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The Theory and Practice of
Steam Reforming By:
Gerard B. Hawkins
Managing Director, CEO
Contents
Steam reforming reactions Steam reforming catalyst Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre-and post-reforming
Steam Reforming of Methane
CH4 + H2O CO + 3H2 (Steam Reforming))
CO + H2O CO2 + H2 (water Gas Shift)
• Overall strongly endothermic • Need to get large amounts of heat in
– narrow-bore steam reformer tubes
Steam Reforming of Heavier Hydrocarbons
CnHm + nH2O nCO + (n+m/2)H2
Still endothermic Easier than methane More prone to carbon formation
Contents Steam reforming reactions Steam reforming catalysts
• catalyst activity • catalyst development and testing • importance of gas and htc
Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre - and post reforming
Steam Reforming Catalyst Steam reforming can be done without
catalyst, but needs very high temperatures • partial oxidation
Modern steam reforming catalyst use nickel on a ceramic support • with or without promoters and stabilisers • precious metals offer alternatives to Ni
Supports must be strong; inert; thermally and chemically stable
Catalysts lower the temperature at which steam reforming occurs at a high rate
Steam Reforming Catalyst Activity
Reaction highly endothermic • may be limited by process of getting
heat in to reactant sites
Process may also be limited by diffusion
Activity Testing
Define some measure of reaction • exit methane
Measure for a range of catalysts under fixed conditions • flow, temperature pressure, catalyst
Reactants
Products Reaction
Gas Film
• Two types: - molecular diffusion - Knudsen diffusion
Diffusion Effects
Bulk Gas Bulk Gas
Diffusion Processes
Molecular diffusion, Dm • determined by rate at which molecules collide
with each other • depends on pressure • independent of pore radius
Knudsen diffusion, Dk • determined by the rate at which molecules
collide with pore walls • depends on pore radius
Check for Knudsen Diffusion
Mean free path of molecules must be greater than pore radius for Knudsen diffusion to dominate • at 700oC (1290oF), mean free path is 100 Angstrom
Typical pore radius for steam reforming catalyst is 150 - 1000 Angstrom • Not Knudsen regime
Steam Reforming Catalyst Activity
Intrinsic activity (chemical reaction only) Extrinsic activity (includes heat and mass
transfer effects) Steam reforming dominated by extrinsic
effects Influence of pressure significant
Pressure bar (psi)
Catalyst B
Catalyst A
1 (14.5)
10 (145)
20 (290)
Pressure Dependence
Adsorption
Desorption Adsorption
Dehydrogenation
Surface Reaction * *
OH2 C + 2H2 CH4 *
H2O CO + H2 CH4
Surface Science
Photo of XPS
Activity Testing
Techniques exist to measure intrinsic activity • plug-flow reactors and CSTR systems • tests for mass/heat transfer limitations
Quantify other effects explicitly • measure htc • measure diffusional effects
Activity Testing
Intrinsic activity measurements Bench-scale for screening Scale-up to include heat/mass transfer
effects
Activity Testing Microreactor Semi-tech
Steam Reforming Catalysts
Require • high geometric surface area (gsa) • high heat transfer coefficient (htc) • low pressure drop (pd)
Balance of properties Cubes; rings; optimised shapes
Nickel crystallites
No further reaction Reaction zone
Catalyst Pellet Pore
Reactants
Products
Effect of gsa
Steam Reformer Tubes
Need to get a lot of heat in • narrow bore tubes
High temperatures and pressures • tubes in creep region • tubes will fail by rupture • tube life very sensitive to temperature
850 (1560)
900 (1650)
950 (1740)
1000 (1830)
Temperature oC (oF)
0.1
0.2
0.5
1
2
5
10
20 Design
Effect of Tube Wall Temperature on Tube Life
+ 20oC (+ 36oF)
Top Fired Reformer
Distance Down Tube m (ft)
Tube
Wal
l Tem
pera
ture
D
eg C
(Deg
F)
0 1 2 3 4 5 6 7 8 9 10 11 12
BASE CASE BASE CASE WITH TWICE SURFACE AREA BASE CASE WITH TWICE HEAT TRANSFER
840
800
760
720
(1544)
(1472)
(1400)
(6) (12) (18) (24) (30) (36)
Effect of Catalyst Design Variables on Tube Wall Temperature
Tube Wall
Bulk Process Gas Temp. 715oC (1319oF)
1200oC (2192oF)
830oC (1526oF)
775oC (1427oF)
Fluegas
Outside tube wall temperature
Inside tube wall temperature
Gas film Temperature Profile Top-fired reformer, 40% down
Tem
pera
ture
Deg
C (D
eg F
)
Tube Wall Temperature Limit
Poor stability
Good stability
Days on Line 0 1,000 500 100 200 300 400 600 700 800 900
925 (1697)
900 (1652)
875 (1607)
850 (1562)
Effect of Catalyst Stability on Tube wall Temperature
Contents Steam reforming reactions Steam reforming catalysts Equilibrium considerations
• equilibrium curves • effect of process variables
Carbon formation Poisoning Steam reformer modelling Pre-and post-reforming
Methane Steam Equilibrium
CH4 + H2O CO + 3H2
P [CH4] P [H2O] Kms =
P [CO] P [H2] 3
– equilibrium tables
– equilibrium curves
Equilibrium curves (methane)
508
203
102
Equi
libriu
m %
CH
4 (d
ry b
asis
)
Pres
sure
(psi
g)
Pres
sure
(bar
g)
Steam Ratio
2.0
3.0
4.0
5.0
(Illustration only - limited accuracy) 35
14
7
Equilibrium curves (methane)
Pressure : 30 bar (435 psi)
Temperature : 850°C (1562°F)
Steam:Carbon Ratio : 3.5
What is exit CH4 at these conditions?
Equilibrium value 5.6% CH4
(Illustration only - limited accuracy)
Steam Ratio
2.0
3.0
4.0
5.0
100
50
20
10
5.0
2.0
1.0
35
14
7
508
203
102
Equi
libriu
m %
CH
4 (d
ry b
asis
)
F[CH4 ] F[H2O] 1 K ms = F[CO ] F[H2]3 Pt2
F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
Equilibrium Considerations
CH4 + H2O CO + 3H2
Effect of Pressure
• Exit methane proportional to pressure squared • lower exit methane at lower pressures • overall plant economics dictate higher
pressures, typically 20 bar (300 psi)
CH4 + H2O CO + 3H2
F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
Effect of Steam- to- Carbon Ratio
• Exit methane inversely proportional to steam • lower methane requires more steam • actual value depends on overall plant design
• s/c ratio typically 5-6 on older plants • s/c ratio typically 3 on newer plants
CH4 + H2O CO + 3H2
F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
• Exit methane proportional to Kms • Kms approx inversely proportional to temperature
• lower methane requires higher temperatures • limited by tube metallurgy
Effect of Temperature
CH4 + H2O CO + 3H2
F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
Temperature Pressure Steam/Carbon Ratio
Exit Temperature
Exit Pressure
Steam/Carbon Ratio
Exit Gas Composition (% dry)
850 800 850 850 850 850
1562 1472 1562 1562 1562 1562
30 30 20 35 30 30
435 435 290 508 435 435
3.5 3.5 3.5 3.5 3.0 4.0
73.35 70.68 74.76 72.67 72.15 74.26
(°C)
(°F)
(atas)
(psi)
5.35 9.31 3.35 6.30 6.70 4.36 12.18 9.73 13.09 11.78 12.79 11.59 CO
CH 4
CO 2
H 2
9.12 10.28 8.80 9.25 8.36 9.78
Effect of Temperature, Pressure, S/C Ratio
Feedstock Refinery Off Gas
Methane Butane Naphtha
C/H Ratio CH6 CH4 CH2.5 CH2.2
Exit Gas CH4 CO CO2
H2
6.67 8.14 4.45
80.74
5.35 12.18 9.12 73.35
4.29 14.17 12.36 69.16
4.01 14.73 13.77 67.49
All at exit temperature 850 Deg C (1562 Deg F) Exit pressure 30 atas (435 psi) Steam/carbon ratio 3.5
Effect of Feedstock
70
60
50
40
30
20
10
0
Methane Feedstock Exit Temperature 850 C (1472 F) Exit Pressure 30 atas (435 psi)
Steam/Carbon Ratio 3.5
New
Old
CH4
CO CO2
H2
Catalyst activity
Com
posi
tion
(% d
ry)
Effect of Catalyst Activity
Approach to equilibrium
The system is not actually at equilibrium, but close to it
A measure of catalyst performance is the Approach to Equilibrium, ATEms • ATEms = 0 when at equilibrium • the bigger ATEms, the further from
equilibrium
Temperature oC (oF)
770 780 790 800 810 820
2
4
6
8
10
12
Met
hane
slip
(%)
(1418) (1454) (1436) (1472) (1490)
Exit CH4
Approach to Equilibrium
(1508)
ATE
Equilibrium Temp Gas Temp
0 0.2 0.4 0.6 0.8 1 200
(392)
400 (752)
600 (1112)
800 (1472)
Fraction down tube
Tem
pera
ture
o C (o F
)
Gas Temp Eq'm Temp
Approach to equilibrium
Contents
Steam reforming reactions Steam reforming catalysts Equilibrium considerations Carbon formation
• formation and removal reactions • role of alkali • range of catalysts
Poisoning Steam reformer modelling Pre-and post-reforming
Carbon Formation
Depends on: - feedstock - operating conditions - catalyst
Carbon Deposition
Carbon
Catalyst surface
1 mm (40 thou)
Carbon Formation
CH4 C + 2H2 (Thermal Cracking)
CO + H2 C + H2O (CO Reduction)
2CO C + CO2 (CO disproportionation “Boudouard”)
Carbon Formation
Direction of reaction determined by process gas conditions
Generally, CO reduction and Boudouard
are carbon removing Generally, cracking restricted to top half
of reformer
pH22
pCH4
10
1.0
0.1
550 600 650 700 750 800
Carbon Formation Zone
No Carbon Formation
Deposition rate < removal rate
Deposition rate > removal rate
1100 1200 1300 1400 (°F)
100
Carbon Formation Removal Reactions
Temperature (°C)
100
10
1.0
0.1 550 600 650 700 750 800
0.6 0.5
0.4
0.3 Carbon Formation Zone
Temperature (°C)
Proportion of tube length from inlet
1100 1200 1300 1400 (°F)
Carbon Formation - Inside Reformer Tube
pH22
pCH4
No Carbon Formation
100
10
1.0
0.1 550 600 650 700 750 800
0.6 0.5
0.4
0.3 Carbon Laydown Zone
1100 1200 1300 1400 (°F)
Carbon Formation - Hot Band
Carbon Formation Zone
Temperature (°C)
pH22
pCH4
No Carbon Formation
Carbon Formation
C + H2O CO + H2 (CO Reduction - in reverse!)
Catalyzed by OH-
800
100
10
1.0
0.1
0.6
0.5
0.4
0.3
550 600 650 700 750
Increasing Potash Content
1100 1200 1300 1400 (°F)
Carbon Formation - Effect of Alkali
Carbon Formation Zone
Temperature (°C)
pH22
pCH4
No Carbon Formation
Role of Alkali
Reduces likelihood that carbon will be formed
Enables carbon to be removed readily Incorporation into support must be done
correctly • Release rate not too fast/slow • Effect on activity
Fraction Along Tube
Inlet Outlet
Non-Alkalised Catalyst
Rings
Optimised Shape
Inside Tube Wall Temperature
920 (1688)
820 (1508)
720 (1328)
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
Tem
pera
ture
o C (o
F)
Carbon Formation
Methane feed/Low heat flux
Increasing Alkali Addition
0
Methane feed/High heat flux Propane, Butane feeds (S/C >4)
2-3
Propane, Butane feed (S/C >2.5) Light naphtha feed (FBP < 120oC, 248oF)
4-5
Heavy naphtha feed (FBP < 180oC, 356oF)
6-7
Role of Alkali
K2O wt%
Feedstock Natural Gas Reforming
Non- alkalised
Associated Gas Ref Lightly
alkalised
Dual Feedstock Reforming Moderately alkalised
Naphtha Reforming
Heavily alkalised
Non-alkalised Low alkali Moderate alkali High alkali
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
Typical Steam Ratios for Catalyst/ Feedstock Combinations
Alternatives to Alkali • Precious metals can also be used instead
of Ni as the catalyst – Significant higher activity and hydrogenation
activity yields lower carbon formation rates – Platinum, Ruthenium …etc – Effective “ultra”-purification essential
• Lanthanum used in addition to Ni – Helps also with the removal of carbon
• Magnesium/Ni – Also suppresses carbon formation rates – However, magnesium not stable with steam
Contents Steam reforming reactions Steam reforming catalysts Equilibrium considerations Carbon formation Poisoning
• sulphur • sintering
Steam reformer modelling Pre-and post-reforming
Sulfur Poisoning
Most common poison Severe levels (.5ppm) can lead to rapid
catalyst deactivation “Normal” levels (20-30ppbv) leads to very
slow deactivation Sulfur equilibrium depends on
temperature
(752) 400 500 600 700 800 900 0
0.2
0.4
0.6
0.8
1
Rel
ativ
e C
atal
yst
Dea
ctiv
atio
n
(932) (1112) (1292)
Temperature oC (oF)
(1472) (1652)
Sulfur Poisoning
Sulfur Poisoning
Complex; some disagreement in literature, particularly at low levels
Low level Sulfur will lead to increased twt
with time Other deactivation mechanisms also
operate
Sulfur Poisoning - Precious Metals Reforming
• Precious metals require ultra-low poison levels
– Typically <5 ppbv – Use specialised purifcation absorbent
downstream of ZnO • Typical S slip 1-2 ppbv
Catalyst Sintering
Initial rapid sintering Slower subsequent sintering Temperature dependent Both Ni crystallites and support sinter
Photos of Catalyst Sintering
Fresh Catalyst Sintered Catalyst
Contents
Steam reforming reactions Steam reforming catalysts Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre-and post-reforming
Steam Reforming Modelling
Detailed simulation models can be developed for
• reformer design • evaluation of performance • prediction of changes
Steam Reformer Types
Cylindrical (limited to small plants) Top-fired Side-fired Terraced wall Bottom-fired (relatively rare) Heat exchange type (relatively new)
Top-Fired Steam Reformer
Terrace Wall Steam Reformer - Schematic
Model Results
Input reformer details Model output: gas temperatures and
compositions down tube Radial effects considered also
Temperature Deg C
0.0
0.5
1.0
Frac
tion
Dow
n Tu
be
Process Gas
Tube Wall
Furnace Gas
400 600 800 1000 1200 1400 1600
Temperature Deg F 750 1500 2250 3000
Temperature Profiles
Fraction Down Tube
Composition Wet mol%
Composition Wet mol%
0.0
0.2
0.4
0.6
0.8
1.0 1.5 1.0 0.5 10 20 30 40 50 60 70 80
C2
CH4
H2O
C4+ C3
CO2
CO H2
Composition Profiles
Contents Steam reforming reactions Steam reforming catalyst Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre-and post-reforming
• pre-reforming concept • retrofitting and new plants • post-reforming concept • retrofitting
Pre-reforming
Low temperature adiabatic steam reforming
Wide range of feedstocks Requires highly active, high nickel
catalyst Exo/endothermic, depending on feedstock Converts all heavy hydrocarbons to
methane
Tem
pera
ture
475 deg C (890 deg F)
410 deg C (770 deg F)
0 100 50
NG Pre-reformer Temperature Profile
Percentage Down Bed
450 Deg C (842 Deg F)
500 Deg C (932 Deg F)
Percentage Down Bed
Tem
pera
ture
Naphtha Pre-reforming temperature Profile
Reformed
Gas
Steam
Pre-reformer
Desulphurised Feed
Incorporation of a Pre-reformer
Post-reforming
Heat exchange type of steam reformer Uses steam reformer exit gas as heating
medium for fresh feed Compact design
• small footprint Uses conventional catalyst No extra fuel firing needed
• no increase in Nox emissions Typically allows 25 % increase in rate
Gas Heated Reactor
Shell
Shift
Internals
Steam Reformer
Heat Exchange Reformer
Reformed Gas
Desulphurised Feed
Steam
Incorporation of a Post-reformer
Summary Steam reforming reactions Steam reforming catalyst Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre- and post-reforming