How to Select the Right Molecular Sieve for Natural Gas Drying

Natural gas extracted from wellheads is almost always saturated with water vapor. When the gas is compressed, transported, and processed—especially in liquefied natural gas (LNG) facilities operating below -160°C, this moisture becomes a serious operational hazard. Even trace amounts of water can form hydrates that block pipelines, accelerate corrosion when coexisting with CO2 or H2S, and cause freezing blockages in cryogenic equipment.

 

Although glycol systems and silica gel also have their applications, molecular sieves are the only technology capable of widely achieving stable pipeline dew points below -20°C or liquefied natural gas (LNG) feed requirements of <0.1 ppmv water.

 

1.Core Selection Principle: Molecular Sieving

Molecular sieves operate on the principle of size exclusion: molecules smaller than the pore size enter and are adsorbed, while those larger than the pore size pass through. However, pore size is not the sole determining factor. Molecular polarity is equally important—highly polar molecules like water are more readily adsorbed than nonpolar molecules, even when their diameters are similar.

 

Typical pore size of A-type molecular sieve

Type	Pore diameter	Balanced cation	Characteristics
3A	0.3nm	K+	Block molecules larger than H2O
4A	0.4nm	Na+	Common gas drying
5A	0.5nm	Ca2+	Adsorbed water+n-alkanes CO2、H2S

 

2. Model selection: Match aperture according to application

For natural gas and LNG dehydration, the following three models have practical significance:

2.1 3A molecular sieve - preferred for LNG and hydrocarbon rich gas streams

Aperture: 3 Å - allows water molecules (with a kinetic diameter of approximately 2.65 Å) to enter, while blocking larger hydrocarbon molecules such as ethylene, ethane, propane, and butane.

Why is this important: During the dehydration process of LNG feed gas, hydrocarbon co adsorption competes with water molecules for active sites, thereby reducing the adsorption capacity of water. During the high-temperature regeneration process (250-320°C), the adsorbed hydrocarbons will crack and form carbon deposits, causing permanent deactivation of the molecular sieve.

The main advantages of 3A molecular sieve applied to LNG are:

* Achieve an extremely low dew point below -70°C to meet LNG feed specifications;

* Compared to 4A, the co-adsorption of hydrocarbons is reduced by 30-40%;

* Reduce the risk of hydrocarbon cracking during the regeneration process;

* Longer service life in hydrocarbon rich gas streams.

 

The following are recommended operating conditions for 3A:

* LNG pre-treatment (feed gas for liquefaction unit)

* Natural gas flow containing high C3+hydrocarbon components

* Olefin drying (ethylene, propylene)

* Offshore platforms with expensive renewable energy costs

 

2.2 Selection of 4A Molecular Sieve Pipeline Natural Gas Dehydration Standards

Pore size: 4 Å - efficiently adsorbs water while also removing small amounts of CO₂, ammonia, and SO₂.

Application scope:

* Pipeline natural gas dehydration, target dew point -10°C to -20°C

* Gas gathering station and compression station

* Dehydration of Liquefied Petroleum Gas (LPG)

* Conventional natural gas processing with low moisture requirements

Important limitation: In gas streams containing high C3+heavy hydrocarbons, 4A molecular sieves will co adsorb these hydrocarbon molecules. During the regeneration process, they will crack and form carbon deposits, gradually reducing their adsorption capacity.

Selection rule of thumb: If your airflow contains more than trace amounts of C3+hydrocarbons, even for pipeline applications, 3A is technically a better choice.

 

Application Scenario:

* Biogas purification (simultaneous removal of H₂S and water)

* Small LNG system with integrated deacidification function

* Hydrogen purification

The working condition requires simultaneous dehydration and limited removal of acidic gases in a single bed layer.

Explanation on substitutability: 5A cannot completely replace 4A for most gas drying applications. Although 5A can also adsorb water, its larger pore size may simultaneously adsorb other molecules, which may reduce adsorption efficiency and product gas purity. For applications that require selective removal of water without co adsorption of hydrocarbons or acidic gases, 3A or 4A is still a suitable choice.

Application scenarios	Recommend type	Reason
LNG cryogenic gas	3A	Prevent hydrocarbon co-adsorption and carbon deposition
Pipeling gas	4A	Economic efficiency
Pipeline gas containing heavy hydrocarbons	3A	Prevent hydrocarbon cracking during regeneration process
BIogas, sulfides	5A or 13X	One bed layer removes two types of pollutants
Air pre-purification	13X	Effective adsorption of CO2 with larger pore size

 

The correct molecular screening method can not only significantly reduce operational energy consumption during natural gas dehydration, but also effectively extend equipment life and avoid safety risks caused by hydrate freezing and blockage. Welcome to contact us anytime for technical solutions.