steam reforming - practical operations
DESCRIPTION
Reaction Chemistry Typical Reformer Configurations Catalyst Design Criteria Carbon Formation and Prevention Catalyst Deactivation Steaming Reforming Catalysts Monitoring Reforming Catalysts Catalyst Loading Reduction & Start-Up Tube Wall Temperature MeasurementTRANSCRIPT
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Steam Reforming Practical Operation
C2PT Catalyst Process Technology
By Gerard B Hawkins Managing Director, CEO
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Contents
Reaction Chemistry Typical Reformer Configurations Catalyst Design Criteria Carbon Formation and Prevention Catalyst Deactivation Steaming Reforming Catalysts Monitoring Reforming Catalysts Catalyst Loading Reduction & Start-Up Tube Wall Temperature Measurement
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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
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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
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Reformer is a Heat Exchanger
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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
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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
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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
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Top Fired Reformer
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Side Fired Reformer
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Terrace Walled Reformer
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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
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Heat Transfer - Top Fired
Radiative heat flows
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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
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Typical Reforming Configuration
Steam
Steam
Steam + Gas
Steam Reformer
500°C
870°C
1200°C
3% CH4
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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
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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)
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Typical Primary Reformer Catalyst Loading
Load
ed L
engt
h - 1
2.98
m
Hal
f Loa
d D
ip -
7.38
m
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
-Z10
2 6.
49m
VS
G-Z
101
6.49
m
Ammonia Plant
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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
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0 100 200 300 400 500 600 700 800 900
0 0.2 0.4 0.6 0.8 1
Fraction down tube
Tem
pera
ture
(°C
)
Gas T
Eqm. T
ATE
Approach to Equilibrium
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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
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High and Stable Activity
Low methane slip
Lower tube wall temperature
Reduced Fuel usage
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Low Pressure Drop
Savings in Compression Power / Fuel
Possible Throughput increase
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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
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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
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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
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Diffusion into and out of Catalyst
Bulk Gas
Gas Film
Ni Sites
CO H O 2
H 2
CH 4
Catalyst Support
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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
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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
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Un-sintered Catalyst
0.001 mm (1/25 thou)
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Sintered Catalyst
0.001 mm (1/25 thou)
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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
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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
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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
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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
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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
Tube
Wal
l Tem
pera
ture
(°C
)
Base case with twice GSA
Base case with twice heat transfer
Base case
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Heat Transfer and Pressure Drop
1 2 3 4
1 2 3 4Voidage 0.49 0.6 0.58 0.59Relative PD 1 0.9 0.9 0.8
Relative HTC 1 1.3 1.1 1
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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
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VULCAN VSG-Z101 VULCAN VSG-Z102
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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
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Breakage Characteristics
Contraction of tube - some readjustment - some breakage
Cold Cold Hot Initial catalyst level
Expansion of tube - some settling
All catalysts show breakage with time No support can withstand tube forces
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Pressure Drop due to Catalyst Breakage
Rel
ativ
e pd
(%)
% 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
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Breakage Characteristics
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Breakage Characteristics
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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
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Packing Characteristics
Extended External Surface Area - "Cogs"
Void
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Catalyst Support
Three types commercially available
– Alpha Alumina
– Calcium Aluminate
– Magnesium Aluminate Spinel
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Catalyst Support - Bulk Chemistry
AlphaAlumina
CalciumAluminate
MagnesiumAluminateSpinel
Structure Corundum Spinel-like Spinel
Stability toSintering
ExtremelyStable
RelativelyStable
RelativelyStable
ChemicalStability(Hydrolysis)
Inert Stable ‘Free’ MgOHydrationunderSteamingConditions
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Catalyst Support - Surface Chemistry
AlphaAlumina
CalciumAluminate
MagnesiumAluminateSpinel
Surface Area Low Higher Higher
Basicity Inert Basic Sites Most BasicSupport
SurfaceInteractionwith Ni / NiO
No ChemicalInteraction
ModeratelyReactiveSurface
Somebonding ofNi 2+ ions
MostReactiveSurface
Strongestbonding ofNi 2+ ions
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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
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Catalyst Support - Reduction Temperatures
Alph
a Al
umin
a
Cal
cium
Alu
min
ate
Temperature (°F)
Temperature (°C)
800 1000 1200 1400 1600
400 500 600 700 900
Magnesium aluminate spinel material usually supplied pre-reduced
Mag
nesi
um A
lum
inat
e Sp
inel
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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
POW
ER O
UTA
GE
CA
TALY
ST C
HA
NG
E A
T 58
4 D
AYS
RED
UC
ED
CA
TALY
ST C
HA
NG
E A
T 28
0 D
AYS
PD L
IMIT
RA
TE
RED
UC
ED
CA
TALY
ST C
HA
NG
E A
T 42
1 D
AYS
0 0 0 0 200 400 200 200 200
TWT LIMIT
RA
TE 2
5%
CA
TALY
ST C
HA
NG
E A
T 25
8 D
AYS
600 700 800 900
MAX
IMU
M T
UB
E W
ALL
TEM
PER
ATU
RE
(°C
)
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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
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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
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Carbon Formation - Heavy Feeds
Hydrocarbon Feed
Intermediates (Olefins, Paraffins,
CH4, & H2)
Carbon
CH4, H2, CO2 & CO
Catalyzed Partial Decomposition
Thermal Cracking
Polymerization
Steam Reforming
Carbon Gasification
H2O
H2O
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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
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Carbon Formation and Prevention
Giraffe Necking
Hot Tube Hot Band
Reformer tube appearance - Carbon laydown
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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
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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
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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 -
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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
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Carbon Formation and Prevention
Increasing potash addition
Methane feed/Low heat flux
Methane feed/High heat flux Propane, Butane feeds (S/C >4)
Propane, Butane feed (S/C >2.5) Light naphtha feed (FBP < 120 °C)
Heavy naphtha feed (FBP < 180 °C)
K2O wt% 0
2-3
4-5
6-7
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Methane Cracking
100
10
1.0
0.1
Temperature (°C )
(pH2) 2 pCH4
Carbon Formation Zone
No Carbon Formation
CH4 2H2 + C
550 600 650 700 750 800
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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
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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
650 700 650 700 O
(pH2) 2 pCH4
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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
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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
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Carbon Formation and Prevention
Fraction Down Tube Top 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
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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
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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
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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
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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
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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
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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
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Natural Gas Reforming Catalysts
Associated Gas Reforming Catalysts
Dual Feedstock Reforming Catalysts
Naphtha Reforming Catalysts
Un-alkalised Lightly Alkalised Moderately Alkalised
Heavily Alkalised
VSG-Z101 VSG-Z102 VSG-Z102 VSG-Z102
Naphtha 3.0 – 3.5
Light Naphtha 6.0 – 8.0 3.0 – 4.0 2.5 – 3.0
Butane 4.0 – 5.0 2.5 – 3.5 2.0 – 3.0
Propane / LPG 3.0 –4.0 2.5 – 3.0 2.0 – 2.5
Refinery Gas 6.0 – 10.0 3.0 – 4.0 2.0 – 3.0 2.0 – 2.5
Associated Gas 5.0 – 7.0 2.0 – 3.0 2.0 – 2.5
Natural Gas 2.5 – 4.0 1.5 – 2.0 1.0 – 2.0
Pre-reformed Gas
2.0 – 3.0 1.0 – 2.0 1.0 – 2.0
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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
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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
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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
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Sulfur Poisoning
Pellet S S S
S S
S
S S
S S
Nickel
CH4
H2O
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Sulfur Poisoning
Depends upon the catalyst temperature
Occurs in the cooler regions
Upper section of tubes vulnerable
Also depends upon sulfur exit the HDS
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Effect of sulfur coverage on activity
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Coverage
Act
ivity
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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
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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
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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
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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
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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
Res
idua
l wt %
of p
otas
h
VSG-Z102
Comp. A
Comp. B
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Steaming Reforming Catalysts
Carbon Removal By Steaming
50
0
100
150
200
1.2
0 0.2 0.4 0.6 0.8 1
1.4 1.6
Time (hours) 0 5 10 15 20
Residual C
arbon (%)
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5 10 15 20 25 30 35 2.5
0
3
3.5
4
4.5
Time on line (months)
HDS Problem
Catalyst Steamed M
etha
ne s
lip (m
ol %
dry
)
Design
Steaming Reforming Catalysts
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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
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Sulfur Levels in Discharged Catalyst
0
100
200
300
0 10 20 Distance Down Reformer Tube (m)
sulfu
r (pp
m w
t)
Before Steaming
After Steaming
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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
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Reformer Catalyst Monitoring
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Reformer Catalyst Monitoring
Monitor frequently (daily)
Exit Methane Tube Wall Temperature (TWT) Tube Appearance
Monitor Less Frequently
Pressure Drop Approach to Methane Steam Equilibrium
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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
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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.
.
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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
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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.
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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)
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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)
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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
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Catalyst supplied in pre-weighed socks
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Sock slightly narrower than tube bore End of sock folded over
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Lowered down tube on rope ‘Tugged’ to release fold Free fall <0.5m allowable
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Vibrated with hammer after each sock
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PD measured empty / ½ full / full Adjusted to ± 3 - 5%
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Outage Adjusted
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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
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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
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Catalyst Charging - Unidense Method
Support grid
Charging chute
Loading rope with flexible springs
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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
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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
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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
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Reduction & Start-Up
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Introduction
Start-up Procedures Warm-up Catalyst Reduction Feed Introduction
Shut-down
Case Studies
Contents
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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
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As supplied - NiO on support Active species - Ni Crystallites Reduction process needed:-
NiO + H2 ⇔ Ni + H2O
Introduction - Catalyst Reduction
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400 500 600 700 800 100
200
300
500
700
Temperature °C (°F)
Partial Pressure of H2O / Partial Pressure H2 Eq
uilib
rium
Con
stan
t
Reducing Conditions
Oxidising Conditions
(752) (932) (1112) (1292) (1472)
Introduction - Catalyst Reduction
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Faster at high temperature
Slower in presence of steam
Thermodynamically, very little hydrogen needed
Support also affects ease of reduction
Introduction - Catalyst Reduction
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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
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Introduction
Start-up Procedure Warm-up Catalyst Reduction Feed Introduction
Shut-down
Case studies
Start-Up Procedure
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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
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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
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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
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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
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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)
Tube
Life
( ho
urs
)
800 900 1000 1100 1200 (1500) (1650) (1830) (2010) (2200)
5 bar 30 bar
Start-Up Procedure - Tube Life
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Introduction
Start-up Procedure Warm-up Catalyst Reduction Feed Introduction
Shut-down
Case studies
Start-Up Procedure - Catalyst Reduction
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Reduction with Hydrogen
Reduction with Natural Gas
Reduction with other sources of hydrogen
Higher hydrocarbons Ammonia Methanol
Start-Up Procedure - Catalyst Reduction
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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
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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
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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
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Natural Gas
Will be converted to carbon oxides + hydrogen in reformer
May crack thermally to give carbon
Catalyst Reduction with Natural Gas
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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
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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
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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
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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
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Introduction
Start-up Procedure Warm-up Catalyst Reduction Feed Introduction
Shut-down
Case studies
Start-Up Procedure - Feed Introduction
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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
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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
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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
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Introduction
Start-up Procedure Warm-up Catalyst Reduction Feed Introduction
Shut-down
Case Studies
Case Studies
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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
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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
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Introduction
Start-up Procedure Warm-up Catalyst Reduction Feed Introduction
Shut-down
Case Studies
Case Studies
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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
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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
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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
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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!
30 m
inut
es
Case Studies - No. 1
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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
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Ammonia Plant
LTS reduction loop included steam reformer
CO2 released from LTS reduction Carbon formed in steam reformer
Case Studies - No. 2
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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
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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
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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
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Importance of Tube Wall Temperature Measurement
Need accurate information
Tube life Artificial limitation on plant rate
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Effect of Tube Wall Temperature on Tube Life
850 900 950 1000 (1560) (1650) (1740) (1830)
Temperature °C (°F)
Tube
Life
(Yea
rs) Design
+ 20°C
20
10
2
5
1
0.5
0.2
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Tube Wall Temperature Measurement
Contact
- Surface Thermocouple
"Pseudo-contact“
- Gold Cup Pyrometer
Non-contact
Disappearing Filament Infra Red Optical Pyrometer Laser Pyrometer
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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
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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
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Infra-red Pyrometer
Easy to use
Need to correct for emissivity and reflected radiation
Inexpensive
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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
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Gold Cup Pyrometer
Excludes all reflected radiation
Approximates to black body conditions
High accuracy / reproducibility
BUT
- Limited access - Awkward to use
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Gold Cup Pyrometer
Tube Furnace Wall
Water Cooling
To Recorder
Gold Cup Lance
*
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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
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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)
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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
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Background Temperature Measurement
NORTH
A
a 2
a 1
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Comparison of Infra-red Pyrometer and Calculated Tube Wall Temperature
Measurements Te
mpe
ratu
re (°
C)
Tem
pera
ture
(°F)
Fraction Down Tube 0 0.2 0.4 0.6 0.8 1
950 900 850 800 750
1742 1652 1562 1472 1382
Uncorrected Pyrometer
Corrected Pyrometer Calculated
= Gold Cup Measurements
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Tube Wall Temperature Measurement -Conclusions
IR Pyrometer typically reads high
Top-fired reformer 32°C Side-fired reformer 50°C
IR Pyrometer with Gold Cup "calibration“
Top-fired reformer 2°C Side-fired reformer 16°C
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Classroom Exercise 2 - PROBLEM Reformer exit gas composition (dry %)
H2 73.19 N2 + Ar 1.11 CH4 3.04 CO 15.55 CO2 7.11 Total (dry) 100.00 H2O 41.34 Total (wet) 141.34
Reformer exit pressure 18.11 barg Reformer exit temperature 875°C
Calculate the approach to equilibrium
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Classroom Exercise 2 - ANSWER Exit Pressure (ata) = (18.11 / 1.013) + 1 = 18.88 ata pCH4 = 3.04 x 18.88 = 0.4061 ata 141.34 pH2 = 73.19 x 18.88 = 9.7766 ata 141.34 pH2O = 41.34 x 18.88 = 5.5221 ata 141.34 pCO = 15.55 x 18.88 = 2.0771 ata 141.34 pCO2 = 7.11 x 18.88 = 0.9497 ata 141.34
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KWGS = pH2 . pCO2 pH2O . pCO
= 9.7766 x 0.9497 5.5221 x 2.0771
= 8.09 x 10-1
From Tables TWGS = 875°C Reformer exit temperature = 875°C
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KMS = pCH4 . pH2O pCO . (pH2)3
= 0.4061 x 5.5221 2.0771 x (9.7766) 3
= 1.15 x 10-3
From Tables TMS = 874°C
ATE = 875 - 874 = 1°C
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