Corrosion Due to Elemental Sulfur in Sour Gas Production and Claus - - PowerPoint PPT Presentation

Peter D. Clark Director of Research, Alberta Sulphur Research Ltd. and Professor Emeritus of Chemistry, University of Calgary and

• N. I. Dowling

Senior Research Scientist, Alberta Sulphur Research Ltd. Contact: pdclark@ucalgary.ca

Corrosion Due to Elemental Sulfur in Sour Gas Production and Claus Sulfur Recovery Systems

MESPON 2016 Abu Dhabi, United Arab Emirates, October 9 – 11, 2016

Wet Sulfur Contact Corrosion of Carbon Steel

The Safety Moment: Iron Sulfide and Fire

Fe + S8 FeS H2O FeS O2 (Air) Fe2O3 + SO2 + Energy (∆H = -1,226 kJ) The Fe2O3 becomes red hot igniting any flammable material (sulfur) Result: Fires inside the Claus plant (catalyst beds), sulfur pits, tanks, sulfur forming plants and more

Electrochemical Mechanism of Sulfur Corrosion

steel sulfur iron sulfide ‘mackinawite’ S-deficient FeS(1-x) “non-stoichiometric” FeS is pyrophoric when dry and finely divided steel sulfur iron sulfide ‘mackinawite’

Mechanistic Overview of Steel / Sulphur Corrosion

non-stoichiometric e- conducting FeS layer

2e-

Effect of Moisture and Steel / Sulfur Contact

• Contact and moisture are essential for corrosion -

(free-standing H2O) (2-3 wt% moisture)

Effect of Temperature on The Rate of Wet Sulfur Contact Corrosion

> 20oC most

• f the time

PANAMA CANAL TRANS-ATLANTIC TRANS-PACIFIC

Partial Oxidation of FeS and Formation

• f Sulfuric Acid

FeS + O2 (Air) Fe2 (SO4)3 Fe2 (SO4)3 + H2O 2 Fe3+ [6 H2O] + 3 SO4

2- [6 H2O]

Partial

• xidation

Solvolysis Fe3+ (H2O)6 Fe2+ (H2O)5 OH + [H+]

• Iron sulfate forms acidic solutions (pH ≈ 1-2) which corrode steel

S8 Deposition within a Sour Gas Flow Line+

Two Examples of Sulfur Deposition in Sour Gas Production Facilities

S8 Deposition in a Gas Plant Inlet Separator*

+ Photograph courtesy of John Morgan, John M. Campbell & Company * Photograph courtesy of Mark Townsend, Burlington Resources

Sulfur Deposition Arising from Oxidation of ppm Level H2S

Sources: 1. Chesnoy, A.-B., and Pack, D.J., S8 Threatens Natural Gas Operations, Environment, OGJ, V.95, No.17, pp.74-79, Apr. 28, 1997.

• 2. Courtesy of PG&E, Technological and Ecological Services (TES), San Ramon, California

Natural Gas Distribution Regulator Cage 2 200-mm ANSI 600 Turbine Meter 1 Magnified Image of Elemental Sulfur on Restrictor Valve Surface 2

Elemental sulfur confirmed by SEM/EDX analysis

In Situ Formation of Sulfur in Sour Gas Equipment

FeS Protective Coating

H2S S8 O2 (Air) (ppmv) High P Sour Gas CH4 / H2S / CO2 (S8 / H2O) 2 Fe2+ (S) + ½ O2 2 Fe3+ + O2- 2 Fe3+ + H2S 2 Fe2+ + 2 H+ + 1/8 S8 2 H+ + O2- H2O

• The protective FeS coating becomes a catalytic layer
• The reaction is fast at high P; the amount of sulfur (H2O) formed

depends on amount of O2 ingress

Sulfur Formation and Corrosion in Amine Plants

Steam Condensate

• Sulfur is formed at FeS layer in the contactor and then transported around

the amine loop

• Degradation occurs in the regenerator; ionic species enhance corrosion

~130°C CH4 H2S – CO2 ~40°C FeS CH4/H2S- CO2 O2 (ppmv) R3 NH HS / HSx +

• Contactor: H2S + ½ O2 H2O + 1/8 S8 R3 NH HSX

Regenerator: R3 NH HSX R CO2 H + HS2O3 / HSO4

+

• pH >10
• Amine

R3N, H2S

• +

“Combustion” of Steel with Sulfur in the Claus Furnace

S8 Process gas Acid gas Air Ceramic brick

• O2 is very rapidly consumed by H2S in the flame
• Steel is oxidized by sulfur forming a mixture of FeS and FeS2

Fe + S2 FeS2 H2S H2 + ½ S2 Fe + H2S H2 + FeS FeS2 ½ S2

Brick failure

The Chemical Function of the WHB Ferrule

Ceramic ferrule H2O T < 400°C Steam T < 400°C H2S, S8, H2 H2O, N2 Furnace gases (T > 1,000°C)

• The alumina ferrule is chemically inert to all species
• Steel (Fe) is relatively inert to sulfur and other species < 400°C.

S8 S8 Steam H2O

Hot Purge (CH4 combustion)

The Importance of Purging Sulfur From a Claus Unit During Shutdowns

• Ceramic brick and catalyst retains sulfur after unit shut down
• Conditions must be maintained to prevent condensation of sulfur

in places other than the condensers

Corrosion in Off-Gas Line Below Water Dew Point

• Inadequate heating at line support allows water condensation
• Rapid Fe/S corrosion: Fe + 1/8 S8 FeS
• Aqua Claus reaction: 2 H2S + SO2 [H2SxOy] 3/8 S8 + 2 H2O
• Highly acidic aqueous solution is formed

H2O(l) H2O(l)

Tail Gas Insulation TGU Incinerator Line Support (“heat” sink) S8 + H2O(l) FeS Corrosion Products

Field Pictures of Corroded Claus Tail Gas Line

Pictures provided to ASRL by:

Mechanisms for Deterioration of Concrete in Sulfur Pits

Liquid S8 130ºC Concrete

• Migration of H2S, SO2, O2, H2O and S (vap) into internal pore

structure of the concrete followed by chemical reactions.

H2S + S8 H2Sx

Solid/ liquid S8 T ºC = 90→130

AIR

Liq S8 H2S S8 SO2 O2 O2 N2 (H2O)

Formation of Sulfur Inside the Concrete Detailed Chemistry H2S + O2 SO2 + H2O

1/8 S8(vap) + ½ O2 SO2

2 H2S + SO2 3/8 S8 + H2O “CaO” CaSO4, CaS2O3

Lower density, higher volume unconsolidated products.

Concrete pore structure

“H2 SO4” “H2 S2O3”

Concrete Claus chemistry intermediates [strong acids]

Secondary Corrosion Processes at Concrete Pit Reinforcing Steel

Fe Fe CaO CaO Fe2 (SO4)3 Fe2O3, H2SO4 Fe H2O, S8 Fe FeS O2, H2O

Secondary Corrosion Enhanced Sulfur Formation at Steel

Fe + H2SO4 FeSO4 + H2 CaO + H2SO4 CaSO4 + H2O 2 H2S + SO2 3/8 S8 + 2 H2O H2S + ½ O2 1/8 S8 + H2O Fe2O3 Fe2O3

Primary Corrosion in Sulfur Tanks – Air Drafted Systems

Air / SO2 / S8(vap) To scrubbers Air (H2O) Insulation

• Poor roof insulation (or poor heating)

may result in inner roof temperature

• f < 100°C
• S8 solid deposition and water / H2SXOY

condensation (from SO2 / H2O)

• Fe / S8 corrosion

Fe + 1/8 S8 FeS

• Acid corrosion

2 Fe + H2SXOY 2 Fe SXOY + H2 Steam coil Steam ~ 140°C Liquid S8 SO2 SO2 N2 S8(vap) O2

H2O or H2SXOY

Consequences

Secondary Corrosion on Sulfur Tank Roofs – Air Drafted Tanks

Air (H2O) < 100°C Insulation T = 140°C S8(liq) SO2 SO2 N2 O2 Air / SO2 / S8(vap) To scrubbers O2 [H2O] FeS corrosion layer Oxidation of FeS 2 FeS + 3/2 O2 Fe2O3 + 2/8 S8 2 FeS + SO2 2 FeO + 3/8 S8 Continued Sulfur Corrosion Fe + 1/8 S8 FeS (H2O)

• Without roof heating, T may fall to < 100°C, allowing H2O or H2SXOY

condensation.

• Partial oxidation of FeS may reform S8 at surface.
• Corrosion at “cool” roof surface may result from condensed acids (H2SXOY),

sulfur deposited or formed by chemical reaction.

Air Steam coil Steam ~ 140°C S8(liq)

Rupture of Steam Coils in Sulfur Tanks

Air / SO2 (H2S) (H2S) Condensate / Steam

• Mechanical erosion of FeS layer leads to thinning of carbon steel coils and

eventual rupture Steam Coil Corrosion Fe + H2S FeS + H2 S8 (H2S) FeS Fe Mechanical erosion Fe "Clean surface" H2S (S8(liq)) Fe New FeS S8(l) inlet FeS

Shipping Sulfur By Rail

S8 S8 Steel Box Aluminum Box [Don’t do it!]

• Polymer coating to prevent

iron-sulfur corrosion OR

• Keep sulfur dry by adding

a cover or roof

• Aluminum will melt if S8

catches fire

• Al/S8 react explosively at T
• f burning sulfur to form

Al2S3

• Al2S3 reacts with water

producing H2S Al2S3 + 3 H2O Al2O3 + 3 H2S

Aluminium (mp=660°C)

Thiobacilli H2SO4 S8 S

8

H2SO4 H2O H2O Corrosion: FeS / Sulfur layer may become H2S saturated > 1000 ppmv. FeS + H2SO4 FeSO4 + H2S

denser than air — remains in the bottom of the hold.

Corrosion and Acidity Generation in a Ship Hold

• a Potentially Deadly Combination -

Sulfur Loading to Limewashed Hold

Development of Zinc-modified Limewash

Chemistry: Advantage:

• SUCCESSFULLY FIELD TESTED IN SHIP TRIAL

COMBINE LIMEWASH AND Zn2+

Improved barrier plus additional benefit from release of Zn2+ in event of acidity build-up

Mitigation of Corrosion by Zn2+:

Fe + 1/8 S8 FeS Zn (OH)2 + FeS ZnS + Fe (OH)2

• ZnS is a perfect insulator stopping e-transfer at iron surface

Effect of Soluble Zn2+ on Wet Sulfur Corrosion

solution phase addition of Zn2+

soluble Zn2+ inhibits S corrosion at concentrations even as low as 1 x 10-2 M Inhibition works by in-situ formation of insoluble ZnS barrier at steel / S contact area

Zn2+ + S2- ZnS ( stoichiometric )

ASRL Member Companies 2016 - 2017

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