Unit 2 Amine Sweetening Unit
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Module No. 12 : Gas processing Unit No. 2- Amine gas sweetening unit Page 1/14
description
Amine
Transcript of Unit 2 Amine Sweetening 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
2.0. COURSE OBJECTIVE 3
2.1 INTRODUCTION 4
2.2 AMINE GAS SWEETENING UNIT 6
2.2.1 Functions of the Amine Sweetening Unit Equipment 6
2.3 PROCESS DESCRIPTION 9
2.4 GAS SWEETENING 10
2.4.1 Amine Contactor 10
2.4.2 Amine Flash Drum 11
2.5 AMINE REGENERATION 12
2.5.1 Regenerator Reboilers 12
2.5.2 Regenerator Overhead Condenser and Reflux Accumulator. 13
2.5.3 Amine Filter and Reclaiming Section 13
2.5.4 Lean Amine Recirculation Pumps and Cooler 13
2.5.5 Amine Filters 13
2.5.6 Amine Reclaimer. 14
2.5.7 Corrosion Inhibitors 14
2.5.8 Amine make-up 14
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2.0. COURSE OBJECTIVE
On completion of this unit the trainee will be able to:
Describe and name the important components of the sweetening process.
Describe how and why amines are used.
Describe the operating principles of the amine contactor.
Describe the operating principles of the amine regenerator.
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2.1 INTRODUCTION
Figure 2-1 Gas Sweetening Process Flow Diagram
The feed for the gas treating unit comes from the gas compression unit and condensate stripping unit. The low pressure gas, intermediate pressure gas, and high pressure gas have been combined into a single HP gas line. The gas has also had any entrained liquids removed. This is called natural gas.
The natural gas feed contains impurities, such as hydrogen sulphide (H2S). carbon dioxide (CO2) and carbonyl sulphide (COS). These are called "acid gases". All of these acid gases are impurities in natural gas. Natural gas which contains these impurities is called "sour gas".
The impurities in sour gas must be removed so that an NGL product meets specifications. When the acid gases have been removed it is, called "Sweet Gas".
The presence of H2S and CO2 in the plant would cause the following problems:
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There would be severe corrosion problems; especially where free water is present.
Natural gas consists of the following hydrocarbons:
Methane - C1
Ethane - C2
Propane - C3
Butane- C4
Natural gas also contains small amounts of heavier hydrocarbons, such as pentane, hexane and heptane. A mixture of these natural gas hydrocarbons which are heavier than butane is called natural gasoline C5+
In this unit we will study the Gas Sweetening Process, (Amine Unit).
Changing sour gas into sweet gas is called "sweetening". In this unit we will look at an amine gas sweetening unit.
To remove the acid gases from the feed gases, the feed gases are contacted with an "amine solution". An amine solution is an alkaline solution which attracts and absorbs acid gases like H2S and CO2 There is a chemical reaction between the amine solution and the acid gases. This is called an "absorption process".
This process takes place in a column called an "amine contactor". The sour gas comes into contact with the amine solution. When the sour gas is contacted with the amine solution the acid gases are removed but the hydrocarbons remain in the gas.
When the sweetened gas leaves the top of the column it contains less then 16 ppm (parts per million) H2S and less than 50 ppm CO2. The amine which comes out from the bottom of the column has absorbed a lot of acid gases (H 2S and CO2). It is called "rich amine". The acid gases must be removed from the rich amine, solution so that it can be used again.
Removing the acid gas from the rich amine is called "regeneration". This takes place in a column called an "amine regenerator". The regenerated amine is called "lean amine".
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2.2 AMINE GAS SWEETENING UNIT
The following terms are used in the amine sweetening process:
Sour Gas: Natural gas which contains H2S and CO2
gases.
Sweet Gas: Natural gas after H2S and CO2 have been removed.
Acid Gas: Any gas that forms an acid when mixed with water.
Gas Sweetening: A process which removes the acid gases from the gas stream.
Amine: A general term for a number of chemicals which are used to absorb the acid gases from the gas stream.
Absorption: To take one fluid inside another fluid and trap it. E.g., H2S and CO2 are absorbed into the liquid amine solution.
Regenerate / Strip: To remove the absorbed fluids from the absorption chemical. E.g., when the amine solution is regenerated H2S and CO2 are stripped from the amine solution.
Amine Concentration The amount of pure amine in the amine water solution
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2.2.1 Functions of the Amine Sweetening Unit Equipment
Feed Gas Filters
These remove hydrocarbon liquid droplets and fine solid particles from the feed gas.
These materials could cause foaming of the amine solution later in the process.
Amine Contactor Column
This allows close contact between the lean amine and the sour gas stream to ensure:
All the acid gases are absorbed by the amine solution.
The gas which leaves the top of the contactor is sweet (less than 16 ppm H2S). This means that it contains less than 16 parts H2S in 1,000,000 parts of natural gas.
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Gas Outlet Mist Extractor
This device removes any free liquids (water / amine) from the sweet gas stream. This reduces amine losses.
Rich Amine Flash Drum
This separates the hydrocarbon and sour gas vapours from the rich amine by decreasing the pressure and velocity of the rich amine stream.
Light vapours separate and leave from the top of the flash drum.
Rich amine fluid flows out of the bottom outlet of the flash drum.
Amine Filters
These remove impurities from the lean amine solution. The impurities would cause the amine solution to foam (bubble) which would damage the process equipment. The solution passes through a mechanical filter first and a carbon filter after that.
Mechanical Filter
This removes all the dirt and other solid particles from the amine solution.
Carbon Filter (uses activated carbon)
This removes the entrained hydrocarbons from the amine. It does this by passing the amine solution through activated carbon. Activated carbon is carbon that has been treated to increase it's surface area. This gives more contact with the hyd rocarbons. It makes it easier for the carbon to remove the hydrocarbons.
Regenerator / Stripper Column
This allows close contact between the rich amine and the hot steam from the reboiler. The hot steam does the following:
Increases the temperature of the rich amine.
Strips the acid gases from the rich amine.
Carries the acid gases up to the top of the column. These hot sour gases leave the column as a vapour.
The stripped amine leaves the bottom of the column as lean amine.
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Reflux Condenser and Reflux Accumulator
The hot sour gases from the top of the regenerator flow to the reflux condenser. The reflux condenser cools them down until all the water vapour from the stripping steam condenses.
The fluids are separated into liquid and gas in the reflux accumulator. All the liquids are pumped back to the regenerator column. The acid gases go to the sulphur plant for processing, or to flare.
Regenerator Reboiler
This supplies the heat to boil some of the water from the amine solution. This is used as stripping steam for the regenerator column.
Hot oil provides heat for the reboiler.
Amine Reclaimer
This takes a small amount of the lean amine and purifies it by increasing its temperature. When all the liquid changes to vapour it goes back into the regenerator column. Any solids in the amine remain in the vessel. The solids are "blown down" from the bottom of the vessel as waste material
Lean Amine Recirculation Pump
This increases the pressure of the lean amine from the regenerator stripping column so that it equals the pressure in the amine contactor, (so it can enter the contactor column).
Lean Amine Fin Fan Cooler
This controls the lean amine temperature so that it is 50 to 101C cooler than the temperature of the sour gas when it enters the contactor column. At this temperature:
The lean amine will absorb the acid gases.
The hydrocarbons will not condense or be absorbed by the solution.
If the amine is too hot it will not absorb the acid gases.
If the amine is too cold it will absorb the hydrocarbons.
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2.3 PROCESS DESCRIPTION
AMINE DESCRIPTION
The most common amines used in gas sweetening units are:
MEA - (Monoethanolamine).
DEA - (Diethanolamine).
TEA / MDEA - (methyldiethanolamine).
DIPA or ADIP - (Di-propanolamine)
DGA - (Di-Glycol Amine)
Recall that natural gas which contains impurities like H2S and CO2 is called sour gas.
In the gas sweetening process, H2S and CO2 are removed from the natural gas (sour gas) feed. These gases must be removed because they cause problems in NGL processing. H2S is also very poisonous.
An amine solution is a liquid agent that is used to sweeten the sour natural gas.
The gas sweetening process depends on the following principles:
1. At a low temperature and high pressure, the amine reacts with H2S and CO2
and will easily absorb these gases.
2. At a high temperature and low pressure, H2S and CO2 Will easily separate from the amine.
The amine gas treating plant can be understood *best by dividing it into two parts:
Gas Sweetening
Amine Regeneration.
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2.4 GAS SWEETENING
See figure 2-1
Figure 2-1 Gas Sweetening Process Flow Diagram
2.4.1 Amine Contactor
The purpose of the amine contactor is to sweeten the sour gas using an amine solution. The acid gases are removed when they are contacted with the amine solution.
The feed gases for the contactor column come from the condensate stripper and compression unit. The combined feed gases first flow to the feed gas filter. The feed gas filter removes hydrocarbon droplets and small solid particles. Both types of material may cause the amine solution to foam (bubble) if they are not removed.
The filtered sour gas enters the contactor column under the bottom tray (1st tray) at near ambient temperature. It flows up the inside of the column.
The lean amine solution enters-the contactor at- the-- top of the column.
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Inside the amine contactor, the amine solution flows down the column. it fills each tray and flows down onto the next tray below. The sour gas flows up the column.
As the amine flows down the column, and the sour gases flow up the column, both fluids must pass through the holes on each tray. This flow in opposite directions is called counter-current. The countercurrent flow allows close contact between the amine solution and the sour gas.
As the amine solution makes contact with the sour gas,, it absorbs the H2S and C02 from the sour gas. The low temperature and the high pressure in the column ensure that the H2S and CO2 are absorbed by the amine.
As the amine flows down through each tray, it absorbs more and more H2S and CO2- When the amine solution reaches the bottom of the column it-is nearly saturated (full) with the H2S and CO2 which it has absorbed from the sour gas. This saturated amine is called "Rich Amine". It must be regenerated before it can be used again.
When the gas reaches the top of the column nearly all the sour gases have been removed. The gas now consists mostly of hydrocarbons. Hydrocarbon gas that contains little or no H2S and CO2 is called "Sweet Gas". The sweet gas leaving the top of the amine contactor flows to the Natural Gas Liquid (NGL) Recovery Plant for processing.
The chemical reaction that occurs when the amine absorbs H2S and CO2 is an exothermic reaction. This means it produces heat. Therefore, the temperature of the rich amine at the bottom of the contactor is higher than the temperature of the lean amine that enters at the top. The exothermic reaction in the contactor increases the temperature of the amine solution.
The amine solution in the contactor column can get very hot.
Tray No.8 is a chimney tray. All the liquid failing onto this tray is drawn off and pumped through the side cooler. The cool liquid is fed back onto tray No.8. This cooling stops the acid gases boiling out of the amine in the contactor.
2.4.2 Amine Flash Drum
As the amine flows through the contactor, it absorbs some hydrocarbons along with the H2S and CO2. The hydrocarbons dissolve easily in the rich amine. However they are not chemically bonded to the amine so they can be separated easily.
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The amine flash drum separates these hydrocarbon gases from the rich amine. The flash drum receives rich amine from the amine contactor column. As the rich amine flows into the hydrocarbon flash drum, its pressure and velocity drop.
This pressure drop causes the dissolved hydrocarbons to flash off (separate) from the rich amine. The hydrocarbon gases leave the top of the flash drum. These hydrocarbon gases are sent to the flare. The rich amine solution flows from the bottom of the amine flash drum to the amine regenerator, where H2S and CO2 are removed.
2.5 AMINE REGENERATION
The function of the amine regeneration system is to:
Separate H2S and CO2from the rich amine.
Return lean amine to the amine contactor column.
The amine regenerator converts rich amine solution back into lean amine solution. This conversion is called regeneration.
Inside the regenerator, the pressures are much lower than the pressures in the amine contactor. Also, the temperatures are higher. The rich amine is regenerated at low pressure. This helps the acid gases and the amine solution to separate easily.
The rich amine solution enters the regenerator near the top of the column. It is heated by hot vapours rising through the column. As the temperature of the amine increases, the amine releases more and more of the absorbed acid gases. These released acid gases flow out of the top of the column.
2.5.1 Regenerator Reboilers
Remember, heat is created when the amine absorbs H2S and CO2 in the amine contactor (exothermic reaction). In the amine regenerator the reaction is completely opposite (endothermic reaction). The acid gases and the amine solution separate easily when heat is added. The chemical reaction absorbs the heat. The amine releases the acid gases at a temperature of 100 C and above.
The reboiler at the bottom of the amine regenerator supplies the heat to the column. Part of the lean amine bottoms product goes through the reboilers where it is heated to a high temperature. The heated amine returns to the bottom of the column as a vapour. These hot vapours rise up the column and strip the sour gas out of the rich amine. These vapours then go to the overhead condenser.
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2.5.2 Regenerator Overhead Condenser and Reflux Accumulator.
The acid gases are carried out of the top of the column by the steam. The vapours are cooled in the overhead condenser. The temperature drop causes the steam to condense. The acid gases are separated from the condensate in the regenerator reflux accumulator.
The sour condensate from the reflux drum is pumped back to the top of the column. The condensate cools the top of the column. The cool reflux condenses the hot amine vapours in the gas stream inside the column. This prevents any amine from leaving the top of the column.
Some of the cool, sour condensate also passes into the reclaimer to control the temperature of the reclaimer. From there it passes back to the regenerator.
The sour gas leaves the top of the reflux drum on pressure control. It flows to the sulphur recovery plant for processing or to the flare to be burned.
Make-up water can be added to the process at the reflux accumulator.
2.5.3 Amine Filter and Reclaiming Section
The amine filter and reclaiming section cleans the lean amine solution before returning it to the amine contactor column.
2.5.4 Lean Amine Recirculation Pumps and Cooler
The recovered lean amine solution flows from the bottom of the regenerator through the recirculation pump. Downstream of the pump, the lean amine divides into two streams. The main stream, (90 % of the lean amine solution) returns to the amine contactor through the lean amine cooler.
The cooler (heat exchanger) reduces the temperature of the lean amine before it re-enters the contactor column.
2.5.5 Amine Filters
The second amine stream from the regenerator., (10 % of the flow) flows to the amine filters. As amine circulates in the system, it picks up corroded particles from the process pipelines. The amine also picks up solids from the sour gas. These particles are removed in the amine filters.
From the filters, the lean amine flows through the recirculation pumps. The pumps return the filtered amine to the contactor, through the lean amine cooler. A small stream of filtered amine is sent to the amine reclaimer.
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2.5.6 Amine Reclaimer.
As the amine solution circulates through the system, salt compounds are formed. The amine reclaimer removes these unwanted salt compounds from the amine solution.
The amine reclaimer is a kettle type reboiler (heat exchanger). The amine reclaimer vapourises the amine solution. The heat that is applied to the amine solution reverses the chemical reactions that caused the unwanted salt compounds.
The heat in the amine reclaimer causes the compounds to break down into pure amine and sour gases. The vapours produced in this reaction leave the top of the reclaimer and are returned to the amine regenerator column.
The chemical impurities cannot boil. They collect at the bottom of the amine reclaimer. After a period of time the amine reclaimer is taken out of service and the impurities are removed. They are drained manually to the sewer drain system.
Approximately 10 % of the lean amine solution flow from the amine filters is continuously recycled through the amine reclaimer. (This is approx 1 % of total amine flow). The rest of the flow goes to the suction of the amine recirculation pump.
2.5.7 Corrosion Inhibitors
Corrosion inhibitors help to prevent corrosion of the process equipment.
These chemicals can be injected into different parts of the process.
2.5.8 Amine Make-Up
Some amine is lost during the process. It is replaced from an amine storage tank. This amine make-up enters the line leaving the regenerator. It is added upstream of the amine circulating pump.
NOTE:
A second method used to remove acid gases is with hot potassium carbonate solution.
This is used instead of an amine solution.
The process is very similar to the process described above.
Because the hot potassium carbonate does not remove all the acid gases it is followed by an amine process to remove the final traces of acid gases.
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