Theory and Practice of Steam Reforming

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 catalysts
• catalyst activity
• catalyst development and testing
• importance of gas and htc
 Poisoning
 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

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)
TubeWallTemperature
DegC(DegF)
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

TemperatureDegC(DegF)
Tube Wall Temperature Limit
Poor stability
Good stability
Days on Line
0 1,000500100 200 300 400 600 700 800 900
925
(1697)
900
(1652)
875
(1607)
850
(1562)
Effect of Catalyst Stability on
Tube wall Temperature

Contents
• equilibrium curves
• effect of process variables
 Poisoning

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
Equilibrium%CH4(drybasis)
Pressure(psig)
Pressure(barg)
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
Equilibrium%CH4(drybasis)

F[CH4 ] F[H2O] 1
Kms =
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[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[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[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 11.59CO
CH4
CO
2
H2
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
Composition(%dry)
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
Methaneslip(%)
(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
TemperatureoC(oF)
Gas Temp Eq'm Temp
Approach to equilibrium

Contents
• formation and removal reactions
• role of alkali
• range of catalysts
 Poisoning

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

pH2
2
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)

10
0
10
1.0
0.1
550 600 650 700 750 800
0.6
0.5
0.4
0.3
Temperature (°C)
Proportion of tube
length from inlet
1100 1200 1300 1400 (°F)
Carbon Formation - Inside Reformer
Tube
pH2
2
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
Temperature (°C)
pH2
2
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)
pH2
2
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
TemperatureoC(oF)
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
 Poisoning
• sulphur
• sintering

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
RelativeCatalystDeactivation
(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
 Poisoning

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)

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
FractionDownTube
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
 Poisoning
• 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

Temperature
475 deg C
(890 deg F)
410 deg C
(770 deg F)
0 10050
NG Pre-reformer
Temperature Profile
Percentage Down Bed

450 Deg C
(842 Deg F)
500 Deg C
(932 Deg
F)
Percentage Down Bed
Temperature
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
 Poisoning
 Pre- and post-reforming

Theory and Practice of Steam Reforming

Theory and Practice of Steam Reforming

Recomendados

Recomendados

Mais conteúdo relacionado

Mais procurados

Mais procurados (20)

Semelhante a Theory and Practice of Steam Reforming

Semelhante a Theory and Practice of Steam Reforming (20)

Mais de Gerard B. Hawkins

Mais de Gerard B. Hawkins (20)

Último

Último (20)

Theory and Practice of Steam Reforming