Natural gas coming out of a well is not automatically ready to enter a sales pipeline.
It can contain water, heavier hydrocarbons, carbon dioxide, hydrogen sulfide, nitrogen, sand, compressor oil, and other contaminants.
Some of those components reduce gas quality.
Others create corrosion problems.
Hydrogen sulfide can also make the gas extremely hazardous.
Before the gas can be transported or sold, unwanted acid gases often need to be removed.
That process is called gas sweetening.
One of the most common ways to sweeten natural gas is with an amine unit.
The basic idea is surprisingly simple.
Sour gas contacts an amine solution.
The amine absorbs hydrogen sulfide and carbon dioxide.
Sweet gas leaves the top of the contactor.
The contaminated amine is then heated so it releases the acid gases.
The regenerated amine is cooled and sent back to the contactor to be used again.
That continuous cycle can operate for years.
The difficult part is keeping every part of the cycle working properly.
What Is Sour Natural Gas?
Sour gas contains hydrogen sulfide, commonly written as H2S.
The term can also be used more broadly when discussing gas that requires acid gas treatment.
H2S is especially important because it is highly toxic and corrosive.
Raw gas can also contain carbon dioxide, written as CO2.
CO2 is not normally described as sour in the same toxic sense as H2S, but it is still an acid gas and often needs to be removed.
A natural gas stream containing both H2S and CO2 may therefore need significant treatment before sale.
What Does Sweet Gas Mean?
Sweet gas has had enough hydrogen sulfide removed to meet the required specification.
Depending on the pipeline or customer, carbon dioxide may also have a maximum allowable concentration.
The exact sales specification is not universal.
Different pipeline systems and gas contracts can require different limits.
The goal of the amine unit is therefore not simply to remove some H2S.
It has to produce gas that consistently meets the required product specification.
Why Is Hydrogen Sulfide Dangerous?
H2S deserves special respect.
It is toxic even at relatively low concentrations.
At higher concentrations it can quickly become fatal.
People sometimes associate H2S with a rotten egg smell, but smell is not a reliable warning method.
Exposure can reduce the ability to detect the odor.
That means workers should rely on proper gas detection, procedures, respiratory protection requirements, and site controls rather than their nose.
H2S can also contribute to corrosion and materials problems.
A facility handling sour gas therefore needs equipment and procedures suitable for sour service.
Why Remove Carbon Dioxide From Natural Gas?
CO2 can create several problems.
It takes up pipeline capacity without providing the same heating value as methane.
A gas stream containing large amounts of CO2 therefore contains less useful fuel energy per unit volume.
CO2 can also contribute to corrosion when water is present.
When carbon dioxide dissolves into water, the resulting environment can become corrosive to carbon steel.
Removing CO2 can therefore improve gas quality while helping manage corrosion risk.
What Is an Amine?
In gas processing, amine normally refers to an aqueous chemical solvent used to absorb acid gases.
The solvent is mostly water plus a selected amine compound at a controlled concentration.
Several amines are used in the industry.
Examples include:
MEA
DEA
MDEA
DGA
Different solvents have different advantages.
Some remove acid gas very aggressively.
Some are more selective toward H2S.
Some require more regeneration energy.
Some are better suited to particular gas compositions or treating requirements.
There is no single best amine for every plant.
What Is MDEA?
MDEA means methyldiethanolamine.
It is widely used in natural gas processing.
One advantage of MDEA is that it can be formulated and operated to remove H2S selectively while allowing more CO2 to remain in the gas when that is desirable.
Selective H2S removal can reduce circulation and regeneration requirements in certain applications.
Other plants need deep CO2 removal as well.
The solvent choice and operating conditions therefore depend on the actual gas specification.
What Is MEA?
MEA means monoethanolamine.
It reacts strongly with acid gases and has been used for many years in gas treating.
It can provide effective removal, but regeneration energy and solvent degradation are important considerations.
The appropriate amine is selected by engineers based on the feed gas, pressure, contaminants, product specification, plant size, and economics.
Operators usually work with whatever solvent the plant was designed to use.
What Is an Amine Contactor?
The contactor is the tall vessel where sour gas meets lean amine.
It is also called an absorber.
Gas normally enters near the bottom and flows upward.
Lean amine enters near the top and flows downward.
This creates countercurrent contact.
Freshest amine contacts the cleanest gas near the top of the tower.
As the gas rises, H2S and CO2 transfer from the gas into the liquid amine.
Sweetened gas then leaves the top.
Rich amine leaves the bottom.
Why Does Gas Flow Up While Amine Flows Down?
Countercurrent flow improves mass transfer.
The sourest gas entering the lower section meets amine that has already absorbed some acid gas but can still continue treating.
As the gas becomes cleaner while rising, it eventually contacts the freshest lean amine near the top.
That final contact helps achieve the required gas specification.
This is a common design principle in absorption towers.
What Is Lean Amine?
Lean amine is regenerated amine containing relatively little absorbed acid gas.
It enters the contactor ready to remove H2S and CO2.
The word lean refers to acid gas loading.
It does not mean the solution contains very little amine.
A plant might operate with a substantial amine concentration while still calling the solution lean because its H2S and CO2 loading is low.
What Is Rich Amine?
Rich amine has absorbed acid gas from the sour gas.
It leaves the bottom of the contactor carrying H2S and CO2.
The rich amine then travels through the regeneration section of the plant.
The objective is to remove the absorbed acid gases so the solvent becomes lean again.
The same amine can then return to the contactor.
What Happens to Rich Amine After the Contactor?
The exact arrangement varies, but rich amine commonly passes through several pieces of equipment before reaching the regenerator.
These can include:
A flash vessel
Filters
A lean rich heat exchanger
A pressure control system
Other conditioning equipment
Each component has a purpose.
The rich amine contains more than dissolved acid gas.
It may also contain dissolved hydrocarbons, entrained gas, solids, corrosion products, and other contaminants.
What Is an Amine Flash Tank?
Rich amine leaves the absorber at relatively high pressure.
Reducing that pressure can release dissolved hydrocarbons and gas.
A flash tank provides a place for those gases and any separated hydrocarbon liquid to leave the amine before regeneration.
Removing hydrocarbons helps reduce problems in the regenerator.
Flash gas may be used as fuel or routed to another suitable system depending on the plant.
Why Is Hydrocarbon Contamination Bad for Amine?
Hydrocarbon contamination can contribute to:
Foaming
Poor separation
Solvent losses
Equipment fouling
Unstable operation
If oil or condensate enters the amine system, the plant can become difficult to control.
This is why good inlet separation upstream of the amine contactor matters.
The amine unit treats gas.
It is not meant to function as the primary liquid separator for large quantities of condensate or oil.
What Is the Lean Rich Heat Exchanger?
Hot lean amine leaves the regenerator.
Cooler rich amine is traveling toward the regenerator.
A heat exchanger transfers heat between these two streams.
The hot lean amine is cooled.
The rich amine is warmed before entering regeneration.
This reduces the amount of heat the reboiler has to supply and reduces the amount of cooling later required for the lean solvent.
The exchanger is therefore an important energy recovery device.
What Is the Amine Regenerator?
The regenerator removes H2S and CO2 from rich amine.
It is sometimes called the stripper.
Rich amine enters the tower.
Heat supplied by the reboiler drives acid gases out of the solution.
The acid gas travels upward and leaves near the top.
Regenerated lean amine collects near the bottom.
The process essentially reverses what happened in the contactor.
The contactor loads the solvent.
The regenerator unloads it.
Why Does Heating Amine Release H2S and CO2?
Absorption is favored by conditions in the contactor.
Regeneration changes those conditions.
Heating the solution and stripping acid gas from it shifts the equilibrium so H2S and CO2 leave the solvent.
Steam generated from the water in the amine solution helps carry acid gases upward through the regenerator.
This is why the reboiler is such an important part of the amine plant.
Without adequate regeneration, the amine returns to the contactor carrying too much acid gas.
What Is the Amine Reboiler?
The reboiler supplies heat to the regenerator.
The heat source may be steam, hot oil, direct firing, or another suitable system.
The reboiler heats the amine solution and generates stripping vapor.
Operators monitor conditions such as:
Temperature
Liquid level
Heat input
Regenerator pressure
Gas quality
Lean amine quality
Too little heat can result in poor regeneration.
Too much heat can create other problems, including increased energy use and possible solvent degradation.
What Happens to Acid Gas Leaving the Regenerator?
The overhead gas can contain H2S, CO2, water vapor, and smaller amounts of other components.
What happens next depends heavily on plant design and acid gas quantity.
The stream may go to:
A sulfur recovery unit
An acid gas injection system
An incineration system
A flare under applicable operating conditions
Another approved treatment or disposal process
Large sour gas plants often recover elemental sulfur from H2S rather than simply disposing of the acid gas.
What Is a Sulfur Recovery Unit?
A sulfur recovery unit converts hydrogen sulfide into elemental sulfur.
The Claus process is widely used for this purpose.
Instead of treating H2S only as waste, the plant converts much of its sulfur content into a recoverable solid or liquid sulfur product.
Sulfur can then be used in industries such as fertilizer and chemical manufacturing.
The amine unit and sulfur recovery unit therefore often operate as connected systems.
What Is Acid Gas Injection?
Some facilities compress acid gas and inject it into a suitable underground formation.
This can provide a disposal option for H2S and CO2 where geological and regulatory conditions allow it.
Acid gas compression is demanding service because the gas can be corrosive and hazardous.
Water management, materials selection, compression, and injection well performance all become important.
Why Is Lean Amine Cooled Before Returning to the Contactor?
Amine absorption generally works better when the lean solvent enters the contactor at an appropriate temperature.
If the amine is excessively hot, acid gas absorption performance can deteriorate.
The lean rich exchanger removes part of the heat.
A lean amine cooler removes additional heat before the solvent returns to the absorber.
However, making the amine unnecessarily cold can also create operating problems.
Temperature needs to be controlled, not simply minimized.
Why Should Lean Amine Be Warmer Than the Inlet Gas in Some Operations?
If amine is too cold relative to the gas, heavier hydrocarbons can condense into the solvent.
That hydrocarbon contamination can increase foaming risk.
Plants therefore often maintain a suitable temperature relationship between lean amine and incoming gas.
The exact target depends on plant design and feed conditions.
This is another reason amine temperature is an operating variable rather than just a cooling problem.
What Is Amine Circulation Rate?
Circulation rate is the amount of amine solution moving through the treating system.
Higher sour gas rates or higher acid gas concentrations can require more treating capacity.
Increasing circulation can provide more solvent to absorb H2S and CO2.
But simply turning the circulation rate higher is not always the correct solution.
Excess circulation increases:
Pump energy
Heating demand
Cooling demand
Hydraulic loading
Potential entrainment
Regenerator load
The goal is enough circulation to meet gas specification without unnecessarily overloading the plant.
What Happens If Amine Circulation Is Too Low?
The solvent may not have enough capacity to remove the required acid gas.
Sweet gas H2S or CO2 can increase.
The plant may go off specification.
The problem can become more obvious when feed gas flow or acid gas concentration increases.
Operators should therefore compare circulation rate with both gas rate and feed composition.
Can Increasing Amine Circulation Fix High H2S in Sales Gas?
Sometimes.
But not always.
High H2S at the contactor outlet can be caused by:
Low circulation rate
Poor lean amine regeneration
High inlet H2S
High gas rate
Poor tower contact
Foaming
Contaminated solvent
Incorrect temperature
Damaged trays or packing
Poor amine distribution
Instrumentation errors
Before increasing circulation aggressively, determine which limitation is actually controlling performance.
What Is Amine Foaming?
Foaming occurs when gas and amine form a persistent frothy mixture inside the contactor or regenerator.
Some amount of gas and liquid contact is normal.
Stable foam is not.
Foaming can reduce effective tower capacity and cause amine to be carried out with the gas.
It can also create unstable pressure drop and poor treating performance.
A plant that normally operates smoothly can become difficult to control very quickly when severe foaming starts.
What Causes Amine Foaming?
Common contributors include:
Hydrocarbon contamination
Compressor oil
Solids
Corrosion products
Degradation products
Certain chemical contaminants
Poor filtration
Excessive gas velocity
Contamination from upstream operations
The exact cause can be difficult to identify.
Antifoam chemical may temporarily reduce symptoms, but repeated foaming deserves a search for the contamination source.
What Are Signs of Amine Foaming?
Operators may notice:
Increasing contactor differential pressure
Unstable liquid levels
Amine carryover
Poor sweet gas quality
Sudden solvent losses
Changing regenerator behavior
Unexpected liquid in downstream gas equipment
The pattern often develops quickly.
Comparing tower differential pressure with normal values can provide an important clue.
Why Does Contactor Differential Pressure Matter?
Gas has to flow upward through trays or packing while amine flows downward.
That creates a normal pressure drop.
If the tower begins flooding or foaming, resistance to gas flow increases.
Differential pressure rises.
A rising tower differential pressure combined with poor gas quality can indicate a hydraulic problem rather than simply insufficient chemical strength.
What Is Flooding in an Amine Contactor?
Flooding occurs when gas velocity or liquid loading becomes too high for the tower to maintain proper countercurrent flow.
Liquid begins accumulating.
Gas has difficulty moving upward.
Differential pressure can increase sharply.
Mass transfer performance becomes unstable.
Possible causes include excessive gas flow, high liquid circulation, foaming, damaged internals, or restrictions.
Reducing one problem by increasing amine circulation can therefore make another problem worse if the tower is already near its hydraulic limit.
Why Are Amine Filters Important?
The amine system collects contaminants over time.
Filtration helps remove suspended solids and other material that can contribute to:
Foaming
Erosion
Fouling
Corrosion
Equipment plugging
Plants may use different types of filtration, including mechanical filters and carbon treatment.
Filter condition can provide useful information about what is happening inside the amine system.
Rapidly plugging filters may indicate that a larger contamination or corrosion problem exists.
Why Does Amine Turn Dark?
Healthy amine solution does not necessarily remain perfectly clear forever.
Darkening can indicate accumulation of:
Corrosion products
Degradation products
Hydrocarbon contamination
Solids
Other contaminants
Color alone does not provide a complete diagnosis.
Laboratory analysis is much more useful.
But a major change in appearance can be a clue that solvent condition is changing.
What Are Heat Stable Salts?
Certain acidic contaminants react with amine and form salts that are not removed easily through normal regeneration.
These are commonly called heat stable salts.
They remain in the circulating solution and can contribute to corrosion, reduced solvent capacity, and operating problems when concentrations become excessive.
Plants monitor amine chemistry and may use reclaiming or other solvent management methods when needed.
Why Does Amine Corrosion Occur?
Amine systems can become corrosive under certain conditions.
Contributing factors can include:
Acid gas loading
Temperature
Heat stable salts
Solvent degradation
High velocity
Contaminants
Poor operating control
Corrosion products can then circulate through the system and contribute to fouling and foaming.
This can create a cycle where one problem generates another.
Why Is the Reboiler Temperature Important?
The reboiler needs enough heat to regenerate the solvent.
But excessive temperature can damage the amine solution.
Thermal degradation can create unwanted compounds that remain in circulation.
Operators therefore maintain the reboiler within the plant’s approved operating range.
The correct temperature depends on solvent type, pressure, concentration, and equipment design.
More heat is not automatically better regeneration.
Why Can High Regenerator Pressure Hurt Performance?
Regeneration works by encouraging acid gases to leave the solvent.
Higher pressure can make that separation less favorable.
A restriction in the overhead system can therefore affect lean amine quality.
If regenerator pressure rises unexpectedly, operators may investigate:
Overhead piping
Condensers
Valves
Acid gas handling equipment
Downstream pressure
The regenerator is affected by whatever happens after the acid gas leaves it.
What Is the Reflux System?
Water vapor and some amine related components leave the regenerator overhead with the acid gas.
The overhead stream is commonly cooled.
Water condenses.
A reflux accumulator separates the condensed liquid.
Some of that liquid is returned to the regenerator.
This helps control water balance and reduce amine loss.
The remaining acid gas continues to its downstream destination.
Why Does Amine Concentration Matter?
The solvent must contain the correct amount of amine and water.
If concentration becomes too low, treating capacity may suffer.
If it becomes too high, other operating concerns can appear depending on the solvent.
Water is not simply an unwanted diluent.
It is part of the designed solvent system.
Operators monitor concentration and add water or amine according to the plant’s solvent management program.
Why Does the Amine System Lose Water?
Water can leave with:
Sweet gas
Acid gas
Hydrocarbon liquids
Drainage
Maintenance activities
Leaks
Higher temperatures can increase water vapor losses.
The plant therefore needs makeup water.
Water quality matters because contaminants introduced with makeup water can accumulate in the solvent system.
Why Does an Amine Plant Lose Solvent?
Solvent losses can occur through:
Foaming
Gas entrainment
Hydrocarbon carryover
Leaks
Draining
Maintenance
Regenerator overhead losses
Poor separation
A sudden increase in amine makeup should be investigated.
The solvent did not disappear.
It went somewhere.
Finding that path can reveal a larger equipment or process problem.
Why Can Amine Reach the Sales Gas System?
Liquid amine can be carried out of the contactor with the treated gas.
This is called amine carryover.
Potential causes include:
Foaming
High gas rate
Damaged mist elimination equipment
Flooding
High liquid level
Tower internal problems
Carryover can contaminate downstream equipment and increase solvent consumption.
A downstream separator or filter catching unexpected amine is evidence worth investigating.
Why Does Inlet Separation Matter So Much?
The amine contactor works best when the incoming gas is actually gas.
Free hydrocarbon liquid and water should normally be removed upstream.
Liquid entering the absorber can contaminate the amine and increase foaming.
A poorly functioning inlet separator can therefore create problems that appear to originate inside the amine plant.
When a treating problem begins suddenly, check what changed upstream.
What Happens If the Amine Unit Goes Down?
If the gas is sour enough that treatment is required for sales specification, losing the amine unit can limit or stop gas production.
Untreated gas may not be allowed into the sales pipeline.
Depending on facility design, the operator may need to:
Reduce production
Shut in wells
Divert gas
Use temporary treating equipment
Follow another approved operating plan
A relatively small treating plant can therefore control the production capacity of an entire field.
Why Is Gas Sweetening Different From Gas Dehydration?
Sweetening removes acid gases such as H2S and CO2.
Dehydration removes water vapor.
An amine unit and glycol dehydrator therefore perform different jobs.
A gas plant may need both.
For example, gas might first be sweetened with amine and then dehydrated with glycol before entering a pipeline.
The correct process order depends on the facility design.
Is Amine the Only Way to Sweeten Natural Gas?
No.
Other gas treating technologies include:
Physical solvents
Solid absorbents
Membranes
Specialized scavengers
Hybrid solvents
Other chemical processes
The preferred technology depends on:
Gas flow
Pressure
H2S concentration
CO2 concentration
Product specification
Gas composition
Facility size
Economics
Disposal options
Amine remains common because the solvent can be continuously regenerated and reused.
What Is an H2S Scavenger?
A scavenger is a chemical that reacts with H2S.
Scavengers can be useful for smaller gas streams, temporary operations, polishing service, or applications where a full regenerative amine plant is not practical.
Unlike a regenerative amine system, many scavenger processes consume chemical as H2S is removed.
That can become expensive at large acid gas loads.
The economics therefore change with gas rate and H2S concentration.
Why Is Gas Sweetening Important for Pipelines?
A transmission pipeline wants gas that meets its quality specification.
Excess H2S creates safety and corrosion concerns.
Excess CO2 reduces heating value and can contribute to corrosion when water is present.
Off specification gas can therefore be rejected.
The producing company may have plenty of gas underground but still be unable to sell it if the treating system cannot meet specification.
How Should an Operator Troubleshoot High H2S in Sweet Gas?
Avoid changing several variables at once.
Start by confirming the reading.
Then compare current conditions with normal operation.
Check:
Inlet H2S concentration
Gas rate
Amine circulation
Lean amine temperature
Lean amine loading
Contactor differential pressure
Regenerator temperature
Regenerator pressure
Filter condition
Foaming indicators
Solvent concentration
Recent process changes
Ask what changed immediately before gas quality became worse.
That question often shortens troubleshooting dramatically.
A Simple Amine Plant Example
Imagine sour gas enters an amine plant containing H2S and CO2.
The gas first passes through an inlet separator to remove free liquid.
It enters the bottom of the amine contactor.
Lean amine enters from the top.
As the two streams contact each other, acid gases move into the amine.
Sweet gas leaves the top and continues toward dehydration or sales.
Rich amine leaves the bottom.
Pressure is reduced.
Hydrocarbon gas may flash off.
The rich amine is heated in the lean rich exchanger.
It enters the regenerator.
The reboiler supplies heat.
H2S and CO2 leave overhead as acid gas.
Lean amine leaves the bottom.
It transfers heat to the incoming rich amine.
A cooler reduces its temperature further.
Filters remove contaminants.
The circulation pump sends the regenerated lean amine back to the contactor.
Then the entire cycle begins again.
That is the core of amine gas sweetening.
Frequently Asked Questions
What does gas sweetening mean?
Gas sweetening is the removal of acid gases, especially hydrogen sulfide and often carbon dioxide, from natural gas.
Why is natural gas called sour?
Gas containing hydrogen sulfide is commonly called sour gas.
Why is hydrogen sulfide removed?
H2S is highly toxic, corrosive, and usually restricted by gas quality specifications.
Why is carbon dioxide removed?
CO2 reduces gas heating value, occupies pipeline capacity, and can contribute to corrosion when water is present.
What is an amine unit?
An amine unit uses a circulating liquid solvent to absorb H2S and CO2 from natural gas and then regenerates the solvent for reuse.
What is lean amine?
Lean amine is regenerated solvent containing relatively low acid gas loading.
What is rich amine?
Rich amine is solvent that has absorbed H2S and CO2 from sour gas.
What is an amine contactor?
It is the vessel where sour gas contacts lean amine and acid gases are absorbed into the liquid.
What is an amine regenerator?
It is the tower where rich amine is heated so absorbed H2S and CO2 are released.
Why does an amine plant have a reboiler?
The reboiler provides the heat needed to regenerate the solvent.
What happens to H2S removed by the amine?
It leaves the regenerator in the acid gas stream and may go to sulfur recovery, acid gas injection, or another approved handling system.
What causes amine foaming?
Hydrocarbons, compressor oil, solids, corrosion products, solvent degradation products, and other contaminants can contribute to foaming.
How do you know an amine contactor is foaming?
Possible signs include rising differential pressure, unstable levels, poor gas quality, amine carryover, and sudden solvent losses.
Why are amine filters important?
They remove contaminants that can contribute to foaming, corrosion, fouling, and poor plant performance.
Can increasing amine circulation always fix high H2S?
No. Poor regeneration, foaming, contamination, high gas rate, tower flooding, temperature problems, or damaged internals may be the real cause.
Why is lean amine temperature important?
Temperature affects acid gas absorption and hydrocarbon condensation. The solvent needs to enter the contactor within the appropriate operating range.
Why does an amine plant need an inlet separator?
The separator removes free liquids that could contaminate the amine and cause foaming or other problems.
Is gas sweetening the same as dehydration?
No. Sweetening removes H2S and CO2. Dehydration removes water vapor.
Is amine the only method used for gas sweetening?
No. Membranes, physical solvents, scavengers, solid media, and other technologies can also be used.
What is the most important thing to understand about an amine plant?
The contactor and regenerator are two halves of the same cycle.
The contactor loads the solvent with acid gas.
The regenerator removes that acid gas.
If either side of the cycle performs poorly, the plant can produce off specification gas even when the other side appears to be operating normally.