Sulfur+recovery (1)

‫شرکت راه اندا زی و بهره ربداری صناعی نفت( ایکو )‬

‫‪Common course‬‬ ‫)‪(process‬‬ ‫‪Sulfur recovery‬‬

‫تح ق‬ ‫مدرییت آموزش‪ ،‬یق و توسعو‬ ‫شهریور‪1391‬‬

Sulfur recovery

Objectives: Upon completion of the unit the trainees should be able to:  Explain classical Claus process steps.  Describe the effect of pressure, temperature and catalyst on the process plant efficiency.  Know the main equipment and systems of the Claus plant.

Contents: 1. General introduction 1.1. Function of the unit 1.2. Battery limits of the unit 1.3. Plant capacity and inter connecting 1.4. Feed stock characteristics and capacity 1.5. Properties of H2S and sulfur 2. General description 3. Unit description 3.1. Claus section 3.2. Incineration section 3.3. Water and steam system 3.4. Sulfur degassing and storage section.

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Sulfur recovery

SULFUR RECOVERY-CLAUS PROCESS 1. GENERAL INTRODUCTION These documents are extracts from the process package prepared by Elf ep and Lurgi Bamag according to the design basis for sulfur recovery units for the South Pars field development phases 2 & 3, onshore facilities. The process package concerns the Claus units to be installed at Assaluyeh Refinery for recovery of sulfur from acid gas produced by MDEA sweetening units. The sulfur facilities of each phase consist of two identical trains, with a capacity of 103 t/d each. The sulfur recovery units are designed to achieve an overall recovery of 95%. The absolute amount of sulfur emission of 95% corresponding to the design case. Concerning the process arrangements proposed, they have been selected to cope with the specificities of the design basis with regard to the leanness of the gas feeds (between 21.3 and 31.7% vol. H2S) and the wide range of operating cases (40-105% of design case). 1.1. Function of the Unit The sulfur recovery units SRU’s are designed to recover commercial liquid sulfur from acid gas streams produced in the amine regenerators. The feed gas contains H2S, CO2 and some organic sulfur compounds (mainly mercaptans), together with a small amount of hydrocarbons and water vapor the SRU’s shall recover sulfur from this stream down to the required specification. Sulfur recovery units will consist of two identical sulfur recovery trains, each including. - Air and acid gas feed pre-heaters, - Thermal reaction furnace, - Three Claus converters in series, - One sulfur degassing section. There is one incinerator for two trains. The incinerator is designed to incinerate the tail gas at 540C prior to release to atmosphere.

1.2. Battery Limits of the Unit The sulfur recovery units battery limits include: - Acid gas inlet facilities (flow control, K.O. drum) - All equipment necessary for the sulfur recovery operation - Tail gas incinerator and chimney - Fuel-gas inlet facilities

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Sulfur recovery

- Steam recovery and Boiler Feed Water (BFW) system facilities including BFW purge, cooling and steam condensate return pumps - Sulfur degassing facilities and daily storage. - Sulfur export pumps to main liquid sulfur storage. Some items will be common to several SRU trains, as further indicated in process descriptions.

1.3. Plant Capacity and Interconnecting - The total sulfur recovery units capacity is the capacity resulting from processing of 2000 MMSCFD of reservoir fluid (dry basis). This capacity is to be treated in 4 parallel identical trains, each processing 25% of the acid gas from the gas treating units. - The corresponding acid gas flow rate per SRU train in normal operation is given in this section with the feedstock composition. A margin of 5% should be added for the design of each train to take into account the possible fluctuations due to the control system (load sharing between trains). - The sulfur recovery units interconnecting philosophy with upstream and downstream units is pictured in Figure 1. Basically, two sulfur recovery trains are fed from a common acid gas header receiving the acid gas of two gas treating trains. The acid gas from this common acid gas header shall be routed to an acid gas knock-out drum, also common to 2 trains. The acid gas flow is controlled by flow control for the first train and by pressure control for the second train. The liquid sulfur from the sulfur recovery unit’s storage and degassing pits (1for 2 trains) will be sent to a common liquid sulfur storage, from which it is processed in several parallel sulfur forming trains.

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Sulfur recovery

Figure 1 Acid Gas and Liquid Sulfur Interconnecting Piping Philosophy

1.4. Feedstock Characteristics and Capacity The three acid gas qualities, used as basis of the design, correspond to the following cases: - Design case, corresponding to the maximum H2S-content in the plant (31.6% H2S), and therefore, to the maximum sulfur production rate (98 t/d). - Sensitivity case, corresponding to the minimum H2S content in the plant main gas feedstock, and therefore to a reduced sulfur production capacity (84.6 t/d) and H2S concentration (26.8% H2S) in the feed to the SRU. The turn down case is related to the operation of the gas treating units at their minimum capacity (40%) corresponds to the minimum H2S concentration (21.3% H2S) that can be expected in the SRU acid gas feed stock and the minimum sulfur rate (33.7 t/d) in continuous operation. These cases represent the extremes of the expected acid gas compositions. The unit shall operate the acid gas feeds from this three cases. The definition and composition of design and operating cases are given below. - Acid gas feed compositions and conditions at battery limit Pressure and temperature of acid gas are the same for all cases, design values are:

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Sulfur recovery

Pressure (max.) Temperature

bar g

0.8

C

55

Sulfur recovery units feedstock characteristics Normal Operation

Turndown

Design case

Sensitivity case

Operation

(High H2S)

(Low H2S)

1.8 max.

1.8 max.

1.8 max.

55 max.

55 max.

55 max.

134.5*

115.9*

46.3*

B.L. Conditions  Pressure

bar abs

 Temperature (C) H2S normal flow rate per train

(kmole/h)

Composition

(dry mole %

kmole/h* mole %

kmole/h* mole %

kmole/h*

basis) H2 S

34.2

134.5

29.0

115.9

23.1

46.3

CO2

64.7

254.3

69.9

279.4

74.8

150.0

COS

-

-

-

-

-

-

Mercaptans

Traces

0.1

Traces

0.1

Traces

0.1

Hydrocarbons

1.0 max.

4.1

1.1 max.

4.3

2.1 max.

4.2

393.0

100.0

399.7

100.0

200.6

Total train (dry basis) 100.0 *

Saturation @ GTU Saturation @ GTU Saturation Water Content

reflux conditions

drum reflux

@

GTU

drum reflux drum conditions

conditions

* The molar flow rates indicated are for normal capacity. For SRU design, a margin of 5% shall be added in each case.

1.5. Properties of H2S and Sulfur - Properties of gaseous H2S Density relative to air :1.19 Heat value:3820 cal/kg Spontaneous ignition:260C Explosive mixture between 4.3 and 46% by volume in air (0C-760mm) Soluble in water (water can absorb as much as 3 times its volume at ambient temperature)

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Sulfur recovery

Mud tend to absorb H2S Agitation or temperature increase produces toxic gas liberation At high concentration, gas is less odorous (therefore you cannot rely on your sense of smell) After a 10 minutes exposure, the nose cannot perceive the odor. Burns with a blue flame to evolve SO2 Colorless

- Properties of sulfur 124 C

10.94 cp

132 C

8.66 cp

150 C

7.09 cp

156 C

7.19 cp

159.9 C

7.59 cp

159.1 C

9.48 cp

159.4 C

14.45 cp

160 C

22.83 cp

160.3 C

77.32 cp

165 C

500.00 cp

Density: 1.78 g/cm3 at 150C (302 F) Melting point: 119C (246 F) Normal handing temperature for liquid sulfur: 145C-155C (293-311F) - Final SRU product specification: Purity on dry basis:

Min 99.8% wt.

Color:

Bright

yellow

(in

solid

form) Organic matter:

500 ppm wt max.

Ashes:

200 ppm wt max.

H2S content:

10 ppm wt max.

2. GENERAL DESCRIPTION OF SRU PROCESS. - Classical Claus Process Step

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Sulfur recovery

Sulfur recovery by the Claus process involves conversion of a poisonous waste by-product of natural gas treating operations, namely acid gas into saleable sulfur and a relatively innocuous off gas suitable for discharge to the atmosphere. A net credit of the process is useful heat in the form of steam generation. In chemical terms, the object is to oxidize the H2S in the acid gas to S plus H2O while still burning any hydrocarbons present to sulfurfree products without soot formation. The overall net result of the process may be represented by the following equation: 

2 H2S + O2

2 H2 O + S 2

This is not a reaction which occurs at any one place; the process is carried out by burning one third of the acid gas stream mixed with the total amount of air required according to the above equation, producing SO2 and H2O. H2S

+ 3/2 O2  SO2

2 H2S + SO2

+ H2O

+ 124 kcal/mole

 3/n Sn + 2 H2O - 11 kcal/mole for S2 gas

(1) (2)

+ 35 kcal/mole for S8 liq The reaction (1) takes place in the reaction furnace by burning one third of the total acid with air. The reaction (2) between H2S and SO2 to form sulfur begins immediately in the combustion zone of the reaction furnace, but requires further contact of the process gas with a Claus catalyst at controlled temperatures, in converters following in series, to carry the reaction towards completion. The sulfur vapor formed by the reaction (2) is condensed and recovered as liquid sulfur after each step of the reaction (reaction furnace and catalytic converters).

- COS-CS2 Formation Acid gas containing hydrocarbons and carbon dioxide may undergo side reactions to form carbonyl sulfide (COS) and carbon disulphide (CS2). The main reactions capable of forming these compounds that occur in the reaction furnace are the following: CH4

+ 2S2

 CS2 + 2H2S

H2S

 S

+ H2

CO2

+ H2

 CO

CO

+ ½ S2  COS

+ H2 O

The sulfur compounds undergo partial hydrolysis in the catalytic converters tanks to the high water vapor content in the process gas. This hydrolysis is most noticeable in the first converter tanks to the higher temperature. These hydrolysis reactions are the following: COS

+

H2O  H2S

+ CO2

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Sulfur recovery

CS2

+ H2O 

COS +

H2S

- H2S/SO2 Ratio The control of the H2S to SO2 ratio, as close as possible to the exact stoichiomeric ratio of 2 throughout the unit is essential for maximum conversion and thus minimum sulfur losses in the tail gases. The air flow rate for oxidation of one third of H2S must be set at the best to optimize the reaction (2): -an air deficiency entails additional sulfur losses of H2S -an air excess entails additional sulfur losses of SO2 - Principle of Sulfur Degassing Process Elementary sulfur produced by the Claus process always contains dissolved H2S and chemically combined H2S under the form of polysulphides H2SX. H2SX  H2S + (x-1) S The degassing process can be schematized as follows: H2Sx sol

 H2S sol. + (x-1) S  H2S gas + (x-1) S

Polysulphide H2 S Decomposition degassing As H2S is removed from the system, the above equilibrium is driven to right resulting in further breakdown of hydrogen polysulphides with subsequent degassing of more H2S. The decomposition reaction is promoted by the use of the AQUISULF catalyst.

3. UNIT DESCRIPTION 3.1. Claus Section The unit is schematized on the attached process flow diagram. The acid gas from the sweetening unit is routed to the acid gas knock-out drum 108-D-101. The liquid carry over is removed and sent back to acid gas removal unit or water treatment. The acid gas K.O. drum 108-D-101 is common to two trains. The acid gas flow to the two trains is controlled by flow control for the first train, and by pressure control for the second train. The process air for reaction furnace is supplied by air blowers 108-K-101 A/B. The second blower is common spare for two trains. Prior to entering reaction furnace 108-H-101, the acid gas and the air are preheated up to 220C by HP steam in the 108-E-107 and 108-E-108 exchangers.

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Sulfur recovery

The acid gas then passes through the reaction furnace burner 108-H-101 where it is burnt with a controlled amount of air: The air to acid gas control is ensured by means of two control valves in parallel on the air circuit: - one main air valve which controls the primary air flow rate, - one trim air valve which adjusts the balance of air. The main valve is controlled by an air to acid gas flow ratio controller (feed forward control). The trim valve is actuated from a cascade control system from the H2S/SO2 analyser, which presets the trim airflow a controller (feed back control). When the signal to the trim air flow controller entails a full opening or closing of the trim air valve, an incremental signal is sent back to the main air to acid gas ratio valve ratio controller to decrease or increase its ratio accordingly. This control system device allows to keep the trim air valve position within its operational range at all times. Due to the high variations of acid gas composition the reaction furnace burner is fitted with fuel gas tips to burn continuously a small amount of fuel gas. The fuel gas burner is acting as pilot flame to maintain acid gas flame in case of perturbation. The fuel gas flow is set to 50-75 Nm3/h, and will be adjusted from acid gas feed to achieve a reaction furnace temperature up to 925C. The unit is provided with an acid gas bypass line to split a small part of acid gas feed (10-15%) downstream reaction furnace to be routed directly to the first condenser 108-E101.That allows to burn the acid gas on reaction furnace burner with a higher air to gas ratio than Claus ratio and then to achieve a higher flame temperature to get a good flame stability.

The acid gas bypass is used only during start-up or turn down period or when acid gas quality coming from sweetening unit is less than 28%.The process gas is cooled in passing through the reaction furnace boiler 108-B-101 where LP steam is generated. The process gases flow to the 1st condenser 108-E-101 where the gases are cooled and sulfur formed in the reaction furnace is condensed. LP steam is generated in the shell side of condenser 108-E-101. The condensed sulfur is removed from 108 E 101 through a bottom seal drain to the sulfur degassing pit 108-T-101.Then, the process gases are routed the 1st catalytic stage. The process gas passes through the auxiliary burner 108-H-102, where they are reheated up to 235C before entering the first catalytic reactor 108-R-101.

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Sulfur recovery

During normal operation (design and sensitivity case) the auxiliary burner is operated by burning a split stream of acid gas feed. The acid gas is burnt under substoichiometric conditions to avoid oxygen going through catalytic beds. During turn down operation the variation and lower H2S-content in acid gas make the control of substoichiometric combustion difficult. For turn down operation the auxiliary burner is operated with fuel gas instead of acid gas, in order to get a correct control of the substoichiometric combustion. When the H2S- content of the acid gas is decreasing below 25%, fuel gas will be progressively introduced to the auxiliary burner.The temperature controller of process gas at the outlet of auxiliary burner 108-H-102 is setting the air amount to the auxiliary burner. The air flow rate feeding the burner is controlling in cascade mode the acid gas and / or fuel gas demand via a ratio controller according to the substoichiometric ratio defined for both fuels. The gases flow downwards through the catalytic bed where the sulfur formation reaction and hydrolysis reaction of COS and CS2 are carried on. The catalytic bed is composed of a first layer of AM 4.8 promoted catalyst on the top, and an under layer of CR 3S activated alumina catalyst. In the reactor 108-R-101, additional sulfur is produced and carried over in vapor phase by the process hot gases. The exothermic Claus reaction results in a temperature rise across the reactor. The inlet temperature should be set in such a way that the outlet temperature exceeds 300C in order to favour the hydrolysis of COS and CS2. The hot gases leaving the 1st reactor are cooled in the 2nd condenser 108-E-102 by generating LP steam. The condensed sulfur flows through a seal pot to sulfur degassig pit 108-T-101. The gases leaving the second condenser 108-E-102 are reheated up to 215C passing through the tubes of the re-heater 108-E-105. Gases are heated by condensing saturated HP steam in the shell side. The catalytic bed in the 2nd reactor 108-R-102 consists of CR 3S activated alumina.The hot gases leaving the second converter are cooled in the 3rd condenser 108-E-103 by generating LP steam. The condensed sulfur flows through a seal pot to the sulfur degassing pit.Process gases leaving the third condenser are reheated up to 200C by means of HP steam, in the exchanger 108-E-106, prior to entering the 3rd catalytic reactor 108-R-103. The process gases pass downwards through the catalyst bed of third reactor 108-R-103 loaded with CR3S, activated alumina, on the top and CRS 31, titanium dioxide, underneath. Then the gases are cooled to 130C in the last condenser 108-E-104 in order to

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Sulfur recovery

condense the sulfur and to decrease sulfur losses by vapor pressure in the tail gas. The last condenser ensures cooling of gases by generating very low LP steam, which is condensed in an air cooler. Prior to being incinerated, the gases through the final separator 108-D-105 at the outlet of the last condenser, where entrained droplets of liquid sulfur are retained. The residual H2S and SO2 contained in the tail gas are analyzed to control the H2S/SO2 ratio through the trim air valve at unit inlet.

3.2. Incineration Section The tail gases leaving the final condenser 108-E-104 are routed together with the tail gases from the second train to the incinerator, where all remaining sulfur compounds are converted into SO2 by incineration at 540C before release to atmosphere through the stack. This temperature is reached by burning fuel gas with a slight excess of air, which is introduced into the incinerator burner by natural draft.

3.3. Water and steam systems The sulfur unit is producing LP steam only in reaction furnace boiler and sulfur condensers. LP steam is send to LP steam network at B.L..HP steam from B.L. is used to heat up air and acid gas feeds and to reheat process gas at the inlet of 2nd and 3rd reactors. HP condensate from air and acid gas pre-heater is recovered in HP condensate drum 108D-103 and then send to steam drum 108-D-102. HP condensate from 2nd and 3rd stage reheaters are directly routed to steam drum 108-D-102. To be adequate for sulfur reheating, the pressure of LP steam is reduced to 2.5. bar g before feeding steam jackets (liquid sulfur lines) and unit tracing system. LP steam is desuperheated by injection of boiler feed water. LP condensate from both units are recovered in LP condensate flash drum 108-D-106 and cooled down by recirculation through air cooler 108-A-102. By pump 108-P-105 LP condensate production is routed to B.L.

3.4. Sulfur degassing and storage section The sulfur degassing pit is common for two trains. The liquid sulfur produced in both units in all sulfur condensers is routed to sulfur degassing and storage pit 108-T-101.The degassing pit includes 2 compartments working in series.

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Sulfur recovery

The liquid sulfur is routed to the first compartment of degassing section, where sulfur is sprayed in the gas space by a re-circulating pump 108-P-102. Liquid sulfur flows to the second compartment by an opening at the bottom of dividing wall, where the sulfur degassing is completed by spraying with the second re-circulating pump

108-P-103. The degassed liquid sulfur is transferred by level control to the storage

section by means of a control valve on the discharge line. An injection of AQUISULF catalyst is carried out at the suction of both re-circulating pumps. The catalyst is injected from catalyst tank 108-T-102 at preset flow rate by the volumetric pump 108-P-104. To remove the H2S released during the process, the gaseous phase of the pit is swept by air. This sweeping gas is discharged to incinerator by a steam ejector 108-J-101. A continuous analyzer monitors the H2S content in the air.

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