High-pressure adsorption of methane, carbon dioxide, and nitrogen on zeolite 13X was measured in the pressure range (0 to 5) MPa at (298, 308, and 323) K and fitted with the Toth and multisite Langmuir models. Isosteric heats of adsorption were (12.8, 15.3, and 37.2) kJ/mol for nitrogen, methane, and carbon dioxide respectively, which indicate a very strong adsorption of carbon dioxide. The preferential adsorption capacity of CO 2 on zeolite 13X was much higher than for the other gases, indicating that zeolite 13X can be used for methane purification from natural gas or for carbon dioxide sequestration from flue gas.

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A vacuum pressure swing adsorption (VSA-PSA) process is studied for the removal of carbon dioxide in a contaminated stream of natural gas to achieve fuel grade methane. The adsorbent used was zeolite 13X (CECA) where CO2 is strongly adsorbed. A Skarstrom-type cycle comprising pressurization with product, feed, countercurrent blowdown, and countercurrent purge was employed. A mixture having 60% CH4/20% CO2/20% N2 was used, and two different temperatures were evaluated in a single-column VSA-PSA unit. Under the conditions tested, CO2 was removed to levels lower than 2% as required by fuel grade methane with methane recovery higher than 80% without recycle. This separation process also helps in the CH4−N2 separation. A bidisperse (macropore−micropore) model also including distributed energy balances in gas, solid, and column wall considering heat and mass transfer resistance at the gas−solid interface was used to simulate the VSA-PSA behavior and compare with experiments. Also, some scale-up considerations are considered and evaluated by simulations of the process.

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This work focuses on the production of pipeline grade methane from landfill gas (LFG). Vacuum pressure swing adsorption technology using a kinetic adsorbent, Carbon Molecular Sieve 3K (Takeda), was employed for the separation of methane−carbon dioxide mixture. Adsorption equilibrium and kinetics of methane and carbon dioxide are reported at 298, 308, and 323 to model the adsorption-based process. A four-step Skarstrom-type cycle was employed comprising pressurization, feed, counter-current blowdown, and counter-current purge with product. Co-current pressurization with feed stream and counter-current pressurization with product were evaluated. The separation of a mixture of CH4 (55%)−CO2 (45%) was tested using two different four-step cycles:  pressurization with feed stream, feed, blowdown, and purge with product and pressurization with product, feed, blowdown, and purge with product. The results indicate that purity of methane higher than 96% can be obtained with recovery higher than 75%. The difference of the performance in the non-adiabatic and adiabatic cases was also studied. It was observed that the temperature in the column increases allowing a faster and more important desorption of carbon dioxide in the blowdown step, retaining more CO2 in the column and improving methane purity and recovery.