Study On Adsorption And Separation Of CH₄/CO₂ By Modified Sapo-18

With the rapid development of adsorbents and adsorption processes,adsorption process has become one of the most widely used separation methods in industry.The adsorption process is usually carried out on a packed column filled with adsorbent particles,i.e.a fixed bed adsorber.There are many kinds of adsorbents.Activated carbon,silica gel,mesoporous materials,activated alumina and zeolite are common adsorbents for removing carbon dioxide from natural gas.Among them,zeolite molecular sieves have unique surface chemical properties and good thermal stability.SAPO-18 zeolite with AEI structure has great potential in adsorption and separation.Therefore,SAPO-18 zeolite was synthesized by hydrothermal method and further modified.At the same time,the synthetic zeolite was characterized by XRD and SEM,and the performance of the synthesized zeolite in removing carbon dioxide from natural gas was investigated.Firstly,SAPO-18 zeolite was successfully synthesized by hydrothermal method.The effects of crystallization time,Si/Al ratio,crystallization heating rate and the amount of template on the synthesis of SAPO-18 zeolite were further investigated.Then sodium nitrate,potassium nitrate,magnesium nitrate and calcium nitrate were used to provide Na⁺,K⁺,Mg²⁺,Ca²⁺ions for ion exchange of SAPO-18 molecular sieves with different silicon content.Gas chromatography was used to draw the adsorption penetration curve and desorption performance curve,and the adsorption of carbon dioxide in methane was obtained.It was found that Ca-SAPO-18 molecular sieves with high silicon content had a large amount of carbon dioxide adsorption on these molecular sieves,which was 1.9875 mmol/g.Under the same reaction conditions,by comparing SAPO-18 molecular sieves with different silicon content,it is found that the adsorption time increases with the increase of silicon content,which indicates that with the increase of silicon content,the isomorphic substitution of silicon atoms will be triggered,the skeleton negative electricity will be increased,and more sites will be provided for ion exchange.After ion exchange,the surface of zeolite has higher electric field gradient,which will produce stronger interaction with adsorbate molecules.At the same time,the adsorption properties of molecular sieves modified by different ion exchange with the same silicon content were studied.By comparing the adsorption penetration curves of Na-SAPO-18,K-SAPO-18,Mg-SAPO-18 and Ca-SAPO-18 molecular sieves,it was found that the relationship of adsorption time was Ca²⁺>Mg²⁺>Na⁺>K⁺.Because the cation radius plays a dominant role in all interactions for monovalent cations,the size of Na⁺is smaller than K⁺,and the interaction is inversely proportional to rⁿ,so the adsorption effect of Na⁺with smaller size is better.However,for divalent cations,polarizability plays a dominant role,and larger polarizability can increase the induced energy（proportional to polarizability）.The polarizability of Ca²⁺is about five times that of Mg²⁺,so the adsorption effect of Ca²⁺with higher polarizability is better.Divalent cations have double charges,so the induced energy of divalent cations（proportional to q²）is much larger than that of monovalent cations,so the adsorption effect of divalent oxygen ions is better than that of monovalent cations.At the same time,the desorption experiments of these molecular sieves show that the modified SAPO-18 molecular sieves only have physical desorption at lower temperature,which indicates that the modified SAPO-18 molecular sieves are easy to desorb and regenerate.It has outstanding advantages in adsorbing carbon dioxide in methane and is a good adsorbent for carbon dioxide.