Adsorption equilibrium and charge/discharge characteristics of methane on MIL-101

For developing metal organic frameworks (MOFs) suitable for the storage of nature gas by adsorption, the MIL-101 (Cr) sample was synthesized by solvothermal method, with which the characterization by adsorption of nitrogen at 77.15 K and the adsorption equilibrium and charge/discharge of methane were conducted. The adsorption equilibrium data of methane on the sample were measured volumetrically at temperature range of 293-313 K within a pressure range of 0-100 kPa and 0-7 MPa, respectively. The limit isosteric heat of adsorption was determined by employing the Henry's law using the adsorption data at very low pressure region, and the absolute adsorption amounts of methane on the sample were determined via nonlinear fit of the adsorption data at high pressure range by using Toth's equation. Isosteric heats of methane adsorption were then calculated through Clausius-Clapeyron equation and Toth's potential function. The charge and discharge tests of methane were performed at a flow rate range of 10-30 L/min on a 3.2 L conformable vessel packed with samples about 940 g. The results show that the mean limit isosteric heat is 23.89 kJ/mol, and the average relative error of the result predicted by the Toth equation is about 1.06%. The mean isosteric heat of adsorption determined by Clausius-Clapeyron equation and Toth's potential function is about 15.51 kJ/mol and 13.56 kJ/mol, respectively. The results also reveal that the total amount of charge/discharge at the flow rate of 10 L/min and 30 L/min is about 347 L/338 L and 341 L/318 L, respectively, which are in correspondence with the ratios of discharge about 98.3% and 94.1%. It suggests that the isosteric heat of methane adsorption determined by Clausius-Clapeyron equation is more reasonable for practical applications, and slower charging/discharging with a smaller flow rate is beneficial to increasing the total amount of charge/discharge and the discharging of the adsorbent bed.