Cryogenic adsorption of methane and carbon dioxide on zeolites 4A and 13X

The utilization of adsorption processes operating at low temperatures can be interesting in the context of production of liquefied natural gas (LNG), where they can constitute a lower energy alternative as hybrid technologies with cryogenic distillation. This paper provides the necessary parameters to design an adsorption process for selective removal of CO₂ from methane at low temperatures to satisfy LNG specifications, with particular emphasis on a temperature swing adsorption (TSA) process. Adsorption equilibrium of CH₄ and CO₂ on commercial zeolite 4A and zeolite 13X is reported at cryogenic temperatures: 198, 208, 223, 248, and 279 K. Carbon dioxide is much more adsorbed than methane, and CO₂ isotherms are extremely steep at low temperatures. In the studied low-temperature range, it was observed that zeolite 4A has a very different behavior toward CH₄ and CO₂; adsorption of methane is entirely controlled by diffusion (kinetic control), while adsorption of CO₂ is mostly controlled by the shape of the isotherm (equilibrium control). Adsorption breakthrough curves of a mixture of 1.5% CO₂ and 98.5% CH₄ were measured in the zeolite 4A adsorbent at 204 K to identify transport phenomena at such low temperatures and verify if adsorption equilibrium can be described on the basis of pure component data. Experiments were performed at different total pressures (1 and 10 bar) and different flow rates.