Carbon Molecular Sieve

Carbon molecular sieve is the working material in a PSA nitrogen generator, making nitrogen from ordinary compressed air on site. That nitrogen is used to blanket and purge tanks and reactors in chemical and oil-and-gas plants, to keep oxygen away from packaged food, to hold an inert atmosphere in electronics and metal heat treatment, and for laser cutting, tyre filling and mine inerting. Anywhere a plant needs its own steady nitrogen supply, CMS in a PSA unit is the common way to make it.

By speed, not by capacity, which is what sets it apart from a zeolite sieve. The carbon pores are sized so that oxygen molecules slip inside faster than the slightly larger nitrogen molecules. In a PSA generator, air is held on the bed for only a few seconds, and in that short window the oxygen is drawn into the carbon while most of the nitrogen sweeps past and leaves as product gas. The pressure is then dropped to release the oxygen and refresh the carbon, and a second bed takes over so the nitrogen flow stays steady. It is the difference in diffusion rate, not a hard size cut-off, that does the separating.

Grade and purity trade against each other. We stock CMS-220, 240, 260 and 280, and the higher the number the more nitrogen a given charge makes at a set purity. Purity itself is set by how the generator is run: a slower, gentler cycle gives higher purity but less nitrogen, a faster cycle gives more nitrogen at lower purity. For inerting and blanketing a middle purity is plenty and the output is high; for electronics or analysis the cycle is tuned for the highest grades at a lower flow. Tell us the nitrogen purity and the flow you need and we will point to the grade and the loading.

They are opposite tools. A zeolite oxygen sieve holds nitrogen by capacity and lets oxygen through, so it makes oxygen; carbon molecular sieve holds oxygen by speed and lets nitrogen through, so it makes nitrogen. CMS is carbon, not an aluminosilicate, and it separates on diffusion rate rather than pore size alone. Regeneration is by pressure, not heat: at the end of each short cycle the bed is vented to atmosphere, the oxygen comes back off, and the carbon is ready again, with no thermal step in normal PSA service. Kept free of oil and liquid water from the compressor, a CMS charge lasts for years.