Study On Adsorption Performance Of Modified Molecular Sieve For Dichloromethane Waste Gas

In this paper,13X molecular sieve was employed as an adsorbent to purify the volatile dichloromethane waste gas.A set of dynamic adsorption experimental equipment was performed to investigate the factors affecting the adsorption capacity of molecular sieve during the adsorption process.Considering the fact that the large amount of water vapor was in the actual industrial waste gas,the hydrophobic modification of the molecular sieve was carried out using liquid phase silanization method and liquid phase dealuminating silanization method.The hydrophobic effect of the modified molecular sieve and the adsorption capacity to dichloromethane under different humidity conditions were explored.The main contents are as follows:（1）The effects of packing height,gas flow rate,gas temperature,packing void fraction,relative humidity and other factors on the adsorption of dichloromethane onto 13X molecular sieve were investigated.The results shown that:changes in packing height did not affect the shape of the penetration curve;the saturated adsorption capacity increases at first and then decreased with the increase of gas flow rate.With the increase of temperature,the equilibrium of adsorption desorption tended to the direction of desorption,and the saturated adsorption amount decreased.The higher the porosity and relative humidity,the smaller the saturated adsorption capacity.On the basis of these experiments,three dynamic adsorption models were used to fit the penetration curves of different influencing factors.The results showed that the theoretical values calculated by the three models were in good agreement with the experimental values,which indicated that the three models could well predict the dynamic adsorption process of dichloromethane gas onto 13X molecular sieve.（2）The molecular sieves were modified by liquid phase silanization method,and the influence of different deposition solvents and the deposition amount of trimethylchlorosilane on the hydrophobic effect of molecular sieves were investigated.The optimal modification conditions were determined as follows:cyclohexane was the deposition solvent,and the deposition amount of trimethylchlorosilane was 64mg/g.Scanning electron microscope and energy spectrometer,X-ray diffraction,Fourier transform infrared spectrometer was used to characterize the molecular sieve analysis before and after modification.And the results showed that after modification,hydrophobic methyl groups attached to the surface of the molecular sieve,which improved the hydrophobic effect of the molecular sieve.The hydrophobic effect of the modified molecular sieve was evaluated by static water adsorption experiment.The results shown that the water absorption capacity of the modified molecular sieve decreased from 30.95%to 21.10%.In addition,the best modified molecular sieve adsorption performance was explored via different humidity adsorption experiments,and the results shown that the saturated adsorption capacity of dichloromethane increased by 24.45 mg/g,18.28 mg/g and-9.07 mg/g under condition of RH as 95%,65%and 25%,respectively compared with the original molecular sieve.This indicated that the moisture resistance of the modified zeolite is enhanced,but saturated adsorption capacity decreased under low humidity because of the occupy of some adsorption sites by the silane reagents.（3）The hydrophobic modification of the molecular sieve was further studied via liquid phase dealuminization silanization method,and the influence of acid type and acid concentration on the hydrophobic effect of the modified molecular sieve was investigated.The best dealuminizating reagent was determined as hydrochloric acid,and the best hydrochloric acid concentration was 0.3 mol/L.Under this condition,the best modified molecular sieve was prepared and the adsorption experiment was carried out.The characterization results of the modified zeolite shown that hydrochloric acid dealuminization can increase the Si-Al ratio of the zeolite,and also reduced the specific surface area of the zeolite.At the same time,a large number of hydroxyl nests generated by dealuminization were replaced with hydrophobic Si-（CH₃）₃groups by silane reagents,which enhanced the hydrophobicity of the molecular sieve and partially repaired the framework of the molecular sieve.Static water adsorption experiments shown that the water absorption capacity of the modified zeolite was reduced to 13.66%after liquid phase dealuminization silanization.Dynamic adsorption experiments with different humidity shown that the saturated adsorption capacity of dichloromethane of the modified molecular sieve at95%,65%and 25%humidity increased by 27.15 mg/g,16.58 mg/g and-12.09 mg/g,respectively,compared with that of the original molecular sieve.It can be found that the modification effect of dealuminization silanization is better than that of silanization alone at high humidity.But in general,the molecular sieves modified by these two methods have better moisture resistance and better treatment effect for high humidity dichloromethane industrial waste gas.