
860
880
900
920
940
DAYS ON LINE
ICI RINGS
COMP A
SHAPE
COMP B
SHAPE
COMP B
SHAPE 2
ICI SHAPE
POWEROUTAGE
CATALYSTCHANGE
AT584DAYS
REDUCED
CATALYSTCHANGE
AT280DAYS
PDLIMITRATE
REDUCED
CATALYSTCHANGE
AT421DAYS
0 0 0 0200 400200 200 200
TWT
LIMIT
RATE25%
CATALYSTCHANGE
AT258DAYS
600 700 800 900
MAXIMUMTUBEWALLTEMPERATURE(°C)
58. Carbon Formation and Prevention
Carbon formation is totally unwanted
Causes catalyst breakage and deactivation
Leads to overheating of the tubes
In extreme cases carbon formation causes a
pressure drop increase
59. Cracking
CH4 ⇔ C + 2H2
C2H6 ⇔ 2C + 3H2 etc
Boudouard
2CO ⇔ C + CO2
CO Reduction
CO + H2 ⇔ C + H2O
If carbon formation rate is faster than removal
rate then carbon will be deposited
61. Effect of Carbon Formation
1. Physical poisoning
-Carbon covers the catalyst surface
2. Pressure drop increase
- Usually only in severe situations
- Carbon fills catalyst bed voids
-Carbon formed in catalyst pores will weaken
or break catalyst
3. Hot tubes
- Carbon laydown on the inside of the tube
wall
- Lower catalyst activity
62. Carbon Formation and Prevention
Giraffe
Necking
Hot TubeHot Band
Reformer tube appearance - Carbon laydown
63. Carbon Formation and Prevention
Under normal conditions carbon gasification by
steam and CO2 is favored
i.e. gasification rate > C formation rate)
Problems of carbon formation may occur when:
i) steam to carbon ratio is too low
ii) catalyst is not active enough
iii) higher hydrocarbons are present
iv) tube walls are too hot (high flux)
v) catalyst has poor heat transfer characteristics
64. Carbon Formation and Prevention
Methods of preventing carbon formation:
– Use more active catalyst
– Use better heat transfer catalyst
– Reduce levels of higher hydrocarbons
– Increase the steam ratio
– Use a potash doped catalyst (VULCAN-
series) which reduces probability of carbon
formation
65. Alkali greatly accelerates carbon removal
Addition of potash to the catalyst support reduces
carbon formation in two ways:
a) increases the basicity of the support
b) promotes carbon gasification (aids
adsorption of water)
C + H2O ⇔ CO + H2
Potash is mobile on the catalyst surface
Level of potash required depends on feed and heat
flux
Potash doped catalyst is only needed in the top half
of the reformer tube
Carbon Formation and Prevention
OH
-
66. Increasing the content of alkali (potash) allows:
Higher heat flux for light feeds
Heavier hydrocarbons in feed
Lower steam to carbon ratios
Faster carbon removal during steaming
Carbon Formation and Prevention
69. Methane Cracking - Kinetic Limitation
Carbon Formation
Zone
No Carbon
Formation
Deposition rate
< removal rate
Promoted by alkali
Deposition rate
> removal rate
Promoted by acid
550 600 650 700 750 800
Temperature (°C )
100
10
1.0
0.1
(pH2) 2
pCH4
70. Methane Cracking - Kinetic Limitation
0.6
0.5
0.4
0.3
Fraction of
tube length
from top
550 600 750 800
Temperature C
100
10
1.0
0.1
Carbon Formation
Zone
No Carbon
Formation
Deposition rate
< removal rate
Deposition rate
> removal rate
650 700650 700
O
(pH2) 2
pCH4
71. Methane Cracking - Basic Catalyst
Support
0.6
0.5
0.4
0.3
0.25
More basic
support
550 600 800
Temperature C
100
1.0
0.1
No Carbon
Formation
650 700 750
10
Carbon Formation
Zone
O
(pH2) 2
pCH4
72. Methane Cracking - Increased Potash
Content
Carbon
Formation Zone
Increasing
Potash
Content
550 600 800
Temperature C
100
1.0
0.1
No Carbon
Formation
650 700 750
10
0.6
0.5
0.4
0.3
0.25
O
(pH2) 2
pCH4
73. Carbon Formation and Prevention
Fraction Down TubeTop Bottom
Non-Alkalised
Catalyst
Ring Catalyst
Optimised Shape
(4-hole Catalyst)
Inside Tube Wall
Temperature
920°C
820°C
720°C
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Alkalised
Catalyst
Carbon Forming
Region
74. Carbon Formation and Prevention
For light feeds and LPG etc. using lightly
alkalized catalyst (VSG-Z102)
- Potash is chemically locked into catalyst
support
- Potash required only in the top 40-50% of
the reformer tube
- Catalyst life influenced by
Poisoning
Ni sintering
Process upsets etc.
VSG-Z101
VSG-Z102
75. Carbon Formation and Prevention
For heavy feeds, potash needs to be mobile
Utilize VSG-Z101 series catalyst
Removes carbon on the catalyst surface and
inside of the tube wall
Potash slowly released by a complex reaction
VSG-Z101
VSG-Z102
76. Carbon Formation and Prevention
Potash release mechanism (VSG-Z102 series)
K2O-Al2O3-SiO2
CaO-Al2O3
MgO-Al2O3
CO2 + H2
CaO-Al2O3-SiO2
CaO-MgO-SiO2
K2CO3
H2O
2KOH + CO2
Note:- MgO is chemically
locked into catalyst support
material
Catalyst Support Material
77. Carbon Formation and Prevention
VSG-Z102 Series catalysts
Catalyst life determined by residual potash
remaining in the catalyst
Minimum amount typically 2-3 %wt at bottom of
potash promoted catalyst bed - but minimum
level depends on feedstock type and
operational severity
78. Carbon formation by polymerization
– Suppressed by having NiO / MgO solid
solutions as the active catalyst component
– Need to increase total NiO content to
overcome loss of steam reforming activity
– Zirconia addition also further enhances
catalyst activity
Carbon Formation and Prevention
79. Potash promotion
Lowers hydrocarbon carbon cracking rate
Increases carbon removal rate
VSG-Z102 series can remove carbon from tube
wall ("mobile" potash)
VSG-Z102 series contain some NiO/MgO solid
solutions to lower polymerisation activity
Greatly facilitates carbon removal during
steaming operation (after severe carbon
formation)
Summary
82. Catalyst Activity Die Off
2 major factors:
Poisoning by sulfur
Affects upper tube and tube temperatures
Thermal sintering
Affects lower tube and approach
Some effect on upper tubes and tube
temperatures
83. Poisons
Many poisons affect reforming catalysts
Halides, phosphates, sulfur
Heavy metals, alkali metals etc
Major poisons are:
Sulfur ex feedstock
Phosphate ex BFW
Metals ex BFW or liquid feed
84. Sulfur Poisoning
Nickel is a very good sulfur adsorbent
Sulfur sticks to the nickel surface
Do not need a lot of sulfur to give problems
Can totally deactivate a reforming catalyst
86. Sulfur Poisoning
Depends upon the catalyst temperature
Occurs in the cooler regions
Upper section of tubes vulnerable
Also depends upon sulfur exit the HDS
88. Sulfur Poisoning
Sulfur poisoning is reversible
Steam the catalyst for 24 hours
Steam slowly strips off the sulfur
Re-oxidizes the catalyst in addition
May lose some activity permanently
90. Steaming Reforming Catalysts
Steaming of the catalyst is required when there is:
Severe carbon formation
Loss of steam
Incorrect steam to carbon ratio operation
sulfur poisoning
sulfur poisoning
Poor performance of the desulfurization
system
91. Steaming Reforming Catalysts
Isolate hydrocarbon feed
Maintain steam flow at highest possible level
Adjust reformer firing to achieve 750°C reformer
exit temperature or higher if possible
Monitor carbon oxides or H2S in the exit gases
Addition of a small amount of nitrogen into the
steam facilitates reliable sample analysis
92. Potash promoted catalyst
Accelerates carbon gasification during
steaming
Required since carbon laydown often occurs in
the top section of the reformer tubes where high
temperatures needed for steaming are not
easily achieve
Design of catalyst needs to ensure potash
release is controlled during steaming
Release rate for VSG-Z102 series is
approximately double the normal rate during
steaming (24 hours of steaming ages the
catalyst by 48 hours)
Steaming Reforming Catalysts
93. Effect of Steaming - Alkalized Catalyst
Steaming Temperature °C (equivalent to 1 year operation)
Potash Retention - Steaming Test
500 550 600 650 700 750
0
0.5
1
1.5
2
2.5
3
Residualwt%ofpotash
VSG-Z102
Comp. A
Comp. B
95. 5 10 15 20 25 30 35
2.5
0
3
3.5
4
4.5
Time on line (months)
HDS
Problem
Catalyst
Steamed
Methaneslip(mol%dry)
Design
Steaming Reforming Catalysts
96. Sulfur removal
Catalyst performance can be restored
High reformer inlet temperature during
steaming is important for successful sulfur
removal
Need to monitor the H2S slip during steaming
Steaming Reforming Catalysts
97. Sulfur Levels in Discharged Catalyst
0
100
200
300
0 10 20
Distance Down Reformer Tube (m)
sulfur(ppmwt)
Before Steaming
After Steaming
98. Steaming Reformer Catalyst
MgO in catalyst support must not be "free"
otherwise during steaming, the MgO will
hydrolyse
MgO + H2O Mg(OH)2
Hydration of the MgO causes rapid loss of
catalyst strength and severe catalyst break-up
and high reformer pressure drop
VSG-Z102 series catalyst contain MgO that is
chemically locked into the the catalyst support -
No hydration
100. Reformer Catalyst Monitoring
Monitor frequently (daily)
Exit Methane
Tube Wall Temperature (TWT)
Tube Appearance
Monitor Less Frequently
Pressure Drop
Approach to Methane Steam Equilibrium
101. Methane Slip
– Dependant on throughput, heat load & catalyst
activity
– If these conditions vary then exit CH4 will vary
Approach to Equilibrium
– ATE defined as Difference between Actual
Temperature & Equilibrium Temperature
– Better guide to catalyst activity
– ATE increases as catalyst activity decreases
Reformer Catalyst Monitoring
102. Reformer Catalyst Monitoring
Tube Wall Temperature
– Dependant on catalyst loading, catalyst
activity & physical catalyst condition
– As maximum TWT is approached, rate must be
reduced. In worst case catalyst will need to be
changed
Appearance
– A good indication of how reformer is operating
– Tubes should look cool. Poor catalyst
performance will mean tubes looking hot.
.
103. Pressure Drop
–PD will increase with time due to physical
blockage/breakage of catalyst
–Too high PD will result in throughput
limitation
–Should back-calculate PD at design
conditions (independent of throughput)
Since PD α (velocity) 2
Normalised PD = Measured PD 100
% design( (2
104. Approach to Equilibrium (ATE)
The approach to equilibrium (ATE) at any point
along the catalyst bed is the difference between
the actual gas temperature and the equilibrium
temperature corresponding to the gas
composition.
The ATE can be used as a good measure of the
performance of the catalyst when the operating
temperature of the reactor is held constant, and
when the reaction is equilibrium limited, such as
with primary reforming.
105. Calculation of ATE
Steam Reforming Reactions
CH4 + H2O ⇔ CO + 3H2 Methane Steam (MS)
CO + H2O ⇔ CO2 + H2 Water Gas Shift (WGS)
Since the WGS reaction is so fast it can be
assumed to be at equilibrium under reformer exit
conditions
This means then the equilibrium temperature for
this reaction (TWGS) can be used as a reliable
estimate of the actual reformer exit temperature
(Measurements are unreliable)
106. For the WGS reaction the equilibrium constant
(KWGS) can be calculated
Then equilibrium tables can be used to determine
the equilibrium temperature for this reaction (TWGS)
For the MS reaction the equilibrium constant (KMS)
can also be calculated and equilibrium tables then
used to determine the equilibrium temperature for
this reaction (TMS)
107. The ATE can then be calculated as the difference
between TWGS (equal to the actual reformer exit
temperature) and TMS
GBHE uses a computer program to calculate ATE
108. Example Calculation of ATE
Reformer Exit Composition
% v/v
H2 68.0
N2 1.6
CH4 9.6
CO 10.2
CO2 10.6
Total 100.0 Dry
H2O 76.6
Total 176.6 Wet
Reformer Exit
Pressure = 31.6 ata
Reformer Exit
Temperature
(TWGS) = 796°C
109. pCH4 = 9.6 x 31.6 = 1.7178 ata
176.6
pH2 = 68.0 x 31.6 = 12.1676 ata
176.6
pH2O = 76.6 x 31.6 = 13.7065 ata
176.6
pCO = 10.2 x 31.6 = 1.8251 ata
176.6
110. Example Calculation of ATE
KMS = pCH4 . pH2O
pCO . (pH2)3
= 1.7178 x 13.7065
1.8251 x (12.1676) 3
= 7.161 x 10-3
From Tables TMS = 792°C
ATE = 796 - 792 = 4°C
112. Catalyst Handling
Catalysts are expensive & should be treated with
care at all stages of:
– Handling on arrival
– Storage
– Charging
– Storage in vessel before start-up
Careful & detailed supervision at all stages is
essential
Safety: proper equipment is essential both for the
safety of workers & to prevent damage to catalyst
113. Handling & Storage
On Arrival
– Use suitable fork lift truck or crane to transfer to
storage
– Don't drop drums off tail board of lorry
– Don't roll drums
– Inspect drums for damage & repair broken lids
Storage
– Store under cover (long term storage)
– Avoid damp / wet conditions
– Store drums in upright position
– Stack no higher than 4 drums
– Catalyst not affected by extremes of
temperature
– (-50°C to +50°C) provided kept dry.
114. Catalyst Loading
If loading is poor, variety of flows in tubes
Each tube has different exit temperature
Each tube has a close approach
Methane slip not linear with temperature
Mixture of all tubes far from equilibrium
Made worse by the flow imbalance
115. Base Case
Reformer Exit: 20 ata 870°C design
10 °C approach to equilibrium
Maldistribution
10 °C approach to equilibrium
Tube 1: 105% flow 850°C Exit T
Tube 2: 95% flow 890°C Exit T
116. Base Case Maldistribution Case
Tube 1 Tube 2
Exit Temperature (°C) 870 850 890
Relative Flow (%) 100 105 95
Approach to Equilibrium (°C) 10 10 10
Methane Slip (% dry) 3.583 4.698 2.687
Average Methane Slip (% dry) 3.583 3.743
Average Approach to Equilibrium (°C) 10 13.1
117. Catalyst Charging - Tubes
Inspect empty tubes
Check pressure drop on tubes both empty and
full
'Sock' or 'Unidense' method recommended
Avoid excessive hammering and vibration
Final PDs should be within 5% of mean
Better to discharge tubes with high PDs rather
than over-vibrate tubes with low PDs
Weighing is a useful check on charged bulk
density, but not essential
124. Norsk Hydro technology
- available through Hydro Agri Europe
Simple & fast loading technique
No pre-socking and no tube vibration required
Applicable to a range of catalyst types & reformer
designs
Offers high uniform catalyst density
Catalyst Charging - Unidense Method
125. Charging Technique
Weighed amount of catalyst is poured into the
tube & the loading rope is gradually pulled out of
the tube as the catalyst layer builds up.
The brushes with flexible springs reduce the
speed of the catalyst particles so that breakage
is avoided.
This results in a loading without bridges & voids,
hence there is no need for tube vibration /
hammering.
Catalyst Charging Unidense Method
126. Catalyst Charging - Unidense Method
Support grid
Charging
chute
Loading rope with
flexible springs
127. Benefits
Reduced loading time
Reduced possibility of bridging / less hot spots
Contributes to lower tube wall temperatures and
prolonged tube life
Narrow pressure drop variation in tubes
Slightly higher PD than sock method
Minimal further settling / PD increase
Catalyst Charging - Unidense Method
129. Precommisioning / Periods of Shutdown
Completely close reactor after charging
Box up under N2 if necessary
After commissioning leave temperature points
connected and check regularly during shutdown
periods
Check drains regularly
After shutdown keep under positive N2 pressure
(natural gas OK for sulfur removal catalysts)
On decommissioning Nickel containing catalysts
must be purged free from carbon oxides before
temperature falls below 250°C
130. Normally
Process feed on flow control
Process steam on ratio control from feed
rate
Purge fuel / flash gas to fuel header
Fuel header on pressure control
Fuel to reformer on flow control
◦ Adjusted to maintain reformer exit temperature
133. Steam reformer is complex
heat exchanger
chemical reaction over catalyst
combustion, leading to steam generation
Common symptoms of poor performance
high exit methane slip
high approach to equilibrium
high tube wall temperature
high pressure drop
Need properly active catalyst
Introduction
134. As supplied - NiO on support
Active species - Ni Crystallites
Reduction process needed:-
NiO + H2 ⇔ Ni + H2O
Introduction - Catalyst Reduction
136. Faster at high temperature
Slower in presence of steam
Thermodynamically, very little hydrogen needed
Support also affects ease of reduction
Introduction - Catalyst Reduction
137. Extreme danger of local overheating!
Requires high temperature
- fire steam reformer
Requires reducing conditions
- supply H2 or reducing gas
- re-circulation or once-through
Since little or no steam reforming is taking place,
less heat is required to warm up gas
50% steam rate, with 5:1 steam:H2 ratio requires
1/7 fuel of normal operation
Introduction - Catalyst Reduction
139. Air warm-up possible, but not for previously
reduced catalyst (possible carbon)
Purge plant of air with N2
(Care: must be free of hydrocarbons and carbon
oxides)
Heat reformer above condensation temperature
Add steam when exit header temperature 50°C
above condensation temperature
(low pressure favours good distribution and
lowers this temperature)
Increase steam rate to 40 - 50 % of design rate
(min 30%)
Stop N2 circulation
Start-Up Procedure - Warm Up
140. Rapid warm-up minimises energy usage / time
Limited by mechanical considerations of steam
reformer
Assess effect on plant equipment
thermal expansion of inlet/exit pipes
reformer tube tensioners
reformer tubes
refractory linings
Traditionally: 50°C per hour
Modern material: 100°C per hour
Catalyst: 150 - 170°C per hour
Start-Up Procedure - Warm Up
141. If upstream pipe-work cold, good practice to warm
up by steam flow to vent to prevent carry-over of
water.
Steam Steam
Reformer
Cold Pipe-work
Start-Up Procedure - Warm Up
142. Temperatures referred to are true catalyst
temperatures at exit of tube
Measured temperatures during normal operation are
10 -100°C cooler due to heat losses
Most catastrophic failures of tubes in top-fired
furnaces occur during start-up
Cannot rely on plant instrumentation during start-up
lower flows than normal
higher heat losses than normal
fewer burners can give severe local effects
Frequent visual inspection of reformer tubes and
refractory essential during start-up
Start-Up Procedure - Warm Up
143. Effect of Pressure and Temperature
1
10
100
1,000
10,000
100,000
1,000,000
10,000,000
Tube Wall Temperature °C (°F)
TubeLife(hours)
800 900 1000 1100 1200
(1500) (1650) (1830) (2010) (2200)
5 bar30 bar
Start-Up Procedure - Tube Life
145. Reduction with Hydrogen
Reduction with Natural Gas
Reduction with other sources of hydrogen
Higher hydrocarbons
Ammonia
Methanol
Start-Up Procedure - Catalyst Reduction
146. H2 or H2-rich gas can be added at any time to the
steam when plant is free of O2
Steam : hydrogen ratio normally 6:1 - 8:1
Get tube inlet temperature as high as possible
Increase exit temperature to design value
>700°C
Hold for 2-3 hours
Catalyst Reduction with Hydrogen
147. Hydrogen must be free of poisons (S, CI)
Special consideration must be given to the
presence in impure hydrogen sources of:
carbon oxides
hydrocarbons
Also applies to nitrogen (or inert) source used for
purge/warm-up
Catalyst Reduction with Hydrogen
148. Recirculation loop may include HDS unit (at
temperature)
Carbon oxides above 250°C (480°F) methanate over
unsulphided CoMo catalyst:
temperature rise 74°C per 1% CO converted
temperature rise 60°C per 1% CO2 converted
If H2 contains > 3 % CO or > 13 % CO2 or a mixture
corresponding to this then by-pass the HDS system
Catalyst Reduction with Hydrogen
149. Natural Gas
Will be converted to carbon oxides + hydrogen in
reformer
May crack thermally to give carbon
Catalyst Reduction with Natural Gas
150. Warm-up as before (N2 then steam)
Introduce natural gas at 5% of design rate
Slowly increase gas rate to give 7:1 steam:carbon
over 2-3 hours
Simultaneously increase reformer exit temperature
to design level i.e. >700°C
Increase inlet temperature as much as possible (to
crack natural gas to give H2)
Monitor exit methane hourly
Reduction complete when methane reaches low,
steady value (4 to 8 hours)
Catalyst Reduction with Natural Gas
151. E.g. propane
Increased possibility of carbon formation
Much greater care needed
Longer time periods needed
More precision in all measurements needed
Hydrogen addition recommended if possible
Purification issues
- Desulfurization
- Methanation of carbon oxides
Catalyst Reduction with Higher
Hydrocarbons
Not normally recommended
152. Crack ammonia in ammonia cracker
Crack ammonia in steam reformer
inject liquid ammonia upstream of steam
reformer
bypass HDS
Procedure as for hydrogen reduction
Exit temperature 800°C (1470°F) to maximise
ammonia cracking
Catalyst Reduction with Ammonia
153. Uncommon Procedure
Methanol decomposes to give H2 and CO
Regulate flow of liquid methanol to give
6:1 - 8:1 steam:hydrogen ratio exit steam reformer
Do not recycle exit gas (potential methanation of
carbon oxides)
Catalyst Reduction with Methanol
155. Introduce feedstock at high steam:carbon ratio (5:1
for natural gas; 10:1 for higher hydrocarbons)
Steam reforming will give small increase in inlet
pressure, cooling of tubes, and lower exit
temperature
Need to increase firing to maintain exit temperature
Then increase feedstock flow
Increase pressure to operating pressure
Adjust steam:carbon ratio to design
Start-Up Procedure - Feed Introduction
156. Increase flow of natural gas to design
steam:carbon ratio (2 hours)
Maintain exit temperature
Check that exit methane stays low
(reducing steam:carbon ratio will increase
methane slip and heat load)
if not, hold at 7:1 steam : carbon for 2 hours
Increase throughput to design level
Increase pressure to design level
Always increase steam rate before feed rate
Start-Up Procedure - Feed Introduction
157. Shorter re-reduction recommended
Typically 4-6 hours for heavy feeds
Not essential to carry-out reduction with natural
gas or light off-gas feedstock
Start up at 50% design rate, high steam:carbon
ratio
Start-Up Procedure - Restart
159. Reduce tube exit temperature to 750°C
Decrease feed and steam flows in stages to 40%
design
- always decrease hydrocarbon flow first
- adjust firing to keep exit temperature steady
Keep steam flow constant, shut off hydrocarbon
feed
- adjust firing to maintain exit temperature
- purge system of hydrocarbons
Decrease exit temperature to 550°C at 100°C per
hour
Shut-down
160. Add flow of N2 and continue cooling
Shut off steam 50°C above condensation
temperature
Continue cooling with N2 flow
When catalyst below 50°C tubes may be emptied
Shut-down
162. Large modern top-fired steam reformer
Significant tube failures during start-up
Caused by overfiring at start-up due to a number
of coincident factors
Case Studies - No 1
163. Site steam shortages requiring conservation of steam
Pressure to avoid a shut-down (due to low product
stocks)
Burner fuel usually from two sources, mixed:
one low calorific value
one high calorific value
At time of incident, all high calorific value
(unexpectedly) fuel received
Operators had seen many shutdown/start-ups during
past two years
Case Studies - No. 1
164. Plant trip (loss of feedstock to reformer) due to
valve failure
Feedstock to reformer not isolated adequately by
valve
Setpoint on reformed gas pressure not reduced
Steam introduced for plant restart at reduced rate
All burners lit (deviation from procedure)
Reformer tubes remained at normal operating
pressure of 16 barg
Case Studies - No. 1
165. Steam reformer tubes "looked normal"
Nearly 3x as much fuel going to burners
than there should have been
High calorific value fuel added an extra
15% heat release
First tubes rupture
High furnace pressure (trip bypassed)
Oxygen in flue gas dropped to zero
Flames seen from peep holes
Normal furnace pressure
Visual inspection revealed "white hot
furnace and tubes peeling open"
Emergency Shutdown Activated!
30minutes Case Studies - No. 1
168. Reformer exit gas temperature on panel never
exceeded 700°C
Cannot use this instrumentation as a guide to tube
temperature
Reformer start-up at normal operating pressure
Tube failure temperature 250°C lower than
normal for start-up
All burners lit
Far too much heat input resulted in excessive
temperatures
Tubes Fail Rapidly!
Case Studies - No. 1
169. Ammonia Plant
LTS reduction loop included steam reformer
CO2 released from LTS reduction
Carbon formed in steam reformer
Case Studies - No. 2
170. LTS reduction with closed loop circulation
Normally condenser, compressor and pre-heat coil
This time included steam reformer, pre-heater and
waste heat boiler
Steam reformer fired to TWT of 900°C
LTS reduction liberates CO2
By 2am, LTS reduction almost complete
- 50 % CO2 in recirculation gas
- also some H2 present
Case Studies - No. 2
171. Steam reformer pushed to give apparent LTS
temperature of 200°C
Due to instrument error, in fact 380°C
Between 5am - 7 am, steam reformer PD
increased
Tubes looked hot
Reformer steamed for 18 hours
No reduction in PD
Plant shutdown
Case Studies - No. 2
Reformer catalyst black and badly broken up - due
to severe carbon formation
172. Check to ensure that recirculation loops do not
contain high levels of carbon oxides
Case Studies - No. 2
CO2 can shift in LTS to CO
CO2 + H2 CO + H2O
CO in presence of H2 gives carbon
CO + H2 C + H2O
CO can methanate in steam reformer (if some
catalyst reduction due to presence of H2 is seen)
forming CH4
CO + 3H2 CH4 + H2O
This cracks to form carbon
CH4 C + 2H2
174. Importance of Tube Wall Temperature
Measurement
Need accurate information
Tube life
Artificial limitation on plant rate
175. Effect of Tube Wall Temperature on
Tube Life
850 900 950 1000
(1560) (1650) (1740) (1830)
Temperature °C (°F)
TubeLife(Years)
Design
+ 20°C
20
10
2
5
1
0.5
0.2
176. Tube Wall Temperature Measurement
Contact
- Surface Thermocouple
"Pseudo-contact“
- Gold Cup Pyrometer
Non-contact
Disappearing Filament
Infra Red Optical Pyrometer
Laser Pyrometer
177. Surface Thermocouples
Continuous measurement, by conduction
"slotting" can weaken tube wall
Spray-welding leads to high readings
Short, unpredictable lives (6 -12 months)
Not commonly used for steam reformer tubes
178. Disappearing Filament
Hand held instrument
Tungsten filament superimposed on image of
target
Current through filament altered until it
"disappears“
Current calibrated to temperature
Range 800-3000°C
Very operator sensitive
Largely displaced by IR
179. Infra-red Pyrometer
Easy to use
Need to correct for emissivity and reflected
radiation
Inexpensive
180. Laser Pyrometer
Laser pulse fired at target and return signal
detected
Can determine target emissivity
Must correct for background radiation
High spacial selectivity
Very accurate for flat surfaces
181. Gold Cup Pyrometer
Excludes all reflected radiation
Approximates to black body conditions
High accuracy / reproducibility
BUT
- Limited access
- Awkward to use
183. Accurate Temperature Measurement
Combination of IR pyrometer and Gold cup pyrometer
Gold cup pyrometer allows calculation of emissivity
Full accurate survey of reformer possible with IR
pyrometer
184. Temperature Measurement Corrections
epyrometer (Tm)4 = etube (Tt )4 + rtube (Tw)4
Measured True Averaged
target target background
temperature temperature temperature
e = emissivity
r = reflectance
= (1-e)
185. Accurate Temperature Measurement
(Tm)4 = etube (Tt )4 + (1 - etube) (Tw)4
- Set IR Pyrometer emissivity at 1
- Measure Tm and Tw with Pyrometer
- Measure Tt with Gold Cup
- Calculate etube