Molecular Sieve 3A

Selectivity by Pore Size

The SorbiTech 3A grade is the potassium exchanged form of the 4A sieve, a crystalline aluminosilicate with the empirical formula 0.4 K₂O · 0.6 Na₂O · Al₂O₃ · 2 SiO₂ · 4.5 H₂O. Replacing sodium with the larger potassium ion narrows the pore opening from about 4 Ångström to about 3 Ångström. Only water and ammonia can enter; ethane, propylene, methanol, and all heavier molecules are excluded at the pore mouth. The bed therefore dries the stream to single digit ppm water without coadsorbing valuable hydrocarbon or losing capacity to organic fouling.

SorbiTech manufactures the active crystal and binds it with a clay matrix into 1.6 to 2.5 mm and 2.5 to 5.0 mm beads. The bead form gives a low pressure drop in deep adsorber beds and a controlled particle size distribution that resists fluidisation through pressurisation cycles.

Why Potassium Exchange Matters

Size selectivity is the central engineering reason operators specify 3A over wider pore sieves. In ethanol dehydration a 4A bed would adsorb both water and ethanol, lose net ethanol yield to the regeneration vent, and accumulate the heavier component in the pores until capacity collapses. A 3A bed holds water alone, so the regeneration off gas is essentially pure water vapour and the ethanol stream is preserved.

In olefin and cracked gas service the same logic applies. Ethylene, propylene, and butadiene cannot enter the 3 Ångström pore, so coke precursors do not laydown inside the crystal during regeneration. The result is a sieve that holds its water capacity over many years where a 4A bed would lose capacity within twelve to eighteen months.

Cycle Performance and Regeneration

Equilibrium water capacity is 22 percent by weight or more at 25 °C and 17.5 mbar water partial pressure. In a temperature swing unit a properly sized SorbiTech 3A bed delivers outlet water dew points below minus 70 °C with an adsorption half cycle of 8 to 12 hours. Regeneration uses a dry hot purge at 175 to 315 °C, followed by a cooling step that returns the bed to adsorption temperature before the next cycle.

• Adsorption pressure: up to 150 barg
• Adsorption temperature: minus 20 to plus 60 °C
• Regeneration temperature: 175 to 315 °C, ramped
• Purge ratio: typically 8 to 12 percent of dry product
• Half cycle time: 8 to 24 hours, set by inlet water load

Service Life and Coke Resistance

Service life in clean gas duty is three to five years for a properly sized SorbiTech bed protected from liquid carryover. The crystalline structure tolerates several thousand temperature swing cycles before mass transfer zone broadening forces a changeout. The limiting factors are mechanical attrition from poor flow distribution, hydrothermal damage from saturated steam during upsets, and an inlet coalescer that fails to remove free liquids. The 3 Ångström pore protects the sieve from organic fouling, which is the failure mode that drives most early changeouts in wider pore service.

Loading and First Cycle Commissioning

A fresh SorbiTech 3A bed is loaded under dry nitrogen to prevent ambient humidity from saturating the sieve before commissioning. The loading routine follows a controlled bulk density target of 0.66 kg/L, with the bed settled by vibrating the vessel wall in a defined pattern as the layers build up. A non uniform load leads to channelling and accelerates breakthrough.

The first regeneration runs at the standard conditions but with a longer hold time, typically 8 hours instead of 4, to remove residual moisture trapped during loading and to bring the bed to the as commissioned reference baseline. SorbiTech commissioning engineers log the outlet dew point through this first cycle as the comparison record for every subsequent campaign.

Forms and Supply

Supplied as 4×8 mesh beads (1.6 to 2.5 mm) and 8×12 mesh beads (2.5 to 5.0 mm), manufactured at the SorbiTech plant under an ISO 9001:2015 system and released against a certificate of analysis per batch. Third party verification by SGS or Intertek is available on request. Standard packaging is 25 kg drums, 150 kg drums, or 850 kg big bags; Halal and Kosher eligibility is held on the standard grades.

Order the SorbiTech 3A media alone for an existing dryer, or loaded and commissioned with the matching twin tower TSA dehydration unit engineered by SorbiTech. Within the SorbiTech Group, the 3A grade is the default desiccant for natural gas dehydration and is delivered inside the twin tower TSA package, with sector context under oil & gas and the full molecular sieves family.