Preparation And Characterization Of Adsorbents For CH₄/N₂ Adsorption Separation

With the situation of lacking oil,insufficient natural gas and deteriorating environment,the utilization of methane through purification and separation,which can reduce the energy crisis and environmental pollution from the direct emissions of coal bed methane and other low quality sources of methane gas in a large number,has a very vital significance in our country.It is difficult that separate methane from methane-nitrogen mixture gas.Although the pressure swing adsorption?PSA?has the superiority in separation of CH₄/N₂,the lack of high selectivity adsorbents restricts the industrial application.Based on the advantages and adsorption characteristics of zeolite and carbon adsorption materials as adsorbents used for the difficult separation system CH₄/N₂,this paper studied the effect of carbon content,surface modification and crystallization time on the pore structure and surface properties of novel mesoporous composite zeolite X/activated carbon,and further get the effect of carbon content,pore structure,surface properties and crystallization time to CH₄/N₂ adsorption separation ability.Study the pore structure influence on CH₄/N₂ separation by changing the preparation conditions of microporous carbonaceous adsorbent,in order to achieve effective separation of CH₄/N₂.The main research contents and conclusions include:1.Series of X/AC composite material with different carbon content was prepared by adjusting ratio of pitch and elutrilithe via 6 steps?blend-extrusion-dry-carbonization-CO₂ activation-hydrothermal crystallization?from elutrilithe as raw material combined with some pitch and SiO₂.Then was treated with ammonium chloride solution of solid-liquid ratio of 1g:25mL in248K.The crystalline,pore structure and surface properties of the samples were investigated by X-ray diffraction?XRD?,adsorption isotherms of N₂ at 77K.The adsorption isotherms of CH₄ and N₂ were tested at 298K and 100KPa.The experiment showed that the adsorption capacity of CH₄ and N₂ was mainly determined by the content of activated carbon,the structure and surface properties of the activated carbon.Among them,the adsorption capacity of CH₄was mainly determined by the content of activated carbon and the volume of the micropores,while the N₂ adsorption capacity was mainly determined by the ratio of zeolite X and the alkali content of the surface.It is worth mentioning that under 298K,100KPa,HAX-3 can achieve optimum CH₄/N₂ separation selectivity of 3.4,meanwhile maintain a high CH₄ adsorption capacity of 13.4mL.g^-1,which is expected to separate CH₄/N₂ by PSA;the CH₄ adsorption amount of HAX-2 reached 10.9 mL.g^-1,and CH₄/N₂ selectivity was 3.1.The point of modification of the zeolite X/activated carbon composite material have a major effect on the modification of zeolite X was proved by the modification of the carbon precursor and 13X zeolite.2.Take the samples 50AC,67AC as carbon matrix,and keep other synthesis conditions unchanged,choose crystallization time of 6h,12h,18h,24h to get two series of samples PwAX-t?Pw=50,67;t=6,12,18,24?.Study the effects of rystal and pore structure on the adsorption of CH₄ and N₂ separation ability.The results showed that the crystallization process is the main stage of crystal formation,with prolonging the crystallization time,relative crystalline degree increased,particle size increased,the micropore specific surface area,total pore volume and micropore volume increase,up to 12h can obtain X zeolite/activated carbon composite molecular sieve,up to 24h zeolite X relative crystallinity degree reached a peak.Compared to the sample PwAX-12 and PwAX-6,CH₄?N₂ adsorption capacity increased significantly.CH₄ equilibrium adsorption amount of PwAC-12 reached a relatively high level,prolong the crystallization time from 12h,CH₄ equilibrium adsorption amount slowly increase while N₂ equilibrium adsorption capacity still increase significantly due to the continuous formation of the X zeolite.The change trend of CH₄,N₂ adsorption equilibrium quantity lead to CH₄/N₂ separation selectivity firstly increased and then decreased,67AC-n have the same trend with 50AC-n,both reached maximum when 12h,respectively were3.2(CH₄,N₂ adsorption capacity were 10.6 mL.g^-11 and 3.4 mL.g^-1,3.4(CH₄,N₂adsorption equilibrium quantity were 13.7 mL.g^-11 and 4.0 mL.g^-1),the results reached the ideal result by NH₄Cl treated composite,but also reduce the crystallization time,eliminating the need for modification of the tedious step.3.Series of carbon material were prepared via blend-dry-carbonization-CO₂activation steps from petroleum asphalt and corn starch elutrilithe as raw material,the carbonization process is 450?remained 2h,then 850?remained2h.Fixed activation temperature is 830?,series of activated carbon sample T6,T12,T24 were obtained with different activation time?6,12,24h?;fixed activation time was 24h,series of samples T750,T780,T800 and T830 by adopt different activation temperature?750,780,800,830??and study the effect of activation time and activation temperature on CH₄ and N₂ adsorption separation ability.The results show that the sample T24 and T830 have high CH₄equilibrium adsorption capacity,and the separation ratio is 3.7,which is potential to be used in the industrial PSA process.