Effect Of Pore Structure Of Activated Carbon On Adsorption And Separation Performance Of CO₂ And CH₄

As a kind of porous carbon material,activated carbon,with rich pore structure,is widely used in the field of adsorption and separation of CO₂ and CH₄.However,the mechanism of the effect of pore structure on the adsorption and separation performance of activated carbon is still unknown.In order to research the influence of pore structure on adsorption capacity and separation performance,nine activated carbon samples with different pore structure were selected as the research material and CO₂ and CH₄ were used as adsorbents,the adsorption isotherms and adsorption kinetics curves of pure components on the samples were measured by gravimetric method.In addition,in order to comprehensively investigate and compare the selective adsorption ability of different samples for CO₂/N₂,CH₄/N₂ and CO₂/CH₄ system,the selectivity,selection parameter and adsorption heat of three binary mixed system of CO₂/N₂,CH₄/N₂ and CO₂/CH₄ were calculated based on the adsorption isotherms of N₂,CO₂ and CH₄.The main conclusions are as follows:1.Study on the adsorption and separation performance of CO₂ on ACs（1）The Toth model was used to fit the adsorption isotherms of CO₂ on ACs,and the Q_s,0,b₀,Q₀ and t₀ were obtained respectively.Q₀ is less than 40k J/mol,indicating that the adsorption of CO₂ belongs to physical adsorption.（2）The adsorption kinetic curves of CO₂ at the pressure of 0.5-1k Pa and 45-60k Pa were fitted by the first-order kinetic equation.It can be found that the adsorption rate of POR series is generally faster than that of MIC series at 0.5-1k Pa,which indicates that the introduction of mesoporous promotes the adsorption rate of CO₂.However,there is no significant difference in the adsorption rate at 45-60 k Pa,indicating that the pore distribution have little effect on the adsorption rate at 45-60 k Pa.（3）By establishing a kind of functional relationship between q_a of CO₂ at the pressure of10k Pa,100k Pa,500k Pa and 1000k Pa and V_<1nm and V_1-2nm of the samples,it can be concluded that q_a of CO₂ is mainly related to the micropore volume of the ACs,in which the influence of the small micropore volume is dominant.With the increase of pressure,the influence of big micropore volume becomes more and more obvious.In addition,the q_eff of CO₂ in the pressure range of 10-100k Pa,10-500k Pa and 10-1000k Pa are established as a function of V<1nm and V1-2nm,and the results are consistent with the qa.（4）Through the results of IAST and Henry selectivity at 298K and a binary-gas ratio of15/85 CO₂/N₂,it is found that the Henry selectivity is close to the IAST selectivity at 10 k Pa,which indicates that the adsorption of CO₂ in low-pressure region conforms to Henry’s Law.With the increase of pressure,the selectivity decreases.The selectivity of the samples still keeps above 8 at 1000 k Pa,which indicates that these samples have high separation ability as adsorbents.The pore structure of MIC-L series is favorable to the IAST selectivity of 10k Pa and 100k Pa and Henry selectivity.The selectivity of IAST at 500k Pa and 1000k Pa are not significantly affected by pore structure.（5）By calculating the selection parameter of CO₂/N₂ system,it can be concluded that the pore structure of the samples has little effect on the selection parameter at the pressure of100k Pa,500k Pa and 1000k Pa.（6）By comparing the isosteric heat of CO₂ adsorption of MIC and POR series,it can be found that the isosteric heat of adsorption of all seven samples is in the range of 19-30k J/mol,indicating that the adsorption of CO₂ on ACs belongs to physical adsorption.2.Study on the adsorption and separation performance of CH₄ on ACs（1）The Toth model was used to fit the adsorption isotherms of CH₄ on ACs,and the Q_s,0,b₀,Q₀ and t₀ were obtained respectively.Q₀ is less than 40k J/mol,indicating that the adsorption of CH₄ belongs to physical adsorption.（2）The adsorption kinetic curves of CH₄ at the pressure of 45-60k Pa were fitted by the first-order kinetic equation.It can be found that the adsorption rate of POR-S series is generally faster than MIC series and POR-B,which indicates that the introduction of small mesoporous promotes the adsorption rate of CH₄.（3）By establishing a kind of functional relationship between q_a of CH₄ at the pressure of10k Pa,100k Pa,500k Pa and 1000k Pa and V_<1nm and V_1-2nm of the samples,it can be concluded that q_a of CH₄ is mainly related to the micropore volume of the ACs,in which the influence of the small micropore volume is dominant.With the increase of pressure,the influence of big micropore volume becomes more and more obvious.At the pressure range of 10-100k Pa,10-500k Pa and 10-1000k Pa,the q_eff of CH₄ is established as a function of V_<1nm and V_1-2nm,and the results are consistent with the q_a.（4）Based on the adsorption isotherms of CH₄ and N₂,the IAST selectivity of CH₄/N₂ ratio of 15/85 and 85/15 were calculated at the pressure of 10k Pa,100k Pa,500k Pa and 1000k Pa.It is found that when the feeding ratio is 15/85,the IAST selectivity of the samples are always maintained above 3.5,indicating that the samples have high separation performance.For the CH₄/N₂ system with a ratio of 85/15,the selectivity at 1000k Pa is close to 1,indicating that the separation performance of the samples are poor.（5）By calculating the selection parameter of CH₄/N₂ system of a ratio of 15/85,it can be concluded that the selection parameter of MIC series is higher than POR series at the pressure of 100k Pa.Besides,the pore structure of the samples has little effect on the selection parameter at the pressure of 500k Pa and 1000k Pa.（6）By comparing the isosteric heat of CH₄ adsorption of MIC and POR series,it can be found that the isosteric heat of adsorption of samples is in the range of 19-22k J/mol,indicating that the adsorption of CH₄ on ACs belongs to physical adsorption.3.Comparison of adsorption and separation of CO₂ and CH₄（1）By comparing the adsorption capacity of CO₂ and CH₄ on the samples,it is found that the CO₂ adsorption capacity of the samples are better than that of CH₄ due to the small size and high polarity of CO₂.Besides,the adsorption heat and interaction force generated by CO₂adsorption are higher than those of CH₄.Because the adsorption heat and interaction force are affected by the polarity of adsorbates.（2）By calculating the selectivity of the samples to CO₂/CH₄ mixture,it is concluded that the Henry selectivity of the samples are all greater than 1,indicating that the interaction force between the samples and CO₂ is higher than that of CH₄.In the CO₂/CH₄ system,because the polarity of CO₂ is stronger than that of CH₄,it will be preferentially adsorbed on the strong adsorption sites of the samples.For the CO₂/CH₄ system,the presence of large mesopores is conducive to the Henry selectivity and IAST selectivity at 10k Pa.But the IAST selectivity of samples at 100k Pa,500k Pa and 1000k Pa has little difference.（3）For the CO₂/CH₄ system,the selection parameter of POR series is generally higher than MIC at the pressure of 100k Pa.As for 500k Pa and 1000k Pa,the pore structure of the samples has little effect on the selection parameter.