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NIOC – IFP SEPTEMBER 2003
SULPHUR RECOVERY
Joëlle CASTEL
GAS PROCESSING AND SULPHUR TECHNOLOGIES
TECHNIP FRANCE
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PART 2
SULPHUR RECOVERY
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Sulphur Recovery Units (SRU)
H2S : toxic impurity
H2S or SO2 rejection to atmosphere is subject to stringent regulations
Objective : process the acid gas produced in the amine units to convert H2S and other sulphur compounds into elemental sulphur
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ACID GAS
PROCESS OVERVIEW
GAS SWEETENING UNIT
SULPHUR RECOVERY UNIT
TAIL GAS TREATMENT UNIT
INCINERATOR
SOUR GAS
SWEET GAS
FLUE GAS
SULPHUR
SULPHUR
CLAUS Unit
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FEEDSTOCKS TO SRU
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SRU Feedstocks main characteristics
NATURAL GAS TREATMENT
Large SRU capacity
Sulphur production as high as several thousands tons/day
Examples : Qatar North Field : 200 t/d S
Astrakhan : 4 * 2100 t/d S
OGD1: 3 * 600 t/d S
Stable operating conditions
H2S content can be low, when the natural gas contains CO2 (example South Pars Field)
Possible presence of heavy hydrocarbons at SRU inlet (aromatics)
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SRU Feedstocks main characteristics
REFINERY UNITS
Lower sulphur production capacity
Up to a few hundreds tons/day sulphur
LEUNA refinery 420 t/d sulphur
Bukhara refinery 20 t/d sulphur
H2S rich acid gas feed (no or little CO2)
Operation with variable acid gas capacity
Large flexibility and turn down requirement
Presence of ammonia in significant quantities
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Sulphur Recovery Efficiency
Varies according to the specific local regulations and the capacity of the sulphur plant
from 95% to 99.9+ %
The environmental regulations are becoming more and more stringent in most countries
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Main applications
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Main Applications
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CLAUS SULPHUR RECOVERY UNIT
Old and proven technology (before 1900)
principle
Partial oxidation of H2S into SO2
Claus reaction to form sulphur
Recovery of sulphur in the liquid form
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SULPHUR PRODUCT CHARACTERISTICS
Bright yellow solid at ambient temperature
Liquid sulphur density : approx. 1.8 @ 140 °C
Non soluble in water
Poor heat transfer capability
Melting point 118°C
Viscosity :
special behaviour vs temperature with a maximum around 180°C
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CLAUS REACTION
2 H2S + SO2 3/n Sn + 2 H2O
Exothermic reactions
CLAUS REACTION
H2S PARTIAL OXIDATION
3 H2S + 3/2 O2 2 H2S + SO2 + H2O
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H2S Conversion – Temperature impact
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Typical arrangement of a Claus Unit
H2S partial oxidation in a combustion chamber:
Claus thermal reactor or reaction furnace
The Claus reaction begins in the combustion chamber
Progress of the Claus reaction toward sulphur production at lower temperature on catalyst beds (several catalytic stages with intermediate condensation of liquid sulphur)
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SULPHUR RECOVERY PLANT TYPICAL FLOW SCHEME
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Side reactions
CxHy + (x+y/4) 02 xCO2 + H20
2NH3 + 02 N2 + 3H20 + DH
2HCN + 02 N2 + H20 + 2CO2
CH4 + 2S2 2H2S + CS2
CO2 + H2 CO + H20
CO + 1/2 S2 COS
H2S 1/2 S2 + H2
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Side reactions
CO and CS2 hydrolysis
COS + H20 H2S + CO2
CS2 + 2H20 2H2S + CO2
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Hydrocarbons in acid gas
Air requirement increased
Risks of incomplete combustion
Carbon deposits
Catalyst fouling
“black” sulphur
Consequences
Sulphur recovery efficiency drop
Plugging
Catalyst deactivation
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Hydrocarbons in acid gas Aromatic compounds effect (BTEX)
BTEX compounds destruction is difficult
BTEX may have detrimental effects on catalysts activity even at low concentration (> 20-50 ppm at catalyst bed inlet)
Low catalyst life
Reduced sulphur recovery efficiency
Corrective actions:
Solvent selection in the upstream unit (chemical solvent preferred)
Keep a high flame temperature in the Claus unit thermal reaction chamber
1150 – 1200°C
Residence time in combustion chamber > 1.5 seconds
High efficiency burner
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Effect of Ammonia in acid gas feedstock
Ammonia may lead to the formation of ammonium sulfides and sulfates
bouchages
corrosion
sulfatation des catalyseurs
Corrective action
High flame temperature to ensure ammonia destruction
> 1250°C
Use of specially designed high efficiency burners
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Conceptual design of a sulphur recovery plant
Combustion scheme
Acid gas preheating or not
Air preheating or not
Presence of impurities (NH3, BTEX)
Type of burner
Combustion chamber residence time
Waste heat boiler steam level
Catalytic reactors inlet gas reheat mode
WHB by-pass
In line burner
External heat preheater
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Claus unit operating parameters
Pressure
Air to acid gas ratio
Flame temperature
Catalytic reactors temperature
Type of catalysts
Incineration
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Combustion Chamber operating pressure
The unit exhaust is at atmospheric pressure
Operating pressure is therefore function of pressure drop
generally 0.4 à 0.7 bar eff in the burner i.e. 0.5 à 0.8 bar eff at unit battery limit
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Air to acid gas ratio
The control of the air/acid gas ratio is fundamental for the sulphur recovery performance
Optimum H2S/SO2 = 2
Air flow rate is controlled by a continuous analysis of the H2S/SO2 ratio in the process gas at unit
H2S/SO2 = 2 +/- 0.1
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Claus reaction : impact of air control
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Flame temperature
Function of the composition of the acid gas feed (CO2 content)
Shall be above 950 – 1000°C as a general rule to ensure a stable burner flame (1150-1250 °C if BTEX or NH3 are present)
No specific constraint if H2S > 55 – 60% vol in feed gas
Otherwise:
air and/or acid gas preheat
Partial acid gas by-pass (except when BTEX are present)
Fuel-gas co-firing
Use of oxygen enriched air
Catalytic oxidation
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Temperature of catalytic reactors
Compromise between
Claus reaction kinetics (favoured by high temperature)
Conversion rate (favoured by low temperature)
The temperature must always be kept above the sulphur dew point
typically 210 to 280°C at reactor inlet
Temperature increase on catalytic beds
~ 100 °C in the first reactor
~ 25 à 50 °C in the second reactor
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Gas reheat at reactor inlet
Hot gas bypass (first reactor)
Apparently simple but:
Low flexibility
Reduced conversion rate
Requires valves operating at hich temperature
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Gas reheat at reactor inlet
In line burners (auxiliary burners)
Simple and flexible but
Risk of excess oxygen at reactors inlet
Part of the acid gas by-passes the main burner (combustion of HC less efficient)
Fuel gas can be used in the burners
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Gas reheat at reactor inlet
Preheat with external heating medium
The best solution for high conversion rates
Higher cost
Increased pressure drop
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CATALYSTS
Generally speaking, activated alumina is used
In the first catalytic reactor, possible use of COS hydrolysis catalysts (catalysts enhancing COS nd CS2 conversion ): titanium oxide catalysts (more active but more expensive)
Vendors examples :
AXENS (PROCATALYSE) CR3S, CRS 31, CRS 21
ALCOA
UOP
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Performances
Sulphur recovery efficiency for rich H2 feed
94 à 96% with 2 catalytic reactors
95 à 97.5 % with 3 catalytic reactors
Heat recovery (typically steam generation)
Typically 3 TONS Steam /TON produced sulphur
Large units – MP Steam WHB (20 – 40 bars)
– LP Steam (4 bars) at condensers
Small units – LP steam only
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SULPHUR QUALITY
Color : bright yellow (solid state)
Purity : S > 99.9 % wt
ash < 0.05 % wt
organic matters < 0.03% wt
H2S content
upstream degassing approx. 250 ppm wt
Downstream degassing < 10 ppm poids
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EQUIPMENT : Burners
Ammonia destruction
Requires a high efficiency burner
Low ammonia content
single burner
air and acid gas preheat
Average content
Single high efficiency burner
air and acid gas burner
High ammonia content
Two burners (or dual burner)
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EQUIPMENT: Duiker burner
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EQUIPMENT: John Zink burner
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EQUIPMENT: Lurgi burner
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View of burner (LURGI)
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View of burner (LURGI)
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Combustion chamber control (typical)
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Equipment : combustion chamber and waste heat boiler
Combustion chamber :
carbon steel with refractory lining
Equipped with temperature measurement devices (optical pyrometers)
Waste heat boiler with process gas on tube side and steam generation on shell side:
Natural thermosiphon circulation
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Equipement: Combustion Chamber
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Equipement : aste Heat Boiler
Steam risers and BFW Downcomers
Sulphur Outlet
Tube Sheet Refractory (ceramic ferrules in tubes)
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Equipement: catalytic reactors
Conception :
low pressure drop
Even distribution of the process gas
generally horizontal reactors
Carbon steel with refractory lining
Equipped with temperatures measurement devices (thermocouples)
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Equipement : catalytic reactors
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Equipement: Sulphur Condensers
Single pass exchangers:
process gas on tube side
Steam generation on shell side
Equipped with a KO drum with mesh to avoid sulphur droplets carry over
Lightly slopped to enhance sulphur drainage
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Incineration (thermal oxidiser)
High incineration temperature required for the complete conversion of H2S and sulphur compounds to SO2
H2S < 10 ppm vol
temperature 650 to 800°C
excess air in flue gases : 2 à 5% vol
Natural draft incineration (no air blower)
less expensive
excess air control is uneasy
Forced draft incineration (with air blowers)
more expensive but excess air can be controlled
preferred option if the incinerator is equipped with waste heat recovery
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Thermal oxidiser with WHB
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Sulphur degassing
Required to remove the H2S and poly sulfides dissolved in sulphur (safety during storage and transport)
Maximum allowable H2S content < 10 ppm wt
Principle :
sulphur agitation by circulation or stripping
with or without a catalyst
the degassing offgas is sent to the incinerator via air sweeping
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Sulphur storage and transport
Liquid sulphur storage:
concrete pit
metallic tank
Storage temperature 125 – 140°C
significant viscosity increase when T > 145°C)
Liquid sulphur export by truck, rail or ship tankers (pipeline on short distances)
Solidification for storage and transport (slates, pastilles, granules)
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Sulphur solidification
Slates
Drawback : low mechanical resistance
Pastilles or granules are more regularly shaped and resistant
ROTOFORM Sandvik (pastilles)
ENERSUL GX (granules)
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Pastilles
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Rotoform process
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Granulation – Procédé Enersul GX
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Bulk sulphur storage
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Operating problems
Catalyst beds deactivation
Incomplete combustion of hydrocarbons
Sulfatation
fouling
Loss of sulphur recovery efficiency
Inactive catalyst
Poor COS or CS2 hydrolysis efficiency
Temperature increase at sulphur condensers
Poor H2S/SO2 ratio control
Sulphur mist carry-over
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Operating problems
Plugging
Sulphur solidification
Corrosion
Operation below sulfuric acid dew point (water condenstaion
Poor insulation and heat tracing
Sulphur « fires » in reactors
Excess oxygen
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Improvements in Claus units
New catalysts
COS and CS2 hydrolysis
Direct conversion of H2S to S by oxidation (SuperClaus and uroClaus)
Direct conversion by reduction (Procédé PROCLAUS)
High efficiency burners (NH3, BTEX)
H2S/SO2 analysers
Use of enriched air or oxygen
Better control of start-up and shut-down operations
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Claus tail gas composition (typical
(1) (2)
H2S 1.05 0.79
SO2 0.52 0.40
COS 0.07 0.01
CS2 0.02 –
S vapor 0.05 0.01
S liquid
N2 43.34 65.07
CO2 29.39 0.93
H2O 25.56 32.67
(1) Lean acid gas H2S = 37% mol TES = 91%
(2) Rich acid gas H2S = 95% TES = 96%
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Tail Gas treatment
Objective
Enhance the sulphur recovery efficiency to limit SO2 rejection to ambient air
Environmental standards more and more stringent
ex : 250 mg SO2/Nm3 flue gas i.e. SRE > 99.9%
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The various types of Claus tail gas treatment processes
Catalytic processes
Solvent based processes with recycle of H2S or SO2
REDOX processes in liquid phase
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REDOX type process
Principle (iron chelate)
H2S + 2 Fe3+ ——–> 2 Fe2+ + 2 H+ + 1/n Sn
1/2 O2 + 2 Fe2+ + 2 H+ ——–> 2 Fe3+ + H2O
Suitable only for low capacities
Sulphur difficult to market
High Capital Cost
Characteristics
Complex Operation
Chemical Make-Up
A
O
Sulfur
Air
Vent Gas
Sour Gas
Sweet Gas
Catalytic solution
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HP SULFINT process (IFP)
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Catalytic TGT processes
Process based on Claus reaction at lower temperature
Sulfreen process and its alternates
HydroSulfreen
DoxoSulfreen
CBA (Cold Bed Adsorption) process
MCRC process
Clinsulf process
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Catalytic TGT processes
Direct oxidation processes
Superclaus process
Euroclaus process
BSR/Hi-Activity high activity
BSR/Selectox
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TGT – Sulphur production in liquid phase
Clauspol process (IFP)
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TGT – Absorption and recycle of H2S or SO2
H2S recycle processes
Hydrogenation of tail gas residual sulphur compounds to H2S
H2S absorption in selective amine wash (MDEA)
H2S stream recycle to upstream Claus unit
Type SCOT, RAR, Resulf, etc…
SO2 recycle processes
Tail Gas residual sulphur compounds oxidation to SO2
SO2 absorption with solvent (amine, physical solvent)
SO2 stream recycle to upstream Claus unit
Type Cansolv process, Solinox process, Clausmaster process
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Relative cost of Sulphur Recovery Plants with TGT
Recovery Relative
Technology Level Cost
2-Stage Claus 95-96% 1.0
3-Stage Claus 96-97% 1.1-1.2
1+2-Stage Claus/CBA 97-99% 1.2-1.3
2+2-Stage Sulfreen 98.5-99.5% 1.3-1.4
3-Stage Claus + TGCU 99.5-99.95% 1.8-2.0
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