A Piecewise Loading Bed Based On The Vacuum Swing Adsorption Performances Of Silica Gels

Volatile organic compounds(VOCs)are primary sources of atmospheric pollution.Their huge emissions cause enormous harm to both the human health and the ecological environment.Adsorption,a kind of recovery method,has the advantages of high recovery efficiency and no secondary pollution.It is often combined with condensation,distillation or other processes in the recovery of VOCs.Three kinds of silica gels(SG)with different pore size distributions were tested for removal and recovery of o-xylene by vacuum swing adsorption(VSA).The adsorption mechanisms of o-xylene on different kinds of silica gels have been revealed through the mathematic modeling of adsorption isotherms.The strong adsorption potential in micropore leads to a high equilibrium adsorption capacity at low pressure of o-xylene,while the adsorption of mesopore comprises the inner surface adsorption at low pressure and the capillary condensation at high pressure.The analysis of adsorption kinetics indicates that SG with micropore shows a steep concentration front and a long breakthrough time due to the strong adsorption force field,negligible transfer resistance and compressive effect of favorable isotherm.While the reverse capillary condensation and mass transfer resistance cause a disperse front and a short breakthrough time for mesoporous SG.However,the fast desorption rate makes mesoporous SG more cost-saving compared with SG with micropore.Accordingly,to further improve the front slope and utilization of mesoporous SG in the adsorption process,a small amount of SG with micropore was loaded at the end of the fixed bed.It is proved that the slope of the front could be effectively increased in the piecewise loading bed compared with single mesoporous SG.The tests of 5 times VSA cycles show that the breakthrough time and adsorption capacity of the piecewise loading bed increases by 55.76% with single mesoporous SG,when the ratio of microporous SG and mesoporous SG is 2:3.And the energy consumption was observed to exhibit an obviously decrease by 28.8%.These results prove that the feasible piecewise loading method could recover the VOCs gas in a much more energy-saving way.