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KHABAROVSK REFINERYHYDROPROCESSING PROJECT

PROCESS THEORY

APRIL 29th – MAY 3rd 2013, MADRID, SPAIN

TRAINING COURSE

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CONCEPTUAL BLOCK DIAGRAM

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CLAUS PROCESS DESCRIPTION

CLAUS PROCESS

Modified Claus Sulphur Recovery Process foresees two process steps: Step No.1, thermal step:

The acid gas is burnt in the thermal reactor where only one third of H2S has to be oxidised (substoichiometric conditions) to SO2.

Ammonia (NH3) in the SWS sour gas is burnt almost completely to N2.

Step No.2, catalytic step:

The SO2 formed in the combustion step reacts with the unburned H2S to form elemental sulphur and water.

• PURPOSE OF CLAUS PROCESS:

To remove hydrogen sulphide and sulphur compounds from acid gas, producing elemental sulphur.

As a second effect, NH3 content of the SWS stream will be highly reduced.

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FEED STREAMS TO CLAUS SECTION

AMINE ACID GAS (AG): Hydrogen sulphide Hydrocarbons Water

SWS SOUR GAS (SWS): Hydrogen sulphide

AmmoniaWater

COMBUSTION AIR: Oxygen Inerts

PROCESS STREAMS CHEMICAL SPECIES

CompositionH2O % mol 8.41H2S % mol 91.37NH3 % mol 0.08CH4 % mol 0.02C3H8 % mol 0.04H2 % mol 0.08

Total % mol 100

AMINE ACID GAS SOUR WATER STRIPPER ACID GAS

Composition

H2O % mol 27.68H2S % mol 36.38NH3 % mol 35.94CH4 % mol 0.00C3H8 % mol 0.00H2 % mol 0.00

Total % mol 100.00

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CLAUS REACTIONS

THERMAL STEP

Main Oxidation H2S + 1.5O2 H2O + SO2

CATALYTIC STEP

Conversion 2H2S + SO2 1.5S2 + 2H2O

MAIN REACTIONS

Overall REACTION

Total Balance 3H2S + 1.5O2 3H2O+ 1.5S2

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CLAUS REACTIONS

NH3 decomposition 2NH3+ 3/2O2 N2+ 3H2O

AMMONIA DESTRUCTION

Ammonia (NH3) in the SWS sour gas is burnt almost completely to N2.

Incomplete destruction of ammonia in the reaction furnace can lead to the formation of ammonium salts in cooler downstream part of the Unit (plugging).

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SIDE REACTIONS

The following side reactions can occur in the thermal step:

H2S H2+ 0.5S2 H2S Dissociation

CnH2n+2 + O2 nCO + (n+1)H2O CO Formation/HC combustion

CnH2n+2 + O2 nCO2 + (n+1)H2O CO2 Formation/HC combustion

CO2 + H2S COS + H2O COS Formation

CO2 + 2H2S CS2+ 2H2O CS2 Formation

CLAUS REACTIONS

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SIDE REACTIONS

COS AND CS2 FORMATION DEPENDS ON CO2 CONTENT AND HYDROCARBONS CONTENT, WHICH ARE INCLUDED IN THE PROCESS GAS FED TO THE CLAUS THERMAL REACTOR.

COS AND CS2 FORMATION

CO2 +H2S COS+H2O

CO2 +2H2S CS2+2H2O

CLAUS REACTIONS

PROCESS GAS FROM WHB

1.9427.110.0353.410.000.006.043.020.010.000.000.000.000.008.430.000.000.000.000.000.000.000.000.000.00

Stream DescriptionComponent %mol H2

H2OCON2

O2 CO2

H2SSO2 COSCS2

CH4 C2H6 C3H8 i-C4H10 S2-vapS4-vapS6-vapS8-vapS1-LIQ

NH3 n-C4H10

i-C5H12

n-C5H12

n-C6H14

MDEA

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ELEMENTAL SULPHUR SPECIES

Elemental sulphur vapour can exist as four separate species, hence it is important to consider the reactions:

S2 S4

S4 S6

S6 S8

S8 Sliq

Most of the sulphur vapour formed in the thermal reactor exists as S2. As the temperature of the process gas decreases, the sulphur shifts partially to S4 and then to nearly all S6 and S8.

HIGH TEMPERATURE LOW TEMPERATURE

S2 S4 S6 S8

The liquefaction of sulphur – which is produced in thermal reactor and Claus reactors- is performed in the sulphur condensers, from

where it is separated by means of hydraulic seals.

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INSIGHT ON CLAUS PROCESS - 1

OVERALL REACTION: 3H2S + 1.5O2 3H2O+ 1.5S2

IMPORTANT PARAMETERS

REACTANTS RATIO IN THE CATALYTIC STAGEH2S/SO2=2

MINIMUM FLAME TEMPERATURETO HAVE STABLE COMBUSTION AND COMPLETE AMMONIA DESTRUCTION

MAIN OBJECTIVE: TO DRIVE THE OVERALL REACTION TO NEAR COMPLETION

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CLAUS THERMAL STEP - overview

The conversion of the oxidation reaction is affected not only by the reactants’ ratio and by the temperature, but also by the residence time.

CLAUS BURNER: To reach a suitable temperature. CLAUS THERMAL REACTOR: To provide the proper residence time at high temperature in order to obtain the desired conversion. CLAUS BURNER CONTROL SYSTEM:To ensure that the reactants are in the proper ratio.

WASTE HEAT RECOVERY (CLAUS BOILER)To recover the heat available in the process gas from the thermal reactor and to produce steam.

HEAT RECOVERY

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The combustion of AAG and SWS AG must be carried out with the proper amount of oxygen in order to obtain a ratio of H2S/SO2=2 in the tail gas from the Claus section. COMBUSTION CONTROL SYSTEM

REACTANTS RATIO

The amount of oxygen to be fed to the Claus Thermal Reactor is evaluated and controlled by DCS facilities.

DCS facilities have been foreseen to perform:

substoichiometric combustion of H2S fed to the thermal reactor H2S/SO2=2 in the tail gas from the Claus Section.

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In the Feed-forward part:

the required quantity of air is calculated by measuring the individual acid gas flows and

multiplying these flows with their required ratios air/acid gas;

the resulting air demand signal sets the flow control system in the main air line supply,

through main control valve

In the Feed-back part:

the flow control system is adjusted by the H2S/SO2 analyzer controller located in the Claus

tail gas line;

the feed back control ensures an H2S/SO2 ratio equal to 2 in the tail gas, in order to obtain

the optimum sulphur recovery efficiency of the unit


COMBUSTION AIR CONTROL SYSTEM

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The stability of the combustion in the Thermal reactor is strongly dependent on the temperature of the flame.

The flame temperature depends mainly on the composition of the acid gas: the higher is the concentration of H2S, the higher is the temperature of the flame.

COMBUSTION STABILITY

1000 1000 1100 1100 1200 1200 1300 1300

HCHC

1400 1400 1500 1500 900900

NH3 NH3

BTXBTX

Flame Temp. (°C)

The minimum adiabatic flame temperatures to achieve flame stability and impurities destruction in a Claus burner are summarized here below:


The minimum temperature that guarantees a stable flame inside the Thermal Reactor and complete ammonia destruction is 1420°C.

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Temperature of the Claus Thermal Reactor 1st zone shall be kept at 1450°C in order to have the almost complete Ammonia destruction (to be burnt as N2).

The oxidation reaction of Ammonia is:2NH3 + 1.5O2 N2 + 3H2O

AMMONIA DESTRUCTION

This is due to the fact that the same amount of air finds a minor amount of Amine AG in the 1st zone and leads to the same SO2 but within less flowrate (1st zone higher temperature). The final temperature after the 2nd zone does not depend on the bypass ratio.

The temperature control of Claus Thermal Reactor 1st zone is achieved by means of a partial bypass of the Amine AG from the 1st to the 2nd zone.

The more the bypass, the more the exothermic reactions in the first zone will proceed, thus causing an increase of temperature in the first zone.


H2S/SO2 in process gas at Flame adiabatic

at Thermal Reactor outlet temperature, °C

2.0 about 1330

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CLAUS SECTION OVERVIEW

AAG

THERMAL STEP

1450°C

CATALYTICSTEP

LIQUID SULPHUR

TAIL GAS

H2S/SO2=2

FLOW RATE IN ORDER TO HAVE H2S/SO2 =2 IN TAIL GAS

COMBUSTION AIR

SWS

Stream Description

Component %molH2

H2O

CON2

O2 CO2

H2SSO2

TAIL GAS

2.18

36.59

0.03

60.04

0.00

0.01

0.68

0.34

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CLAUS CATALYTIC STEP

PURPOSE To drive the Claus conversion reaction to near completion producing liquid sulphur.

CLAUS CATALYTIC STAGE

The Claus catalytic conversion is performed in 2 stages (two reactors and one shell), each stage includes: process gas preheating

catalytic reaction

sulphur condensation

CLAUS CONVERSION OF H2S AND SO2 TO SULPHUR

HYDROLISIS REACTION OF COS AND CS2

CLAUS CONVERSION OF H2S AND SO2 TO SULPHUR

1st Claus Reactor

2nd Claus Reactor

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CLAUS CATALYST ARRANGEMENT

1ST CLAUS REACTOR

The typical arrangement for the 1st Claus Reactor is a double catalytic bed, where the top two-thirds of the bed is the Claus reaction catalyst (Activated Alumina).

The hydrolysis catalyst is placed at the bottom layer (one-third) of the 1st Claus Reactor, where the best temperature for the hydrolysis is achieved. OPERATING TEMPERATURES TIN ~240 °C, TOUT ~306 °C

In the 2nd Claus Reactor there is only the Claus catalyst.

OPERATING TEMPERATURES TIN ~206 °C, TOUT ~228 °C

2ND CLAUS REACTOR

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EQUILIBRIUM REACTION The conversion of H2S and SO2 to elemental sulphur is an equilibrium reaction.

CLAUS CATALYST The Claus reaction is supported by a specific Alumina Catalyst.

EXOTHERMIC REACTION The reaction is exothermic, it’s favored at low temperature: there is an increase of temperature through the catalytic bed.

2H2S + SO2 2H2O + 3/x Sx + 557 kcal/Nm3 of H2S

CLAUS REACTION

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HYDROLISIS REACTION in 1st CLAUS REACTOR

HYDROLYSIS REACTION

COS and CS2 react with water to form Hydrogen Sulphide and Carbon Dioxide.

It’s an exothermic reaction, so it is thermodynamically favoured by low temperature

CATALYTIC REACTION The reaction yield is enhanced by a special catalyst (titania catalyst) which promotes the hydrolysis of COS and CS2 at high temperature.

HIGH CONVERSION The reaction, if performed on the special catalyst at high temperature, is practically complete.

COS + H2O => CO2 + H2S

CS2 + 2H2O => CO2 + 2H2S

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CLAUS REACTORS TEMPERATURE

The reactor inlet temperatures are automatically controlled by acting on the

Hot gas by-pass for the first Claus Reactor and on the Claus Heater for the

second Claus Reactor.

The temperature must be higher than the dew point temperature in order to avoid Sulphur condensation in the catalytic bed with temporary catalyst deactivation.

OPERATING TEMPERATURE

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SULPHUR DEGASSING STEP

SAFETY The presence of H2S and H2SX during sulphur transport and handling represents a danger for safety and environmental problem

Liquid sulphur produced from the sulphur recovery unit contains about 300 ppmw H2S, part simply as dissolved H2S and part in the form of polysulphides (H2Sx). The combination of sulphur atoms and H2S is called ‘polysulphide’.

Cooling and agitation of the sulphur accelerate the release of H2S, and often occur during storage, loading, and transport of the sulphur. As H2S is released, an explosive mixture of air and H2S may be formed.

Necessity to degas the liquid sulphur to reduce H2S content to a safety value of 10 ppm wt. (to remove the dissolved hydrogen sulphide and hydrogen polysulphide from the liquid sulphur).

DEGASSING STEP

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Below 120°C the margin between operating and sulphur solidification

temperature of 115°C would become too small.

Above 155°C degassing is less effective due to the increased sulphur viscosity.


The Degassin Process removes H2S from sulphur through two mechanisms. Some of the H2S and H2Sx are oxidized to sulphur, some is oxidized to SO2, and some H2S is stripped from the sulphur.

H2Sx => H2S + S(x-1)

LIQUID SULPHUR CONTAINS H2S AND H2SX DISSOLVED:TOXICITYEXPLOSION HAZARD

NECESSITY OF LIQUID SULPHUR DEGASSING PACKAGE.SAFETY VALUE: 10 PPM WT.

Air stripping to sweep H2S from liquid sulphur.

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Degassing is achieved inside the Sulphur Pit by means of a packed tower where liquid sulphur is contacted with air stream. The Stripping Air to the Degassing Column package is sent from the Combustion Air Blower.

Liquid Sulphur and process air flow through the DEGASSING COLUMN filled with packing. Air is fed to the bottom part of the contactor by means of a distributor to ensure a good mixing with flowing sulphur.

The overhead gas is sent to the Incinerator.


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TGT SECTION

PURPOSE:

To reduce all sulphur compounds in the tail gas from Claus Section into H2S by the reducing action of Hydrogen

To absorb the residual H2S from the Tail Gas with Amines

To recycle the acid gas obtained by amine regeneration to the Claus section

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TGT SECTION

HYDROGENATION REACTIONS Sulphur Dioxide and sulphur vapours are reduced to H2S :

SO2 + 3H2 H2S + 2H2O (SO2 reduction)

Sx + xH2 xH2S (Svapor reduction)

CO, COS and CS2 react with water as the following:

CO + H2O H2 + CO2 (CO shift)

COS + H2O H2S+ CO2 (COS hydrolysis)

CS2 + 2H2O 2H2S + CO2 (CS2 hydrolysis)

HYDROLISIS REACTIONS

FORMATION OF H2S, CO2

FORMATION OF H2S, H2O

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TAIL GAS REDUCTION

HIGH CONVERSIONTo obtain a high conversion of both hydrogenation and hydrolysis reaction, two main parameters are important:

The presence of reducing reactant (H2) in the process gas fed to the reactor

Minimum inlet temperature to activate the catalyst

PREPARATION OF THE FEED TO THE HYDROGENATION REACTOR

Hydrogen injection is expected upstream of TGT hydrogenation reactor.

MINIMUM INLET TEMPERATURE

TGT HEATER

The TGT Heater will allow the preheating of the feed to the reduction reactor.

SPECIAL CATALYST FOR BOTH REACTIONS. CATALYST ACTIVE AT

280/330°C (SOR/EOR conditions)

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TAIL GAS COOLING STAGE

TAIL GAS TO ABSORBER

TAIL GAS FROM CLAUS SECTION

QUENCH WATERQUENCH TOWER

HYDROGENATION REACTOR

TGT HEATER

To cool the Tail Gas before feeding it to the TGT absorber, since absorption is carried out at low temperature

PURPOSE

INDIRECT COOLINGThe Tail Gas is firstly cooled down in the TGT Gas/Gas Exchanger.

TAIL GAS COOLING

DIRECT COOLINGThe Tail Gas is contacted with quench water with the purpose to saturate the tail gas and then to cool the tail gas. During the cooling step, heat and condense of sour water are removed from the system.

TGT GAS/GAS EXCHANGER

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ABSORPTION STAGE

AMINES IN SOLUTION (aqueous medium) dissociate to form

FORMATION OF WEAK BASIS

H2S, CO2 IN SOLUTION (aqueous medium) dissociate to form

FORMATION OF WEAK ACID

WEAK ACID + WEAK BASIS

FORMATION OF SALT BY CHEMICAL COMBINATION OF ACID/BASE WITH REMOVAL OF ACID COMPOUNDS

PRINCIPLE OF ABSORPTION:

H2S + H2O H3O+ + HS-

CO2 + H2O H+ + HCO3-[AMINE] + H2O OH- + [AMINE]H+

The absorption of acid compounds within amine solution is ensured by the correct inlet temperature of the amine coming from regeneration and correct MDEA %wt.

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ABSORPTION STAGE

H2S ABSORPTION ISTANTANEOUS REACTION

CO2 ABSORPTION

HIGH RATE REACTION FOR PRIMARY AND SECONDARY

AMINES (MEA, DEA)

LOW RATE REACTION FOR TERTIARY AMINES (MDEA)

The most commonly used in the industrial processes are Primary Amine RNH2 MEA (Monoethanolamine) Secondary Amine R2NH DEA (Diethanolamine) Tertiary Amine R2NCH3 MDEA (Methil diethanolamine) where R= CH2CH2OH

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ABSORPTION STAGE

PRIMARY SECONDARY TERTIARY Selectivity for H2S respect to CO2 increases

CO2 Absorption with MEA or DEA

CO2 + 2 [AMINE] [AMINE] + + [AMINE]CO2-

CO2 Absorption with MDEA (or tertiary amine) is not direct

CO2+H2O+ R2NCH3 HCO3- + R2NHCH3

+

Low THigh T

Low T

High T

SELECTION OF THE RIGHT SOLVENT

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ABSORPTION STAGE

TAIL GAS FROM CLAUS SECTION:LOW H2S CONCENTRATION

SELECTIVE ABSORPTION favoured by MDEA

High concentration of CO2 with reference to H2S concentration

Necessity of SELECTIVE ABSORPTION

MDEA: TERTIARY AMINE HIGHLY SELECTIVE TOWARD H2S

MDEA solution (50%) has been adopted for TGT section.

MDEA,WHY?

TAIL GAS FROM QUENCH TOWER

3.06

5.90

0.00

88.92

0.00

0.06

2.06

Stream Description

Component %mol

H2

H2O

CO

N2

O2

CO2

H2S

TAIL GAS FROM TGT ABSORBER

3.12

5.98

0.00

90.80

0.00

0.06

0.03

Stream Description

Component %mol

H2

H2O

CO

N2

O2

CO2

H2S

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ABSORPTION AT LOW TEMPERATURE

DESORPTION AT HIGH TEMPERATURE

AMINE PROCESSES ARE REGENERATIVE

The absorption reactions are EQUILIBRIUM REACTION:

The direct reaction is favoured at LOW temperature

The reverse reaction is favoured at HIGH temperature

ABSORPTION - REGENERATION STAGE

The desorption of acid compounds from amine solution is realised by mean of heat input: the stripping of the rich amine solution is ensured with the amine vapours produced in the Regenerator Reboiler (LPS used to heat amine solution and generate vapour).

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INCINERATION SECTION

PURPOSE To transform all the Sulphur compounds contained in the tail gas to SO2

To discharge the flue gas to the atmosphere via a stack

THERMAL OXIDATION WITH EXCESS OF OXYGEN

LOW OXYGEN CONTENT WILL NOT FAVOUR THERMAL OXIDATION TO SO2

HIGH OXYGEN CONTENT WILL FAVOUR SO3 AND NOX FORMATION.

BEST COMPROMISE

OXYGEN EXCESS IN THE FLUE GAS FROM THE STACK

~2% VOL (WET BASIS) MIN.

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THERMAL OXIDATION

All Sulphur compounds will be transformed to SO2 by thermal oxidation at high temperature using excess of oxygen.

POSSIBLE REACTIONS

The possible reactions in the Thermal Incinerator are:S + O2 SO2

H2S + 1.5 O2 H2O + SO2

COS + 1.5 O2 CO2 + SO2

CS2 + 3 O2 CO2 + 2SO2

SO2 + 0.5O2 SO3

Oxidation reactions, not regarding S-compounds, are:H2 + 0.5 O2 H2OCO + 0.5O2 CO2 and the complete oxidation of fuel gas.

OPERATING CONDITIONS

Oxygen excess required 2 vol.% min; Operating temperature 650°C (normal condition);Fuel gas sustaining combustion

INCINERATION SECTION

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ABSORPTION AT LOW TEMPERATURE

DESORPTION AT HIGH TEMPERATURE

AMINE PROCESSES ARE REGENERATIVE

All the reactions involved are EQUILIBRIUM REACTIONS:

The direct reaction (absorption), being an exothermic reaction, is favoured at LOW temperature

The reverse reaction (regeneration) is favoured at HIGH temperature and occurs at the boiling point of the solution in the stripping column (Regenerator).

REGENERATION (ARU) SECTION

The desorption of H2S from amine solution (DEA at 25 %wt) is realised by mean of heat input: the stripping of the rich amine solution is ensured with the amine vapours produced in the Regenerator Reboiler (LPS used to heat amine solution and generate vapour).

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H2S and NH3 are soluble in water.

The solubilisation of H2S and NH3 in water is favoured at LOW temperature

The separation is favoured at HIGH temperature.

SW STRIPPING (SWS) SECTION

The separation of H2S and NH3 from sour water solution is realised in the Sour Water Stripper by mean of heat input: the stripping steam is produced in the Stripper Reboiler (LPS is used to heat the sour water and generate vapour).

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