Steam Reforming - Common Problems

Gerard B. Hawkins Managing Director

WWW.GBHENTERPRISES.COM

The aim of this presentation is to • Discuss some of the common problems that

occur on steam reformers • Analyze problems associated with ◦ Catalyst ◦ Tubes ◦ Reformer Design WWW.GBHENTERPRISES.COM

Many ageing plants Problems generally

increase towards end of plant life

Loss of corporate knowledge

De-manning Result is that

number of problems will increase

Plant Reliability

Life of Plant


• Many problems • Varied causes and effects • Not always easy to detect • Can reduce efficiency significantly • Can affect plant financial profitability


• Some typical ones include ◦ Poisoning ◦ Carbon formation ◦ Tunnel problems ◦ Air leaks ◦ Tube failures

• Less common ones include ◦ Tunnel Port Effect ◦ Flue gas Mal-distribution



• There are many poisons but most common is sulfur - must not rule out ◦ Chlorides ◦ Heavy metals - Arsenic/Vanadium ◦ Phosphates

• Will reduce activity • Raises tube temperatures • Can lead to hot bands • And hence carbon formation


• Hot bands are formed due to ◦ Loss of activity ◦ Poor heat transfer ◦ Localized high voidage ◦ Too low a steam to carbon ratio

• Once formed they will get worse • On Top Fired reformers occur about 1/3 down the

tube ◦ Not so much of a problem on Terrace Wall or Side Fired

furnaces as inside tube temperatures are lower


Weld

Hot Band


• All combustion at top of furnace • High heat transfer rate at this point • Measured by heat fluxes ◦ Top fired between 80-140 kW/m² ◦ But some in range 140-160 kW/m²

• Side Fired/Terrace Wall have multiple fuel combustion points ◦ Heat Fluxes are lower

• Therefore Top Fired more prone to carbon formation


• Can remove by steaming • Need exit temperature above 700°C = 1300°F • No feed - Only steam • Continue for at least 12 hours • Monitor for CH4 and CO2 exit reformer • Check process condensate for sulfites and

sulfates • Add nitrogen as carrier if required • If really bad may need an air burn • If even worse - a new charge of catalyst


Hot Band Hot Tube Settling Giraffe Necking

Tiger Tailing


• Can eventually lead to tube failure of affected tube

• Can run with failed tube ◦ Provided leak is not too

severe ◦ No direct impingement on

adjacent tubes • Can nip each tube to

allow continued operation

Nipped Tube


Nipped tubes run much hotter

Eventually fail Normally fall

over Need to monitor Shut down if

there is a problem

Coffin


Occurs if nipping tubes but no firing adjustment

Single Tube Failure Failures spread along row

Failed tubes Nipped Tubes


Continues to move along row Jumps across to adjacent rows

Spreads along adjacent rows WWW.GBHENTERPRISES.COM

• Absolutely vital to get right in primary reformers • If not right then will get a spread of temperatures • A case is shown below - problems on loading

Poor Loading


• Next charge used a dense loading technique • Achieved a very even loading

Good Loading - UnidenseTM


-20

-15

-10

-5

0

5

10

15

20

-20 -15 -10 -5 0 5 10 15 20

Pressure drop variation (%)

Flow

var

iatio

n (%

)

-50-40

-30-20-100

102030

4050

TWT

varia

tion

(oC

)


Catalyst Deactivation Physical processes

• Tube expansion/contraction • Expansion during start-up • Contraction during cooling stresses pellets

At shutdown Stressed Pellets

After loading

During operation


Initial catalyst level

Cold Hot

Expansion of tube - some settling

Cold

Contraction of tube - some readjustment - some breakage

Hot Zone


• Catalyst will break in service during ◦ Trips ◦ Steaming

• Will lead to high resistance to flow • Some tube will have less flow ◦ Therefore will appear hotter



Fluegas Fans


Double Fan can lead to variation of TWT along reformer


• This occurred on Methanol Plant in Western Europe

• Plant ran for almost 30 years • Tubes at the hot points had been replaced 3 times

during the plant life ◦ Average life of these tubes was 7years

• Tubes at the cold points were replaced once or never at all ! ◦ Average life of these tubes was 25-30 years

• Overall cost plant money since tube life not utilized fully


Can also get mal-distribution due to • Poor fuel header design ◦ Causes fuel mal distribution between cells

• Poor feed header design ◦ Causes feed mal distribution between cells

• Also affects side fired type reformers


Poorly Balanced

Cell 1 55 869

1596 1.5 11 20

Cell 2 45 810

1490 3.7 11 20

Well Balanced

100 842 1548 2.3 12 21

Mixed Gas 100 840 1544 2.6 16 29

Fuel flow (% of average) Exit temp (oC) Exit temp (oF) Exit CH4 (mol % dry)

ATE (oF) ATE (oC)

Parameter


• Used to collect flue gas in Top Fired reformers • Used to collect the flue gas such that furnace

operates in ‘Plug Flow’ regime • Have proven to be a problem ◦ Mechanically - they have collapsed ◦ Damage can lead to localized mal-distribution ◦ Tunnel port effect

• Some plants have removed them ! ◦ Reduces flue gas side pressure drop ◦ Allows uprate of plant


Effect on Flow

Effect on Temperature WWW.GBHENTERPRISES.COM

• Kellogg plant in India • Coffin collapsed • Side wall and roof bricks rested on the manifold • Manifold deviated from normal position • Induced additional stresses • Manifold did not fail - problem rectified at shut

down just after failure


With Coffins

Fluegas flow patterns

Tubes Coffins

Without Coffins

Area of Hot tubes


• Due to preferential flow of flue gas to the extraction end ◦ Have more flow ◦ Therefore more heat available ◦ Therefore high temperatures

Distance

Tem

pera

ture


• Common problem on uprated plants • Lack of ID (Fluegas) fan capacity leads to high

box pressures ◦ -2 or -3 mm water gauge ◦ Normally -10 mm ◦ Safety issue - flames can pass out of box

• Lack of FD (Combustion Air) fan capacity can lead to low excess air/oxygen levels ◦ Leads to afterburning ◦ In worst case high CO levels in duct


• Classic example is a South American Methanol Plant ◦ Tightly designed plant - 2000 mtpd but operating at

2200 mtpd + ◦ Both fans limited ◦ One end of reformer was at positive (+) pressure ◦ Flames emitted from peepholes

• Outer lane cool - inner’s hot • Afterburning in centre • CO inlet duct • CA ducting symmetrical • Poor CA distribution


• Tube Failures - covered in Tube Design ◦ Can nip the tube

• Tunnel Port Effect ◦ Can be overcome by Installation of appropriate catalyst - high heat transfer/activity Design of the tunnel ports

• Weld position - on top fired reformers avoid the hottest point of tube

• Movement of coffin walls


• All streams should be symmetrical

• Including ◦ Feed, fuel, effluent and combustion

air headers/ducts ◦ Prevents mal distribution of process

flows ◦ Prevents variations in operating

conditions Tube and exit temperatures

Preferential Flow Path


Flow prefferentially passing to this end of reformer

Flow starved at this end

Distance Down Reformer

Tem

pera

ture


• Must ensure good sealing between the tube and the refractory

• Otherwise air will be sucked into the furnace

• Increased excess O2 levels

• Inefficient plant operation • Normally pack with

refractory rope and blankets



• Mainly affects Top Fired furnaces • Two types ◦ #1 - direct impingement of flame on tube This is the worst since tube wall temperatures will be raised

the most Can lead to rapid tube failure ◦ #2 - impingement of hot flue gas on tube Normally observed as shimmering on tube surface Will lead to premature failure in ‘long term’

• Both raise tube wall temperatures


• Causes ◦ Fluegas Mal-

distribution ◦ Poor burner design ◦ Blockage of ports in

burners ◦ Mis-alignment of burner

• Can remove burners on line

• Allows for repair • Must be careful


• Cause by a localized lack of combustion air • The fuel is not fully combusted • Fuel moves on and when it meets oxygen is

combusts ◦ The flue gas/fuel mixture is above auto-ignition

temperature • Usually observed on surface of the tubes ◦ Can damage tubes


• Causes ◦ Lack of Forced Draft (Combustion Air) fan capacity ◦ Poor combustion air header design ◦ Poor burner design ◦ Fuel gas composition deviations ◦ Poor balancing of furnace combustion air

• Can rectify but must identify cause


If distance from inlet of tube to the surface of the catalyst is too short then can get milling of the catalyst

Allow 200 mm for top entry

Allow 150 mm for side entry

Top entry Side entry


From a South American Reformer

Had thoroughly wetted catalyst On restart pressure drop very

high Catalyst badly damaged Water had rapidly vaporised

inside pellets Blew them apart


On heating catalyst can be a variety of colors

Carbon

Normal

Overheated


1200oC

1100oC

1000oC

900oC

800oC

700oC


If tubes receive differential amounts of heat from adjacent burner rows

Each side expands differently Tubes will bow This increases stress on

outside of bow GBHE recommends change

tube if more than ID outside of the centre line of the lane


• Damage to refractory ◦ Flame impingement ◦ Poor installation

• Gas tracking behind refractory ◦ Usually due to anchor failure ◦ Refractory moves away from wall ◦ Casing becomes hot - cooling by steam lances

etc


• Condensation due to ◦ passing valves ◦ dead legs ◦ too early steam or feed introduction

• Ammonia formation ◦ Problems on demin train ◦ Environmental

• Boxing up reformer ◦ Avoid as can slowly cook tubes


• Stress Corrosion Cracking ◦ Due to condensation - can

eliminate by design modification Tube tops - insulation Tube bottoms - use hot

bottom design

Failure Point


• Over tensioning of tubes - premature failure

• Pigtail failure ◦ Operate in creep regime - will fail

• Header failure - Again in creep regime

• Poor burner maintenance ◦ Must clean regularly

• Metal dusting of burner tips ◦ Methanol plants only

Failure


• Modifications to coffins/ports in coffin ◦ Can lead to flow mal-distribution

• Wind changes ◦ Changes temperature - up to 20°C seen

• Leaks in air pre-heaters ◦ Worst on rotary air pre-heaters ◦ Have seen leaks on static heaters as well

• Fouling of duct coils ◦ High DP and low heat transfer


• Gas composition analysis has a number of problems

• Sample shifting ◦ Normally affects only CO and CO2 ◦ In worst case can increase CH4 ◦ Problem if using CH4 as a constraint in a model

• Hydrogen by difference ◦ Errors in other components will be included in H2

• Inerts (N2 and Ar) poor measurement ◦ Can affect fitting programs


• Typical temperature losses are • Exit tubes 1-3°C • Sub headers 3-10°C • Main headers 5-15°C

• Inlet secondary • Top Fired Reformers 10-20°C • Foster Wheeler Reformers 15-35°C • Side Fired Reformers 15-35°C

• Inlet WHB, • Top Fired Reformers 10-20°C • Foster Wheeler Reformers 15-35°C • Side Fired Reformers 15-35°C


Main cause of catastrophic tube failure • This one was at North American Methanol Plant • Plant trip (loss of feedstock to steam reformer)

due to valve failure • Feedstock to steam reformer not isolated

adequately by valve • Set point on reformed gas pressure not reduced • Steam introduced for plant restart at reduced rate • All burners lit (deviation from procedure) • Tubes at 16 bara


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"

30 m

inut

es


Reformer exit gas temperature on panel never exceeded 700°C (1290°F) Cannot use this instrumentation as a guide to tube

temperature Reformer start-up at normal operating pressure

Tube failure temperature 250°C (450°F) lower than normal for start-up

All burners lit Far too much heat input resulted in excessive

temperatures


Date post:	02-Nov-2014
Category:	Technology
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Steam Reforming - Common Problems

Technology