Studies Of Adsorption Equilibrium Of Methane On Typical Materials

Natural gas is considered as an ideal fuel due to its environmental protectiveness and rich reserves.Currently,natural gas is mainly stored in a compressed form(CNG)as well as liquefied one(LNG),however,these two methods have some disadvantages.Due to a relatively higher storage density under a moderate pressure,adsorbed natural gas(ANG)has been being considered as a promising method for storage of natural gas.However,application of ANG technique also limits by lacking of an efficient storage medium.For development of an adsorbent for ANG,this paper carries out the researches in following aspects:(1)Synthesis and characterization of metal organic frameworks(MOFs).MOF-5 and MOF-199 were respectively synthesized by mechanochemical and solvothermal methods.The prepared samples were firstly tested for measuring adsorption and desorption data of nitrogen at77K by Micromeritics 3Flex to determine the specific surface areas and pore volumes,and then were undergone the analysis of the crystal structure,apparent morphology and thermal stability of MOFs by employing X-ray diffraction(XRD),scanning electron microscopy(SEM)and thermogravimetric analysis(TG).Volumetric method was used to set up a test rig for measuring adsorption equilibrium data covering a wide temperature-pressure range.Results show that the characteristic peaks of the prepared MOF-5 and MOF-199 samples are similar to those reported in the literatures,and the samples all have excellent thermal stabilities.The particle size of MOF-5 synthesized by mechanochemical method is only 1/50 of that synthesized by solvothermal,but the specific surface area of MOF-5 is only 2/5 of that of MOF-199.(2)Adsorption equilibrium analysis of methane on the MOFs.Adsorption isotherm of methane on MOF-5 and MOF-199 were measured by Micromeritics 3Flex automatic micropore adsorption instrument and the test rig over a temperature range of 283K-303K and a pressure respectively within 0-100kPa and 0-10MPa.Comparisons were made as per the prediction accuracies between experimental data and those predicted by Langmuirian equations.Results show that Toth equation has the highest prediction accuracy,with a mean relative error of between the results from predictions and those from experiments respectively 0.98%and 1.53%for MOF-5 and MOF-199.The isosteric heat of methane adsorption on MOF-5 and MOF-199 is20.51-26.78kJ/mol and 13.88-18.92kJ/mol.It suggests that the energy heterogeneity of the MOF-5 surface is stronger than that of the MOF-199,and the interaction energy between methane molecules and MOF-5 is accordingly stronger than that on MOF-199.(3)Analysis of adsorption equilibrium of methane on carbon-based materials.77K nitrogen adsorption isotherm was measured via Micromeritics 3Flex automatic microporous adsorption instrument to characterize the structures of coconut shell activated carbon SAC-01 and graphene sheets layers GS(3D)as per the specific surface area,pore volume and pore size distribution(PSD).Adsorption isotherms of methane on SAC-01 and GS(3D)were also tested under the same temperature-pressure range as that on the MOFs.Based on the adsorption equilibrium data at extremely low pressure range,Henry law constants and 10-4-3 potential function were employed to determine the interaction energy between methane molecule and carbon atoms on the surface of carbon-based materials.Evaluation of prediction accuracy of the Langmuirian adsorption models was performed on high-pressure adsorption equilibrium data,which were then used to determine the corresponding absolute adsorption amount for calculation of the isotheric heat of methane adsorption on SAC-01 and GS(3D).Results show that the specific surface areas of SAC-01 and GS(3D)are 1507m~2/g and 2062m~2/g,respectively.The interaction energy between methane molecules and carbon atoms on the surface of SAC-01 and GS(3D)are67.19K?k and 64.23K?k.Toth equation also has the highest prediction accuracy,with an average relative error between the results from predictions and the experimental data about 0.25%and2.29%for SAC-01 and GS(3D).Results also show that the isosteric heat of methane adsorption on SAC-01 and GS(3D)was 17.01-20.91kJ/mol and 12.44-18.41kJ/mol,respectively.