Visualizing Uphill Diffusion in LTA-4A

Many natural gas reserves contain nitrogen in concentrations ranging to about 20%. To meet pipeline specifications, the nitrogen level must be reduced to below 4%. A large majority of nitrogen removal facilities use cryogenic distillation, but such units are economical only for large capacity wells. For smaller reserves pressure swing adsorption technology has economic benefits, especially because the feed mixtures are available at high pressures. It is desirable to use adsorbents in pressure swing adsorption (PSA) units that are selective to N2. For most known adsorbents, the adsorption selectivity for separation of N2/CH4 mixtures is in favor of CH4 due to its higher polarizability. One practical solution is to rely on diffusion selectivities by using microporous materials, such as LTA-4A zeolite. The suitability of LTA-4A zeolite for kinetic separations was first suggested by H.W. Habgood in 1958. Nitrogen is a “pencil-like” molecule whose cross-sectional dimension is significantly lower than that of methane. Within cage-window structure of LTA-4A, the diffusivities of N2 are significantly higher compared to that of CH4. The kinetic selectivity helps to override the adsorption selectivity that favors methane. The primary objective of this presentation is to examine the mixture separation characteristics of LTA-4A using the Maxwell-Stefan diffusion model. I demonstrate the occurrence of uphill transport of N2, and transient overshoots, in quantitative agreement with the experiments of Habgood (1958). The origins of the overshoots can be traced to thermodynamic coupling effects that emanate from sizable off-diagonal contributions of the matrix of thermodynamic correction factors. If thermodynamic coupling effects are neglected, the overshoots are not realized.I also perform breakthrough simulations in a fixed bed adsorber, to demonstrate that kinetic separations with LTA-4A are achievable only at cryogenic temperatures.For theoretical background please consult my papers (1) 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. (2) Krishna, R. Fundamental Insights into Intra-Crystalline Diffusional Influences on Mixture Separations in Fixed Bed Adsorbers. Chem Bio Eng. 2024, 1, 53-66. https://doi.org/10.1021/cbe.3c00057. (3) 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. (4) 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.... (5) Krishna, R. A Maxwell-Stefan-Glueckauf Description of Transient Mixture Uptake in Microporous Adsorbents. Sep. Purif. Technol. 2018, 191, 392-399. https://doi.org/10.1016/j.seppur.2017.... (6) Krishna, R.; Van Baten, J. M. Investigating the Non-idealities in Adsorption of CO2-bearing Mixtures in Cation-exchanged Zeolites. Sep. Purif. Technol. 2018, 206, 208-217. https://doi.org/10.1016/j.seppur.2018... also Uphill Diffusion on my YouTube channel    / @rajamanikrishna250   LTA 4A