Downsizing Zeolite L/T Crystal Size And Study On Gas Enhanced Mass Transfer And Separation Performance

Our country has abundant reserves of coalbed methane（CBM）resources.The enrichment of CH₄ in low-concentration coalbed methane can not only reduce the probability of mining disasters and avoid the emission of CH₄ greenhouse gases but also enable the effective use of resources.In this paper,the enrichment of CH₄ in low-concentration coalbed methane（the separation of CH₄/N₂）and the capture of CO₂ in the flue gas of power plants（the separation of CO₂/N₂）have been explored in detail.In recent years,pressure swing adsorption（PSA）has shown great competitive advantages in the gas separation industry due to its convenient start-up and shutdown,high degree of automation and low energy consumption,and the key to its efficient operation is the development of high-performance adsorbents.Zeolite stand out among many porous materials due to their wide material source,low price,simple synthesis and excellent gas separation performance.At present,the gas separation application of industrial molecular sieves encounters bottlenecks.Our group’s previous research found that zeolite materials have a low mass-transfer rate when used for methane and nitrogen separation.The low gas adsorption and mass transfer rate will inevitably increase the energy cost of industrial gas separation.How to strengthen the gas mass transfer process is still a problem that needs to be solved urgently.The choice of zeolite pore structure and changing the crystal morphology or size can effectively alleviate the diffusion limitation.In this paper,two strategies were proposed.On the one hand,three different morphology of zeolite L were synthesized,and the effect of crystal size reduction on CO₂/CH₄ and CO₂/N₂ gas separation performance was studied;on the other hand,the organic template method successfully reduced the crystal size of zeolite T from 12×2μm to 1.5×1μm.The effect of the reduction of the zeolite T crystal size on the adsorption,separation,and kinetics of CH₄/N₂ was studied.The main research contents and conclusions as follows:（1）Three different morphology of zeolite L were synthesized:cylindrical（C-L:3×2μm）,disc-shaped（D-L:0.2×1.2μm）and nano（N-L:40×25 nm）.The basic characterization of the sample was carried out by powder X-ray diffraction（PXRD）,field emission scanning electron microscope（SEM）,77 K nitrogen adsorption and desorption test,thermogravimetric analysis and Inductively coupled plasma-optical emission spectroscopy（ICP-OES）tests.The BET specific surface area,external specific surface area,total pore volume and microvolume of N-L are（427.2 m²/g,90.7 m²/g,0.382 cm³,0.167 cm³/g）much higher than that of C-L（279.8m²/g,13.1 m²/g,0.098 cm³/g,0.098 cm³/g）,and D-L（308.6 m²/g,31 m²/g,0.116 cm³/g,0.112 cm³/g）,N-L has a richer pore structure,which may be related to the mesopores produced by the accumulation of small crystals.The CO₂ adsorption capacity of N-L（70.7 cm³/g）is 20.9%higher than that of C-L（58.5 cm³/g）.The kinetic experiment proves that,compared with M-L and D-L,N-L has a faster diffusion and mass transfer rate of gas molecules due to a shorter diffusion path,and the time to reach gas adsorption equilibrium under the same pressure is shorter.Dynamic breakthrough experiments further proved that N-L can effectively capture CO₂ in power plant flue gas and biogas,and as a high-performance adsorbent with great application potential.（2）The synthesis time of zeolite T was successfully reduced from 7 d to 2 d,and the crystal size was reduced from 12×2μm to 1.5×1μm by using tetramethylammonium hydroxide（TMAOH）as the organic structure directing agent.The basic information of all samples was characterized by powder X-ray diffraction（PXRD）,field emission scanning electron microscopy（SEM）,77 K nitrogen adsorption and desorption test,and Inductively coupled plasma-optical emission spectroscopy（ICP-OES）tests.The influence of the amount of organic template added on the synthesis of T-type molecular sieve was explored,and when the amount of TMAOH added wasα=1.5（T-1.5）,the highest crystallinity and relative smaller crystal size（1.5×1μm）was obtained.The crystal sizes of M-T and T-1.5 are 12×2μm and 1.5×1μm,respectively.The external specific surface area（59 m²/g）and mesopore volume（cm³/g）of T-1.5 are higher than M-T（9.7 m²/g,0.016 cm³/g）.Compared with M-T,single-component adsorption capacity of CH₄ and N₂ on T-1.5 increased by 21%and 27%,respectively.The gas adsorption kinetics shows that CH₄ can reach adsorption equilibrium faster on T-1.5.The Dynamic breakthrough experiments further proved that T-1.5 has longer penetration separation time.