Date post: | 01-Nov-2014 |
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Steam Reforming - Poisons
Gerard B. Hawkins Managing Director
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The aim of this presentation is to • Introduce the various poisons • Indicate actions that need to be taken if
catalyst is poisoned • Effect of ultra purification
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Common poisons include • Sulfur • Chlorides and other halides • Metals including arsenic, vanadium, mercury,
alkali metals (including potassium) • Phosphates • Organo-metalics
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Reduced catalyst activity In primary reformer this means • Reduced reaction • Reduced reaction heat load • High tube/process gas temperatures • More susceptible to carbon formation • Hot bands
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Hydrocarbon Feed
Hydrogenation Chloride
Removal
Sulfur
Removal
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Steam reforming catalyst requirements Poison Limit Effect
Sulfur <0.1 ppmv Poison
Chlorides <0.1 ppmv Poison
As/V/Pb/Hg <5ppbv Poison
Olefins <1-2 vol% Carbon
• Process gas feed to reformer (dry basis)
• Excludes Pre Reforming catalysts
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For Pre Reformers must be lower than for primary reformers
Sulfur specified at 25 ppb or less But needs to be checked using a space
velocity calculation Chlorides and Sodium are also poisons Silica is also an issue
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• Sulfur Feedstock • Chlorides/halides Feedstock • Arsenic Vetrocoke • Vanadium Benfield • Mercury Feedstock • Alkali metals Steam/BFW • Phosphates Steam/BFW
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Natural Gas
Associated Gas/LPG
Refinery Off Gas (ROG)
Naphtha
Hydrogen Sulfide X X X X
Mercaptans (low) X X X X
Carbonyl Sulfide X X X
Mercaptans (high) X X
Thiophenes X
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• Vast majority of poisoned reformers are affected by Sulfur
• Many problems with sulfur analysis
• At more than 5 ppm will cause severe and rapid deactivation
• At 20-30 ppb will lead to slow deactivation
• Nickel is excellent sulfur absorbent
• At high enough levels will completely de-active catalyst
PelletSSSS
SSS
S S S
Nickel
CH
H O
4
2
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Sulfur less ‘sticky’ at high temperatures Only affects upper parts of tube Little affect lower down - usually hot bands are
so bad that catalyst is changed out prior to affecting bottom part of tube
Poisoning is generally reversible - can steam May lose some activity Particularly if low inlet temperature
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Sulfur can be passed to reformer if • ZnO saturated - difficult to test • COS in feed gas and not hydrolyzed over
ZnO • Organics in feed with no CoMo/NiMo • Plants with NG bypass to secondary • Bypass around HDS/ZnO ◦ DP Tappings ◦ Bypass lines ◦ Incorrect valves open
• Leaks on HDS interchangers WWW.GBHENTERPRISES.COM
COS not as common as H2S, mercaptans
COS + H2O H2S + CO2
Reacts over CoMo/NiMo at typical conditions
or
Over ZnO that contains some alumina
NOTE: COS is not removed by amine systems
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Zinc Oxide will remove some COS provided that there is alumina in support
As with VSG-S201- series Competitors have no alumina Organic sulfur compounds (mercaptans)
pass through zinc oxide
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• Difficult to set up analysis • Tests are difficult • Tests not accurate • Looking at low levels
• Generally looking for sulfur at detection limit of the laboratory equipment
• Limit is 10 ppb • ZnO will slip about 10 ppb
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• Problems of sampling • Pipe work must be of stainless
• Sulfur absorbs into Carbon Steel • Will absorb into stainless but at lower rate
• Short pipe runs • Prevent absorption
• Sample keeps for four hours • In a stainless steel sample bomb
• Use plastic sample bags if sample to be transported
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• Some sulfur is removed during shut down • All shut downs include a steam out
• Bulk and surface different readings
• use surface
• Better to conduct full steam out • Test condensate for sulfur compounds
• Or smell condensate (beware)
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Of benefit to most plants Effect most pronounced in tough conditions ◦ Low S/C operation where Carbon formation is likely ◦ Prereformers which are very sensitive ◦ GHRs where deactivation impacts heat transfer ◦ Heavy feed reforming where poisoning and carbon
formation determine life ◦ Heavily stressed reformers
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Together with ZnO at regular operating conditions ◦ ZnO to remove bulk of sulfur (H2S) ◦ Followed by a layer of
Ultrapurification for polishing Cannot replace ZnO completely since
it has a lower saturation capacity Catalyst requires reduction prior to
use Can be pyrophoric on discharge -
similar to LTS
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Unfortunately, measuring low ppbv levels of sulfur is difficult
So proof has to come from lab/field work We wanted to test the concept So we chose a plant where we can measure
the effect of deactivation, a reformer we know suffers from deactivation, and where LTS catalysts are known to pick up sulfur
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• Smaller, slower decrease of GHR UA expected in theory with Cu/Zn
• Historical evidence from plant that UA settles to lower than SOR value
UA v Days online
0.8
0.85
0.9
0.95
1
1.05
0 200 400 600
Without With
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Arsenic is a very virulent poison If a reformer has been poisoned by arsenic
then must clean tubes thoroughly If this is not done then arsenic will poison
then next batch of catalyst And continue to do so
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• Most common as HCI or highly mobile ion
• Remove less than 5ppb
• Accelerates sintering in catalyst metal crystallites
• Found in: • Feed storage locations
• Crude and distillate oils
• Certain refinery processes
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Effect of Chloride on ZnO Sulfur Removal Catalyst
1. Fresh ZnO 2. Poisoned ZnO
HClZnOCrystallites
Catalyst Pores
ZnCl2 blocks
catalyst surface
and pores to prevent sulfur absorption
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Location Elemental Mercury
Concentration (Micrograms/m 3 )
South America 69 –119 Far East 58 –193 North Africa 0.3-130 Groningen 180 Middle East 1-9 Eastern US Pipeline
0.019-0.44
Midwest US Pipeline
0.001-0.10
North America 0.005-0.040
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Most steam reforming catalysts can handle olefins
Typically between 1-2% Can lead to rapid formation of hot bands If more than this then need to be treated in
HDS
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