| Recently, environmental issues caused by the greenhouse effect have become the focus of international attention. It is found that both CO2and CH4are important greenhouse gases. In addition, CH4is not only a cleaning and great efficiency fuel but also an important industrial chemical. So the recovery and utilization of CO2and CH4have significant economic and environmental benefits. CO2and CH4are mainly from flue gas (CO2/N2gas mixture), landfill gases (CO2/CH4gas mixture) and coal-bed gases (CH4/N2gas mixture), therefore, the recovery and purification of CH4and CO2involve the effective separation of CO2/N2, CO2/CH4and CH4/N2gas mixtures. Adsorption process as a promising technology to separate gas mixtures has a lot of advantages, for example, low energy consumption, easy operation, inexpensive devices, wide application and process automatization. In this paper, zeolite ZSM-5, a new kind of hydrophobic material, is used to separate CO2/N2, CO2/CH4and CH4/N2, respectively, and the feasibility to recover CO2and CH4from flus gases, landfill gases and coal-bed gases by adsorption technology is evaluated.Single adsorption equilibrium isotherms of CH4, N2and CO2on zeolite ZSM-5are investigated experimentally and theoretically. The experimental data of single adsorption equilibrium isotherms are measured by magnetic suspension balance (the gravimetric method). It is found that zeolite ZSM-5can adsorb CO2, CH4and N2with different adsorption capacity. At100kPa and293K, the adsorption capacities of CO2, CH4and N2on zeolite ZSM-5are1.578mmol/g,0.608mmol/g and0.171mmol/g, respectively. Furthermore, the experimental data are fitted with the calculated results of Sips model, Toth model, MSL model, DSL model and Virial model, respectively, and the relative parameters of models are obtained by the nonlinear regression method. Among five models, Sips model gives the best fitting result, and the averaged relative deviations for CH4, N2and CO2are all less than5%. The model parameters obtained by fitting the experimental results for single adsorption equilibrium will be significant for the prediction of binary competitive adsorption equilibrium isotherms of CO2/N2, CO2/CH4and CH4/N2gas mixtures.Based on the investigation of single adsorption equilibrium isotherm, binary competitive adsorption equilibrium of CO2/N2, CO2/CH4and CH4/N2gas mixtures are studied experimentally and theoretically. The total adsorption capacity and each component adsorption capacity of gas mixtures on zeolite ZSM-5are measured by magnetic suspension balance (the gravimetric method) and by adsorbent packed bed (the breakthrough curve method), respectively. Binary competitive adsorption equilibrium models are developed, namely Sips-IAST model, IRC model and MSL model, where the model parameters are obtained from single adsorption equilibrium model. Then, the experimental data are compared with the predicted results by three kinds of the competitive adsorption models, and it is found that Sips-IAST model has the best accuracy. According to the experimental data, the adsorptive selectivities of CO2/N2, CO2/CH4and CH4/N2on zeolite ZSM-5are calculated and compared. It is found that the adsorptive selectivities of both CO2/CH4and CH4/N2on zeolite ZSM-5are higher than that of literatures. The influence of temperature, pressure and gas composition on adsorptive selectivities is investigated. The results show that the increase of temperature and pressure is not favor the adsorption separation of three kinds of gas mixtures, and the increase of strongly adsorbed component amount in gas mixtures is not favor the adsorption separation of CO2/N2but it seems has no effect on the separation of CO2/CH4and CH4/N2. |
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