Adsorption Performance And Industrial Application Of Zeolites For VOCs

Volatile organic compounds(VOCs)are vital precursors of ozone(O3)and fine particulate matter(PM2.5),which play an important role in the formation process of complex atmospheric pollution.High volume and low concentration are common characteristics of industrial VOCs exhaust gas.The use of suitable adsorbents for high-efficiency concentration pretreatment is the key to improve efficiency and reduce cost of subsequent advanced treatment of catalytic combustion.Activated carbon is not resistant to high temperature and has potential safety hazards.On the contrary,zeolites can effectively overcome this shortcoming and have strong hydrophobicity and large adsorption capacity.However,there are various kinds of zeolites,so it is of great significance to screen out zeolites with high use value and conduct deep research on the adsorption performance and industrial application of zeolites for VOCs.Y zeolites(A1 and C3)with high adsorption capacity and ZSM-5 zeolites(B1 and B2)with strong hydrophobicity were selected as main subjects,and the Brunner-Emmet-Teller(BET)and X-ray diffraction(XRD)measurements were used to characterize the zeolites.Toluene,isopropanol,butyl acetate,cyclohexanone,butanone and styrene were selected as the main pollutant probes.The adsorption performance and industrial application of zeolites for VOCs was evaluated from both static and dynamic aspects.The Yoon-Nelson model was used to estimate the adsorption time of toluene and butyl acetate on zeolites.Results of BET tests demonstrated that the specific surface area of Y zeolites and ZSM-5 zeolites was 600 and 370 m2/g,respectively,and the pore volume was 0.529 and 0.250 cm3/g,respectively.Results of XRD illustrated the pore order of zeolites was pretty good.Results of adsorption experiments revealed that the dynamic adsorption quality was about 20%-30%of the static adsorption quality.Results of the adsorption performance of zeolites for VOCs in the range of 40 to 80 ℃ showed that the adsorption temperature should not exceed 70 ℃.Breakthrough curves indicated the breakthrough time of VOCs on zeolites was about 35%-60%of adsorption equilibrium time,and the breakthrough adsorption quality was about 70%-80%of equilibrium adsorption quality.Competitive adsorption of water vapor,butyl acetate and cyclohexanone to toluene was also studied.Results illustrated that when the relative humidity rised from 0 to 40%,the adsorption quality of toluene decreased by 63%for Y zeolites and 30%for ZSM-5 zeolites,respectively.At the same time,the coexistence of butyl acetate or cyclohexanone with toluene could reduce the toluene adsorption quality of zeolites by about 35%-45%.The error between the adsorption time of toluene and buty 1 acetate on zeolites obtained by Yoon-Nelson model and the experimental values was less than 10%.The desorption activation energy of zeolites for toluene,isopropanol and butanone was measured by temperature programmed desorption(TPD)to be about 50-70 kJ/mol,and for butyl acetate the desorption activation energy was more than 100 kJ/mol.Besides,the desorption activation energy of Y zeolites for cyclohexanone and styrene was more than 115 kJ/mol,while the desorption activation energy of ZSM-5 zeolites for these two pollutants was about 70 kJ/mol.Due to the influence of desorption activation energy,the desorption efficiency of VOCs on zeolites was different in the process of sweeping and heating,which ranged from 3%to 50%.After five cycles of adsorption and desorption,the reuse rate of zeolites for toluene,isopropanol and butanone was more than 90%,while for butyl acetate the reuse rate was about 60%-70%.Besides,the reuse rate of Y zeolites for cyclohexanone and styrene was about 20-40%,while that of ZSM-5 zeolites was more than 60%.As a result,ZSM-5 zeolites were more suitable for adsorbing exhaust gas containing ketone or styrene than Y zeolites.