steam reforming - a comprehensive review

www.gbhenterprises.com

Gerard B. Hawkins Managing Director

Equilibrium, Approach & Kinetics Carbon Laydown ◦ Potash Doping

Catalyst Loading Pigtail Nippers GHR & AGHR Secondary Reforming Metal Dusting

Equilibrium, Approach and Kinetics

Steam Reforming limited by Heat transfer Catalyst activity Kinetic rate Equilibrium

Approach to Equilibrium

770 780 790 800 810 820 2

4

6

8

10

12 M

etha

ne s

lip (%

) Gas Exit T Eq'm T

ATE

(1418) (1454) (1436) (1472) (1490) Temperature Deg C (Deg F)

Exit CH 4

Approach to Equilibrium CH4 + H2O <=> CO + 3H2

Approach Tms = Actual T gas - EquilibriumT gas (A.T.E.)

measured calculated •Measure of catalyst activity

–If ATE = O, system at equilibrium

–As catalyst activity decreases, ATE increases

Kinetics

•Reversible action, zero rate at equilibrium

•Reforming reaction very fast

•Limited to pellet surface only-diffusion limit

•Depends upon catalyst GSA

•Depends upon gas composition

•Exponential with temperature

•C2, C3 & C4 considered as not reversible

Diffusion Limitation inactive active

Pore

Catalyst pellet

CHH OHCOCO

422

2

CO H2 CO2

nickel crystal

Steam Reforming Catalysis 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 CO and H2

Kinetics - Natural Gas

•Kinetic model used for natural gas feeds

•Methane kinetics well validated

•C2 ,C3, & C4 kinetics ok for natural gas tail only

•NOT to be used for LPG feeds

•Kinetics for steam reforming reactions

•Shift reaction taken to be at equilibrium

•Shift very fast compared to reforming

Kinetics - Natural Gas

•Form of equation as below:

R[CH4] = K.GSA.Ract.exp(T).P[CH4].(Kp’-Kp)

P[H2O]

R[CH ] = Rate of methane reaction T = Temperature K = Constant P[X] = Partial pressure Ract = Catalyst relative activity Kp' = Equilibrium constant of gas GSA = GSA Kp = Equilibrium constant at T

Geometric Surface Area

• GSA for short –Area per unit volume –Typically 200-500 m2/m3

• Important as apparent activity is a very strong function

• Function of –shape –size –number

Kinetics

•Major points of interest

–Steam is a poison

–Kp’- Kp can be changed to approach

–GSA term is included

–Relative activity term is included

Kinetics •For higher hydrocarbons

•R[C2H6] = K.GSA.Ract.exp(T).P[C2H6]

•R[C3H8] = K.GSA.Ract.exp(T).P[C3H8]

•R[C4H10]= K.GSA.Ract.exp(T).P[C4H10]

•Simple first order kinetics-non reversible

•Hence limit to natural gas tail only

Catalyst Types

VSG-Z101: Gas reforming

Catalyst Applications

• VSG-Z101

–Other light feeds - refinery offgas –Light duties e.g. side fired reformers –Use up to butane at 4.0 S:C ratio –Use in top fired reformers at tube top

Steam Ratios for Catalyst/Feedstock Combinations

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

Pre - reduction

• This will maximize the activity of a catalyst • The start up will be easier and quicker • Catalyst should remain more active at

tube top • Useful for low inlet temperature reformers

Catalyst Support - Reduction Temperatures

Alp

ha A

lum

ina

Cal

cium

Alu

min

ate

Mag

nesi

um A

lum

inat

e Sp

inel Temperature

(Deg F)

Temperature (Deg C)

800 1000 1200 1400 1600

400 500 600 700 800 900

Magnesium Aluminate spinel material usually supplied pre-reduced

Carbon Laydown

Carbon Formation

•Carbon formation formed by side reactions

•Totally unwanted due to damage caused

•Catalyst break up and deactivation

•Catalyst tubes overheated - hot bands

–Premature tube failure

–Catalyst activity reduction

–Pressure drop increases

Carbon Formation

•Carbon forms when

–Steam ratio is too low

–Catalyst has too little activity

–Higher hydrocarbons are present

–Tube walls are hot - high flux duties

–Catalyst has poor heat transfer coefficient

Carbon Formation - Types

•Carbon cracking

•Boudouard

•CO Reduction

Carbon Formation

•Cracking

–CH4 <=> C + 2H2

–C2H6 => 2C + 3H2 etc

•Boudouard

–2CO <=> C + CO2

•CO Reduction

–CO + H2 <=> C + H2O

Heavier Feedstocks

•Steam reforming

–Not practical to increase steam to carbon ratio using gas reforming catalysts

–Carbon formation more problematic

Need promoters to limit carbon and/or increase its removal once formed

Carbon formed not just by cracking but also by polymerisation of intermediates

Naphtha Carbon Formation

Alkalized Catalysts

Heavier Feedstocks •Carbon removal (heavy feed reforming)

–Potash (alkali) incorporated into catalyst support

Inhibits cracking rate

Accelerates carbon gasification

Needs to be “mobile” to remove carbon on the inside tube wall surface - Potash released by complex chemical

Release reaction controlled by temperature

Required only in the top section of the reformer tube - Lower section of catalyst absorbs liberated potash

Heavier Feedstocks •Potash Addition (Heavy feed reforming)

–Reduces catalyst activity

Need extra Ni

–MgO/NiO solid solution

Low Polymerization activity - reduces carbon formation

MgO must be “fixed” so as to avoid hydration of the “free” MgO to magnesium hydroxide - weakens catalyst pellet severely (cannot steam catalyst!)

Carbon Formation

•Use of potash to prevent carbon formation

•Increases the rate of carbon removal

•Does not stop the formation from cracking

•Potash catalyzes the rate of steam gasification

•Balance of kinetics altered to favour removal

•Can steam with care

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

Role of Alkali - Lightly Alkalized For light feeds and LPG etc using lightly alkalized catalyst

–Potash is chemically locked into catalyst support

–Potash required only in top 30-50% of the reformer tube

Catalyst life influenced by

–Poisoning

–Ni sintering

– Process upsets etc

Lightly alkalized

Non-alkalized

Carbon Formation and Prevention

Top Fraction Down Tube Bottom

Non-AlkalisedCatalyst

Ring Catalyst

Optimised Shape(4-hole Catalyst)

Inside Tube WallTemperature

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

AlkalisedCatalyst

Carbon FormingRegion

Optimized Shape VSG-Z101

Potash and Activity

•However, potash is a catalyst poison

–Potash does reduce activity

•At low levels 2%, the effect is minimal

•Use a gas type catalyst in bottom of tube

•Use naphtha catalyst in top of tube for carbon

•Therefore no carbon and low exit approach

Potash Levels in Heavy Feed Steam Reforming Catalyst

0 20 40 60 80 1000

1

2

3

4

5

6

7

% Down Reformer Tube

wt% Potash

2 year

1 year

0 years

Potash Promoted Non-akalised

Must watch the interface temperature At >650°C Potash leaching too high Leads to ◦ Fouling of WHB etc ◦ Loss of carbon resistance ◦ Hot banding etc

Naphtha Loading

02468

10121416

180 220Final boiling point Deg C

% R

educ

tion

in n

apht

ha

load

ing

% reduction

200 210 190

Naphtha Loading

0

5

10

15

20

25

30

10 20Aromatic content (%)

Red

uctio

n in

nap

htha

load

ing

12 14 16 18

When to Use and Why

• Used when major feedstock variations – NG to LPG

• Feedstock flexibility – When 650oC limit is reached

• Problem of Potash Leaching

Catalyst Loading

Sock Loading - Measurements

• Key part of charging procedure • Aim to pack catalyst to uniform voidage • Measure pd

– Not outage in tube at any one time – Not weight per tube – Not catalyst density – After 50% – After full loading

• Use defined and consistent procedure throughout

DP Measurement

• Use VSC Pressure Drop Rig • If too high then

– suck out catalyst and recharge • If too low then

– Vibrate tube – Top up if outage too great

Pressure Drop Measurement

• Fixed flow of air (choked flow through orifice) • Mass flow rate through orifice function of

– Upstream pressure – Orifice diameter (known) – Temperature (known)

• Downstream pressure is measure of pd

Measurement of Pressure Drops

PD rig

Inlet pigtail

Exit pigtail

4a. Exit pigtail

Empty tube

PD rig

4b. Catalyst

catalyst

PD rig

4c. Inlet pigtail

catalyst

Digital Pressure Drop Instrument

Sock Loading - Vibration

• Electric or pneumatic vibrators – rotating cams are noisy

• Soft-faced shot-filled hammer –need consistent blows

“UNIDENSE” Method • Developed by Norsk Hydro

– licensed to a number of organisations • Tried in a number of plants

– ammonia, hydrogen and DRI • Leads to “denser” packing

– less pd variation • more uniform gas flows

– easier procedure • shorter loading time (70%)

– slightly higher pd • effect on throughput?

Pigtail Nippers

Pigtail Nipper

This hydraulic device designed by ICI is operated at a safe distance from the leaking tube and squeezes the pigtail flat with the plant still operating.

Allows furnace to stay on line.

No thermal cycle.

Pigtail Nipper

Pinched Pigtail with Clamp in Place

Pinched Tube in Steam Reformer

Row of steam reformer tubes

Pinched tube

Gas Heated Reformer (GHR) Advanced Gas Heated Reformer (AGHR)

GHR Based Reforming

Secondary Reformer

Steam + Gas

Air / Oxygen

GHR

LCM Flowsheet

Purifier

Saturator GHR Secondar

y Converter

Preheater

Purge to fuel

Topping Column

Refining Column

Process condensate

water

Fusel oil

Natural gas

Oxygen Steam

Refined methanol

Purge

Crude methanol

GHR History

•Developed for ammonia process - LCA •Early 1980's - Paper exercise •Mid 1980's - Sidestream unit at Billingham •Mid 1980's - LCA design developed •Late 1980's - ICI Severnside plants start up •1991 - BHPP LCM plant designed •1994 - BHPP plant start up •1998 - AGHR Start Up •1998 - MCC Start Up

GHR Shellside Design

•Shellside heat transfer usually poor •Minimize tube count with expensive alloys •Tubes are externally finned •Designed as double tubes - Sheath tube •Produces much smaller tube bundle •Allows scale up to higher capacities

GHR Tubesheets Gas/Steam Hot

gas Twin tubesheets

Refractory

Syngas

GHR and Secondary Arrangement

Normal Operating Conditions

SecondaryReformer

GHR

Syngas

Gas/steam425`C

701`C

975`C

515`C

742`C

21,000 Nm3/Hr

Oxygen30`C

1200`C

2,590 Nm3/Hr

43.7 Barg 39.2 Barg

38.6 Barg

37.9 Barg

22.0% Methane

16.6% Methane

0.4% Methane

40.6 Barg

LCA GHR

LCM GHR Internals

Advanced GHR

-Shellside heat transfer enhancement

-Non bayonet design

-Hot end tubesheet

-Sliding seal system

Uprate Capabilities

•GHRs can be used in parallel to existing primary reformer •Potential to uprate capacity by 40% •Severely impacts steam system •Most applicable to hydrogen plants •No changes to radiant or convection sections of reformer / fans burner etc. •New WHB may be required •Rest of plant must be uprated

Fluegas Heated Reformers

FHR

Combustion chamber

Natural gas & Steam

Fuel

Air

Fluegas to stack

Combustion air compressor

Fluegas recycle compressor

Fluegas power recovery turbine

Syngas

GHR

Secondary Reforming VSG-Z201/202/203

Keys to Good Performance

•Burner Design •Mixing Space •Catalyst

Poor Mixing Performance

•Good mixing is absolutely essential

•Poor mixing in combustion zone gives high approach and high methane slip

•Problem is poor mixing can not be differentiated from poor catalyst performance UNLESS thermocouples are in the bed.

•Bed temps will show divergence •Bed temps will show odd behaviour

Catalyst Bed Sizing

•Based on a space velocity technique •Wet Gas Space Velocity (WGSV) •Uses exit flow with steam included •See attached graph •Modify space velocity by catalyst GSA •See table for relative catalyst GSA

WGSV Chart

0

5

10

15

20

25

0 2000 4000 6000 8000 10000Wet Gas Space Velocity (Nm³/m³)

App

roac

h (°

C)

Two types LCM style ◦ Single ‘pipe’

Ring burner

Metal Dusting

Metal Dusting Key features

•Catastrophic carburization •Occurs at "low" temperatures 700 - 450°C •Induction period sometimes required •Often local pitting - pits coalesce •Can have general corrosion •Can be very rapid 3mm/year •Carbon formation occurs

Metal Dusting

Mechanism of Metal Dusting Initiation

•Gas has propensity to deposit carbon 2 CO => CO2 + C (Boudouard) CO + H2 => H2O + C (CO Reduction) •Oxide film breakdown exposes active Fe, Ni, Co sites •Carbon deposits at active sites

Metal Dusting and GBHE

•GBHE have great experience •GBHE have a solution

•Proven to work in operation