CH4/N2 Adsorptive Separation on Zeolite X/AC Composites

Hindawi Journal of Chemistry Volume 2019, Article ID 2078360, 9 pages https://doi.org/10.1155/2019/2078360 Research Article CH4/N2 Adsorptive Separation on Zeolite X/AC Composites Cai Long Xue, Wen Ping Cheng , Wen Ming Hao, Jing Hong Ma , and Rui Feng Li College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China Correspondence should be addressed to Wen Ping Cheng; and Jing Hong Ma; Received 26 September 2018; Accepted 21 November 2018; Published 2 January 2019 Academic Editor: Philippe Trens Copyright © 2019 Cai Long Xue et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A series of zeolite X/activated carbon (AC) composites were prepared from the same starting materials at various activation time. The corresponding modified samples were obtained by being treated with diluted NH4Cl solution. The relationship between porosity development, surface properties, and CH4/N2 adsorption performance was investigated. The increase of micropore volume is beneficial to the improvement of CH4 and N2 adsorption capacity, but more sensitive for CH4. In addition, the polar functional groups of zeolite X/AC composites may enhance CH4 adsorption capacity. More importantly, both developing micropore structure and surface modification contributed to enhance the adsorption selectivity αCH4 /N2 . As the optimum sample of these studies, HZAC(24) showed CH4 adsorption capacity of 17.3 cm3/g and the highest adsorption selectivity αCH4 /N2 of 3.4. The CH4 and N2 adsorption isotherms of all samples can be well fitted by the Langmuir–Freundlich model. HZAC(24) showed an excellent cyclability of adsorption/desorption of CH4 with a neglectable capacity loss after subsequent cycles. Moreover, HZAC(24) displayed relatively rapid adsorption kinetics. These properties of zeolite X/AC composites are essential for the adsorptive separation of CH4 from N2 in the pressure swing adsorption (PSA) process. 1. Introduction The coal bed methane (CBM) is an unconventional gas with a main composition of CH4, N2, and CO2, which reserve is about two times higher than that known to natural gas [1]. Nowadays, due to the CH4 content in the drainage gas of coal mine is only 20–45%, CBM is usually extracted into the atmosphere, which not only is a waste of energy source but also pollutes the environment as CH4 is one of the major contributors to the global warming with 20 times higher global warming potential than that of CO2 [2, 3]. So, it is of great significance to develop and utilize the CBM. The content of CH4 higher than 80% is required for the application of chemical raw stock and 90% for merging into the civil gas system [4]. For pipeline quality natural gas, the impurities of N2 and CO2 content should not exceed 4% and 2%, respectively [5, 6]. So, CH4 separation from N2 and CO2 is one of the important industrial separation processes [7, 8]. However, CH4 and N2 possess extremely similar physicochemical properties and kinetic diameter. Therefore, it is a really large challenge to enrich CH4 from the mixture of CH4 and N2 such as CBM. Generally, membrane [9], cryogenic [10, 11], and adsorption separation [12] are applied to separate the CH4 and N2 mixture. Membrane separation has the drawbacks of low selectivity and the strongly dependence of the membrane which is liable to damage and block; thus, it is not economical in scale separation [13, 14]. Cryogenic separation requires high-energy consumption and is helpless for low flow rates. In comparison with other methods, adsorption separation with easy operation, lower energy requirement, lower operational cost, running continuous at ambient temperature, and so on attracts increasingly attention. During the past three decades, there has been a rapid growth in the development of adsorption-based technologies for separation and purification of different gas mixtures, so that it can be applied to medium-scale CH4/N2 separation. However, it is still a big challenge as for the larger-scale CH4/ N2 adsorption separation due to the lack of satisfactory adsorbent with high adsorption capacity and selectivity. Many materials have been developed for gas selective separation. Zeolites, as one of the candidates for enriching CH4 from gas mixture, have shown great prospect and can potentially be used in the pressure swing adsorption (PSA) process. In order to obtain an appropriate zeolite for gas adsorption separation, the effect factors, including the pore structure of zeolite and the strength of the electric fields 2 caused by the presence of exchangeable cations in the frameworks, have been investigated [5, 14]. It has been found that zeolite X [15–21] is one of the most suitable zeolite adsorbents for adsorption and separation due to its large pore diameter of 0.74 nm which can accommodate large molecules and low Si/Al ratio with the presence of extra framework of cations that produces electric field which interacts strongly with the high polarizability CH4 molecule or quadrupolar CO2 molecules. Activated carbon [22–24], another promising candidate for gas separation, possesses lots of advantages such as tunable pore size, easy regeneration, and low cost. In general, activated carbons have higher equilibrium selectivity for CH4 over N2 but smaller adsorption capacities for CH4 than zeolites. As a consequence, in order to integrate the advantages of both zeolite and activated carbon in industrial application, increasingly attentions have been concentrated on the synthesis of the novel porous material of zeolite/AC composites in recent years [25, 26]. Meanwhile, the environmental application such as wastewater treatment as well as gas separation using of zeolite/AC composites has been explored preliminarily [27, 28]. In our previous work, the zeolite X/AC composites from elutrilithe are prepared by adding pitch powder and precipitate silicon dioxide as an additional carbonaceous and silica source, respectively [25, 29, 30]. The main aim in this work is to introduce amine modifications in a series of zeolite X/AC composites with different activation time in order to increase the interactions with CH4 without improving those with gases. This strategy will improve the potential of these materials to separate CH4/N2 mixtures, making these materials candidates for natural gas upgrading. The effect of pore texture and surface properties of the adsorbents on the adsorption performance of CH4 and N2 was investigated in detail. 2. Experimental 2.1. Preparation of Zeolite X/AC Composites. Zeolite X/AC composites were prepared by the following two steps. First, the locally available Elutrilithe chunk, with major chemical composition of 41.0 wt.% SiO2, 35.5 wt.% Al2O3, and 7.0 wt.% C, was crashed and sieved in order to collect the grains with an average size below 200 meshes. The (...truncated)