3A and 4A Molecular Sieves: Choosing the Right Pore Size

Part of the Zeolite Molecular Sieves: The Complete Guide — this article is one of the detailed pages in the guide.

Direct answer: 3A sieve (0.3 nm pores, potassium-exchanged A zeolite) admits water molecules but excludes CO₂, methane and ethanol — the choice for deep drying and solvent dehydration where nothing but water may be adsorbed. 4A sieve (0.4 nm pores, sodium A zeolite) co-adsorbs water with small molecules like CO₂, giving higher total capacity but less selectivity — the choice for general gas drying and small-stream CO₂ removal. Regeneration for both is thermal, typically 200–350 °C.

1. The pore-size logic

The type number states the effective pore opening in ångströms. Selection is decided by comparing the kinetic diameters of the molecules involved:

Molecule	Kinetic diameter	Enters 3A (0.3 nm)?	Enters 4A (0.4 nm)?
Water	~2.8 Å	Yes	Yes
CO₂	~3.3 Å	No	Yes
Methane	~3.8 Å	No	Yes
Ethanol	~4.4 Å	No	No

3A = potassium-exchanged type A zeolite. Water in, everything else out.
4A = sodium type A zeolite. Water plus CO₂, H₂S, methanol, ethanol vapor, NH₃.

Molecular diameters are textbook values (general technical knowledge); pore assignments are vendor-standard (sorbsieve.com).

2. Typical duties

Duty	Choice	Reason
Ethanol / fuel-ethanol dehydration to ≥99.5% (v/v)	3A	Adsorbs water without holding ethanol
Solvent drying (dries the solvent, not the solvent out of itself)	3A	Size exclusion protects the product
Natural-gas deep drying / LNG pre-liquefaction	3A	Dew points to −100 °C, water below 0.1 ppmv
Olefin cracker-gas drying (<1 ppm water, dew point < −70 °C), refrigerants, LPG	3A	Deep dry without co-adsorbing product
Insulated-glass units	3A	Adsorbs moisture only — 4A would co-adsorb O₂/N₂, creating pressure differences that deform the panes
Compressed-air drying, pipeline-gas dew-point control (−10 to −20 °C), CO₂ removal	4A	Higher total capacity, small-molecule co-adsorption acceptable
Detergent builders (phosphate replacement), water softening	4A	Ion-exchange duty, not gas drying
Air separation pre-purification	4A	CO₂ + water removal ahead of cryogenic or PSA separation

3A deep-drying capability: water below 1 ppm / dew point below −70 °C in cracker-gas service (Chinese catalyst vendors), down to −100 °C claimed for natural-gas deep drying (vendor literature — confirm against Zeochem/UOP datasheets before design). 4A in pipeline service delivers −10 to −20 °C dew points.

3. Water capacity and kinetics

Static water capacity: 3A ≥ 20 wt%; 4A ~20–22 wt% (vendor specifications; GB/T 6287-2021 defines the static water adsorption test method). Roughly 0.2 g water per gram of 4A at 25 °C / 60% RH.
Bulk density (4A reference): ~770 g/L → on the order of 150–170 g water per litre of bed (inference from the two vendor numbers — verify per batch).
Kinetics: in pure water-removal service, 3A is the faster adsorber of water; 4A's higher total capacity comes with co-adsorption of other species.

4. Regeneration

Temperature window: 200–350 °C for 3A/4A/5A; below ~200 °C water does not desorb fully, above ~350 °C the crystal structure can be damaged. Heat-up 25–50 °C/h, hold 2–8 hours; laboratory regeneration commonly 200–350 °C oven, 2–4 hours.
Cooling discipline: after regeneration, cool under dry sweep gas — exposing a hot regenerated bed to ambient air instantly re-humidifies it.
Maximum continuous operating temperature (4A): 400 °C.
Service life: 2–3 years typical (vendor sources conflict at 3–5 years; take the conservative figure and verify per application).

5. Specification points for buyers

Chinese standards: GB/T 10504-2017 for 3A sieves; HG/T 2524-2010 for 4A (sphere, extrudate, powder, activated powder grades); static water adsorption per GB/T 6287-2021.
Always state: equilibrium water capacity at which conditions, crushing strength, and the dew point the bed must hold — not just "3A" or "4A".
13X caution: regeneration temperatures cited for 13X conflict across sources (250–350 °C vs 350–500 °C); use the lower recommended range unless a datasheet says otherwise.