steam reforming - practical operations

Steam Reforming Practical Operation

C2PT Catalyst Process Technology

By Gerard B Hawkins Managing Director, CEO

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

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

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

Reformer is a Heat Exchanger

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

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

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

Top Fired Reformer

Side Fired Reformer

Terrace Walled Reformer

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

Heat Transfer - Top Fired

Radiative heat flows

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

Typical Reforming Configuration

Steam

Steam

Steam + Gas

Steam Reformer

500°C

870°C

1200°C

3% CH4

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

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)

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

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

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

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

High and Stable Activity

Low methane slip

Lower tube wall temperature

Reduced Fuel usage

Low Pressure Drop

Savings in Compression Power / Fuel

Possible Throughput increase

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

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

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

Diffusion into and out of Catalyst

Bulk Gas

Gas Film

Ni Sites

CO H O 2

H 2

CH 4

Catalyst Support

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

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

Un-sintered Catalyst

0.001 mm (1/25 thou)

Sintered Catalyst

0.001 mm (1/25 thou)

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

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

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

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

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

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

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

VULCAN VSG-Z101 VULCAN VSG-Z102

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

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

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

Breakage Characteristics

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

Packing Characteristics

Extended External Surface Area - "Cogs"

Void

Catalyst Support

Three types commercially available

– Alpha Alumina

– Calcium Aluminate

– Magnesium Aluminate Spinel

Catalyst Support - Bulk Chemistry

AlphaAlumina

CalciumAluminate

MagnesiumAluminateSpinel

Structure Corundum Spinel-like Spinel

Stability toSintering

ExtremelyStable

RelativelyStable

RelativelyStable

ChemicalStability(Hydrolysis)

Inert Stable ‘Free’ MgOHydrationunderSteamingConditions

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

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

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

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

)

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

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

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

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


Giraffe Necking

Hot Tube Hot Band

Reformer tube appearance - Carbon laydown


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


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

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


OH -

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



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

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

Methane Cracking - Kinetic Limitation


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

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


No Carbon Formation

Deposition rate < removal rate

Deposition rate > removal rate

650 700 650 700 O

(pH2) 2 pCH4

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


O

(pH2) 2 pCH4

Methane Cracking - Increased Potash Content


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


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


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


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


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


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

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


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

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

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

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

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

Sulfur Poisoning

Pellet S S S

S S

S

S S

S S

Nickel

CH4

H2O

Sulfur Poisoning

Depends upon the catalyst temperature

Occurs in the cooler regions

Upper section of tubes vulnerable

Also depends upon sulfur exit the HDS

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

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

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


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

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)


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


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 (%)

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


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


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

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

Reformer Catalyst Monitoring


Monitor frequently (daily)

Exit Methane Tube Wall Temperature (TWT) Tube Appearance

Monitor Less Frequently

Pressure Drop Approach to Methane Steam Equilibrium

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



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.

.

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

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.

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)

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)

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

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

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

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

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

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.

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

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

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

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

Catalyst supplied in pre-weighed socks

Sock slightly narrower than tube bore End of sock folded over

Lowered down tube on rope ‘Tugged’ to release fold Free fall <0.5m allowable

Vibrated with hammer after each sock

PD measured empty / ½ full / full Adjusted to ± 3 - 5%

Outage Adjusted

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

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


Support grid

Charging chute

Loading rope with flexible springs

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


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

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

Reduction & Start-Up

Introduction

Start-up Procedures Warm-up Catalyst Reduction Feed Introduction

Shut-down

Case Studies

Contents

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

As supplied - NiO on support Active species - Ni Crystallites Reduction process needed:-

NiO + H2 ⇔ Ni + H2O

Introduction - Catalyst Reduction

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)


Faster at high temperature

Slower in presence of steam

Thermodynamically, very little hydrogen needed

Support also affects ease of reduction


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

Start-up Procedure Warm-up Catalyst Reduction Feed Introduction

Shut-down

Case studies

Start-Up Procedure

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

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


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


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


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

Introduction


Shut-down

Case studies

Start-Up Procedure - Catalyst Reduction

Reduction with Hydrogen

Reduction with Natural Gas

Reduction with other sources of hydrogen

Higher hydrocarbons Ammonia Methanol

Start-Up Procedure - Catalyst Reduction

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

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


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


Natural Gas

Will be converted to carbon oxides + hydrogen in reformer

May crack thermally to give carbon

Catalyst Reduction with Natural Gas

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

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

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

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

Introduction


Shut-down

Case studies

Start-Up Procedure - Feed Introduction

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


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


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

Introduction


Shut-down

Case Studies

Case Studies

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

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

Introduction


Shut-down

Case Studies

Case Studies

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

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

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


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


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!


Ammonia Plant

LTS reduction loop included steam reformer

CO2 released from LTS reduction Carbon formed in steam reformer


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


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


Reformer catalyst black and badly broken up - due to severe carbon formation

Check to ensure that recirculation loops do not contain high levels of carbon oxides


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

Importance of Tube Wall Temperature Measurement

Need accurate information

Tube life Artificial limitation on plant rate

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

Tube Wall Temperature Measurement

Contact

- Surface Thermocouple

"Pseudo-contact“

- Gold Cup Pyrometer

Non-contact

Disappearing Filament Infra Red Optical Pyrometer Laser Pyrometer

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

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

Infra-red Pyrometer

Easy to use

Need to correct for emissivity and reflected radiation

Inexpensive

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

Gold Cup Pyrometer

Excludes all reflected radiation

Approximates to black body conditions

High accuracy / reproducibility

BUT

- Limited access - Awkward to use

Gold Cup Pyrometer

Tube Furnace Wall

Water Cooling

To Recorder

Gold Cup Lance

*

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

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)

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

Background Temperature Measurement

NORTH

A

a 2

a 1

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

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

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

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

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

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

steam reforming - practical operations

Technology

removal rate

maintain exit

h2o

gold cup

exit temps

carbon oxides

heavy feeds

natural gas