Steam Reforming - Poisons

transcript

Gerard B. Hawkins Managing Director

WWW.GBHENTERPRISES.COM

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

Common poisons include • Sulfur • Chlorides and other halides • Metals including arsenic, vanadium, mercury,

alkali metals (including potassium) • Phosphates • Organo-metalics

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

Hydrocarbon Feed

Hydrogenation Chloride

Removal

Sulfur

Removal

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

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

• Sulfur Feedstock • Chlorides/halides Feedstock • Arsenic Vetrocoke • Vanadium Benfield • Mercury Feedstock • Alkali metals Steam/BFW • Phosphates Steam/BFW

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

• 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

Nickel

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

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

Over ZnO that contains some alumina

NOTE: COS is not removed by amine systems

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

• 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

• 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

• 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)

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

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

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

• 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

0 200 400 600

Without With

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

• 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

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

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

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

Steam Reforming - Poisons

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