Synthesis Of Nano Zeolite And Its Performance In CH₄/N₂ Adsorption Separation

Zeolites(molecular sieves)are microporous crystal materials with 3-dimensional framework structure composed of tetrahedron of TO4,in which T is usually Si and Al,and can also be doped with P,Ge and Ga atoms to form new molecular sieve materials.Thanks to the uniform pore distribution,large specific surface area,exchangeable cations and good stability,zeolite is widely used in catalysis,adsorption separation,ion exchange and other fields.However,the narrow channels of zeolites also reduce the transport of guest molecules,resulting in some problems of conventional micron zeolites,such as the unavailability of active catalytic sites or adsorption space,carbon deposition and low separation efficiency.Nano zeolites can effectively overcome the disadvantages of micron zeolites because of their larger specific surface area and shorter diffusion path.At present,great progress has been made in the synthesis of nano zeolite,but there are still some problems,such as using expensive and highly polluting organic templates and complex synthesis process.Therefore,the development of green synthesis route to make the synthesis process simple,efficient and environmentally friendly is the research focus of nano zeolite.Coalbed methane is a kind of unconventional natural gas with methane as the main component in coal seams.Due to the low concentration of coalbed methane extracted underground,it is difficult to use,and most of it is discharged into the air,with an annual emission of 21 billion cubic meters(equivalent to 27 million tons of standard coal),resulting in a lot of waste of resources.At the same time,the greenhouse effect of methane is 21 times that of carbon dioxide,and its contribution rate to the current human perception of global warming is 25%,which is known as the second largest greenhouse gas.Therefore,enrichment and recovery of low concentration coalbed methane can not only effectively recover resources,but also help achieve the goals of"carbon peaking"and"carbon neutralization".The key of enrichment technology is the separation of methane and nitrogen.In this work,based on the basic theory of"promoting nucleation",a series of nano zeolites were synthesized by the seed iteration method and the concentred gel conversion method.Through gas adsorption test and mixed gas breakthrough test,the separation performance of nano zeolite for CH4/N2 was investigated in detail.The main contents and conclusions of this paper are as follows:(1)A new method,seed iteration method,was proposed for the synthesis of nano zeolite.In this method,the product induced by seed was iteratedly used to induce the next generation product with effect of controlling the crystal size,and nano K-Chabazite zeolite with stable morphology and properties was synthesized in the 4^th generation.The synthesized nano K-Chabazite zeolite are aggregates of about 500 nm,which is composed of about 50 nm nanocrystals.Compared with the micron K-Chabazite zeolite,the nano K-Chabazite zeolite has higher specific surface area,pore volume(92.45 m²/g and 0.32 m³/g)and gas mass transfer rate(CH4:1.06,N2:1.17).The synthesized K-Chabazite has the highest adsorption capacity for CH4among the zeolite adsorbents,which is 40.6 cm³/g(298 K,1bar).The breakthrogh test of mixed gas shows that the zeolite is also the best zeolite adsorbent for CH4/N2 mixture separation.(2)Nano K-ZK-5 zeolite was synthesized by seed iteration method showing that this method of nano zeolite has certain universality.The synthesized K-ZK-5 zeolite is a kind of aggregates formed by the irregular aggregation of 70-100 nm nanocrystals.The specific surface area and pore volume of nano K-ZK-5 are 339.37 m²/g and 0.27 m³/g,which are much higher than those of conventional micro K-ZK-5(248.65 m²/g and 0.18 m³/g).In addition,nano K-ZK-5 has higher gas diffusion rate and can reach adsorption equilibrium faster.The single gas adorption test showed that the nano K-Zk-5 has the second highest adsorption capacity of CH4among the zeolite adsorbents,which is 31.5 cm³/g(298 K,1bar).The breakthrough test of mixed gas shows that the zeolite has excellent separation performance for CH4/N2 mixture.(3)Nano SAPO-34 zeolite with a thickness of?50 nm and a size of?200 nm has been synthesized by the concentrate gel conversion method.Compared with conventional micron SAPO-34(2?m),the nano sheet SAPO-34 synthesized showed higher specific surface area and pore volume.The adsorption capacity of CH4 reached 25.74 cm³/g,increased by 60%,which was higher than that of most molecular sieve adsorbents.Meanwhile,the selectivity of CH4/N2did not decrease significantly(3.1),and reached the level of commercial adsorbent(>3.0).Through the breakthrough test of CH4/N2(50/50,v/v;298 K),it is proved that the nano SAPO-34 is indeed an effective adsorbent for CH4/N2 separation.(4)The industrial scale-up production of nano K-Chabazite was realized by using 1 m³reactor and seed iteration method,and the morphology of the product does not changed.At 298K and 1 bar,the adsorption capacity of CH4 and the selectivity of CH4/N2 were 36.26 cm³/g and 4.7 respectively,which were close to the small batch samples synthesized in laboratory.Nano K-Chabazite was formed by potassium alginate water column molding method,rapid hardening cement pellet making method and tablet molding method respectively.The results show that:the preparation of core-shell spherical adsorbent by potassium alginate water column molding method is very suitable for the formation of nano K-Chabazite zeolite;when the amount of potassium alginate is 2 wt%,the adsorption capacity of CH4 and the selectivity of CH4/N2 reach ideal levels,which are 30.62 cm³/g and 4.7 respectively,and the compressive strength and bulk density are 5.2 N^-1 and 0.41 g/cm³ respectively.A double bed three-step PSA process for separation of CH4/N2 was developed by using K-Chabazite zeolite as loading adsorbent.The experimental results show that under the optimal operating conditions of inlet gas rate(240 L/h)and adsorption time of 7 mins,the pressure swing adsorption process can effectively enrich the CH4/N2 mixture with 20%CH4 to more than 40%,which considerd to be a very good methane enrichment effect.