Study On Adsorption Of Benzene And Cyclohexane By Activated Carbon

Various toxic and harmful substances,including volatile organic compounds(VOCs),are harmful to the health of human beings,production and living spaces,that is a factor of occupational disease hazard not to be ignored.In such toxic and harmful environments,respiratory protective equipments are particularly necessary.The gas mask can filter the toxic gas in the environment of the chemical industry or dangerous gases leakage site,so as to protect us from the harm of toxic chemicals.In GB 2890-2009"Respiratory protection—Non-powered air-purifying respirators",benzene is used as the test gas of filter element to protect organic gas or vapor.But benzene is a strong carcinogen and is harmful to human beings,so it is necessary to find a substitute for it.Cyclohexane is similar to benzene in physical and chemical properties and is very stable,and has been used as a test gas in the worldwide.Therefore,cyclohexane is very suitable substitute as an alternative to benzene.In this paper,the similarities and differences in adsorption behavior between benzene and cyclohexane on activated carbon were studied,and the results were verified through filter elements experiments.The adsorption mechanisms and mass transfer mechanism of benzene and cyclohexane on activated carbon beds were discussed.Firstly,the adsorption thermodynamics of benzene and cyclohexane on activated carbon was studied.Langmuir and Langmuir Freundlich adsorption isotherm equation was used to fit the static adsorption experimental data of benzene and cyclohexane.According to the Langmuir equation,At 20℃,30℃ and 40℃,the saturated adsorption capacity of activated carbon AC-1 for benzene was 446.43 mg/g,400.00 mg/g and 303.03 mg/g respectively,and the saturated adsorption capacity for cyclohexane was 285.71 mg/g,256.41 mg/g,208.33 mg/g,respectively,.According to the Langmuir-Freundlich equation,the saturated adsorption capacities of activated carbon AC-1 for benzene are 481.55mg/g,425.25 mg/g,341.70 mg/g,and for cyclohexane are 349.82 mg/g,317.53 mg/g,272.25 mg/g,respectively.The results show that the adsorption rate constant of benzene is less than that of cyclohexane,and the adsorption of benzene and cyclohexane on activated carbon is non-homogeneous;the adsorption rate of benzene on activated carbon is less than that of cyclohexane on activated carbon,and the static adsorption capacity of benzene on activated carbon is greater than that of cyclohexane.On the other hand,the adsorption energies of benzene and cyclohexane on activated carbon were calculated and compared.The results show that the adsorption energy of benzene molecule is the lowest when it is inclined adsorption,with the value of-46.182 kJ/mol;the adsorption energy of cyclohexane molecule is the lowest when it is inclined adsorption,with the value of-45.914 kJ/mol.The absolute value of benzene molecule adsorption energy is greater than that of cyclohexane molecule adsorption energy,indicating that under the same conditions,activated carbon is more likely to adsorb benzene molecules.Secondly,the adsorption kinetics of benzene and cyclohexane on activated carbon beds were studied.The effects of different physical properties of activated carbon,different initial concentrations and different specific velocities on the penetration curves of benzene and cyclohexane on the activated carbon fixed bed were investigated,and the experimental data were fitted by Yoon-Nelson equation.The results show that for AC-1～AC-4,the total specific surface areas of activated carbon had a higher positive linear correlation with adsorption capacities than microporous specific surface area.The protection time of benzene/cyclohexane on the activated carbon fixed bed decreases with the increase of initial concentration and specific velocity.Under the same conditions,the protection time of benzene is always longer than that of cyclohexane,and the protection time of cyclohexane is 0.79-0.87 times of that of benzene.Finally,the gas mask filter elements were tested to verify whether the protection time rule of benzene and cyclohexane on activated carbon is applicable to the filter elements.The results show that the protection time of cyclohexane on the filter element is 0.787-0.87 times of that of benzene,which is consistent with that on activated carbon.It can be seen that it is practical to use cyclohexane instead of benzene as the test gases of protection time.