Activated Alumina AA-201

A Dual Duty Alumina Sphere

AA-201 is the SorbiTech smooth sphere activated alumina, produced through controlled hydration and thermal activation of aluminium hydroxide. The chemistry leaves the sphere with a high concentration of surface hydroxyl sites on a developed pore network of 320 to 360 m²/g, and the manufacturing route is tuned to keep the crush strength above 130 N per sphere and the attrition loss below 0.3 percent per ASTM D4058.

The sphere geometry and the high crush strength are what separate AA-201 from an ordinary alumina granule. The smooth surface resists abrasion against neighbouring spheres in a fast cycling bed, and the high single sphere strength tolerates the mechanical load of a deep fixed bed without crushing under its own weight.

Surface Hydroxyl Chemistry

The surface hydroxyl groups carry out two adsorption duties that look unrelated but rest on the same chemistry. For water vapour, the hydroxyls form hydrogen bonds with adsorbed water molecules, holding them on the surface up to a water capacity above 18 percent by weight. This makes the sphere a desiccant for compressed air and gas dryers.

For fluoride in liquid phase, the hydroxyls participate in ligand exchange. A fluoride ion displaces a surface hydroxyl, binds directly to the aluminium, and is retained until the bed is regenerated with dilute caustic followed by acid neutralisation. The same chemistry binds hydrogen fluoride in gas phase, which is why SorbiTech specifies AA-201 as an HF guard bed in alkylation and refrigerant service.

Mechanical Strength in High Velocity Beds

The attrition resistance is the deciding property in heatless air dryers. These dryers cycle every 4 to 10 minutes, repressurise quickly, and run at face velocities of 0.3 to 0.6 m/s. Softer media break into fines under that duty and discharge dust through the downstream system, fouling instruments and damaging seals.

AA-201 sustains the heatless cycle without measurable dusting for five to seven years. The smooth sphere also passes the standard rolling abrasion and drop tests required for offshore and mobile dryers, which is why SorbiTech specifies it for marine air dryers and air brake reservoirs on heavy vehicles.

Capacity, Regeneration and Breakthrough

Water capacity is 18 percent by weight or more, with typical service giving 6 to 9 percent dynamic loading at breakthrough. Thermal regeneration runs at 175 to 250 °C, well below the temperature that would damage the gamma alumina phase. Breakthrough is identified by an outlet dew point rise (gas service) or an outlet fluoride concentration above the WHO guideline of 1.5 mg per litre (water service).

• Bulk density: 0.75 to 0.82 kg/L
• Crush strength: ≥ 130 N per sphere
• Attrition loss: ≤ 0.3 percent (ASTM D4058)
• Defluoridation capacity: 1.0 to 2.5 mg F per gram, depending on pH
• Regeneration cycle: thermal (gas drying) or chemical with NaOH then H₂SO₄ (defluoridation)

Selection Against Competing Adsorbents

For compressed air drying the operator chooses between activated alumina and silica gel. Silica gel reaches a lower pressure dew point at moderate inlet humidity, but dusts heavily under fast cycling and cannot tolerate liquid carryover. The SorbiTech AA-201 sacrifices a few degrees of dew point in exchange for the mechanical strength and the carryover tolerance that the dryer needs over a 24 month service interval.

For defluoridation the alternatives are bone char and proprietary iron and lanthanum oxides. Bone char is cheaper per kilogram but has lower capacity; the iron and lanthanum media reach higher capacity but cost several multiples per kilogram. Activated alumina from SorbiTech remains the technology of record for municipal scale duty because the supply chain, the regeneration chemistry, and the regulatory acceptance are all settled.

Defluoridation Grade and Bulk Supply

Supplied as 3 to 5 mm and 5 to 8 mm spheres by SorbiTech with a certificate of analysis per batch. The defluoridation grade is eligible for NSF/ANSI 61 contact with drinking water; the gas drying grade carries separate documentation. Available as standard media for an existing dryer, as the charge in a SorbiTech dehydration package, or as a defluoridation bed sized by SorbiTech for a municipal plant or community unit. AA-201 is delivered by SorbiTech across community defluoridation programmes and the municipal water sector; the parent activated alumina family covers drying, defluoridation, and impregnated grades.