An air separation unit is a machine with a very clean appetite. Before air can be cooled to separation temperatures, the water, the carbon dioxide and a list of trace hydrocarbons must come off — CO2 freezes in the exchangers and chokes them, and acetylene accumulating in liquid oxygen is a genuine safety hazard. The beds that do this job are the front-end purifiers, and the sieve in them is 13X in its air-pre-purification grade, APG.
What the front end removes
- Water to the analytical floor — compressor after-coolers and knock-outs do the bulk; the purifier finishes.
- CO2 to the low single-digit ppm region commonly demanded by the cold-box design — the line that keeps exchangers unfrozen.
- Trace hydrocarbons, acetylene first among them — removed at the front end so they cannot concentrate in the oxygen circuit.
Why the bed is layered
A single material cannot do that list efficiently, so purifier vessels are almost always composites. The inlet layer is typically activated alumina: cheap, mechanically robust, superb at bulk water, and it takes the brunt of any liquid carry-over. Above it, 13X APG finishes the drying and carries the CO2 and trace-hydrocarbon duty — which is what its larger pore and higher cation density are for. The split between the layers is a design calculation against the inlet analysis, not a rule of thumb.
Cycles and regeneration
Front-end purifiers run as two beds on temperature-swing cycles commonly in the 2–8 hour band: one on-line, one heating, cooling or standing. Regeneration uses waste or dry nitrogen heated to the 250–300 °C class and driven through the spent bed counter-currently. Two disciplines decide whether the bed makes its design life:
- Complete re-heat every cycle. A bed that regenerates cold leaves a residual-load floor that ratchets upward until CO2 breakthrough moves upstream of the cold box.
- Inlet air discipline. Compressor oil aerosol reaching the bed coats internal surfaces and takes capacity permanently. Coalescing filtration upstream of the purifier is part of the adsorbent system, not an accessory.
What makes a grade “APG”
13X APG is 13X built for large, high-velocity industrial beds rather than laboratory duties. The differences are physical: bead fractions in the larger classes (commonly 1.6–2.5 and 2.5–5.0 mm ranges), higher crush strength to survive deep beds and years of thermal cycling, low dust so screens and downstream exchangers stay clean, and a CO2 capacity line measured at realistic purifier conditions. Same crystal, industrial body.
Spec lines for a rebed quote
- CO2 adsorption capacity at stated pressure and temperature — the headline line for this duty.
- Water capacity, for the alumina comparison and the layer split.
- Crush strength and attrition — deep-bed service.
- Bulk density — tonnage against vessel volume, and the layer split you have to load.
- Packed water content on the COA — the material arrives dry.
Figures marked as typical ranges are indicative values compiled from published supplier datasheets and standard adsorption-engineering references. They describe the class of material, not our specification and not a performance guarantee for your unit. Bed design must be confirmed against your own flow, pressure, temperature and dew-point target.
Rebedding or building a front-end purifier? Send the operating envelope:
- Unit scale and inlet air analysis, including CO2 and hydrocarbon traces
- Purifier cycle time, regeneration gas availability and re-heat temperature
- Vessel dimensions and the existing alumina / sieve layer split, for rebeds
- Whether the loading is ours to supervise or yours on site
Send those to our technical desk and you get a grade recommendation, a datasheet for the exact grade proposed, and a quotation with packaging and Incoterms stated — not a generic price list.
Related pages on this site
- 13X APG molecular sieve product page
- Air separation application page
- Li-LSX for oxygen PSA downstream
- Regeneration temperature guide
Frequently asked questions
What does 13X APG remove in an air separation unit?
Water, carbon dioxide and trace hydrocarbons — acetylene in particular— from the feed air before the cold box. CO2 is taken to the low single-digit ppm region commonly demanded by exchanger design.
Why is alumina used together with 13X APG?
The alumina layer handles bulk water cheaply and tolerates liquid carry-over;the 13X APG layer finishes the drying and carries the CO2 and hydrocarbon duty.The layer split is set by calculation against the inlet analysis.
What happens if CO2 breaks through into the cold box?
It freezes out in the exchangers and fouls them — rising pressure drop and shortened warm-up intervals. Breakthrough monitoring upstream of the cold box is standard practice.
Can 13X APG be regenerated in place?
Yes — that is the design. Heated waste or dry nitrogen at the 250–300 °C class, driven counter-currently, restores the bed each cycle for years of service.
How is APG different from ordinary 13X?
The crystal is the same; the body is industrial. Larger bead fractions, higher crush strength, low dust, and CO2 capacity measured at purifier conditions rather than laboratory conditions.
[…] you are specifying a new unit rather than replacing a bed, that layer arrangement is covered in our air pre-purification guide, and the cycle mechanics in CMS for PSA nitrogen […]