Adsorption, separation and regeneration of cation-exchanged X zeolites for LNG purification: Li+, K+, Mg2+and Ca2+

The Li+, K+, Mg2+ and Ca2+-exchanged X zeolites were prepared using ion exchange method for producing efficient adsorbents that could be used in the purification of liquefied natural gas (LNG). The adsorption, separation and regeneration characteristics of the samples were systematically explored and discussed using static adsorption of pure gases, dynamic adsorption of a binary gas mixture (3%CO2 and 97%CH4) and regeneration experiments. The results showed that Li13X had the largest surface area of 520 m2/g, whereas Ca13X possessed the highest micropore volume of 0.25 m3/g with the smaller pore size observed from the pore size distribution. The adsorption capacity of Li13X for CO2 (4.10 mmol/g) was almost the same as that of Ca13X (4.12 mmol/g), which were larger than all the other samples at 120 kPa and 30 degrees C. However, K13X had the highest static CO2/ CH4 separation factor of 46, which was due to the increased basicity of the framework. For the dynamic adsorption, Ca13X possessed the highest CO2/CH4 separation factor of 93, and showed the smallest CH4 adsorption capacity of 1.22 mmol/g. As for the regeneration of the proposed adsorbents, the heat requirement for CO2 in dynamic was not only affected by the isosteric heat, but was also dependent on the competition between CH4 and CO2. Moreover, the static and dynamic vacuum regenerations had the same tendency and mainly depended on the pore size distribution.