Indian Oil Corporation Ltd

Panipat Refinery

Presentation on

Sulphur Recovery
SRU/ TGU:
Process, Operation, Controls & best practices
SULPHUR RECOVERY UNIT
Objectives of the Training Program

Familiar with process, chemistry for SRU.

Discussion of operating variables.

Start-up/Shutdown and Emergency handling.

Troubleshooting.

Operational Safety.

Experience Sharing including Case Studies/Q&As.

 MAIN PRESENTATION:
CONTENT AND SEQUENCE
 INTRODUCTION
 OBJECTIVE OF THE WORKSHOP
 GENERAL GLOBAL PERSPECTIVE
 DESCRIPTION OF THE PROCESS
 CHEMICAL REACTIONS AND CATALYSTS
 PROCESS VARIABLES
 NORMAL PLANT OPERATIONS
 TAIL GAS AND INCINERATOR
 START UP
 EMERGENCY HANDLING
 TROUBLE SHOOTING
 SULPHUR RECOVERY PROCESSES
 OPERATIONS SAFETY/ HAZARDS
 INTRODUCTION
 Crude oils divided into "sweet" and "sour" crudes, depending
on their sulfur content.

 All crude oils contain some sulfur, ranging from over 5 weight
percent to well below 0.1 weight percent.

 Sulfur in crude oil presents a real challenge to the refiner

because it is corrosive and malodorous, and because most
product specifications severely limit the sulfur content in
petroleum-derived fuels.

 These sulfur compounds can ultimately end up as sulfur

dioxide emissions once the petroleum fuels are burnt AND
WILL ADD TO THE EMISSIONS.
 INTRODUCTION-contd
 Over the years the petroleum refining industry has
developed a number of processes dedicated to the
recovery of hydrogen sulfide and to convert it into
elemental sulfur.

 During this program, a detailed description of sulfur

recovery fundamentals will be presented. The
technologies discussed will include processes such as
Claus units, and tail gas treating.

 Topics ranging from process chemistry and fundamentals

through monitoring and troubleshooting of commercial
operating units are intended to be covered.
 SRU Block
A typical “Sulphur Recovery Unit” complex consists of
process units like:

 Amine regeneration Unit (ARU)

 Sour Water Stripping Unit (SWS)
 Sulphur Recovery Unit (SRU)
 Tail Gas Treating Unit (TGTU)
 Amine Regeneration Unit
The purpose of this unit is to receive rich amine (containing a high amount of
dissolved H2S) from upstream absorbers in PR & PREP, remove the H2S
from it and return the lean amine (containing very low H2S) back to PR &
PRE for further absorption. The H2S thus released is sent to Sulphur
Recovery Unit.

Sl No. Absorber Flow Rate (Kg/hr)

1 CDU LPG Absorber 4676
2 DCU LPG Absorber 10086
3 DCU FG Absorber 5452
4 DCU De-Ethanizer Gas Absorber 78851
5 DCU Light Naphtha Absorber 5512
6 DHDT HP Gas Absorber 130192
7 DHDT LP Gas Absorber 46886
8 HCU Recycle Gas Absorber 107781
9 HCU HP Amine Absorber 0 (Gas to be routed to DHDT LP Gas Absorber)
10 HCU LPG Absorber 12907
11 HGU Off Gases 0 (Gas to be routed to DHDT LP Gas Absorber)
 Diagram of Amine Regeneration Unit

1.05 KG/CM2
REGENERATOR TO FLARE
92 OC AIR CONDENSER ACID GAS
RICH AMINE 0.95 KG/CM2
FROM UNITS TO SRU

55 OC
104 OC CW
59 OC

AMINE REGENERATOR COLUMN

RICH SOLVENT

REGENERATOR
REFLUX DRUM
FLASH DRUM
RICH/LEAN REGENERATOR
CW SOLVENT EXCH TRIM CONDENSER
40 OC

59 OC 72 OC
LEAN SOLVENT
LEAN SOLVENT AIR COOLER REGENERATOR
TRIM COOLER REFLUX PUMP

127 OC
50 OC RICH AMINE PUMP
LP STEAM

REGENERATOR
REBOILERS

AMINE LP CONDENSATE
126 OC
CIRCULATION 127 OC
TANK REGENERATOR
AMINE 8.0 KG/CM2 40 OC
(785 M3) BOTTOMS PUMP
FILTER
LEAN SOLVENT
LEAN SOLVENT
TO UNITS
PUMP
 Advantages Of MDEA Over DEA

The advantages of an MDEA based system over a DEA based system are
1. Around 15 to 20 % capacity increase if existing DEA based plant is converted to MDEA based
plant with practically no capital investment.
2. Around 15 to 20 % less energy cost compared to DEA system due to low reboiler duty as less
energy is required to break the bond between acid gas and MDEA.
3. Around 15 to 20 % less energy required for pumping as MDEA concentration upto 50 wt %
can be kept compared to 25 wt % limitation in case of DEA, thus circulation rate can be
reduced and pumping requirements will also be reduced accordingly.
4. Higher acid gas loading in rich amine (0.45 to 0.5 m/m in MDEA system) compared to 0.35 to
0.4 m/m in DEA system.
5. Selective absorption of H2S over CO2 in MDEA is better compared to DEA.
6. MDEA freezing point is much lower than DEA, making handling of the solvent much easier.
 Comparison between DEA & MDEA

Parameter Di Ethanol Amine Methyl-Di Ethanol Amine (MDEA)

(DEA)
Structural Formula HN(CH2CH2OH)2 CH3N-(CH2CH2OH)2

Molecular Weight 105.14 119.16

Specific Gravity at
20/20°C 1.092 1.041

Boiling Point, °C at 760 mm Hg 268 247.3

Freezing Point, °C -2 -21

Solubility, at 20°C Complete Complete

in water Complete Complete
water in
Vapor Pressure, mm Hg at 20°C <0.01 <0.01

Viscosity, cP
380
33.8
at 40°C 1.4747
Refractive Index, nD, 20°C 1.4694

Flash Point, °C (°F) 191 138

 Amine Chemistry

H2S + R2CH3N  R2CH3NH+ + HS-

CO2 + R2CH3N + H2O  R2CH3NH+ + HCO3-

(where R = C2H4OH)
H2S Absorption Rate is Fast
CO2 Absorption Rate is Slow

Normal 40 wt% MDEA

 Refinery Sour Water Stripper Unit
The refinery sour water stripper unit is dedicated to treat sour water from non hydro-
processing units, i.e.CDU/VDU, DCU, SRU, HGU & PX-PTA. The H2S and NH3 in the
sour water are stripped away and sent to Sulphur Recovery Unit or sour flare. The stripped
sour water is sent to CDU/DCU or ETP.
Approximate feed flow rates and compositions are
Sl Unit Sour Water H2S ppm NH3 ppm
No. Flow (kg/hr)
1 CDU/VDU 105600 1000 300

2 DCU 15000 6000 2600

3 SRU 21000 120 -

4 HGU 7200 500 1700

5 PX/PTA 2500 150 -

Stripped sour water will contain not more than 50 ppm H2S and 50 ppm NH3.
 Diagram of Refinery Sour Water Stripper Unit
TO FLARE
90 OC
0.9 KG/CM2 SOUR GAS

SOUR WATER
FROM UNITS TO SRU
65 OC

REFINERY SOUR WATER STRIPPER COLUMN

REFINERY STRIPPER
CR AIR COOLER
SOUR WATER 92.4 OC
SURGE DRUM
FEED/BOTTOMS
EXCHANGER
90 OC
71.6 OC

REFINERY STRIPPER
CR PUMP
40 OC 123.6 OC
123.8 OC
SWS FEED PUMPS LP STEAM

STRIPPED REFINERY
WATER STRIPPER
STRIPPED WATER REBOILER
TO ETP
40 OC AIR COOLER

LP CONDENSATE
123.8 OC
STRIPPED WATER
TRIM COOLER
REFINERY STRIPPER
STRIPPED WATER TO CDU/DCU BOTTOM PUMP
 Hydroprocessing Sour Water Stripper Unit
The hydroprocessing sour water stripper unit is dedicated to treat sour water from
hydro-processing units, i.e.HCU and DHDT.The H2S and NH3 in the sour water are
stripped away and sent to Sulphur Recovery Unit or sour flare. The stripped sour water
is sent back to HCU/DHDT or ETP.
Approximate feed flow rates are

Sl Unit Sour Water H2S ppm NH3 ppm

No. Flow (kg/hr)
1 HCU 19900 22855 46610

2 DHDT 16417 13340 26682

Stripped sour water will contain not more than 50 ppm H2S and 50 ppm NH3.
 CW
Diagram of Hydroprocessing Sour Water Stripper Unit
TO FLARE
HYDROPROCESSING 90 OC
SOUR 0.9 KG/CM2
WATER FEED MIX COOLER SOUR GAS
FROM
HCU/ SOUR WATER
DHDT SURGE DRUM TO SRU
65 OC

HYDROPROCESSING SOUR WATER STRIPPER COLUMN

40 OC HYDROPROCESSING
STRIPPER
CR AIR COOLER
94.5 OC

FEED/BOTTOMS
EXCHANGER
90 OC

65.5 OC
HYDROPROCESSING
STRIPPER
123.6 OC CR PUMP
123.8 OC
40 OC
SWS FEED PUMPS LP STEAM
SOUR WATER
STORAGE TANKS
HYDROPROCESSING
STRIPPED WATER CW STRIPPER REBOILER
TO ETP

HYDROPROCESSING STRIPPED LP CONDENSATE

WATER COOLER 123.8 OC

STRIPPED WATER
TO HCU/DHDT
 ‘S’ is removed by Hydroprocessing

Amine
Off Gas ARU
Absorption

Acid
Gas
S+H2 H2S

SRU

Sour
Sour water SWS Gas
 Sulphur Scenario in Panipat Refinery

Panipat Refinery presently has a total crude processing capacity of 15

Million Metric Tonnes per Annum, comprising 25% sweet crude oil and
balance 75% sour crude oil.

Purpose of Sulphur Recovery Unit

1. To remove various sulphur compounds from the petroleum products and
converting them to elemental sulphur, thereby making them cleaner and
reducing Sulphur Dioxide emissions and meeting environmental norms.
2. Solidification and despatch of this elemental sulphur, which is an
important byproduct of the refinery.

Panipat Refinery has two Sulphur Blocks

PR Sulphur Block, commissioned in 1999, having capacity of 230 MT/day.

PRE Sulphur Block, commissioned in 2006, having capacity of 450MT/day.
PRAE(P-15) commissioned in 2011, having capacity of 225 MT/day.
 Design Features
 Capacity: 3 X 225 MT /day
 Process Licensor : Black & Veatch, USA

 Capacity -2 X115 MT /day

 Licensor - Delta Hudson, Canada

 Feed sources :
 H2S (Acid rich gas )from Amine treating unit
 Stripped Gases from Sour water Stripper
 H2S gas from TGT unit
 PRODUCT SPECIFICATIONS
The product sulphur will meet the following specification after
degassing.
State : liquid sulphur
Colour : bright yellow (as solid state)
Purity : min. 99.9 wt% on dry basis
H2S : 10 ppm weight max
 Claus Section
 Friedrich Claus originally patented the Claus Reaction June 30, 1884.
The original process was based on the direct oxidation of H2S over a
catalyst, using air (oxygen), to form elemental Sulfur and water:

Direct Oxidation of H2S

using Oxygen

ELEMENTAL SULFUR & WATER

CATALYST

H2S + 0.5 O2 →S + H2O Oxidation

 Since the oxidation reaction is extremely exothermic, the control of
this reaction was difficult and Sulfur recovery efficiencies were low.
 The basis of the modern Claus Sulfur Recovery process in two steps:
 Step No.1: thermal step;
 Step No.2: catalytic step.
 PROCESS DESCRIPTION
OF SULFUR PLANT
(CLAUS SECTION)

Process Step Process Section

Claus thermal conversion

Claus catalytic conversion Claus Section

Sulfur vapour condensation

 CLAUS SECTION
Thermal Stage Catalytic Stage

• Acid Gas Separator • 1st & 2nd Sulfur

• Sour Gas Separator Condensers
• Thermal Reactor Burner • 1st & 2nd Claus Reactor
• Thermal Reactor • 1st & 2nd Claus Reactor
• Thermal Reactor Waste Reheaters
Heat Boiler • Final Sulfur Condenser
• Thermal Reactor Waste • LLP Steam Condenser
Heat Boiler Steam Drum
• Combustion Air Blower
(one operating, one
spare)
 Typical Temperature Profile
HP STEAM (42 Kg/cm2)

1400 OC LP STEAM
ACID GAS KOD

322 OC 180 OC
REACTION FURNACE WASTE HEAT BOILER

BFW BFW
IST CONDENSER
SOUR GAS KOD

232 OC 208 OC 2ND CONVERTER

IST CONVERTER
TAIL GAS
TO TGTU
132 OC
HP STEAM
HP STEAM
AIR BLOWER 2ND REHEATER
IST REHEATER LP STEAM
TO INCINERATOR HEATED BFW
287 OC 173 OC
233 OC

DEGASSING
CONTACTOR
2ND CONDENSER BFW BFW
FINAL CONDENSER

BFW

DEGASSED SULPHUR TO YARD

SULPHUR PIT
CHEMICAL REACTIONS
& CATALYSTS
 Typical Recovery System

Plant Feed Gas

Acid Gas {H2S + CO2}

H2S Recycle

Tail Stack
Claus S.R. Gas Incinerator
gas
Treat

Sulphur
 Claus Section
 In the thermal step, the Acid Gas containing H2S is
burnt in the Thermal Reactor where only one third of
H2S has to be oxidized to SO2.

 In the catalytic step the SO2 formed in the combustion

step reacts with the unburned H2S to form elemental
Sulfur.

 The main reactions of the Claus process can be written

as follows:
 H2S + 1.5O2 →H2O + SO2 Oxidation
 2 H2S + SO2 →1.5S2 + 2H2O Conversion
 3 H2S + 1.5O2 →3H2O + 1.5S2 Overall
 Claus Section

 During the H2S combustion, part of H2S is dissociated

to H2 and S depending on the temperature level;
 When CO2, CO or hydrocarbons are present in the
Acid Gas feed, side reaction forming COS (Carbonyl
Sulfide) and CS2 are taking place.

 Some side reactions that might occur in the thermal

stage are shown as follow:
 H2S → H2 + 0.5S2 Dissociation
 CO2 + H2S →COS + H2O COS formation
 CO2 + 2 H2S →CS2 + 2H2O CS2 formation
 Claus Section
 Many chemical reactions in modified Claus plants occur
either in the Thermal Reactor or in the Catalytic Reactors:
the most important is Sulfur vapour species transformations.

 Elemental Sulfur vapour can exist as four separate species,

hence it is important to consider the reaction:
S2 ↔ S4 ↔ S6 ↔ S8 ↔ Sliq

 Most of the Sulfur vapour formed in the Thermal Reactor

exists as S2. As the gas passes through the Waste Heat Boiler
and Condenser and therefore the temperature of the process
gas decreases, the Sulfur shifts partially to S4 and then to
nearly all S6 and S8.
 Claus Section
 The purpose of the Main Burner combustion is to reach a suitable
flame temperature while the purpose of the Thermal Reactor is to
provide additional space at high temperatures in which the desired
reaction can progress to a point as nearly approaching equilibrium as
possible.

 For thermodynamic reasons, the maximum equilibrium conversion of

H2S to Sulfur that can be achieved at high temperature is only about
70%. It is considerably better at lower temperature (exothermic
reaction), but the rate of the equilibrium is much lower and reaction
has to be assisted by a catalyst.

 The Main Burner of the Claus Section can operate in two different
ways:
 Acid Gas combustion (normal operation);
 Natural Gas combustion (start-up/shut-down operation);
 Claus Catalyst
The acceptable value for the CO and O2 content in the flue gas are:
 CO = 0.4% by volume max;
 O2 = 0.1% by volume (expect normal operating value)
 O2 = 0.4% by volume max (not prolonged, maximum figure tolerate by
catalyst).

 Thermal Reactor lining has a max operating design temperature of

about 1450 °C, it is necessary to moderate the flame temperature by
means of quench steam.

 The steam rate is controlled in ratio with the natural gas flow rate.

 The suggested maximum adiabatic temperature in the Thermal

Reactor is 1420 °C; the theoretical quantity of steam to moderate the
flame temperature to adiabatic 1420 °C is about 5-6 kg steam/kg
natural gas
Reaction Furnace
 Reaction Furnace with Waste Heat
Exchanger (Modified Claus Process)

Main Process
Air Stream
Steam
Start-up Refractory lining
Fuel Gas

2nd pass tubes

1st pass tubes

Acid Gas
Ceramic
Burner
Ferrule
Sulphur
Saddle
drain BFW
 Reaction Furnace

 Burner
 Flame scanners, igniters, pilots
 Checker wall
 Refractory lining (90-95% alumina)
 Ceramic ferrules
 Temperature measurement
 Skin temperature
 Rain shield
 Claus Section
 During the Acid Gas combustion, all the combustible components, if
any, contained in the Acid Gas, are burnt according to the following
exothermic reactions:
 H2 + 1/2O2 → H2O
 CH4 + 2O2 →CO2 + 2H2O
 C2H6 + 3.5 O2 →2CO2 + 3H2O
 C2H4 + 3O2 →2CO2 + 2H2O
 C3H8 + 5O2 →3CO2 + 4H2O
 C3H6 + 4.5 O2 →3CO2 + 3H2O
 C4H10 + 6.5 O2 →4CO2 + 5H2O
 The listed reactions are practically fully displaced to the right side. A
little part of hydrocarbons is also partially burnt to CO:
 CH4 + 1.5O2 →CO + 2H2O
 C2H6 + 2.5O2 →2CO + 3H2O
 C3H8 + 3.5O2 →3CO + 4H2O
 CO2 + H2S →COS + H2O ;CO2 + 2 H2S →CS2 + 2H2O
 Natural Gas Combustion

 During this operation mode, the adiabatic flame has to be

maintained below the maximum operating temperature of
refractory material lining the Thermal Reactor.

 Quench steam is used to moderate the flame temperature

at values around 1300 °C (not higher than 1400 °C);

 Excess oxygen, if any, shall react with the Sulfur present in

the plant, particularly over the catalyst, according to the
following reaction:
S + O2 → SO2
 Claus Process – 1st Stage
Reaction Furnace: Expectations

Oxidize all hydrocarbons to CO2 and water.

Oxidize all NH3 to Nitrogen.

Convert 1/3rd of the H2S to SO2.

 Natural Gas Combustion
 If the natural gas combustion is carried out with large
oxygen the hydrocarbons contained in natural gas shall
not be completely burnt and some carbon can get
formed.

 Carbon tends to be adsorbed on the catalytic beds of

Claus reactor and then the catalyst gets fouled and poor
quality Sulfur may be produced.

 Operating with O2 deficiency higher than 5% a certain

part of the methane present in the natural gas shall
react according to the equation:
CH4 + 3/2O2 →CO + 2H2O
 Claus Catalyst

 The catalyst used in the Claus Catalytic Reactors is

Alumina. The First Claus Reactor catalyst bed is a
special type particularly active towards the
hydrolysis of COS and CS2.

 The presence of unreacted oxygen in the gas fed to

catalyst must be avoided.
 Claus Catalyst

 Formation of carbon (caused by the uncompleted

combustion of hydrocarbons present in Acid Gas and in
natural gas) and its deposit on the catalyst shall lead to a
poisoning of the catalyst itself and to the production of
fouled Sulfur

 Checks of catalyst pressure drops are also necessary to

detect any catalyst plugging due to Sulfur condensation,
carbon formation or catalyst thermal degradation.
 Claus Catalyst

Standard (activated alumina)

 Alcoa/Discovery
 Alcan
 Porocel

Special properties
 COS/CS2 hydrolysis / O2 scavenger / deactivation
resistance
 Claus Catalyst
Catalyst ageing:
The ageing of the catalyst is due to thermal and
hydro-thermal causes and to sulphation.

a) Thermal and hydro-thermal ageing

 It is known that alumina transforms and loses
part of its catalytic activity when temperatures
exceed 600 °C. The specific surface reduces then,
and the diameter of micropores increases.

 The use of inert gasses (nitrogen) is

recommended
 Claus Catalyst
b) Sulphate poisoning
When activated alumina is brought into contact with
gaseous SO2, the initial phenomenon corresponds to
an absorption of SO2. A fraction of this SO2 is
actually chemisorbed and irreversibly fixed

The chemisorbed SO2 is not chemically stable and

tends to enter into reaction with superficial hydroxyl
groups, building up bound sulphate groups

At higher temperatures, the sulphate formation is

markedly increased. This reaction causes the rapid
ageing of catalysts.
 Precautionary measures to avoid
damage to Claus catalyst

1. Liquid water damages the catalyst causing it to crumble;

it also damages the reactor refractory and causes
corrosions.
2. 100% steam on the catalyst causes its temporary
deactivation due to water absorption.
3. The presence of a high quantity of oxygen in the process
gas fed to the catalyst (more than 0.4% by volume) may
cause catalyst sulphation, due to the formation of
aluminium sulphate. This shall cause the decrease of the
catalyst activity.
4. The maximum safe working temperature of the catalyst
shall not exceed 400 °C.
 Claus Catalyst
 When Sulfur is present in the plant, the oxygen contained
in flue gas can react with the liquid Sulfur at temperatures
higher than 150 °C producing SO2 and SO3 and causing
very high localised and uncontrolled temperatures.

 SO3 is very dangerous because it generates corrosion in

all the parts of the plant; in addition the formation of SO2
on the Claus catalyst causes the progressive deactivation
from sulphation.
 Claus Catalyst
 Any carbon contained in flue gas produced during
natural gas combustion is harmful at any stage of
operation; carbon tends to deactivate the catalysts
itself because it plugs the catalysts micropores.

 Another effect of catalyst poisoning from carbon is

the high-pressure drop that may reduce the plant
capacity.

 Catalyst fouling by carbon is irreversible and

consequently the fouled catalyst needs to be
replaced.
PROCESS DIAGRAM
 Typical Sulphur Lock and Look Pot

GROUND

Flow of liquid Sulphur

PROCESS VARIABLES
 Claus Section normal operation
with Acid Gas
During the normal operation with Acid Gas, the
following variables are highly important:
 Ratio Air/Acid Gas;
 Thermal Reactor temperature;
 Claus Reactors temperature;
 Mass velocity in Sulfur Condensers;
 Steam pressure;
 Feed composition variations;
 Liquid carry over;
 Pressure and temperature;
 Liquid Sulfur quality and yield.
 Process Variables
1. Ratio Air/Acid Gas:
 The control of the Thermal Reactor is achieved by regulating the ratio
of total air (oxygen) to total Acid Gas (H2S) entering the Plant.
 The air entering in the Main Burner of the Thermal Reactor is
described by the following equation:
(total air to Claus Burner)=(main air)+(trim air)
 The control strategy is based on a feedforward control on the main air
and on a feedback control on the trim air to the Claus Burner.
 The Tail Gas analyser measures the H2S and SO2 concentrations in the
Tail Gas; a ratio 2/1 of H2S /O2 is the stoichiometric relationship
between the reactants that produces the maximum products.
 Unfortunately this ratio is not convenient to use as control signal, since
the ratio is non-linear. For this reason, the Air Demand (AD) concept
has been introduced.
 Process Variables

 The Tail Gas analyser control loop will control only about 10%
of the combustion air to the process.

 The Air/Acid Gas ratio can be fixed by the operator and shall be
used to modify the set point of the main air controller.

 In case the Tail Gas analyser is not on stream, laboratory

analysis of H2S and SO2 in the Claus Tail Gas shall be necessary
to set the correct Air/Acid Gas ratio.

 The plant operation with incorrect H2S /SO2 ratio content in

the Claus Tail Gas would lead to a decrease of Sulfur yield.
 Process Variables
Acid Gas Operation:

 During the Acid Gas operation an increase of the

flame temperature is possible if the content of Acid
Gas increases and/or if more air then necessary is
sent to the Thermal Reactor.

 During the Acid Gas operation in case of presence

of hydrocarbons, the flame temperature is also
considerably higher than normal.
 Process Variables
Reactor parameters:

It is important to analyze different combustion air ratios

during Acid Gas combustion (containing about 90% by
vol. H2S)

 The flame temperature should not be used as a

parameter to adjust the Air/Acid Gas ratio during the
operation with Acid Gas, because it is possible to have
the same flame temperature with different air/gas
ratios.

 The only operational parameter to be used to operate

the plant is the Claus Tail Gas analysis.
 Process Variables
 Mass velocity in Sulfur Condensers

o The Sulfur Condensers have been designed with a minimum of mass

velocity, which prevents any operating problems.
o In case of operation below the safe limits Sulfur fogs can be formed; Sulfur
fogs formation originates problems to the Catalytic Reactors and also
decrease the unit performances.

 Steam pressure
o The produced steam pressure can be modified to implement the cooling or
the heating rate of process gas at boilers outlet.

 IMPORTANT:
During Acid Gas operation, the produced steam pres. has not to be
allowed to drop below 1 barg (corresponding to a steam temp. of 121 °C)
to avoid risks of Sulfur solidification.
 Process Variables
 Feed Composition variations
o Load variations should be as smooth as possible to avoid plant upset
and shut-down.

o Acid Gas composition variations can produce a remarkable effect on

the plant operation and/or on Sulfur Recovery Efficiency.

 Hydrogen sulphide
o The concentration of H2S in the Acid Gas determines the achievable
flame temperature, the concentration of H2S is directly proportional
to the flame temperature.

o Fast fluctuation in H2S composition can create upset reducing the

efficiency of the plant due to delay in the control system reaction.
 Process Variables
 Carbon Dioxide
o Carbon dioxide plays the role of diluting process gas, producing COS and CS2
during the Acid Gas combustion.
o The production rate of COS and CS2 is proportional to the concentration of CO2
and hydrocarbons in the feed Acid Gas.

 Steam
o Steam present in the Acid Gas acts as a diluting agent. The increasing of the rate of
the Acid Gas diluting agents implies, in addition to the Sulfur Recovery Efficiency
decreasing, the lowering of the plant capacity due to the increase of the pressure
drops of the system.

 Hydrocarbons
o Hydrocarbons have several negative effects on the Acid Gas combustion;
1. The difficulty in burning hydrocarbons if they are present in massive quantities,
2. The effect of dilution of the process gas with the consequent Sulfur efficiency
decreasing.
3. A further negative effect is the consumption of combustion air according; it is
important to note that while the H2S combustion requires about 0.5 molO2/mol
H2S, the combustion of hydrocarbons requires:
 molO2/mol CH4
 molO2/mol C2H6
 molO2/mol C3H8
 Process Variables
Hydrocarbons (Continued):

 The Thermal Reactor effluent temperature should be

always higher than 1000 °C to ensure the hydrocarbons
complete destruction.

 In case the hydrocarbons content in the Acid Gas

exceeds the tolerance calculated as per the above
system, some carbon can be formed during the Acid Gas
combustion with the consequence of catalyst plugging
and fouling. In addition, note that the carbon formation
can also be caused by deficiency air operation.
 Inert Gases
N2 Ar

Decreases furnace temperature

Decreases plant capacity

Reduces partial pressure of Claus

reactants
Shifts Claus reaction equilibrium
 CO2
Decreases furnace temperature

Decreases plant capacity

Participates in undesirable side

reactions
CO and COS formation
 Ammonia
Increases air demand
Increases furnace temperature
Decreases plant capacity
Possible downstream contamination and
plugging from ammonia salts
 CO

Formed from CO2

Up to 20%of total inlet carbon
Increases with increasing furnace
temperature
Tends to decrease the furnace temperature
Decreases air demand
Participates in undesirable side reactions
 HYDROCARBON
Increase air demand
Increases furnace temperature
Decreases plant capacity
Possible downstream contamination
Participates in undesirable side reactions
 CS2 formation
 Process Variables

 Liquid carry-over
o KO drums are provided upstream the Thermal Reactor burner with
the purpose to remove all the entrained liquids that are separated
and transferred outside the SRU battery limits.

o The effect of liquid entrained to the burner is it creates disturbance

to the Acid Gas flame and the sudden evaporation in the
combustion chamber damages the reactor refractory lining and
also the corrosion of the Acid Gas piping lines.

o In case of impossibility in removing liquids from the Acid Gas

separators, very high level switches shall shut down the Claus
Section. The Acid Gas line is steam traced, the tracing shall be
maintained in operation in all seasons with the purpose to avoid
condensation and corrosion.
 Process Variables
 Pressure and Temperature
o Variation of the pressure in the Acid Gas feed lines upstream the
unit pressure control valves shall correspond to variations of the
load of the unit.

o Small and smooth variations of the Acid Gas load shall be

automatically compensated by adjusting the necessary quantity
of combustion air.

o Very large throughput variations in a very short period of time

require the operator to re-adjust some operating parameters.

o Acid Gas temperature variations shall not have an appreciable

effect on the system if no composition variations are involved.
 Process Variables
 Liquid Sulfur quality and yield
o Liquid Sulfur is collected in an underground Pit from which it is
transferred to the storage system.

o The Sulfur yield is the most important parameter for the definition of
the acceptability of the plant operation.

o The only impurity, which can be present in noticeable quantity, is

carbon. Carbon can be originated by bad combustion of natural gas
during plant heating-up and during the operation with Acid Gas
containing abnormally high quantities of hydrocarbons or in case of
operation with strong process air deficiency.

o The Sulfur produced in the Claus plant is saturated with H2S and
contains Sulfur polysulphides.
 Typical Operating Conditions
Parameters Normal @ Normal Minimum
design during acceptable
capacity Heat-up
Temperatures Thermal Reactor (1st 1350 1350 1000
zone) °C
First Claus Reactor inlet °C 235 205 200
First Claus Reactor outlet °C 300 205 200
Second Claus Reactor inlet °C 210 205 190
Second Claus Reactor outlet °C 236 200 180
Incinerator °C 750 750 600
Claus tail gas °C 135 140 130
Pressures WHB (steam side) barg 50 50 50
Sulfur condenser (steam side) barg 4.5 4.5 4.5
TAIL GAS
CLEAN-UP
 Tail Gas Treating Unit
 The tail gas from the Claus units is heated by superheated HP
steam and mixed with hydrogen.
 Then passed over a catalyst bed in order to convert any SO2
present in the tail gas back to H2S.
 This gas is then cooled and passed through an Amine absorber,
where H2S is absorbed in MDEA.
 The rich MDEA thus formed is regenerated in the amine
regeneration system inbuilt within the TGTU and returned to the
absorber.
 The acid gas released is sent back to SRU for Sulphur Recovery.
 The Tail gas then goes to the Incinerator, where it is burnt off and
the flue gas goes to the stack.
 The main reaction in the hydrogenation reaction is

SO2 + 3H2 H2S + 2H2O

 Precautionary measures to avoid
damage to hydrogenation catalyst

 The hydrogenation catalyst is subject to spontaneous

combustion in the presence of oxygen.
 During the normal operation the Claus tail gas should
not contain oxygen. During unit shut-down it is
necessary to burn off Sulfur and return the catalyst to
its oxide state. Refer to the shut-down.
 The maximum temperature to avoid damage of the
catalyst is 427 °C.
 The oxygen presence shall be avoided in normal
operation and during nitrogen circulation steps.
 The hydrogen concentration in the process gas shall
not be allowed to exceed 6% by volume.
 Diagram of Tail Gas Treating Unit HP STEAM
HP STEAM
HYDROGEN (42 Kg/cm2) SUPERHEATER
39 OC 39 OC
TAIL GAS FROM SRU
FG 815 OC

316 OC
TGTU AMINE ABSORBER COLUMN
39 OC
SUPERHEATED 286 OC BFW
HP STEAM INCINERATOR

STACK
QUENCH COLUMN
INCINERATOR
AIR BLOWER
LP STEAM
HYDROGENATION
REACTOR 177 OC
333 OC REGENERATOR
122 OC AIR CONDENSER ACID GAS
TO SRU

43 OC 55 OC
70 OC

TGTU AMINE REGENERATOR COLUMN

BFW
AMMONIA

REGENERATOR
REFLUX DRUM
115 OC
LEAN SOLVENT
RICH/LEAN
AMINE COOLER
45 OC SOLVENT EXCH
FILTER REGENERATOR
SYSTEM 51 OC REFLUX PUMP 55 OC
LP STEAM
REGENERATOR
REBOILERS

RICH SOLVENT REGENERATOR LP CONDENSATE

PUMPS BOTTOMS PUMP 132 OC
 Process Description
TGU Hydrogenation Reactor Reactions

SO2 + 3H2 H2S + 2H2O

Other Reactions (All Exothermic)

Hydrolysis Due to the Presence of Steam
COS + H2O  H2S + CO2
CS2 + 2H2O  2H2S + CO2
Very Small Amount of COS and CS2 Reduced by Hydrogen
COS + 4H 2 H2S + CH4 + H2O
CS2 + 4H2  2H2S + CH4
Few ppm of Methane Will Be Combusted in the Incinerator
SCOT Process Fundamentals

Process stages:
Tail gas heating and sulphurous compound
reduction to H2S
Cooling and quenching to near ambient
temperatures
H2S absorption, stripping and recycle
 SCOT (99.8% Recovery)
 Shell Claus Offgas Treatment (SCOT):

SCOT Objective – Convert to H2S & Recycle

Claus
SO2 S6
Offgas Catalyst H2S
S8 COS CS2 Absorb
S8-Fog & Recycle
Incineration
 Incinerator Operation
Purpose :
*To oxidise all sulphur compounds in the tail gas
to SO2

H2S + 3/2 O2  SO2 + H2O

2 COS + 3 O2  2 CO2 + 2SO2
CO + ½ O2  CO2
CS2 + 3 O2  2 SO2 + CO2
Sn + n O2  SO2
 Typical Operating Conditions

Incinerator temperature 600-7500C

(1100-13800F)

Stack exit temperature 2400C

“Excess” oxygen : 2 to 6+ mole% in stack

 Operating Issues
NOx and SO3 formation

Usually a result of poor control of operation

 Excess oxygen much higher than required
 Incinerator temperature much higher than
required
NOx is emissions problem
SO3 is a corrosion problem
 Operating Issues

CO destruction

Usually requires very high temperature and

excess oxygen
Usually results in high NOx and SO3
formation
EMERGENCY
SITUATIONS
 Emergency Shutdown

 Steam Failure
Effects:
a) The heating of liquid Sulfur is not possible;
b) The heating of equipment and piping is not
possible;
c) The regeneration of amine solution is not
possible;
d) The quench of In-line Heater flame is not
possible.
 Emergency Shutdown
 Steam Failure-contd

Consequences:
a) An extended failure of the steam network may cause
the solidification of the Sulfur;
b) Shut-down of the Claus Section; Shut-down of the
TGT Section; Possible shut-down of the Incineration
Section for high temperature; Corrosion problems
since steam heating system is part of corrosion
protection strategy.

When the network re-starts working, all steam traps must

be checked to ensure correct operation.
Boiler feed water and demi-water make-
up failure
 Consequences:
a) Shut-down of the Incineration Section;
b) Shut-down of the Claus Section;
c) Shut-down of the TGT Section;
d) Shut-down of the Sulfur Degassing Section;
e) No production of steam.

 Actions to be taken:
a) The problem must be quickly found to permit an immediate restart;
b) Avoid losing completely the water inside the boiler.

IMPORTANT:
In case of complete loss of water inside the boiler do not re-feel
instantaneously the boiler because this will damage the tubes. Wait that
the tubes are cold then start to fill slowly the boiler with hot BFW.
 Emergency Shutdown
 Natural gas failure
Effects:
a) No flame in the Thermal Incinerator;

Consequences:
a) Shut-down of the Incineration Section;
b) Shut-down of the Claus Section;
c) Shut-down of the TGT Section;
d) No production of steam.

Actions to be taken:
The problem must be quickly found to permit an immediate restart.
 Emergency Shutdown
 Electric Power Failure

Actions to be taken following a Plant trip

 Immediately Plant trip, the trip cause must be found and

corrected as quickly as possible;
 The air, natural gas, acid gas and quench steam manual block
valves at burners shall be closed by the operator; the operator
shall also check that the nitrogen purge valves are open;
 Emergency Shutdown
 The temperatures throughout the Unit shall be observed and
if they increase the Unit shall be purged again with nitrogen;
 The Plant shall be restarted in accordance with the procedure
described in this operating manual;
 If it is decided to keep the Plant down for repairs after a shut-
down, the catalysts must be stripped of Sulfur and the
hydrogenation catalyst must be returned to its oxide form,
following a planned shut-down procedure.

IMPORTANT:
During the shut-down phase, it must be constantly reminded
that a rapid change of temperatures is completely undesirable for
the following reasons:
a. Solidification of Sulfur may occur;
b. Thermal expansion or contraction of the equipment may
deform piping and equipment themselves, causing leaks after a
restart;
c. Refractory lining materials may be damaged.
TROUBLESHOOTING
 TROUBLESHOOTING
 If tail gas analyses indicate a high H2S/SO2 ratio
then insufficient air is reaching the Thermal
Reactor.

 This, in turn, prevents complete hydrocarbon

combustion, which may cause carbon deactivation
of the catalyst beds

 The first step would be to increase the air/acid gas

ratio. If the air/acid gas ratio is greater than
normal, operation of the amine system should be
examined to reduce the hydrocarbon content of the
acid gas.
 TROUBLESHOOTING
 Temperature profiles through the catalyst bed
should be checked periodically if the catalyst needs
changing.

 Even in plants operating smoothly at design

conversion levels, catalyst may have slowly lost
surface area and become deactivated.
 TROUBLESHOOTING
 Steam leaks from Boiler or Condensers can cause
plugging and reduced plant efficiency. Moisture
tends to promote catalyst disintegration

 Boiler leak has probably developed due to:

1) low steam production;
2) low boiler effluent temperature;
 TROUBLESHOOTING
Plugged Seal Legs

 The first indication of a plugged seal leg is

increased pressure drop caused by liquid Sulfur
backing into the Sulfur Condenser outlet channel
and then into the effluent Process Gas line, thus
restricting gas flow area.

 Seal legs may plug due to the presence of

refractory dust, corrosion products, catalyst
particles
OPERATIONAL SAFETY
ASPECTS
 H2S Poisoning

 Hydrogen sulphide is both an irritant and an extremely

poisonous gas.
 Breathing even low concentrations of hydrogen sulphide
(H2S) gas can cause poisoning.
 Many natural and refinery gases contain more than 0.10 mol-
percent H2S.
 The current OSHA permissible exposure limits are 20 mol-
ppm ceiling concentration and 50 mol-ppm peak
concentration for a maximum 10-minute exposure.
 The Sulfur Recovery and Tail Gas Treatment Unit gases
contain H2S. These gases must NEVER be inhaled. Also, the
water in the Separators contains dissolved H2S, which will
flash at atmospheric pressure. NEVER drain this water to an
open sewer.
 H2S Poisoning
 One full breath of high concentration hydrogen sulphide gas
will cause unconsciousness and could cause death,
particularly if the victim falls and remains in the presence of
the H2S.

 The operation of any unit processing gases containing H2S

remains safe provided ordinary precautions are taken and
the poisonous nature of H2S is recognized and understood.

 No work should be undertaken on the unit where there is

danger of breathing H2S, and one should never enter or
remain in an area containing it without wearing a suitable
fresh air mask.
 Acute Hydrogen Sulphide
Poisoning
First Aid Treatment of Acute Hydrogen Sulphide Poisoning:
 Move the victim at once to fresh air. If breathing has not stopped, keep the
victim in fresh air and keep him quiet. If possible, put him to bed.

 Secure a physician and keep the patient quiet and under close observation for
about 48 hours.

 In cases where the victim has become unconscious and breathing has
stopped, artificial respiration must be started at once.

 If other persons are present send one of them for a physician.

 Others should rub the patient's arms and legs and apply hot water bottles,
blankets or other sources of warmth to keep him warm.
 Process Safety Concerns

Exceeding plant designed operating parameters

can lead to any of the following events :

• Toxic gas exposure

• Explosions
• Sulphur Fire
• Sulphur solidification
• Corrosion
• Catalyst damage
• Refractory & Equipment Damage
 DANGER

 H2S is extremely poisonous gas (as toxic as hydrogen

cyanide)
Maxm permissible conc.of H2S in air as per OSHA during
8Hrs working day = 10ppm(v)
short-term exposure limits(10 minutes) = 15 ppmv

FATAL :Conc. Of 500-700 ppm in 30 minutes

 H2S is also a flammable gas

 Explosive limit in air LEL (4%)

 Ensure that water does not enter the condenser seal pots
prior to filling with sulphur. Molten sulphur reacts
violently when it comes in contact with water, creating a
hazardous situation.
 Extinguishing Sulfur Pit fires

1. In case of fire in the Sulfur Pit, steam is used to snuff the

fire. This operation shall be done at a safe distance from
the Pit by opening a steam valve. The steam shall break
the bursting disks and shall enter the Sulfur Pit.

2. When the fire is extinguished the steam valve shall be

closed and a new bursting disk shall be installed.
 Fire Protection
 The spontaneous ignition temperature of Sulfur in air is
about 170 °C. When the Sulfur ignites, it is difficult to
extinguish and it can ignite again spontaneously. In
order to minimize the risk of Sulfur fires, all Sulfur
spillage must be removed as soon as they occur.

 When Sulfur fires are extinguished with water, a white

toxic choking acidic fume is released which is extremely
hazardous and inhalation or contact with the skin
should be avoided.

 Sulfur fire can also be extinguished by smothering with

sand. Carbon dioxide fire extinguishers of steam
smothering may also be used.
 SAFE HANDLING OF SAMPLE
 All samples taken on Sulfur plant are considered to be hazardous and the
sampler must wear suitable breathing apparatus and acid resistant
gloves.
 The equipment used must be gas tight and able to withstand
temperatures up to 180 °C.
Handling of gas samples containing H2S :

1) The acid gases are analysed for H2S, H2, hydrocarbons and ammonia
concentration.
2) The Claus tail gas is analysed to check the H2S and SO2 concentration.
3) The reduced gas is analysed to check the H2S.
4) In order to purge the acid gas sample line, the gas may be passed into a
freshly prepared strong caustic solution or sent to flare; acid gas
sampling sent to atmosphere is not allowed.
5) The sample must be taken in a gas tight container.
6) The sampler must have an assistant at hand in case of emergency.
 Handling of liquid Sulfur
 Sulfur collected from the Sulfur Pit or Sulfur
Hydraulic Seals outlet will be hot, about 140 °C
and care must be taken to avoid burns and contact
with the skin. When taking a sample from the
Sulfur Pit a rod with a cup on the end can be used.

 When standing on top of the pit taking a sample

through the inspection hole, the sampler must
wear an anti-gas mask with suitable filter for H2S
protection.
Sulphur - Applications
COMMERCIAL USES OF SULPHUR
 Commercial Uses of Sulphur

Sulphuric Acid – largest chemical produced in the world by tonnage –

200mln tpy

Fertilizers

Making tyres

Vulcanisation of rubber

Black gun powder

 Commercial Uses of Sulphur
Detergents – Cleansing Agent

Medicinal Usage

Matches

Adhesives

Waste Water Processing

Sugar refining

Burnt Sulphur Powder used in dry fruits preservation

Glow Painting
SULPHUR LOADING AT YARD
SULPHUR LOADING AT YARD
Thank you !!