Principles Of Amine Sweetening

Principles Of Amine Sweetening, Amine Sweetening, Amine Sweetening Principles, Gas Sweetening, Acid gases in Natural Gas, Alkanolamines, Absorption, D

1. Acid Gases in Natural Gas

A gas which forms an acid when combined with water is called an acid gas. Hydrogen sulfide (H2S) and carbon dioxide (C02) are acid gases commonly found in natural gas. They occur in varying concentrations depending on the nature of the reservoir from which the natural gas is produced. The acids formed can be very corrosive to equipment and piping. Removal of both H2S and C02 from natural gas is one of the most common treating problems for gas plants.

Hydrogen sulfide must be removed due to its corrosiveness and toxicity. When burned, it forms sulfur dioxide (S02), which is another toxic gas. Carbon dioxide must also be removed from natural gas due to its corrosiveness and lack of heating value. Removing C02 raises the heating value of natural gas and reduces the cost of pipeline transmission.

2. Gas Sweetening

Natural gas which contains acid gases is commonly referred to as sour gas. Gas which contains no acid gases or has had them reduced to an acceptable level by a treating process is called sweet gas. The process of removing acid gases from natural gas is called gas sweetening. Although there are several processes used in the petroleum industry for removing acid gases from natural gas, the most common process utilizes substances known as alkanolamines or simply amines.

3. Alkanolamines

Alkanolamines, which are weak bases, react chemically with acid gases to form salt complexes. These salt complexes can be regenerated so that the amine are again free of acid gases and can be recycled. Amines are nitrogen-hydrocarbon (N-HC) compounds which are categorized as primary, secondary, or tertiary depending on the number of hydrocarbon (HC) groups attached to the nitrogen (N) atom. HC groups will vary depending on the type of amine, e.g., ethanol, glycol, isopropanol, methyldiethanol, etc. The most commonly used amines are :

Monoethanolamine (MEA): A primary amine which has two hydrogen atoms and one HC group attached to the N atom.
Diethanolamine (DEA): A secondary amine which has one hydrogen atom and two HC groups attached to the N atom.
Triethanolamine (TEA) and Methyldiethanol amine (MDEA): Tertiary amines which have three HC groups and no hydrogen atoms attached to the N atom.

4. Absorption

Much like a sponge, amine is used to absorb acid gases from natural gas. By mixing together the sour gas and amine solution in a contactor column, the acid gases are absorbed by the amine solution, thereby removing them from the natural gas.

5. Distillation

In distillation, acid gases are removed from the amine solution by the use of heat. Sufficient heat must be added to the amine solution to break the bonds between the acid gases and the amine. This is generally done by using steam in a stripper column.

6. Heat Transfer

Heat transfer is the process by which heat travels from one substance to another. If two containers of water-one containing cold water, the other hot water are brought into contact with each other, the temperature of the hot water will decrease and the temperature of the cold water will increase. The temperature changes result because of heat transfer from hot to cold. In amine sweetening, it is important to maintain fluid temperatures within relatively narrow ranges to optimize the efficiency of the process. Improper temperature control can cause problems such as inadequate stripping and corrosion. By bringing together “cool” and “hot” amine solutions in a heat exchanger, the process of heat transfer through conducion is accomplished.

7.Basic Process

There are two purposes of an amine sweetening unit. The primary purpose is to remove acid gases from natural gas before putting the natural gas into a pipeline. The other purpose is to remove acid gases from the amine solution so that the amine can be used over and over again in the sweetening process. The process, while it may seem somewhat complicated, is actually quite simple. Sour gas has acid gases removed from it by an absorption process. The amine solution absorbs acid gases from the natural gas, leaving sweet gas. The rich (containing acid gases) amine solution then goes through a process of distillation where the acid gases are removed. The lean (not containing acid gases) amine solution is then sent back to function again in the sweetening of natural gas. In this manner, amine is recycled.

Amine Sweetening Process Flow

A typical amine sweetening unit is shown. It consists of the following components: inlet and outlet separators; contactor column; flash tank; filters; stripper column; reflux condenser, accumulator, and pump; reboiler; reclaimer; lean/ rich heat exchanger; surge tank; circulation pump; and trim cooler. Depending on the type of alkanoiamine used, the amine sweetening unit at your location may vary from this typical unit. However, the process will be very similar to the one shown.

1. Inlet Separator

Unprocessed sour gas flows from its source to a device called a separator or scrubber, where solids and free liquids are removed from the gas. This separator can be an integral part of the treating unit or it can be a separate vessel. With in the separator, baffles aliow most of the liquids to be removed. A mist extractor, located, near the gas outlet, traps liquids entrained in the gas before it leaves the vessel. Efficient removal of solids, such as iron sulfide, from the gas prior to the absorption process can minimize many operational and maintenance problems such as foaming, erosion and high corrosion rates. These problems usually result in amine loss, equipment failure, and difficulty in meeting sweet gas specifications. Some amine sweetening units have an outlet separator in conjunction with the inlet separator. It is used to remove any entrained liquids in the gas stream prior to sales or iniet into further gas processing, e.g., giycol dehydration.

2. Contactor Column

In this vertical pressure vessel, sometimes called an absorber, sour gas comes in contact with amine solution. Contactor columns operate at elevated pressures in the range of 200 - 1000 psig. There are two types of contactor columns commonly used: the tray column and the packed column. In a tray column, which is more commonly used than a packed column, the liquid level is maintained on each tray by a weir on the downcomer. The number of trays or height of packing helps determine the degree of sweetening. Twenty trays are common. Bubble caps, located on each tray, are used for good liquid and gas contact.
In a packed column the amine solution contacts the gas stream by forming a film over the packing. Thus, a large surface is available for C02 and H2S to transfer from the gas to the amine solution. A column can have either random or structured packing. Random packing involves pouring loose packing material into the column. Structured packing involves placing bundles of packing material, referred to as elements, in a fixed orientation within the column. Structured packing allows for better fluid flow and contact between fluids.
Since tray columns are more common, we will discuss flow through this type of cplumn from here on. Sour gas, which is preferably kept at a temperature between 90° and 100° F, enters the bottom of the contactor. It begins to rise through the bubble caps on each tray. At the same time, lean amine, which has entered from the top, flows downward counter-current to the gas flow. The temperature of the lean amine should be kept 10° F, above the natural gas temperature to prevent hydrocarbon condensation.
The gas disperses through the amine in the form of bubbles. A froth is formed. The gas disengages from the froth, travels through a vapor space and up through the liquid on the next tray. Entrained amine solution falls back onto the tray and then flows down through the downcomer to the next tray. Nearly all absorption of H2S and C02 takes place on the trays and not in the vapor space between the trays.
At the top of the contactor, a mist extractor traps entrained liquids from the gas before it enters the gas outlet. This liquid then drops back onto the top tray and rejoins the amine stream. While sweet gas is leaving the top of the contactor, rich amine solution, which contains H2S and C02, is discharged from the base of the contactor.

3. Flash Tank

Rich amine solution leaves the bottom of the contactor and flows through a low pressure flash tank where a large portion of the physically absorbed gases are flashed off. The flash tank should be operated at the lowest possible pressure to maintain amine flow to the stripper. Tank vapors, which are usually sour are recycled to the contactor or flared. The heavier hydrocarbons separate from the amine and form a separate liquid layer on top of the amine solution. They are skimmed off and the amine is drained from the bottom of the tank. From the flash tank, the rich amine stream flows through filters to a heat exchanger.

4. Filters

Filtering the amine solution to remove solid matter and entrained hydrocarbons is essential for proper operation. Filtering can be done several ways. The rich amine can be filtered downstream of the flash tank and upstream of the lean/rich heat exchanger. If filtering is done at this point, normally a full-flow mechanical (sock) filter is used in combination with a side-stream activated carbon filter and mechanical filter. The activated carbon filter is used to remove entrained hydrocarbons whiie the mechanical filters are used to remove solid matter. Filtering can also be done on the lean amine stream just prior to entering the contactor. Solid matter down to the 10-15 micron size range should be removed for good solution control. Some amine sweetening units will have filters on both the rich and lean amine streams.

5. Lean/ Rich Heat Exchanger

From the flash tank, the rich amine stream goes through the lean/ rich heat exchanger, where the rich amine stream is preheated by the lean ami exiting the reboiler. The lean/ rich heat exchanger reduces the heat load on the reboiler by raising the temperature of the rich amine to approximately 190° F. prior to entering the stripper column. The lean/rich heat exchanger is usually a shell and tube type of heat exchanger whit stainless steel tubes. Stainless steel is used because of the corrosiveness of H2S and CO2.

6. Stripper Column

The stripper column or still is commonly referred to as the regenerator because it is in this vessel that acid gases are removed from the rich amine, making the amine solution once again lean. Like the contactor, a stripper column is either a tray or packed column. However, strippers operate near atmospheric pressure.The rich amine solution, which has been preheated in the heat exchanger, enters the stripper near the top and flows downward. The rich amine solution is further heated by steam entering the bottom of the stripper and flowing upward through the descending amine solution.

Steam, generated in the reboiler, adds sufficient heat to the amine solution to strip acid gases from the amine solution. In addition, steam helps to sweep the liberated acid gases out of the stripper.The hot, regenerated (lean) amine solution leaves the bottom of the stripper and flows to the reboiler.

7. Reflux Condenser/Accumulator

Overhead vapors from the stripper, consisting essentially of water vapor and acid gases, pass through the reflux condenser to recover water and the small amount of amine which may have vaporized in the stripper. Some of the water vapor is condensed so it can be separated and pumped back to the stripper for reflux. The condenser effluent then flows to the reflux accumulator for separation of the water and acid gases. The water is returned to the stripper by the reflux pump. Acid gases may be sent either to a flare stack or to a sulfur recovery plant, depending on the amount of H2S present.

8. Reboiler

The stripper bottoms are heated by the reboiler. The heat converts some of the water in the amine solution to a stripping vapor of steam which rises through the stripper to release acid gases from the incoming rich amine solution. In gas treating facilities, reboil heat is usually supplied by a direct-fired heater. This method of heating simply consists of controlled combustion on the inside of an expandable tube that is surrounded by the boiling amine solution. Steam or hot oil can serve as a heat medium if available in sufficient quantity.
From the reboiler, the lean amine solution flows through the lean/rich heat exchanger to the surge tank. In the heat exchanger, the hot lean amine is cooled to a desirable storage temperature by the cool rich amine solution flowing to the stripper.

9. Reclaimer

A reclaimer is not often included in an amine sweetening unit; however, when used it can greatly reduce operating problems. A small portion (1-3 percent) of the lean amine circulation at the bottom of the stripper is semi-continuously fed to the reclaimer where it is boiled. It is distilled and pure lean amine is recovered and returned to the main solution. This process removes contaminants, e.g., hydrocarbons, heat-stable salts and solids, from the lean amine solution. These irreversible degradation products should never exceed 1 percent by weight in the circulating amine solution. The liquid temperature in the reclaimer rises as contaminants accumulate. When the temperature reaches a set temperature, operation is stopped and the reclaimer is drained of accumulated products.

10. Surge Tank

The surge tank is a holding vessel for the cooled lean amine solution until it is pumped to the contactor column. Periodically, water and amine are added to the surge tank to make up for losses of these two liquids during the process. Circulation pumps take their suction from the surge tank and transfer the lean amine solution to the top of the contactor column to begin the cycle again.

11. Trim Cooler

The lean amine solution in the contactor column should be hotter than the sour gas being treated; however, high amine temperatures affect the amount of acid gas removal. A trim cooler is used to lower the temperature of the lean amine solution coming from the surge tank and to control the temperature of the lean amine solution going to the contactor column.

• [accordion]
• 1. Principles of Amine Sweetening
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• 2. Amine Sweetening Initial Design
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• 3. API RP 945-2022 Avoiding Environmental Cracking in Amine Units
• 4. ASTM 350A350M-18 Carbon and Low-Alloy
• 5. Basic Principles and Calculations in Chemical Engineering 9th
• 6. Gas Treating (2014) Absorption Theory and Practice
• 7. Natural Gas Processing - Engineering Design
• 8. PLP P10 2003 Amine Sweetening Process-2nd Ed-Rosen
• 9. PLP P11 2003 Contactors in Amine Sweetening Plant
• 10. PLP P12 2003 Stripper in Sweetening Process-1st Ed-Rosen
• 11. PLP P13 2003 Stabilizing for Oil Condensate