Ultimate Guide to Natural Gas Purification: Methods and Technologies
Nature has blessed us with uncountable resources; some can be used directly, while others need to undergo a purification process. This is true for natural gas as well. To harness its full potential, natural gas must undergo a purification process in a gas processing plant to remove impurities that can hinder its performance. Whether it’s being used for power generation, petrochemical production, or transportation, ensuring its purity is essential.
Natural gas extracted from underground reservoirs does not come in its purest form. It has to undergo industrial separation processes that eliminate impurities, resulting in dry gas suitable for transportation and consumption.
In this guide, we’ll explore the purification processes that natural gas goes through to meet the demands of various industries.
What is Natural Gas Purification, and Why is Removing Its Main Impurities Necessary?
Raw natural gas consists majorly of methane (the main component used for energy) and ethane, but it also contains other impurities such as -
1. Water Vapor (H2O)
The natural gas extraction from the well contains water vapor, which, if not removed, can condense into liquid water, causing issues such as reduced energy content and clogged pipelines.
2. Carbon Dioxide (CO2)
Carbon dioxide can be present in concentrations ranging from 10-20 mol% to as high as 70 mol%. Gas containing high volumes of CO2 cannot be burned safely and can react with H2S and H2O to make corrosive compounds that threaten steel pipelines.
3. Hydrogen Sulfide (H2S) – "Sour Gas"
Natural gas containing high amounts of hydrogen sulfide is also known as sour gas due to its foul smell, toxicity, and flammability. It can pose potential health risks to humans and can damage pipelines.
4. Mercury (Hg)
The concentration of mercury in natural gas can cause problems such as corrosion, equipment damage, poisoning the catalyst, and raising environmental concerns.
It is essential to purify natural gas to ensure safer and more efficient use. This purification not only affects pipelines but also has implications for the environment. For instance, the United States has strict regulations regarding the emissions of harmful gases, which require industries to adhere to specific environmental standards.
Key Equipment Used in Natural Gas Purification and Their Functions
To remove impurities from natural gas, we require the equipment mentioned below. These are important to ensure that natural gas is safe, efficient, and clean before being utilized in houses, industries, and power plants.
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Separators:
The first step towards the purification of natural gas is done using a separator. The gas passes through it and removes liquid like water and heavy hydrocarbons (oily substances) in massive amounts.
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Amine Absorption Units:
Anime absorption units are machines that remove harmful gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S) from natural gas. In brief, this is a process where a chemical called amine is used to clean the gas; when the natural gas passes through the unit, the amine reacts with harmful gases and absorbs them.
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Sulfur Recovery Units (SRUs):
The sulfur recovery unit helps convert hydrogen sulfide into solid sulfur, which is later sold and used in the making of fertilizers, batteries, rubber, and chemicals.
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Cryogenic Separation Units: (Air Separation Units)
The air is separation unit is removing Moisture, Co2 and Hydrocarbon With the help of Molecular Sieve and Alumina Balls, then the cryogenic gas plant uses freezing temperatures to separate natural gas liquids (NGLs) from natural gas. Now, the NGLs include - ethane, butane, and propane; they are valuable and helpful in many industries, so they are sold separately afterward.
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Dehydration system:
Glycol dehydration is a process that is used to remove water vapor from natural gas; it prevents the forming of ice-like substances in the pipeline. The process works in the way that the natural gas passes through a liquid called glycol; it then absorbs the water from the gas and leaves it dry, eliminating the risk of freezing.
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Mercury removal system:
Small size yet significant effect, yes, that's true when we talk about mercury; despite the quantity present in natural gas, the damage is always severe. In LNG plants and cryogenic units, the presence of mercury would do massive damage to aluminum equipment, pipelines and machinery.
The Process of Natural Gas Purification
Natural gas processing is a step-by-step method to remove impurities from the natural gas. Let's take a look at the significant steps -
1. Raw Gas Pretreatment
The first step in natural gas purification is raw gas pretreatment; as the name suggests, it is all about cleaning up the raw gas, removing large impurities, and setting the gas up in the right conditions for the rest of the purification process.
When natural gas is excreted from the ground, it is accompanied by oil, water, and dirt. The unwanted elements are removed using types of equipment’s such as separators and filters.
The pretreatment also involves controlling the temperature and pressure of the gas; natural gas needs to be at the right temperature and pressure for the next steps in purification to work correctly.
2. Acid Gas Removal
Acid gases like hydrogen sulfide and carbon dioxide are removed during the purification process of natural gas. Now, these gases need to be controlled as they pose a serious issue for both the environment and further operations.
Problems such as - corroded pipelines, blockage in equipment, lowering the energy content of natural gas, freezing during specific processes, etc.
Amine absorption is the solution to this problem! The amines react with H2S and CO2, capturing these gases and removing them from the gas stream.
Physical methods such as molecular sieve adsorption or pressure swing adsorption are used for further purification. The following techniques are used when an ultra-pure gas is required, removing almost all sulfur from natural gas.
3. Dehydration
The dehydration process is necessary to prevent hydrate formation in the pipeline because water vapor combines with natural gas to form hydrates(ice-like crystals), blocking the pipelines and causing other serious issues such as loss of gas flow and operational interruptions.
Water in natural gas causes corrosion in pipes and equipment, which damages the machinery and shortens its lifespan with costly repairs.
To protect other processes, like cryogenic separation and sulfur recovery that comes after dehydration, it's essential to remove water from the natural gas.
So now you know why Dehydration is needed! Let's jump onto the Dehydration process. There are four main methods for dehydration-
1. Primary Dehydration with Glycol: Primary Dehydration with Glycol is one of the most common methods to remove water from natural gas. Triethylene glycol (TEG) is a liquid that passes through the gas stream, absorbing the water vapor from the gas. The final gas will have less than 7 lbs of water per million standard cubic feet (lbs/MMscf), which meets the standards for pipeline-quality gas.
2. Molecular Sieves (for higher moisture content): When the water content in the natural gas is high, 4A molecular sieves are used for an efficient purification process, especially in low-temperature conditions.
3. 13X Molecular Sieves are used for deeper drying of natural gas; they are a type of adsorbent material made up of zeolite crystals. Zeolites are porous materials that can trap and hold molecules such as water. The 13X molecular sieves have a high affinity that enables them to remove water even at low concentrations (as low as 0.1 parts per million, or ppm)
4. Activated alumina is used for moderate drying and helps achieve moisture levels like a dew point of -40°C. The activated alumina has a porous structure, high surface area, and high water adsorbing capacity, which makes it the best option for moderate drying. A cost-effective solution to remove bulk water load.
4. Removal of Heavy Hydrocarbons and Mercury
Certain harmful chemicals, such as heavy hydrocarbons and mercury, need to be removed, even if they are in their smallest form. Hydrocarbons (large molecules) solidify in low temperatures, blocking the pipes and equipment.
On the other hand, mercury can corrode aluminum parts in cryogenic heat exchangers, causing them to break down and leading to heavy repair expenses. Molecular sieves (5A,13X) are the most widely used advanced adsorbents to remove these impurities like moisture, CO2 and Hydrocarbons.
The molecular sieves 5A and 13X are very good at trapping both heavy hydrocarbon and mercury. Molecular Sieve is best known for working at low temperatures, a much-needed element in cryogenic systems. They are reusable and cost-effective, making them the best choice for the longer run.
Activated carbon: Activated carbon is also used in the removal of mercury. However, it's always considered as a second choice.
Purity Requirements for Natural Gas in Different Industries
Various industries have different purity requirements for natural gas to ensure that their equipment is not damaged. Second, they want natural gas to be purified in certain ways that fit best for their use. Here is a list of a few industries and their purity requirements.
Different industries have different requirements for natural gas purity depending on what the gas will be used for:
- Pipeline transportation:Needs to be free of water, acid gases, and impurities to prevent corrosion and blockages.
- Power plants need to be dry to avoid corrosion in equipment.
- Petrochemical applications: Needs to be free of sulfur and hydrocarbons to avoid interference with chemical reactions.
- NGL production: Needs to be very dry and free of CO2 to avoid freezing and contamination during the separation process.
- Oil refineries and specialized applications Need specific control over sulfur, moisture, and hydrocarbon composition for efficient and high-quality production.
In natural gas treatment, desiccants such as Alumina Balls and Molecular sieves are the best choice for achieving these purity levels because they are highly effective, selective, and regenerative, which makes them a cost-effective solution for long-term use.
