Adsorption separation of CO2/CH4 gas mixture on the commercial zeolites at atmospheric pressure
Graphical abstract
Introduction
In order to improve the environment and alleviate the energy crisis, more and more attentions are given to the green resource-methane. In general, CH4 often accompanied by CO2 exists in nature, forming gas mixtures, such as natural gas, biogas, landfill gas, and coalbed methane [1]. Removal of CO2 from CO2/CH4 gas mixtures is an important process in upgrading the gas because CO2 can reduce the heating value and leads to pipeline corrosion in the presence of water [2]. So far, zeolites [3], [4], [5], [6], metal organic frameworks (MOFs) [7], [8], [9], [10], [11], [12], active carbons [13], [14], [15], mesoporous molecular sieves [16], [17], [18] and clays [19], [20], [21], [22] have attracted great interest as adsorbents to separate gas mixtures of CO2 and CH4.
In industrial applications, zeolites have attracted a lot of attention because of possessing the superior separation performance and low cost. Relative to many other materials, such as MCM-41, zeolites can adsorb strongly CO2 in atmospheric pressure. In this work, six kinds of the commercially common zeolites (NaX, CaX, NaA, CaA, ZSM-5 and Y) were selected as objects of study. Their adsorption behavior and separation performance were investigated by the analysis method depending on adsorption isotherm and the ideal adsorption solution theory (IAST). The NaX and CaA were selected as two excellent adsorbents for separating mixture gases containing CH4 of 50% and 90%, respectively. The effect of the moisture content in the zeolite and desorption were investigated by a dynamic adsorption measurements. The adsorption mechanisms of CO2 on the NaX and CaA were explored by characterization of the Fourier Transform Infrared Spectroscopy (VERTEX70) and the heat of adsorption. These results will be useful for selecting suitable adsorbents for the separation of gas mixtures in industry as well as theoretical studies.
Section snippets
Adsorption measurements
The pure gas equilibrium isotherms of CO2 and CH4 were measured using a static volumetric adsorption instrument at 30 °C. Experimental setup for dynamic adsorption measurements of binary gas mixture was showed in Fig. 1. It was noteworthy that the sample was heat up to 300 °C (3 °C/min) with blowing inert gas (N2, 99.99%) to wash the instrument and clear the adsorbent in the adsorption system before every measurement. CO2 and CH4 were obtained from Foshan Kodi Gas Chemical Industry Co. Ltd. The
Adsorption equilibrium of pure gas and separation of mixture gas
Pure gases adsorption equilibrium isotherms of CO2 and CH4 on all zeolites at 30 °C were plotted in Fig. 2, Fig. 3. Langmuir–Freundlich model was used to fit two gas adsorption experimental data sets and results were showed in Table 1. Henry’s constant of all zeolites for two gases were listed in Table 1. The constant was directly related to the interaction of molecules with the surface of the adsorbent since at low pressure, molecule-surface forces predominated. It can be observed from Fig. 2
Conclusions
In order to investigate the application of zeolites in the industry of separating CO2/CH4 gas mixtures, a systemic research is present in this paper. Six kinds of the commercially common zeolites (NaX, CaX, NaA, CaA, ZSM-5 and Y) were selected as adsorbents for adsorption experimental of pure gases. Binary component separation was tested experimentally in dynamic adsorption system. Some useful conclusions were obtained as follows:
- (1)
NaX had the greatest adsorption amount of CO2 at 0.1 MPa, and CaX
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities (06101046).
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