1. HYDROPROCESSING PROJECT PROCESS THEORY
KHABAROVSK REFINERY
HYDROPROCESSING PROJECT
PROCESS THEORY
TRAINING COURSE
APRIL 29th – MAY 3rd 2013, MADRID, SPAIN

2. CONCEPTUAL BLOCK DIAGRAM

3. CLAUS PROCESS DESCRIPTION
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.
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.

4. FEED STREAMS TO CLAUS SECTION
PROCESS STREAMS
CHEMICAL SPECIES
AMINE ACID GAS (AG): Hydrogen sulphide
Hydrocarbons
Water
SWS SOUR GAS (SWS): Hydrogen sulphide
Ammonia
COMBUSTION AIR: Oxygen
Inerts
AMINE ACID GAS
SOUR WATER STRIPPER ACID GAS
Composition
H2O
% mol
8.41
H2S
91.37
NH3
0.08
CH4
0.02
C3H8
0.04 H2
Total
100
Composition
H2O
% mol
27.68
H2S
36.38
NH3
35.94
CH4
0.00
C3H8 H2
Total
100.00

5. Main Oxidation H2S + 1.5O2 H2O + SO2
CLAUS REACTIONS
MAIN REACTIONS
THERMAL STEP
Main Oxidation H2S + 1.5O2 H2O + SO2
CATALYTIC STEP
Conversion 2H2S + SO2 1.5S2 + 2H2O
 Overall REACTION
Total Balance 3H2S + 1.5O2 3H2O+ 1.5S2

6. AMMONIA DESTRUCTION NH3 decomposition 2NH3+ 3/2O2  N2+ 3H2O
CLAUS REACTIONS
AMMONIA DESTRUCTION
NH3 decomposition
2NH3+ 3/2O2  N2+ 3H2O
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).

7. CLAUS REACTIONS SIDE REACTIONS
The following side reactions can occur in the thermal step:
H2S  H2+ 0.5S 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

8. CLAUS REACTIONS SIDE REACTIONS COS AND CS2 FORMATION CO2 +H2S COS+H2O
PROCESS GAS FROM WHB
1.94
27.11
0.03
53.41
0.00
6.04
3.02
0.01
8.43
Stream Description
Component %mol H2
H2O CO N2 O2
CO2
H2S
SO2
COS
CS2
CH4
C2H6
C3H8
i-C4H10
S2-vap
S4-vap
S6-vap
S8-vap
S1-LIQ
NH3
n-C4H10
i-C5H12
n-C5H12
n-C6H14
MDEA
SIDE REACTIONS
COS AND CS2 FORMATION
CO2 +H2S COS+H2O
CO2 +2H2S CS2+2H2O
COS AND CS2 FORMATION DEPENDS ON
CO2 CONTENT AND HYDROCARBONS CONTENT,
WHICH ARE INCLUDED IN THE PROCESS GAS
FED TO THE CLAUS THERMAL REACTOR.

9. ELEMENTAL SULPHUR SPECIES
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.
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

10. INSIGHT ON CLAUS PROCESS - 1
OVERALL REACTION: H2S + 1.5O2 3H2O+ 1.5S2
MAIN OBJECTIVE: TO DRIVE THE OVERALL REACTION TO NEAR COMPLETION
IMPORTANT
PARAMETERS
REACTANTS RATIO
IN THE CATALYTIC STAGE
H2S/SO2=2
MINIMUM FLAME TEMPERATURE
TO HAVE STABLE COMBUSTION
AND COMPLETE AMMONIA DESTRUCTION

11. 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

12. INSIGHT ON CLAUS PROCESS - 2
REACTANTS RATIO
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
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.

13. INSIGHT ON CLAUS PROCESS - 3
COMBUSTION AIR CONTROL SYSTEM
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

14. INSIGHT ON CLAUS PROCESS - 4
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
The minimum temperature that guarantees a stable flame inside the Thermal Reactor and complete ammonia destruction is 1420°C.
The minimum adiabatic flame temperatures to achieve flame stability and impurities
destruction in a Claus burner are summarized here below: HC
NH3
Flame Temp. (°C)
900
1000
1100
1200
1300
1400
1500
BTX

15. INSIGHT ON CLAUS PROCESS - 5
H2S/SO2 in process gas at
Flame adiabatic
at Thermal Reactor outlet
temperature, °C
2.0
about 1330
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:
2NH O2  N2 + 3H2O
AMMONIA DESTRUCTION
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.
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.

16. CLAUS SECTION OVERVIEW
AAG
THERMAL STEP
CATALYTIC
TAIL GAS
STEP
1450°C
H2S/SO2=2
LIQUID
SWS
SULPHUR
FLOW RATE IN ORDER TO HAVE
H2S/SO2 =2 IN TAIL GAS
COMBUSTION AIR
Stream Description
Component %mol H2
H2O CO N2 O2
CO2
H2S
SO2
TAIL GAS
2.18
36.59
0.03
60.04
0.00
0.01
0.68
0.34

17. CLAUS CATALYTIC STEP PURPOSE CLAUS CATALYTIC STAGE 1st Claus Reactor
To drive the Claus conversion reaction to near
completion producing liquid sulphur.
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 CATALYTIC STAGE
CLAUS CONVERSION OF H2S AND SO2 TO SULPHUR
HYDROLISIS REACTION OF COS AND CS2
1st Claus Reactor
2nd Claus Reactor
CLAUS CONVERSION OF H2S AND SO2 TO SULPHUR

18. 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

19. CLAUS REACTION 2H2S + SO2  2H2O + 3/x Sx + 557 kcal/Nm3 of H2S
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.

20. HYDROLISIS REACTION in 1st CLAUS REACTOR
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
HYDROLYSIS REACTION
COS + H2O => CO2 + H2S
CS2 + 2H2O => CO2 + 2H2S
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.

21. 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

22. SULPHUR DEGASSING STEP
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’.
SAFETY
The presence of H2S and H2SX during sulphur transport and handling represents a danger for safety and environmental problem
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

23. SULPHUR DEGASSING STEP
LIQUID SULPHUR CONTAINS H2S AND H2SX DISSOLVED:
TOXICITY
EXPLOSION HAZARD
NECESSITY OF LIQUID SULPHUR DEGASSING PACKAGE.
SAFETY VALUE: 10 PPM WT.
Air stripping to sweep H2S from liquid sulphur.
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)
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.

24. SULPHUR DEGASSING STEP
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.

25. 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

26. TGT SECTION FORMATION OF H2S, H2O FORMATION OF H2S, CO2
Sulphur Dioxide and sulphur vapours are reduced to H2S :
SO2 + 3H2  H2S + 2H2O (SO2 reduction)
Sx + xH2  xH2S (Svapor reduction)
HYDROGENATION REACTIONS
FORMATION OF H2S, H2O
 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

27. TAIL GAS REDUCTION
PREPARATION OF THE FEED TO THE HYDROGENATION REACTOR
To 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
HIGH CONVERSION
SPECIAL CATALYST
FOR BOTH REACTIONS.
CATALYST ACTIVE AT 280/330°C
(SOR/EOR conditions)
MINIMUM INLET TEMPERATURE
TGT HEATER
 Hydrogen injection is expected upstream of TGT hydrogenation reactor.
 The TGT Heater will allow the preheating of the feed to the reduction reactor.

28. TAIL GAS FROM CLAUS SECTION HYDROGENATION REACTOR
TAIL GAS COOLING STAGE
To cool the Tail Gas before feeding it to the TGT absorber, since absorption is carried out at low temperature
PURPOSE
TGT HEATER
TAIL GAS FROM CLAUS SECTION
INDIRECT COOLING
The Tail Gas is firstly cooled down in the TGT Gas/Gas Exchanger.
TAIL GAS COOLING
HYDROGENATION REACTOR
DIRECT COOLING
The 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
QUENCH TOWER
QUENCH WATER
TAIL GAS TO ABSORBER

29. ABSORPTION STAGE
H2S, CO2 IN SOLUTION (aqueous medium) dissociate to form
AMINES IN SOLUTION (aqueous medium) dissociate to form
FORMATION OF WEAK ACID
FORMATION OF WEAK BASIS
H2S H2O  H3O HS-
CO H2O  H HCO3-
[AMINE] H2O  OH [AMINE]H+ 
PRINCIPLE OF ABSORPTION:
WEAK ACID + WEAK BASIS
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.
FORMATION OF SALT BY CHEMICAL COMBINATION OF ACID/BASE WITH REMOVAL OF ACID COMPOUNDS

30. ABSORPTION STAGE ISTANTANEOUS REACTION H2S ABSORPTION
HIGH RATE REACTION FOR PRIMARY AND SECONDARY AMINES (MEA, DEA)
CO2 ABSORPTION
LOW RATE REACTION FOR TERTIARY AMINES (MDEA)
The most commonly used in the industrial processes are
  Primary Amine RNH MEA (Monoethanolamine)
Secondary Amine R2NH DEA (Diethanolamine)
Tertiary Amine R2NCH MDEA (Methil diethanolamine)
 where R= CH2CH2OH

31. SELECTION OF THE RIGHT SOLVENT
ABSORPTION STAGE
SELECTION OF THE RIGHT SOLVENT
PRIMARY  SECONDARY  TERTIARY
 Selectivity for H2S respect to CO2 increases
CO2 Absorption with MEA or DEA
CO [AMINE] [AMINE] [AMINE]CO2-
CO2 Absorption with MDEA (or tertiary amine) is not direct
CO2+H2O+ R2NCH HCO3- + R2NHCH3+
Low T
High T
Low T
High T

32. ABSORPTION STAGE MDEA, WHY? TAIL GAS FROM CLAUS SECTION:
LOW H2S CONCENTRATION
TAIL GAS FROM QUENCH TOWER
3.06
5.90
0.00
88.92
0.06
2.06
Stream Description
Component %mol H2
H2O CO N2 O2
CO2
H2S
SELECTIVE ABSORPTION favoured by MDEA
MDEA,
WHY?
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.
Stream Description
Component %mol H2
H2O CO N2 O2
CO2
H2S
TAIL GAS FROM TGT ABSORBER
3.12
5.98
0.00
90.80
0.06
0.03

33. ABSORPTION - REGENERATION STAGE
The absorption reactions are EQUILIBRIUM REACTION:
The direct reaction is favoured at LOW temperature
The reverse reaction is favoured at HIGH temperature
ABSORPTION AT LOW TEMPERATURE
DESORPTION AT HIGH TEMPERATURE
AMINE PROCESSES ARE REGENERATIVE
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).

34. 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.

35. INCINERATION SECTION THERMAL OXIDATION POSSIBLE REACTIONS
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 O2  H2O + SO2
COS O2  CO2 + SO2
CS O2  CO2 + 2SO2
SO O2  SO3
Oxidation reactions, not regarding S-compounds, are:
H O2  H2O
CO O2  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

36. REGENERATION (ARU) SECTION
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).
ABSORPTION AT LOW TEMPERATURE
DESORPTION AT HIGH TEMPERATURE
AMINE PROCESSES ARE REGENERATIVE
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).

37. SW STRIPPING (SWS) SECTION
 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.
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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