Upload
jayant-swami
View
31
Download
1
Embed Size (px)
DESCRIPTION
Steam reforming
Citation preview
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