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Manufacturer 4A vs 3A Molecular Sieves in LNG Dehydration

Table of Contents

Introduction

4A Molecular Sieve
LNG dehydration mainly uses 4A molecular sieves for water and hydrocarbon removal, while 3A suits cases needing zero hydrocarbon adsorption.

In LNG (Liquefied Natural Gas) processing, 4A molecular sieves are more widely used due to superior dehydration performance and ability to adsorb heavy hydrocarbons, while 3A molecular sieves serve in specific cases such as protecting downstream equipment.

4A Molecular Sieve – The LNG Industry Standard

Stronger water adsorption
With a pore size of 4 Å, 4A molecular sieves can adsorb water (2.8 Å) and small molecules such as CO₂ and methanol. They achieve deep dehydration to a dew point below –70 °C, meeting strict pre-liquefaction standards.

Better performance with hydrocarbon-containing gas
LNG feed gas often contains heavy hydrocarbons (e.g., ethane, propane). 4A sieves can adsorb part of these hydrocarbons, reducing the risk of downstream freezing. For example, Dagang Natural Gas Company uses a “high-porosity top layer + 4A molecular sieve bottom layer” configuration to significantly slow dew point degradation.

Proven industrial track record
Projects such as Kunlun Energy’s Huanggang LNG plant and the Inner Mongolia Western Natural Gas project use 4A molecular sieve dehydration, with bed volumes of tens of cubic meters—showing mature, reliable technology.

3A Molecular Sieve – Niche LNG Uses

Catalyst and sensitive equipment protection
With a smaller 3 Å pore size, 3A sieves adsorb only water, excluding unsaturated hydrocarbons like ethylene and propylene. This makes them ideal for cracked gas drying in ethylene production, avoiding hydrocarbon loss.

Lower regeneration energy
Since 3A sieves do not adsorb larger hydrocarbons, regeneration requires less energy. However, in LNG applications, they are rarely used alone.

Selection Criteria – 4A vs 3A

Parameter4A Molecular Sieve3A Molecular Sieve
Pore size4 Å (adsorbs water, CO₂, small hydrocarbon)3 Å (adsorbs water only)
LNG suitabilityMainstream choice for hydrocarbon-rich gasSpecial cases (protecting downstream catalyst)
Typical projectsHuanggang LNG, Inner Mongolia LNGPetrochemical cracked gas drying
4A molecular sieve

Conclusion

LNG dehydration predominantly relies on 4A molecular sieves for their combined water removal and hydrocarbon adsorption capacity. 3A molecular sieves are used only where hydrocarbon adsorption must be avoided. The final choice depends on feed gas composition (especially heavy hydrocarbon content) and process requirements (such as dew point specifications).

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