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Theory and Practice of Steam Reforming

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Steam reforming reactions Steam reforming catalyst Equilibrium considerations Carbon formation Poisoning Steam reformer modeling Pre-reforming Post-reforming
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Page 1: Theory and Practice of Steam Reforming

The Theory and Practice of

Steam Reforming By:

Gerard B. Hawkins

Managing Director, CEO

Page 2: Theory and Practice of Steam Reforming

Contents

Steam reforming reactions Steam reforming catalyst Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre-and post-reforming

Page 3: Theory and Practice of Steam 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

Page 4: Theory and Practice of Steam Reforming

Steam Reforming of Heavier Hydrocarbons

CnHm + nH2O nCO + (n+m/2)H2

Still endothermic Easier than methane More prone to carbon formation

Page 5: Theory and Practice of Steam Reforming

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

Page 6: Theory and Practice of Steam 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

Page 7: Theory and Practice of Steam Reforming

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

Page 8: Theory and Practice of Steam Reforming

Activity Testing

Define some measure of reaction • exit methane

Measure for a range of catalysts under fixed conditions • flow, temperature pressure, catalyst

Page 9: Theory and Practice of Steam Reforming

Reactants

Products Reaction

Gas Film

• Two types: - molecular diffusion - Knudsen diffusion

Diffusion Effects

Bulk Gas Bulk Gas

Page 10: Theory and Practice of Steam Reforming

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

Page 11: Theory and Practice of Steam Reforming

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

Page 12: Theory and Practice of Steam Reforming

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

Page 13: Theory and Practice of Steam Reforming

Pressure bar (psi)

Catalyst B

Catalyst A

1 (14.5)

10 (145)

20 (290)

Pressure Dependence

Page 14: Theory and Practice of Steam Reforming

Adsorption

Desorption Adsorption

Dehydrogenation

Surface Reaction * *

OH2 C + 2H2 CH4 *

H2O CO + H2 CH4

Surface Science

Page 15: Theory and Practice of Steam Reforming

Photo of XPS

Page 16: Theory and Practice of Steam Reforming

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

Page 17: Theory and Practice of Steam Reforming

Activity Testing

Intrinsic activity measurements Bench-scale for screening Scale-up to include heat/mass transfer

effects

Page 18: Theory and Practice of Steam Reforming

Activity Testing Microreactor Semi-tech

Page 19: Theory and Practice of Steam Reforming

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

Page 20: Theory and Practice of Steam Reforming

Nickel crystallites

No further reaction Reaction zone

Catalyst Pellet Pore

Reactants

Products

Effect of gsa

Page 21: Theory and Practice of Steam Reforming

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

Page 22: Theory and Practice of Steam Reforming

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)

Page 23: Theory and Practice of Steam Reforming

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

Page 24: Theory and Practice of Steam Reforming

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

Page 25: Theory and Practice of Steam Reforming

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

Page 26: Theory and Practice of Steam Reforming

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

Page 27: Theory and Practice of Steam Reforming

Methane Steam Equilibrium

CH4 + H2O CO + 3H2

P [CH4] P [H2O] Kms =

P [CO] P [H2] 3

– equilibrium tables

– equilibrium curves

Page 28: Theory and Practice of Steam Reforming

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

Page 29: Theory and Practice of Steam Reforming

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

)

Page 30: Theory and Practice of Steam Reforming

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

Page 31: Theory and Practice of Steam Reforming

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]

Page 32: Theory and Practice of Steam Reforming

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]

Page 33: Theory and Practice of Steam Reforming

• 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]

Page 34: Theory and Practice of Steam Reforming

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

Page 35: Theory and Practice of Steam Reforming

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

Page 36: Theory and Practice of Steam Reforming

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

Page 37: Theory and Practice of Steam Reforming

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

Page 38: Theory and Practice of Steam Reforming

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

Page 39: Theory and Practice of Steam Reforming

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

Page 40: Theory and Practice of Steam Reforming

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

Page 41: Theory and Practice of Steam Reforming

Carbon Formation

Depends on: - feedstock - operating conditions - catalyst

Page 42: Theory and Practice of Steam Reforming

Carbon Deposition

Carbon

Catalyst surface

1 mm (40 thou)

Page 43: Theory and Practice of Steam Reforming

Carbon Formation

CH4 C + 2H2 (Thermal Cracking)

CO + H2 C + H2O (CO Reduction)

2CO C + CO2 (CO disproportionation “Boudouard”)

Page 44: Theory and Practice of Steam Reforming

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

Page 45: Theory and Practice of Steam Reforming

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)

Page 46: Theory and Practice of Steam Reforming

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

Page 47: Theory and Practice of Steam Reforming

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

Page 48: Theory and Practice of Steam Reforming

Carbon Formation

C + H2O CO + H2 (CO Reduction - in reverse!)

Catalyzed by OH-

Page 49: Theory and Practice of Steam Reforming

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

Page 50: Theory and Practice of Steam Reforming

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

Page 51: Theory and Practice of Steam Reforming

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

Page 52: Theory and Practice of Steam Reforming

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%

Page 53: Theory and Practice of Steam Reforming

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

Page 54: Theory and Practice of Steam Reforming

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

Page 55: Theory and Practice of Steam Reforming

Contents Steam reforming reactions Steam reforming catalysts Equilibrium considerations Carbon formation Poisoning

• sulphur • sintering

Steam reformer modelling Pre-and post-reforming

Page 56: Theory and Practice of Steam 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

Page 57: Theory and Practice of Steam Reforming

(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

Page 58: Theory and Practice of Steam Reforming

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

Page 59: Theory and Practice of Steam Reforming

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

Page 60: Theory and Practice of Steam Reforming

Catalyst Sintering

Initial rapid sintering Slower subsequent sintering Temperature dependent Both Ni crystallites and support sinter

Page 61: Theory and Practice of Steam Reforming

Photos of Catalyst Sintering

Fresh Catalyst Sintered Catalyst

Page 62: Theory and Practice of Steam Reforming

Contents

Steam reforming reactions Steam reforming catalysts Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre-and post-reforming

Page 63: Theory and Practice of Steam Reforming

Steam Reforming Modelling

Detailed simulation models can be developed for

• reformer design • evaluation of performance • prediction of changes

Page 64: Theory and Practice of Steam Reforming

Steam Reformer Types

Cylindrical (limited to small plants) Top-fired Side-fired Terraced wall Bottom-fired (relatively rare) Heat exchange type (relatively new)

Page 65: Theory and Practice of Steam Reforming

Top-Fired Steam Reformer

Page 66: Theory and Practice of Steam Reforming

Terrace Wall Steam Reformer - Schematic

Page 67: Theory and Practice of Steam Reforming

Model Results

Input reformer details Model output: gas temperatures and

compositions down tube Radial effects considered also

Page 68: Theory and Practice of Steam Reforming

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

Page 69: Theory and Practice of Steam Reforming

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

Page 70: Theory and Practice of Steam Reforming

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

Page 71: Theory and Practice of Steam Reforming

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

Page 72: Theory and Practice of Steam Reforming

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

Page 73: Theory and Practice of Steam Reforming

450 Deg C (842 Deg F)

500 Deg C (932 Deg F)

Percentage Down Bed

Tem

pera

ture

Naphtha Pre-reforming temperature Profile

Page 74: Theory and Practice of Steam Reforming

Reformed

Gas

Steam

Pre-reformer

Desulphurised Feed

Incorporation of a Pre-reformer

Page 75: Theory and Practice of Steam Reforming

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

Page 76: Theory and Practice of Steam Reforming

Gas Heated Reactor

Shell

Shift

Internals

Page 77: Theory and Practice of Steam Reforming

Steam Reformer

Heat Exchange Reformer

Reformed Gas

Desulphurised Feed

Steam

Incorporation of a Post-reformer

Page 78: Theory and Practice of Steam Reforming

Summary Steam reforming reactions Steam reforming catalyst Equilibrium considerations Carbon formation Poisoning Steam reformer modelling Pre- and post-reforming

Page 79: Theory and Practice of Steam Reforming

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