Unit 5 Sulphur Recovery Unit

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UNITS IN THIS COURSE

UNIT 1

GAS COMPRESSION SYSTEMS

UNIT 2

AMINE GAS SWEETENING UNIT

UNIT 3

NATURAL GAS LIQUIDS (NGL) RECOVERY UNIT

UNIT 4

GAS FRACTIONATION PLANT

UNIT 5

SULPHUR RECOVERY UNIT

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TABLE OF CONTENTS Para

Page

5.0

COURSE OBJECTIVE

3

5.1

INTRODUCTION

4

5.2

FUNCTIONS OF THE SULPHUR RECOVERY UNIT EQUIPMENT

5

5.3

BASIC FUNCTIONS OF THE SULPHUR PLANT

7

5.3.1 5.4

Basic Description of the Reaction that Produces Sulphur

8

DESCRIPTION OF THE FLOW OF GAS THROUGH THE SULPHUR UNIT

9

5.4.1

Acid Gas Feed System

9

5.4.2

Reaction Furnace and Waste Heat Boiler

10

5.4.3

No 1 Condenser

11

5.4.4

No 1 Reheater and No 1 Converter

12

5.4.5

No 2 Condenser

12

5.4.6

No 2 Reheater and No 2 Converter

12

5.4.7

No 3 Condenser

12

5.4.8

No 3 Reheater and No 3 Converter

13

5.4.9

No 4 Condenser

13

5.4.10

Incinerator

13

5.4.11 Storage Pit

Sulphur  13

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5.0

OBJECTIVE Page 5/13

On completion of this unit the trainee will be able to: Describe and identify the most important pieces of equipment used in a sulphur  recovery unit. Describe the functions of each piece of equipment used in the sulphur recovery process.

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5.1

INTRODUCTION Page 7/13

The process used in the sulphur recovery unit is called the "Claus" process. In this process both catalytic and non-catalytic methods are used to combine hydrogen sulphide (H2S) with sulphur dioxide to produce sulphur. Depending on the percentage of H 2S in the feed gas, a portion of the feed is burned in the reaction furnace with a carefully controlled .amount of air. The rest of  the feed flows directly to the first catalytic converter. This type of split flow makes the conversion to sulphur more efficient, (up to 98%). The high and stable temperatures produced by the acid gas burner ensure maximum conversion takes place in the thermal section. Chemically, when H 2S is burned with air it oxidises into sulphur dioxide. If the burning is done using a controlled amount of air, only a portion of H 2S is oxidised producing chemical sulphur, H 2S and SO 2. With the help of a catalyst it is possible to combine the H 2S and SO2. to produce sulphur and water. In this process H 2S is burned under controlled conditions. These conditions produce just the right amount of sulphur dioxide required to catalytically combine with the unburnt H 2S  Approximately 33 % of the H 2S in the feed oxidises to S02. This gives a ratio of H 2S to SO2 throughout the plant of 2:1. This is the ideal ratio for producing sulphur. The catalytic conversion is done in three converters. Unconverted H 2S and SO2 from the reaction furnace is reheated, mixed with hot H 2S from the bypass line and fed to No 1 converter.

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5.2

FUNCTIONS OF THE SULPHUR RECOVERY UNIT EQUIPMENT

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Figure 5-1 Gas Flow through Sulphur Recovery Unit

Acid Gas Knockout Drum •

The acid gas knockout drum is used to separate the liquids (sour. water) from the acid gases.

•

The sour water is pumped to the sour water unit.

Preheater  •

High pressure steam is used in the preheater to increase the temperature of  the acid gases. Increasing the temperature of the acid gases will give good combustion in the reaction furnace.

Reaction Furnace •

In the reaction furnace 2/3 rds (66 %) of the acid gas stream is combined with a controlled amount of air. This is to ensure combustion of 1/3rd of the hydrogen sulphide (H2S) to sulphur dioxide (SO 2).

•

Some of the hydrogen sulphide (H2S) and sulphur dioxide SO 2 are also combined to produce sulphur vapours and water.

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Waste Heat Boiler  Page 13/13

•

The waste heat boiler removes the heat energy that is produced by the combustion of H 2S

•

Boiler feed water is used in the waste heat boiler for cooling the hot gases.

•

The waste heat boiler uses this heat energy from the combustion of H 2S to convert the boiler feed water to high pressure steam for use in the plant.

No 1 Condenser  •

Boiler feed water is used in the condenser to condense (turn to liquid) all of the sulphur vapours.

•

The condenser uses the heat energy from the gases to convert the boiler feed water to low pressure steam.

No 1 Reheater  •

High pressure steam is used in the heat exchanger to increase the temperature of the unconverted acid gases. This ensures a good chemical conversion in the converter.

No 1 Converter  •

The converter is filled with an activated alumina type catalyst. The catalyst increases the speed of the reaction between the hydrogen sulphide (H 2S) and sulphur dioxide (SO2). This chemical reaction produces sulphur vapour and water vapour. This chemical reaction is called an exothermic reaction because it produces heat energy.

No 2 Condenser  •

Boiler feed water is used in the condenser to condense (turn to liquid) all of the sulphur vapours.

•

The condenser uses the heat energy from the gases to convert the boiler feed water to low pressure steam.

No 2 and No 3 Reheater  •

High pressure steam is used in the heat exchanger to increase the temperature of the unconverted gases. This ensures a maximum conversion in the downstream converters.

No 2 and No 3 Converters •

The converters are filled with an activated alumina type catalyst. The catalyst increases the speed of the reaction between the hydrogen sulphide (H 2S) and sulphur dioxide (SO2). This chemical reaction produces sulphur vapour and water vapour. It is an exothermic reaction.

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No 3 Condenser  Page 15/13

•

Boiler feed water is used in the condenser to condense all the sulphur vapours produced in No 2 converter to liquid sulphur.

•

The condenser uses the heat energy from the gases to convert the boiler feed water to low pressure steam.

No 4 Condenser  •

Boiler feed water is used in the condenser to condense the final sulphur  vapours produced in No 3 converter to liquid sulphur.

•

The condenser uses the heat energy from the gases to increase the temperature of the boiler feed water before it flows to the waste heat boiler.

Incinerator  •

5.3

 Any unconverted gas from No 4 converter flows to the incinerator. The incinerator burns all the waste gases at 600'C and changes them to inert products. They are oxidised to SO2 before they are discharged to the atmosphere via the incinerator stack.

BASIC FUNCTIONS OF THE SULPHUR PLANT To take the H2S and convert (change) it to sulphur for sales. To burn all other waste gases to reduce pollution and to protect the environment.

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HOW HYDROGEN SULPHIDE (H2S) IS CONVERTED TO SULPHUR Page 17/13

The hydrogen sulphide (H 2S) is combined with sulphur dioxide (SO 2) to produce sulphur and water in a catalytic reaction. The sulphur which is produced is then condensed and removed in liquid form.

5.3.1

Basic Description of the Reaction that Produces Sulphur  •

2/3 rds (66 %) of the hydrogen sulphide (H 2S) is combined with a controlled amount of air and burned in a reaction furnace.

•

50 % of the hydrogen sulphide (H 2S) gas in the furnace oxidises and becomes sulphur dioxide (SO 2).

•

The remaining hydrogen sulphide (H 2S) is combined with the sulphur dioxide (SO2) with the help of a catalyst to produce sulphur and water.

The chemical reactions which happen are: •

The H2S gas burns to produce SO 2

•

If the H2S and SO2 are in a ratio of 2:1, they will combine to produce sulphur.

THE SULPHUR PRODUCING REACTION HAPPENS IN TWO WAYS Automatically (non-catalytic) Due to the high temperature in the reaction furnace some H 2S + SO2 react to form sulphur + water. Catalytically (with the help of a catalyst) In the converter beds the activated alumina catalyst causes the reaction -to take place at a lower temperature.

THE SULPHUR PLANT PRODUCES ONE MAIN BY-PRODUCT  A large amount of heat energy. This is because the chemical processes a exothermic.

re

This heat energy is used to produce steam at different pressures for use throughout the plant site. Some examples are: • • •

To preheat boiler feed water  To preheat the acid gases To drive steam turbines.

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5.4

DESCRIPTION OF THE FLOW OF GAS THROUGH THE SULPHUR UNIT Page 19/13

5.4.1

Acid Gas Feed System

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Figure 5-2 Acid Gas Knockout Drum

The acid gas flows from the gas sweetening unit to the acid gas knockout drum. In the knockout drum any liquids that are in the gas are removed. Most of these liquids are sour water. The sour water pumped to the sour water unit. Is pumped to the sour water unit. The gas leaves the top of the knockout drum and flows to the preheater. The gas is heated in the pre-heater by high pressure steam. The flow of gas is then split. Two thirds of the gas (66%) flows to the reaction furnace and the rest of the gas flows to the first converter.

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5.4.2

Reaction Furnace and Waste Heat Boiler  Page 23/13

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Figure 5-3 Reaction Furnace / Waste Heat Boiler 

 Approximately 66 % of the acid gas feed to the sulphur unit flows to the reaction furnace (depending on the acid gas strength). A carefully measured amount of air  flows to the reaction furnace. The amount of air depends on the flow of feed to the unit and on the % of H 2S that is in the feed. The amount of air flow is controlled automatically so that it burns approx. 1/3 of the H 2S. The normal operating temperature of the reaction furnace is approximately 1200 °C. The heat in the reaction furnace is used to make high pressure (HP) saturated steam in the waste heat boiler. The HP steam is made from the boiler feed water  that is fed to the waste heat boiler. The steam is separated in the steam drum. The steam leaves the top of the steam drum and flows to the HP saturated steam main.

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5.4.3

No 1 Condenser  Page 27/13

Exhaust gases from the waste heat boiler flow to the first condenser (No 1 condenser). In the first condenser the exhaust gases from the waste heat boiler are used to make low pressure steam (LP). The LP steam is made from the boiler feed water that is fed to the condenser. This process reduces the temperature of the exhaust gases. (See figure 5-1).  Any sulphur gases will be condensed to liquid sulphur. The liquid sulphur flows to a "sulphur seal" which allows liquid sulphur to drain away. However, t he "sulphur seal" maintains a gas seal on the liquid in the sulphur seal pot. This seal prevents H 2S being released to the atmosphere. (See Figure 5-4).

Figure 5-4 Liquid Sulphur Seal Pot

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5.4.4

No 1 Reheater and No 1 Converter  Page 29/13

See Figure 5-1. The unconverted H 2S and SO 2, along with CO 2 and other gases, leave No. 1 condenser and flow to the No. 1 reheater. Here, the temperature of the gases is increased. After this they combine with the other 33 % of hot gases that by-passed the reaction furnace. (This flow doesn't go through the reaction furnace because if all the gas went through the reaction furnace it would lower the temperature of the reaction furnace). The combined gases flow to No 1 converter and down onto a catalyst bed. The catalyst, (activated alumina type), increases the speed of the reaction between the H2S and SO2. This reaction converts the gases into sulphur gas. This reaction is exothermic. Therefore the gases which leave the converter are hotter than the gases which go in to the converter. (See figure 5-1).

5.4.5

No 2 Condenser  Hot gases from No 1 converter flow to No 2 condenser. The heat from these gases is used to make the boiler feed water that flows to the condenser into LP steam. As the hot gases cool they condense to liquid sulphur. This liquid sulphur flows through a liquid sulphur seal pot which is the same as the one at No 1 condenser (see figure 3-4). The combined liquid sulphur flows from condensers 1 & 2 go to the liquid sulphur storage pit.

5.4.6

No 2 Reheater and No 2 Converter  The cooled gases from No 2 condenser flow to No 2 reheater. Here, they are reheated by high pressure steam before they flow to No 2 converter. The reaction in No 2 converter is the same as in No 1 converter. Once again H 2S and SO 2 combine to make sulphur. This reaction is exothermic so the temperature of the gases is increased again. (See figure 5-1).

5.4.7

No 3 Condenser  No 3 condenser is the same as No 2 condenser. Hot gases from No 2 converter  flow to No 3 condenser. Boiler feed water also flows into this condenser. The heat from the gases is used to change the boiler feed water into LP steam. This cools the hot gases and they condense to liquid sulphur. This liquid sulphur flows through a liquid sulphur seal pot which is the same as the one at No 1 and No 2 condensers. The liquid sulphur flows to the liquid sulphur storage pit.

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5.4.8

No 3 Reheater and No 3 Converter  Page 31/13