Influence of temperature on adsorption selectivity: Coal-based activated carbon for CH4 enrichment from coal mine methane

In order to study the effect of temperature on the adsorption separation characteristics of CH4/N-2 mixtures, activated carbons (AC) were prepared from low-rank bituminous coal by KOH activation method, marked as DF-AC and SM-AC. The adsorption isotherms of pure CH4 and N-2 on AC were described by Langmuir-Freundlich model based on the high-pressure adsorption experiments. The adsorption selectivity calculated by ideal adsorption solution theory (IAST) method was used to evaluate the adsorption and separation characteristics of CH4/N-2 binary mixtures. The influence of temperature on adsorption selectivity from 273 K to 373 K (20 K interval) were systematically analyzed. Results show that the adsorption equilibrium states of pure CH4 and N-2 on activated carbons are significantly different, and the CH4 is more sensitive to the variation of temperatures. The isosteric heat of adsorption of CH4 is always larger than that of N-2 under experimental pressure condition. This indicates that the interaction between the CH4 molecules with activated carbons is stronger than that of N-2 molecules. With the temperature increasing from 273 K to 373 K, the separation factors first increase rapidly and then decrease slowly after arriving at the maximum value, while the adsorption selectivity decrease sharply at low pressure and subsequently tended to slowly decrease. When the experimental temperature is higher than 313 K, the adsorption selectivity is increasing monotonically with the increase of adsorption pressure. Under present experimental condition, for CH4:N-2 = 50:50 mixture, the maximum adsorption selectivity of DF-AC (S = 7.06) can be obtained at 273 K and low pressure, while the minimum adsorption selectivity of DF-AC (S = 2.82) is obtained at 373 K and low pressure. The influences of temperature on adsorption selectivity of CH4/N-2 binary mixture on coal-based activated carbons indicate that the lower adsorption temperature is contributed to the CH4 enrichment from low-concentration coal mine methane. And the effect of temperature on adsorption selectivity is greater than that of pressure, which provides a certain guiding significance for the efficient separation of low concentration gas combined with PSA and TSA technologies. (C) 2019 Elsevier B.V. All rights reserved.