| Gas membrane separation technology,which is the so-called "3rd generation gas separation technology" after cryogenic separation and pressure swing adsorption,has advantages such as low energy consumption,simple operation and high separation efficiency.Membrane material,as the core of membrane separation technology,would directly affect the separation performance of membrane-based process.Carbon molecular sieve membrane(CMSM)is a novel carbon based membrane for gas separation,which is obtained from the carbonization of polymers.Compared with commercial polymeric materials,CMSM shows excellent performances in gas permeation and separation,thermal stability and chemical stability,which gives CMSM the potential in the practical applications such as natural gas sweetening,air separation and hydrogen purification.The microstructure of CMSM is an important influencial factor on the separation performance of CMSM.Discovering the microstructural characteristics of CMSM and its influential mechanism on gas separation performance in depth is of great significance to improve the manifestation of CMSM.Herein,in this dissertation,a systematic research on this subject was carried out by means of experiments and molecular simulation.The microstructural characterization and gas separation performance measurement of self-prepared CMSM were carried out.The microstructural evolution of the CMSM and its influencing rules on the gas separation performance was explored.Based on the experimental results,the molecule-scale physical model closer to the actual microstructure of CMSM was constructed.The gas separation process of CMSM was studied by molecular simulation technology,and the influence mechanism of the microstructure on gas separation was analyzed from the microscopic level.The main contents and results of this dissertation are as follows.(1)Preparation and gas separation measurement of CMSM.Based on the machine learning analysis of the previous experimental results,it could be found that among the influential factors,the most effective one for the gas separation performance is the fractional free volume(FFV)of the polymeric precursor.Hereby,the CMSMs were prepared from two commercial polymers with different FFV:PMDA-ODA and PEK-C.The gas separation performance of smallmolecule gases including CO2,CH4,O2 and N2 were measured.Results show that The separation results of CMSM for gas pairs such as CO2/CH4 and O2/N2 exceeds the upper limit proposed by Robeson in 2008,which indicates that the gas separation performance of the selffabricated CMSM is good.With the final carbonization temperature increasing,the gas permeability decreases and the ideal selectivity increases.Compared with the CMSM prepared from PMDA-ODA,the CMSM prepared from PEK-C has higher gas permeability but lower ideal selectivity.The CMSM derived from PEK-C with the final carbonization temperature of 600℃,which shows the CO2 permeability more than 5700 Barrer and the CO2/CH4 selectivity 29.2,follows the selective adsorption-surface diffusion mechanism for gas separation.The other CMSMs follows the molecular sieving mechanism.(2)Experimental study on the effect of microstructure on the gas separation performance of CMSM.Based on the microstructural characterization and gas separation performance measurement,the evolution of the microstructure of CMSM was achieved,and the structureperformance relationship of CMSM between the microstructure and the gas separation performance was preliminary performed.Results show that with the final carbonization temperature increasing,the structures of both precursors gradually change from polymer to amorphous carbon through thermal decomposition and condensation reaction,then turn to the disordered carbon in graphite like microcrystals,and finally the regular aromatic carbon structure are formed.This evolution process leads to the decrease of the average pore size of the CMSM and the enhancement of the sieving effect on larger gas molecules.The CMSMs prepared from both precursors show the carbon structure with the coexistence of amorphous and regulated carbon,where the formed "graphite-like microcrystal" shows the characteristic of imperfect plane,as well as the slit-like pore structure in nanoscale.The CMSM prepared from PMDA-ODA with lower gas permeability and higher gas selectivity shows more regular and dense carbon structure as well as the microporous structure with smaller average pore size than the one prepared from PEK-C.The adsorption capacities of different gases follow CO2>CH4>O2>N2,while the diffusion coefficients follow O2>CO2>N2>CH4.Compared with the variation trend of the gas adsorption coefficient with gas molecular dynamics diameter,the trend of the gas permeability is similar to the one of the gas diffusion coefficient,which indicates that the difference of gas permeability for different gases in CMSM is mainly due to the difference of gas diffusivity.(3)Molecular simulation for the influence of the microstructure on the gas separation performance of CMSM with the two-dimensional single microporous model.Based on the slitlike pore structure of CMSMs and the experimental data of PMDA-ODA based CMSM which shows better separation performance to CO2/CH4,a two-dimensional Zigzag-type single microporous model was established.The influential mechanism of microporous structural factors on CO2/CH4 separation performance of CMSM was studied from the micro perspective using molecular simulation technology.Results show that with the pore size increasing,the adsorption of CO2 in the pore changes from monolayer to multilayer,but the adsorption capacity first increases and then decreases,which is similar to the variation of interaction potential between CO2 and carbon atom.When the pore size is 0.6 nm,the selectivity of the CO2/CH4 mixture in the micropore is highest because the interaction potential interaction potential between CO2 and carbon atom come to a relatively higher value.However,the simulated adsorption selectivity results of O2/N2 in the single micropore are quite different from the experimental results.Although the single microporous model is suitable for the CO2/CH4 mixture with different adsorption capacity,the influence of microstructure on the adsorption separation of the O2/N2 mixture could hardly reflect by the single micropore.Therefore,in order to highlight the influence of microstructural factors on gas separation and improve the applicability,the single microporous model should to be modified to approach the integrally disordered characteristic of CMSM.(4)Molecular simulation based on the three-dimensional carbon structural model for the influence of the microstructure on the gas separation performance of CMSM.Combining with the experimental data of CMSM prepared from PMDA-ODA,the three-dimensional model reflecting the integrally disordered characteristic of CMSM was constructed.The method of disordered accumulation of graphite like microcrystals was used to construct the model and the intrinsic defects of carbon materials,which may give an imperfect plane to the carbon sheets,was taken into consideration.The separation of CO2/CH4 and O2/N2 mixture under different operation conditions was simulated,and the influencing mechanism of microstructural factors on gas separation performance was analyzed.Results show that the the relative error between the simulated adsorption capacity and the experimental data of CO2,CH4,O2 and N2 were less than 5%,which indicated the reliability of the model.In addition to the difference of gas-solid adsorption interaction potential and gas molecular size,the separation performance of CO2/CH4 and O2/N2 by CMSM is related to the of the adsorb distribution of the components in the mixture:The fast gas component in the mixture(CO2 or O2)tends to be adsorbed in the packing gap of graphite like microcrystals with larger pore size,while the slow gas component(CH4 or N2)tends to be adsorbed in the interlayer gap of microcrystals with smaller pore size,which restricts the movement of slow gas component to a greater extent and would promote the separation of fast gas and slow gas in the CMSM. |
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