Preparation Of Biomass Based Activated Carbons By Hydrothermal Carbonization And Its Adsorption And Separation Performance Of CH₄/N₂

The separation of CH₄ and N₂ has always been a difficult problem in gas separation.The key to the separation and purification of CH₄ is to search for a kind of appropriate adsorbent with high adsorption capacity and high selectivity.Activated carbon was widely used in the field of gas adsorption because of its developed and adjustable pore structure and rich and stable surface properties.The adsorption and separation performance of activated carbons is mainly affected by its pore structure and surface properties,which are determined by the types of carbon source and the preparation methods of activated carbons.Therefore,the purpose was expected to adjust the pore structure and surface properties of activated carbons by changing the types of carbon source,preparation methods and preparation conditions,so as to prepare an activated carbon with high CH₄ adsorption capacity and high CH₄/N₂ separation ratio.In this work,biomass material was selected as carbon source,different acid and alkali reagents as auxiliary agents,and direct carbonization method and hydrothermal carbonization method was adopted to prepare activated carbons,mainly investigated the type of carbon sources,carbonization conditions,hydrothermal conditions,acid and alkali solution content and types of the influence on the properties of activated carbons.The surface morphology of activated carbons was analyzed by scanning electron microscopy,and the surface properties of activated carbons were characterized by EDS,FT-IR and Boehm titration.The pore structure of activated carbons was analyzed by 77 K-N₂ adsorption and desorption isotherms and 273 K-CO₂ adsorption isotherms.The adsorption and separation performance of CH₄/N₂ on activated carbons was also studied.The main contents and conclusions were as follows:（1）A handy direct carbonization method was utilized to transform biomass material into activated carbons.The effects of carbonization temperature,carbonization time and the types of carbon sources on the surface properties,pore structure and gas adsorption and separation performance of activated carbons were studied.The results showed that the activated carbons prepared from biomass materials had a high separation ratio of CH₄/N₂,which is above 4.0,but its adsorption capacity of CH₄ was low.The increase of carbonization temperature was beneficial to the formation of micropores in the range of 0.4-1.0 nm,while the oxygen content on the surface decreases.The adsorption capacity of CH₄ and N₂ increased with the increase of micropore at 0.4-1.0 nm,and the separation ratio of CH₄/N₂ increased with the increase of oxygen content.When the carbonization temperature was 600℃and the carbonization time was 2 h,the adsorption capacity of CH₄on walnut shell based activated carbon was 15.9 cm³/g,and the separation ratio of CH₄/N₂ was 4.1;the adsorption capacity of CH₄on microcrystalline cellulose based activated carbon was 17.1 cm³/g,and the separation ratio of CH₄/N₂ was 4.5.（2）Activated carbons were prepared by sulfuric acid assisted hydrothermal carbonization with biomass material as carbon sources.The effects of carbon source types,sulfuric acid content,hydrothermal temperature and hydrothermal time on the surface properties,pore structure and adsorption and separation performance of CH₄ and N₂of activated carbons were investigated.The results show that the micropore volume,specific surface area and CH₄adsorption capacity of activated carbons prepared by hydrothermal carbonization were higher than those prepared by direct carbonization.Activated carbons were prepared by sulfuric acid assisted hydrothermal carbonization using walnut shell as carbon source.The results showed that with the increase of sulfuric acid content,the specific surface area,micropore volume and surface oxygen content of activated carbons firstly increased and then decreased.The specific surface area and micropore volume of activated carbons decreased with the increase of hydrothermal time and temperature.Under the carbonization temperature of 600℃,carbonization time of 2 h,hydrothermal temperature of 180℃,hydrothermal temperature of 10h,and the solid liquid ratio of walnut shell to sulfuric acid of 1∶2∶34,the adsorption capacity of CH₄ increased to 17.1 cm³/g,and the separation ratio of CH₄/N₂ reached as high as 4.4.（3）Walnut shell and microcrystalline cellulose were used as carbon sources to prepare activated carbon by hydrothermal carbonization assisted by acid and base solution.The effects of types of acid and base solution and carbon sources on the pore structure,surface properties and CH₄/N₂ adsorption and separation performance of activated carbons were investigated.When microcrystalline cellulose was used as carbon source,the adsorption capacity of CH₄ and the separation ratio of CH₄/N₂ were not affected by different acid and base solutions.Using walnut shell as carbon source,it was found that the volume of micropores,specific surface area and surface oxygen content of activated carbons prepared by different acid and base solutions were higher than those prepared by direct carbonization method.The acid solution treatment can increase the acid group on the surface of activated carbons,while the alkaline solution treatment can increase the basic group on the surface of activated carbons.At 298 K and 100k Pa,the adsorption capacities of CH₄ and N₂ on activated carbons increased with the micropore volume（0.4-1.0 nm）.The separation ratio of CH₄/N₂ increased with the increase of basic oxygen-containing groups.When the carbonization temperature was 600℃,the carbonization time was 2 h,and the hydrothermal temperature was 180℃,the hydrothermal time was 10 h,the adsorption capacity of CH₄ was 15.6 cm³/g,and the separation ratio of CH₄/N₂ was as high as 4.7.