Study On The Adsorption And Desorption Performance Of Molecular Sieve For VOCs

In recent years,emissions of volatile organic compounds（VOCs）from industrial solvent use account for more than 20%of the total anthropogenic emissions in the country,making it one of the main sources of VOCs in China’s cities.VOCs are important precursors for the formation of ozone（O₃）and secondary organic aerosols（SOA）,which can not only cause serious environmental pollution but also pose a serious threat to people’s health.Among the solvent-using industries,representative enterprises mainly include furniture,paint and ink manufacturing,automobile,electronics and other industries,which are characterized by small and medium-sized enterprises,large in number and covering a wide range,and have a pivotal position in the development of China’s national economy;their VOCs emissions are mainly low in concentration and large in air volume.Among the VOCs treatment technologies,adsorption method has the advantages of economy and applicability,and the core of adsorption method is the selection of adsorbent.Activated carbon is not easy to regenerate and flammable,molecular sieve can make up for these defects.However,there are many types of molecular sieves in the market,and it is important to select the suitable molecular sieve with the actual working conditions.In view of this,four molecular sieves,including Na Y type and ZSM-5type,were selected for this study and characterized by scanning electron microscopy（SEM）,Brunner-Emmet-Teller（BET）,X-ray diffraction（XRD）,X-ray fluorescence analysis（XRF）and Fourier infrared spectroscopy（FTIR）,and the adsorption and desorption performance of molecular sieves on VOCs such as acetone,cyclohexane,isopropanol and ethyl acetate were considered from both static adsorption and dynamic The adsorption and desorption performance of molecular sieve for acetone,cyclohexane,isopropanol,ethyl acetate and other VOCs were considered from both microscopic and macroscopic perspectives,and the adsorption and desorption mechanism of molecular sieve for VOCs was revealed from both microscopic and macroscopic perspectives;the static adsorption process was fitted with kinetics,and the dynamic adsorption process was fitted with Y-N model,Thomas model and adsorption isotherm model to correct the penetration curve errors and obtain guiding parameters.The parameters with guiding significance were obtained;the reuse performance of molecular sieve was measured by adsorption and desorption cycle experiments,and finally the length of mass transfer zone and total mass transfer coefficient were calculated under different conditions,and the competitive adsorption of mixed VOCs was investigated to provide guidance for practical engineering applications.The main findings are as follows.（1）In the SEM test,the molecular sieves were neatly structured and showed a rectangular shape.BET characterization revealed that the specific surface area of the molecular sieves ranged from 228.96 to 395.63 m²/g,and the effective pore size was 1.0547～1.6082 nm.XRD test showed that the pore channels of the molecular sieves were well ordered.FTIR test showed that the functional group types of the four molecular sieves were basically the same,so the effect of the functional groups on the adsorption performance in this The effect of functional groups on the adsorption performance in this experiment was relatively small.（2）In the static adsorption process,the adsorption performance of molecular sieves on VOCs varied.In the kinetic simulation fitting,the quasi-second-order kinetics were basically above 0.99,and their simulation effect was good,indicating that the adsorption was a chemisorption process;in the simulation of the internal diffusion of molecular sieve on ethyl acetate,the fitting effect was good,and the first and second stages had a greater effect on adsorption.In the static desorption process,the molecular polarity and diffusion resistance of VOCs lead to different desorption effects.The saturation adsorption capacity of molecular sieve for dynamic adsorption of VOCs is about 80%of that under static adsorption conditions.The dynamic adsorption penetration curves of molecular sieves for VOCs were all S-shaped,but the adsorption conditions varied between different types of molecular sieves and VOCs;both the physical properties of VOCs and the structure of molecular sieves had a great influence on the adsorption,and a single physical property of VOCs and the structure of molecular sieves could not explain the adsorption process.In the desorption process of100～200℃,the higher the temperature,the better the desorption effect;the pore capacity,effective pore size and Si/Al of the molecular sieve may affect the desorption.After 10adsorption and desorption cycles,the adsorption rate was basically maintained at about50%～60%.（3）The errors between the fitted and experimental values of the Thomas model and the Yoon-Nelson model were within 2%and 10%,respectively,which can provide guidance for practical engineering applications.For the four adsorption isotherm models such as Freundlich,Langmuir,Temkin and D-R,the Langmuir isotherm adsorption model has the best simulation effect;comprehensively,the adsorption of VOCs by molecular sieves is mainly based on chemisorption.（4）The fixed bed height,initial concentration,and temperature affect the length of the mass transfer zone and the total mass transfer coefficient of the substance in adsorption experiments,and high temperatures were found to be unfavorable for adsorption to proceed.In competitive adsorption experiments,the VOCs that occupy a favorable position tend to be the more polar substances.