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Nitrogen-Oxygen Separations

The separation of air to produce N2 and O2 of high purities is one of the most important industrial processes that uses pressure swing adsorption technology.1, 2 The process technologies are geared to either production of purified O2 or purified N2. Cryogenic distillation has been the common technologies for this separation, but adsorptive separations offer energy efficient alternatives. Purified O2 is required for a wide variety of applications that include portable adsorption units for medical applications and in space vehicles. Nitrogen is required in applications where it is desired or necessary to exclude oxygen. Typical industrial applications include preservation of fruit and produce during trucking, the blanketing of fuel tanks of fighter aircraft, the inerting of reactors in a number of pharmaceutical processes, laser cutting. N2 is required for use in laboratory analytical equipment such as GC, LC, LCMS, FTIR, ICP, and in glove boxes. For production of purified O2, cation-exchanged zeolites LTA-5A (also called 5A or NaCaA zeolite containing 96 Si, 96 Al, 32 Na+, 32 Ca++ per unit cell; Si/Al=1), NaX (also called 13 X zeolite, containing 106 Si, 86 Al, 86 Na+ per unit cell; Si/Al=1.23), CaX, LiX, and LiLSX (= low silica LiX zeolite) and can be used as selective adsorbents.2-6 The larger permanent quadrupole of N2 compared to that of O2 is responsible for the stronger adsorption strength of N2 on these zeolites.5 Both O2, and N2 have similar polarizabilities and magnetic susceptibilies. However, the quadrupole moment of N2 is about 4 times that of O2. The Skarstrom cycle can be used, with modifications such as the introduction of a pressure equalization step, for production of enriched O2 using say LiLSX zeolite. For production of purified N2 from air, a different strategy is often employed that rely on diffusion selectivity.7, 8 Diffusion-selective separation are achieved with Carbon Molecular Sieve (CMS), and LTA-4A zeolite. The same basic Skarstrom cycle can be used, with the kinetically selective 4A zeolite or carbon molecular sieve adsorbent, in a nitrogen production process. However, in such a system purging with nitrogen to remove the faster diffusing oxygen from the bed is undesirable since, as well as wasting product, a certain fraction of the slowly diffusing nitrogen will be adsorbed, thus reducing the capacity for oxygen during the next adsorption step. At the end of the counter-current blowdown step, the adsorbent contains both oxygen (fast diffusing) and nitrogen (slow diffusing). Thus, if the bed is simply closed at one end and left for a period of time the oxygen will diffuse out first followed by the nitrogen so the system is, in effect, self purging. For modelling details consult my papers Krishna, R. The Maxwell-Stefan Description of Mixture Diffusion in Nanoporous Crystalline Materials. Microporous Mesoporous Mater. 2014, 185, 30-50. https://doi.org/10.1016/j.micromeso.2.... Krishna, R. Highlighting the Influence of Thermodynamic Coupling on Kinetic Separations with Microporous Crystalline Materials. ACS Omega 2019, 4, 3409-3419. https://doi.org/10.1021/acsomega.8b03480. Krishna, R. Synergistic and Antisynergistic Intracrystalline Diffusional Influences on Mixture Separations in Fixed Bed Adsorbers. Precision Chemistry 2023, 1, 83-93. https://doi.org/10.1021/prechem.2c00003. Krishna, R. Describing the Diffusion of Guest Molecules inside Porous Structures. J. Phys. Chem. C 2009, 113, 19756-19781. https://doi.org/10.1021/jp906879d. Krishna, R. Diffusion in Porous Crystalline Materials. Chem. Soc. Rev. 2012, 41, 3099-3118. https://doi.org/10.1039/C2CS15284C. Krishna, R. Occupancy Dependency of Maxwell−Stefan Diffusivities in Ordered Crystalline Microporous Materials. ACS Omega 2018, 3, 15743-15753. https://doi.org/10.1021/acsomega.8b02465. Krishna, R. Uphill Diffusion in Multicomponent Mixtures. Chem. Soc. Rev. 2015, 44, 2812-2836. https://doi.org/10.1039/C4CS00440J. Krishna, R. Diffusing Uphill with James Clerk Maxwell and Josef Stefan. Chem. Eng. Sci. 2019, 195, 851-880. https://doi.org/10.1016/j.ces.2018.10....