The difference between molecular sieve 3A, 4A, 5A and 13X is pore size. 3A has an effective pore opening of about 3 angstroms (Å), 4A about 4 Å, 5A about 5 Å, and 13X about 10 Å. That single dimension decides which molecules enter the crystal structure and are adsorbed, and which pass through untouched. Capacity, regeneration temperature and application all follow from it.
Each grade is engineered for a specific separation. The correct choice is the pore opening that captures the target molecule while leaving the product stream intact. All are synthetic molecular sieves, crystalline aluminosilicates manufactured to a uniform and precisely controlled pore dimension, and all four are supplied within the same range of molecular sieve adsorbents used for industrial drying and purification.
Comparison of 3A, 4A, 5A and 13X
| Property | 3A | 4A | 5A | 13X |
|---|
| Effective pore size | ~3 Å | ~4 Å | ~5 Å | ~10 Å |
| Zeolite type | Type A, potassium form | Type A, sodium form | Type A, calcium form | Type X, sodium form |
| Adsorbs | Water only | Water, CO₂, SO₂, methanol, ethanol | Water, CO₂, straight chain hydrocarbons | Water, CO₂, mercaptans, larger molecules |
| Excludes | Ethylene, propylene, ethanol | Propane and larger hydrocarbons | Branched and cyclic isomers | Very little, widest opening |
| Typical duty | Drying unsaturated hydrocarbons, LPG, alcohols | General gas and liquid drying, air dryers | n-paraffin separation, hydrogen purification | Air pre-purification, CO₂ removal, oxygen PSA |
| Regeneration temperature | Lower | Moderate | Moderate | Higher |
What the Numbers and Letters Denote
The designation states a dimension, not a ranking. The number gives the approximate pore opening in angstroms; the letter gives the crystal structure.
- 3A, 4A and 5A share the type A crystal framework. The opening differs with the balancing cation: potassium narrows it to roughly 3 Å, sodium gives about 4 Å, and calcium opens it to about 5 Å.
- 13X uses the type X framework, with an opening near 10 Å and a larger internal pore volume.
Functionally, each grade acts as a filter at molecular scale. A molecule smaller than the opening enters and is retained; a molecule larger than the opening passes through, regardless of contact time or bed depth.
Molecular Sieve 3A: Selective for Water
With an opening near 3 Å, molecular sieve 3A admits water while leaving ethylene, propylene and ethanol in the stream. That precision is what the grade is built for.
The 3A zeolite is specified wherever the product itself must be preserved. Typical duties include:
- Drying unsaturated hydrocarbons such as ethylene, propylene and butadiene
- Moisture removal from liquefied petroleum gas (LPG)
- Drying cracked gas in petrochemical processes
- Dehydration of polar solvents and alcohols, with the alcohol retained
- Static drying in sealed systems, including insulating glass units
A second advantage applies to reactive streams. Because unsaturated hydrocarbons stay outside the pores, 3A mol sieve avoids the polymerisation and subsequent coking during regeneration that would otherwise shorten bed life.
Molecular Sieve 4A: Broad Drying Capability
At roughly 4 Å, molecular sieve 4A adsorbs water strongly and additionally takes up carbon dioxide, sulphur dioxide, hydrogen sulphide, methanol and ethanol, while excluding propane and larger hydrocarbons.
That range makes the 4A grade the established desiccant for general duty, where every component of the stream may be dried. It is applied to refrigerant drying, natural gas dehydration, polar solvent dehydration, moisture removal in polyurethane production, and compressed air and gas dryer systems.
Where the requirement is deep drying across the whole stream, zeolite 4A delivers it at the lowest cost per unit of capacity. Where a component must be preserved, the narrower 3 Å opening is the appropriate specification.
Molecular Sieve 5A: Separation by Molecular Shape
At approximately 5 Å, molecular sieve 5A shares the type A structure with 3A and 4A but is supplied in the calcium exchanged form. That exchange widens the opening from about 4 Å to about 5 Å, and the extra angstrom introduces a capability the narrower grades do not have.
Straight chain molecules pass a 5 Å opening while branched and cyclic isomers do not, which allows zeolite 5A to separate by molecular shape as well as size. It is applied to:
- Separation of normal paraffins from branched and cyclic isomers
- Hydrogen purification and PSA gas separation
- Removal of water and CO₂ from air and gas streams
- Drying of natural gas and hydrocarbon liquids
- Protection of downstream catalyst beds
Molecular Sieve 13X: Widest Pores, Highest Capacity
With an opening near 10 Å, molecular sieve 13X admits molecules the type A grades exclude, and its larger internal pore volume gives the highest adsorption capacity of the group for both water and carbon dioxide. The material is designated 13X, zeolite 13X or sodium X interchangeably; all refer to the same type X synthetic zeolite.
The 13X grade is specified where more than moisture must be removed:
- Air pre-purification ahead of cryogenic separation, removing water and CO₂ together
- Oxygen production in PSA systems and zeolite oxygen concentrators
- Removal of CO₂ and moisture from natural gas
- Sweetening duties involving mercaptans and other sulphur compounds
- Drying of refrigerants, solvents and hydrocarbon liquids
Because zeolite 13X adsorbs across a wide molecular range, it is matched to streams where broad removal is the objective. On hydrocarbon streams where a product molecule must remain, the 3 Å opening provides the selectivity instead. Regeneration takes place at a higher temperature, which is accounted for in utility and cycle design.
Selecting the Grade
Selection resolves to four questions, answered in order:
- What must be removed? Water alone indicates 3A or 4A. Water together with CO₂ or sulphur compounds indicates 13X. Separation of straight chain from branched molecules indicates 5A.
- What must remain in the stream? Where unsaturated hydrocarbons or alcohols are to be preserved, the pore opening must exclude them, which specifies 3A.
- What is the outlet specification? A demanding dew point or CO₂ limit affects both the grade and the quantity of adsorbent required, and duties such as semiconductor manufacturing leave no margin on trace moisture.
- What regeneration is available? The temperature the system can reach determines which grades are practical to cycle.
For general drying with no component to preserve, 4A is the standard specification. Where a product molecule must be retained, 3A is correct. Where carbon dioxide accompanies water, 13X is required. A wider grade can perform a narrower duty, so 13X will dry a stream that 4A would handle, but it adsorbs across a broader range and regenerates hotter, so the substitution is made deliberately rather than by default. Grade selection sits within the wider design of a molecular sieve drying and purification system, alongside vessel sizing, cycle design and upstream filtration.
Pore Size Is Not the Only Variable
Once the grade is settled, bead size and shape are matched to the vessel. Particle diameter governs pressure drop, mass transfer and the uniformity of bed loading. Beads and extruded pellets are supplied in common mesh ranges such as 4x8 and 8x12, giving particle diameters of roughly 2.5 to 5.0 mm and 1.6 to 2.6 mm respectively.
Smaller particles adsorb faster and raise pressure drop; larger particles do the reverse. The molecular sieve sizes used in industrial applications are selected against flow rate, vessel geometry and permissible pressure drop, independently of the grade decision.
Regeneration Across the Grades
All of these grades are regenerated thermally, at temperatures that rise with pore opening. 3A regenerates at the lowest temperature of the group, 4A and 5A sit in the middle, and 13X requires the most heat, because water is held more strongly in its wider pores.
Two operational points follow. Regeneration below the required temperature leaves residual moisture and progressively reduces working capacity cycle after cycle. Regeneration substantially above it, particularly for the potassium form, risks the crystal structure. The regeneration profile is confirmed against the datasheet for the specific grade and particle size supplied, rather than applying one figure across every adsorbent in a plant.
Identifying the Grade in an Existing System
The original equipment datasheet or the adsorbent delivery documentation states the grade and particle size in service. Where neither is available, the duty itself is a reliable indicator: the molecules being removed, those that must remain, and the regeneration temperature together narrow the specification to a single grade. From those operating conditions an equivalent grade can be matched for refill or replacement.
Common Selection Errors
- Reading the number as a ranking. 13X is not an upgrade on 3A; each is a different pore opening for a different duty.
- Specifying a wide pore grade on unsaturated hydrocarbon streams. The product is adsorbed alongside the water, where the 3 Å opening would have preserved it.
- Confusing zeolite molecular sieve with carbon molecular sieve. They are distinct materials. Carbon molecular sieve separates nitrogen from air in PSA generators by kinetic diameter and serves a different purpose entirely.
- Disregarding upstream conditioning. Oil carryover and liquid water shorten the life of any adsorbent, however well the pore size is matched.
Position Within the Adsorbent Range
Each desiccant occupies a defined position. Molecular sieve delivers the lowest residual moisture and the only true separation by molecular size, which is why it is specified for final dew point and for size selective duties. Silica gel provides high capacity at elevated humidity and regenerates at low temperature. Activated alumina combines strong mechanical durability with reliable performance in compressed air service.
These strengths are frequently combined rather than compared. Many industrial dryers are loaded in layers, with alumina or silica gel taking the bulk moisture load and molecular sieve polishing the stream to its final specification, so each material works where it performs best.
Specifying the Correct Grade
3A, 4A, 5A and 13X are four pore openings engineered for four duties. Correct specification begins with the molecules present in the stream rather than the grade number, and is confirmed against outlet specification, regeneration capability and particle size. Each sits within the wider molecular sieve adsorption range for drying, separation and purification duty.
Medaad supplies all four grades in beads and extruded pellets with technical documentation for every batch, as a molecular sieve supplier to the UAE, wider Middle East, Africa, Asia and Europe. Operating data is reviewed through our engineering and technical services, and the grade and quantity confirmed before an order is placed.
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