Molecular Sieve Desiccant: 3A, 4A, 5A and 13X Explained

A molecular sieve desiccant is a crystalline aluminosilicate whose pores are all one exact size, so it separates molecules by size as well as adsorbing them. That is what lets it dry a gas to a lower dew point than a bulk desiccant. We supply four types — 3A, 4A, 5A and 13X, with pore openings of 3, 4, 5 and 10 Å — and the type is chosen by the molecules you want to hold back. This page explains what each one adsorbs and how to choose.

What a Molecular Sieve Is

Molecular sieves are adsorbents made of crystalline silicates — sodium aluminosilicate and calcium aluminosilicate — inorganic porous crystalline materials with a uniform microporous structure. That uniformity is the point: unlike a silica gel or alumina, whose pores vary in size, a sieve has a single pore opening, so a molecule larger than that opening is excluded and a smaller one is taken in. Our molecular sieve is supplied as yellowish-brown granules in spherical or rod form.

The Four Types

What Each Type Adsorbs

• 3A — adsorption separation of unsaturated hydrocarbons and cracked gases (ethylene, acetylene, propylene, butadiene), and deep dehydration of gases such as air, ethanol, natural gas and methane.
• 4A — drying equipment for air compressors and LPG stations; adsorption of water, methanol, ethanol, H₂S, SO₂, CO₂, ethylene and propylene; deep drying of gases and liquids; static drying of pharmaceutical packaging and electronic components; dehydrating agent in paints and dyes.
• 5A — n-/iso-paraffin separation, pressure swing adsorption, and co-adsorption of water and CO₂; drying and purification in gas production units; natural gas drying and desulphurisation; ammonia-nitrogen tail-gas purification.
• 13X — catalyst carrier; co-adsorption of water and CO₂ / H₂S and natural gas deodorisation; drying in pharmaceutical and air-compression systems with higher capacity and a lower dew point; deep purification of feed gas in air separation; drying and desulphurisation of natural gas, LPG and liquid hydrocarbons.

How to Choose a Type

Start from the smallest pore that admits the molecules you want to remove. If your goal is dehydration and nothing else, 3A holds water preferentially and excludes larger molecules. If the stream also carries CO₂, H₂S or other small molecules that must come out at the same time, move up to 4A, 5A or 13X, which co-adsorb them. And if you need a size-based separation rather than drying — splitting n- from iso-paraffins, for example — that is a 5A duty. For a gas-drying process specifically, our molecular sieve dehydration units page covers how the bed is regenerated and reused.

Form and What to Specify

• Type — 3A, 4A, 5A or 13X.
• Form — spherical or rod granule, sized to your vessel.
• Duty — dehydration, or a co-adsorption / separation duty.
• Packing and quantity — composite bag or drum; other packaging on request.

Give us the gas or liquid composition, the flow and the target dryness and we will recommend the type and form. Prices are quoted on request. For how the sieve compares against the other adsorbents, see molecular sieve vs silica gel vs alumina and molecular sieve for natural gas drying.