Preparation Of Carbon Molecular Sieve From Coal And Its Separation Performance For CH₄/N₂ By Pressure Swing Adsorption

The exploitation and utilization of low concentration coal bed methane are significant to guaranteeing the coal mines safety production, improving the energy structure, reducing energy waste and environmental pollution and developing low-carbon economy and circular economy. Because of the similarity of CH4 and N2 in physicochemical properties, the lack of an efficient absorbent for the separation of CH4 and N2 by PSA has become the bottleneck of developing this technology in low concentration coal bed methane enrichment.In this paper, carbon molecular sieve(T-CMS) was prepared from anthracite by carbonization-chemical activation-benzene vapor deposition process. The optimal experiments conditions of chemical activation process were determined by single factor experiment: KOH-coal mass ratio was 3:1, activation temperature was 800? and activation time was 90 min. The optimal conditions of carbon deposition were determined by orthogonal experiment: deposition temperature was 600?, deposition time was 10 min and benzene flow rate was 1.2mL/min. At these optimal condition, SBET reached 1847m2/g, microporous proportion accounted for 60.70 percent, average pore size was 0.7041 nm.Adsorption capacities of CH4 and N2 on T-CMS, S-CMS and 13 X zeolite were measured by static volume method. Test results showed adsorption capacity of CH4 were larger than that of N2 on the same adsorbent under the same pressure. With the increase of adsorption pressure, the difference of adsorption capacities between CH4 and N2 was increased. Langmuir adsorption model was used to fit the adsorption capacities experimental data. CH4 was the stronger adsorption constituent according to the fitting parameters of the Langmuir model. Saturated adsorption capacity was in the following sequence : S-CMS>T-CMS>13X zeolite. Separation factors of CH4/N2 of the three absorbents were in accordance with the following order: T-CMS>S-CMS>13X zeolite, among which separation factor of CH4/N2 of T-CMS was 3.097.The breakthrough curves of CH4 on T-CMS, S-CMS and 13 X zeolite were measured on the vacuum and temperature swing adsorption bed made by our laboratory. The breakthrough point of CH4 of T-CMS, S-CMS and 13 X zeolite were( 279 s, 1.25%),( 228 s, 1.25%) and( 165 s, 1.25%), respectively. T-CMS had the optimal separating effect.T-CMS was modified by pro hydrocarbon and low temperature plasma. Pro hydrocarbon reagent included C24, SDB, PEI while plasma modified atmosphere included CH4 and N2. Adsorption capacities of CH4 and N2 of T-CMS-C24, T-CMS-SDB, T-CMS-PEI, T-CMS-P-CH4 and T-CMS-P-N2 were tested and then model-fitting were performed. The separation effects of T-CMS-P-CH4, T-CMS-P-N2 were better than T-CMS which showed low temperature plasma modification was beneficial to the separation of CH4 and N2.Adsorption isotherms of methane and nitrogen on T-CMS at 298.15 K, 313.15 K and 328.15 K were measured by static volume method, respectively. Models including Langmuir, E-L, Freundlich, Toth, Sips, BET, D-R, D-A, F-L were used to fit the experimental data, respectively. The fitting degree of these models were investigated and fitting effect was as follows: D-A > F-L > E-L > Sips > Toth > Langmiur > D-R > Freundilch. Parameters qm that represented the saturated adsorption in Langmuir, Toth and E-L models decreased with the increase of temperature. And that change of temperature had greater influence on nitrogen saturated adsorption. Parameters n represented surface inhomogeneities in E-L, Toth and Sips models increased with the temperature increasing and the change of fractal dimension D in F-L model showed that higher temperatures increased surface inhomogeneities.Through the analysis on the adsorbent thermodynamics, isosteric heat of methane and nitrogen were both decreased with the increase of adsorption capacities and the change was evident on low adsorption capacities. It indicated the surface energy inhomogeneitie of T-CMS. The average isosteric heat of methane adsorption was 11.80 KJ/mol which was comparable to 9.06 KJ/mol of nitrogen, and were both belong to physical adsorption. With the increase of adsorption capacities and the isosteric heat range of N2 was larger than that of CH4.This paper supplies basic experimental data to the preparation of carbon molecular sieve from coal and the enrichment technology of coal bed methane by PSA.