Theory and Operation of Secondary Reformers

By:Gerard B. Hawkins

Managing Director, CEO

Introduction

Purpose Key to good performance Problem Areas

• Catalysts, heat shields and plant up-rates• Burner Guns

Development of High Intensity Ring Burner

Case Studies Conclusions

Secondary Reformer Purpose

Reduce methane slip to very low levels• Around 0.3-0.5 % mol dry

For ammonia plants provide feed point for nitrogen required for ammonia synthesis• And thereby Ensure optimal H/N ratio

Generate heat for transfer for HP steam in Waste Heat Boiler

Typical Reforming Configuration

Steam

SecondaryReformer

Steam

Steam + Gas

SteamReformer

Air / Oxygen500°C

780°C

450°C

1200°C

950°C

10% CH4 0.5% CH4

Secondary Reformer Mechanical Details

Refractory lined pressure shell

Fixed Catalyst bed in lower region

Combustion section in upper region

Water jackets to keep shell cool

Catalyst supported on brick arch

Keys to Good Performance

Three key components

– Burner Design– Mixing Volume– Catalyst

All must be designed correctly to maximize performance

Air/Oxygen

Steam Reformer Effluent

To Waste Heat Boiler

Keys to Good Performance

Again three key components• Burner Design• Mixing Volume• Catalyst - VSG-

Z201/202/203

Since using O2 as oxidant, flame temperature is higher• Failures are much

faster

780°C

540`C

2500°C

1500°C

1100-1200°C

975°C

1500°C

1100-1200°C

1300°C

Note: Oxygen - Methanol Plant Design

Secondary Reformer Operation

•Burner determines mixing performance•Air injected at high velocity•Forces mixing of air and process gas•Combusts only 20% of process gas•Must also mix in other 80%•Should achieve a uniform mixture•Catalyst bed can affect flow patterns

Secondary Reformer Combustion

Gas feed very hot > 630oC

Gas feed contains hydrogen

Gas ignites automatically

Autoignition >615oC

No need for spark or pilot

Must maintain gas above 615oC

Secondary Reforming Reactions

CH4 + 2CO = CO2 + 2H2O2H2 + O2 = 2H2O Exothermic - gives out

heatFlame 2500oC mixed gas 1500oC

Steam reformingCH4 + H2O = 3H2 + CO Endothermic - cools

down gas

Water gas shiftCO + H2O = CO2 + H2Slightly exothermic

Key Components: Catalyst Problems

Catalyst can• Exhibit poor activity

Unlikely• Break up in service

Usually linked to a plant upset• Suffer physical blockage

Alumina vaporization• Become overheated and fuse

Causes increased pressure drop and gas mal-distribution

Key Components: Catalyst Activity

Catalyst is exposed to very high temperatures • Therefore nickel sinters

However once sintered it is very stable Since catalyst operates at high temperature

it is difficult to poison• Poisons will not stick• For ammonia plants will pass through to

HTS and then LTS• For methanol plants will pass through to

methanol synthesis loop

Key Components: Catalyst Activity

VULCAN Series range of catalysts VSG-Z201/202/203• Size - Mini and Standard plus

Elephant • Use as a heat shield• Shape

5-Hole Quadralobe Quadralobe has +20% more

activity than 4-hole• Well proven catalysts that are

high stable and strong• Long lives

Key Components: Catalyst Appearance

White - loss of nickel Coated in white - alumina vaporization Glazed or blue - very high temperatures Pink crystals - synthetic ruby formation

• Cause by high temperatures • A mixture of refractory and transition

metals

Key Components: Mixing Performance

Good mixing is absolutely essential Poor mixing in mixing zone gives high approach

and high methane slip Poor mixing can be due to

• Poor burner design• Insufficient mixing volume• Burner gun failure

Root cause can be checked with CFD but will not detect burner gun failure

Key Components: Burner Gun

If burner gun fails then can lead to • Wall refractory damage• Loss of vessel containment

Poor mixing can lead to zones of high temperature• Leads to high rate of catalyst sintering• Reduction in catalyst activity• Increase in approach to equilibrium (ATE)

Poor mixing can lead to high flow zones• Movement/damage of target tiles or catalyst

bed• Increased ATE

Key Components: Burner Guns

Standard Ammonia secondary burners have• Small number of large holes• Give poor mixing at high rates• High risk of overheating bed• Methane slip rises rapidly at high rates• Burner can be plant limit

Key Components: Burner Guns

For methanol plants remember that oxidant used in oxygen

Gives higher flame temperatures If jet impinges on refractory then

refractory will be damaged much more quickly

Vessel will fail rapidly As oxidant flow is lower than for an

ammonia plant use a different design of burner

Key ComponentsHigh Intensity Ring Burner

The high intensity burner differs from the standard burners• Large number of small holes: Small flames• High degree of mixing: Short mixing distance• Oxidant fed evenly into process gas: Good

Mixing• Insensitive to rate increases• Used in ICI Ammonia plants

Effect of Operational ChangesAir Rate

Name Units Base Case

Increased Air Rate

Plant Rate % 100 100 Air Rate % 100 105 Exit Pressure Bara 39 39 Steam to Carbon to Primary n/a 2.88 2.88 Outlet Temperature °C 1000 1026 Methane Slip mol % 0.41 0.41 H/N Ratio n/a 3.00 2.86 Approach to Equilibrium °C 14.2 45.1

Effect of Operational ChangesPressure

Name Units Base Case

Increased Exit Pressure

Plant Rate % 100 100 Air Rate % 100 100 Exit Pressure Bara 39 40 Steam to Carbon to Primary n/a 2.88 2.88 Outlet Temperature °C 1000 1000 Methane Slip mol % 0.41 0.41 H/N Ratio n/a 3.00 3.00 Approach to Equilibrium °C 14.2 11.3

Effect of Operational ChangesSteam to Carbon Ratio

Name Units Base Case

Decreased Steam to Carbon

Plant Rate % 100 100 Air Rate % 100 100 Exit Pressure Bara 39 39 Steam to Carbon to Primary n/a 2.88 2.78 Outlet Temperature °C 1000 1002 Methane Slip mol % 0.41 0.41 H/N Ratio n/a 3.00 3.00 Approach to Equilibrium °C 14.2 12.2

Key Components: Effect of Poor Mixing

Poor mixing can be illustrates by assuming a secondary reformer with a high zone of high air flow and a zone with low flow

Name

Temperature

Methane slip

Approach

Poor

Units Toomuch air

Too littleair

oC 1034 902

Mol % 0.13 1.89

oC 10 10

Mixed

971

0.9

53

Good

957

0.62

10

Key Components: Catalytic Heat Shield

Bed has to be protected against disturbances

Conventional target tiles or alumina lumps used

Even these can be moved No longer required: can

replace with active catalyst• Additional activity improves

reforming performance Use – VULCAN Series AST

Advanced Support Technology• Large (35mm) 4-hole shape

Key Components Use of CFD for Secondary Reformers

CFD modelling very good for secondary reformers

BUT time consuming and expensive Building up a library of case studies VULCAN Series Catalysts VSG-Z201/202/203

has extensive experience with CFD for secondary reformers• Troubleshooting problems• Designing burner guns• Validation of modifications• Optimization of catalyst quantity

Catalyst Bed

Airgun

Recirculationzones

Case Study 1: Insufficient Mixing Volume

1200 C 1400 C 1500 C 1600 - 2100 C

AirGun

Catalyst Bed

<1200 C 1400 C

Case Study 1:Insufficient Mixing Volume

Case Study 2 Burner Guns In this case, secondary operated well up to 1450 mtpd At rates above this, methane slip rose rapidly Limiting further plant rate increases

00.10.20.30.40.50.60.70.8

1100 1200 1300 1400 1500 1600 1700

Plant rate, mtpd

Met

hane

slip

Catalyst Bed

RecirculationZone

Burner Rings

1200 C 1300 C 1400 C 1500 C 1600 - 2100 C

Catalyst Bed

Burner Rings

<1200 C

Case Study 2:High Intensity Ring Burner

Secondary Catalyst Conclusions All three components must be designed

correctly If there are problems then can change catalyst

type to high activity catalyst – VULCAN SeriesVSG-Z201/202/203 5-hole or Quadralobe• Can achieve large reduction volumes• Allows increase in mixing space• VSG-Z201/202/203 catalysts are well proven,

stable and reliable Good mixing above the catalyst bed is essential Poor mixing gives high methane slip Mixing performance critically depends upon

burner

Secondary Reforming Conclusions

CFD useful for • Troubleshooting • Design, modifications and optimization• VULCAN Series Catalysts can offer this service

GBHE Catalyst Process Technology can recommend the appropriate burner type

• Eliminates problems caused by poor mixing• Optimum burner type opposite plant configuration• But still needs designing correctly• Continued process of improvement to design• Contact your GBHE Catalysts representative for

details