Treatment Of Binary VOCs Mixture Of N-butyl Acetate And P-xylene Using Coupled Temperature And Vacuum Pressure Swing Adsorption

The treatment of volatile organic compounds(VOCs)has become an urgent and important task,because the large emission of VOCs is damaging the atmospheric environment and threating human health.In this paper,the activated carbon adsorption and desorption behaviors of two typical VOCs components(n-butyl acetate and p-xylene)of spraying workshops were investigated.A coupled temperature and vacuum pressure swing adsorption process was applied to concentrate the binary VOCs mixture and improve the utilization of the adsorbent.In this study,it was found that activated carbon adsorbent has developed specific surface area(1102 m~3/g)and a micropore-mesoporous structure.The Yoon-Nelson model shows a good fitting on the kinetic data of single component adsorption and it could be used to predict the adsorption penetration time.The breakthrough curves of binary mixture adsorption show a"roll-up"phenomenon,indicating that competitive adsorption between n-butyl acetate and p-xylene occurs on the surface of activated carbon.Weakly adsorbed n-butyl acetate could be replaced by strongly adsorbed p-xylene on the active site.Based on the single-component adsorption equilibrium data fitted by the Langmuir model,both the extended Langmuir(E-L)model and the ideal solution adsorption(IAST)model can accurately predict the adsorption equilibrium of binary mixture with the average relative error less than 7%and 12%,respectively.The increase of feed flow rate and feed concentration not only enhances the adsorption kinetics and adsorption capacity,but also promotes the competitive adsorption behavior of n-butyl acetate and p-xylene,and improves the selectivity of the two components on activated carbon.The adsorption mechanism studies show that the adsorption of n-butyl acetate and p-xylene on activated carbon is controlled by external diffusion.According to the difference of adsorption behavior,the adsorption bed can be divided into four zones:safe zone,mass transfer zone,pseudo-equilibrium zone and equilibrium zone.In the normal working range of the vacuum pump,there is an operating condition of desorption pressure and the corresponding air stripping,which results in the optimal desorption of VOCs on activated carbon.As the desorption temperature increases from25℃to 65℃,the maximal concentration ratio of n-butyl acetate in the desorbed gas increases from 9.6 to 22.7 and that of p-xylene increases from 9.0 to 18.6,and the total desorption ratio of two components increases from 45.8%to 59.4%.To comprehensively consider the effective utilization of adsorbent and the difficulty of the subsequent recovery,the optimized process conditions were determined as follows:adsorption under the room temperature and atmospheric pressure,desorption with the desorption temperature of 65℃,desorption pressure of 7.0 k Pa and the stripping gas flow rate of 0.85 L/min.