Study On Adsorption Storage Of Methane On Typical MOFs

One of the main obstacles to practical application of adsorbed natural gas(ANG)lies in it’s a less volumetric density of energy than the technique index,development of an efficient ANG adsorbent is still a necessity.Based on the research status of methane adsorption storage on metal organic frameworks(MOFs),this paper tackles with the problems in developing ANG as a natural gas storage method on a diesel engine in terms of formation and solidification of the adsorbent as well as heat transfer enhancement of the adsorbent bed,the researches carried out are as follows:Analysis of adsorption equilibrium of methane on MIL-101.MIL-101(Cr)was selected and synthesized by solvothermal method.Micromeritics 3flex was employed to measure isotherms of adsorption and desorption of nitrogen at 77.15 K on the as-prepared sample.The desorption data were then applied to determine the specific surface area and micropore volume,by using Micromeritics 3flex and Setaram PCT Pro E&E,adosption isotherms of methane on the sample were respectively measured within pressure ranges of 0-0.1 MPa and 0-6 MPa at temperature 293.15-313.15 K.Henry’s law constants were plotted on the adsorption data in low pressure range and then applied to determine the limit isosteric heat of adsorption.Toth’s equation and isosteric heat of methane on the samples were determined by Henry’s law and Toth’s equation.The results show that the average pore size and micropore volume of MIL-101are 1.86 nm and 1.09 cm~3/g,and the specific surface area of MIL-101 determined by BET method is 3141 m~2/g.According to Henry’s law,the average limit isosteric heat of methane on MIL-101 sample is 23.89 kJ/mol,and the average relative error predicted by toth equation is1.06%.The isosteric heat determined by Clausius-Clapeyron(C-C)equation and toth potential function is 14.08-19.37 kJ/mol and 13.06-13.92 kJ/mol.This shows that the methane molecule has a strong interaction with MIL-101,and accuracy of the isosteric heat of adsorption determined by Toth potential function is less than that from the C-C equation due to the introduction of the latent heat of vaporization into the function.Preparation of composite adsorbent of MOFs via the forming and consolidation of expanded natural graphite(ENG).ENG was developed by the expansion of expandable graphite at temperature 600℃with a duration about 30 s.Composite adsorbent samples were prepared by mixing the as-prepared MIL-101 with the ENG at four mass ratios(5%,10%,15%,20%)between the ENG and the MIL-101.The mixing samples were further formed within the mold under pressure 3 MPa.Four prepared samples were undergone structural charaterization and measurement of thermal conductivities.In consideration of that the thermal conductivity of the sample(here denotes as 10%@MIL-101)which was composed by the mass ratio of ENG about10%is equal to the apparent thermal conductivity of the adsorbent bed equipped with honeycomb heat exchanging device,10%@MIL-101 was seleced to undergo the adsorption equilibrium test at the same temperature-pressure range as that on the MIL-101.It was found that the specific surface area and micropore volume of the samples respectively decreased by about31%、45%、48%、50%and 30%、44%、46%、49%when different amounts of the expanded graphite were added,and the thermal conductivity increased to about 3,4,6,7 times in comparing with MIL-101,respectively.Results also show that,within the same test range,the amount of methane adsorbed on 10%@MIL-101 only amounts to 61%-39%of those on MIL-101,and the average limit isosteric heat and isosteric heat of methane adsorption is 18.54kJ/mol and 8.13-16.85 kJ/mol.Researches of charge and discharge characteristics and heat conducting enhancement of methane on the adsorbent bed packed with MIL-101.In order to mitigate the thermal effect of methane on charge and discharge processes on the adsorbent bed,thinking about that full utilization of the space of fuel storage system is essential to the application of ANG,a storage vessel,which is in conformable shape and in a net volume about 2.5 L,was designed for evaluating the performance of the adsorbent in terms of temperature variation and charge and discharge amount as well as the flow rate.The flow rate 10-30 L/min(about 28.30 L/min),which was determined by the fuel consumption rate of a diesel engine whose type is TO7900ET-J under its typical working condition,was used for charge and discharge tests of methane on the vessel respectively packed with MIL-101,10%@MIL-101,MIL-101/honeycomb heat exchanging fins.Results show that,for the adsorbent bed packed with MIL-101,about 10℃temperature fluctuation can be cut down while charging at the flow rate of charge 10 L/min instead of 30 L/min,which accordingly bring about an increment in discharged amount about5.3%.In comparing with that of the adsorbent bed packed with MIL-101,there are about 95.8g(about 41%)decrement in charged amount of methane accumulated within the adsorbent bed packed with composite adsordent;but that on the adsorbent bed equipped with the honeycomb heat conducting fins was about 173.7 g,which is in correspondence with a average temperature fluctuation less than 15℃and a ratio of discharged amount 97.6%.It suggests that,for practical application of the ANG,inserting honeycomb heat transfer fins is a feasible solution while both taking into account of heat conducting enhancement and the amount of methane charged into the adsorbent bed.