CO2, N2 and Ar adsorption on modified zeolites
In this study, CO2 and N2 adsorption on synthetic zeolites namely, 5A and 13X, and acid treated natural zeolites was investigated by using volumetric adsorption device, ASAP 2010. The natural zeolites and its acid treated forms with HCl (1M, 3M and 5M), HNO3 (2 M), and H3PO4 (1.1 M) solutions at 60 oC for 6 or 3 hours were used as adsorbent. The effect of the acid treatment and temperature on the adsorption properties of the zeolites for the CO2 and N2 gases at 5 and 25 oC was studied. Langmuir, Sips, Vrial and Dubinin-Astakhov model equations were applied to the adsorption data in order to determine the affinity and the heterogeneity of the adsorbents.Calorimetric properties of the zeolites were also studied using CO2, N2 and Ar gases through a Tian-Calvet calorimeter, Setaram C80 at 25 oC. Isosteric heat of adsorption of the gases adsorbed on the adsorbents under investigation was determined at 5 oC and 25 oC using Clasius- Clapeyron equation. These results were compared with the heat of adsorptions obtained from the calorimetric studies directly.The zeolite treated with 1.1 M H3PO4, P1 has the highest adsorption capacity (2.24 mmol/g and 0.67 mmol/g) for CO2 and N2 while the natural zeolite has only 2.08 mmol/g and 0.51 mmol/g respectively at 5 oC. Synthetic zeolites 13X and 5A have higher CO2 (6.82 mmol/g and 5.46 mmol/g respectively) and N2 adsorption capacities (0.31mmol/g and 0.91 mmol/g respectively) than natural zeolites at 5 oC. Langmuir b parameter called the affinity constant decreased as adsorption temperature increased for CO2 and N2 adsorption. The Sips model t parameter characterizing the system heterogeneity is higher for CO2 adsorption than N2 adsorption and decreases with increasing temperature. This indicates that the CO2 molecules give more specific interactions than N2 molecules. The pure component selectivities of CO2 over N2 calculated from Langmuir equation, are the highest for NCW zeolite as 408 at 25 oC. This value follows by P1 zeolite which is 151 at the same temperature.The highest differential heat of adsorption value (80.29 kJ/mol) at zero coverage for CO2 also belongs to P1 sample. The heterogeneity parameters of the model equations applied also indicated that P1 sample is more heterogeneous when compared to other acid treated natural zeolites. Finally as expected, for both N2 and CO2 as temperature increased from 5 to 25 oC, the adsorbed amounts decreased when fresh samples were used at each run in the experiments. The temperature difference had a greater effect on N2 adsorption rather than CO2.