| The demand for environmental protection has received increasing attention with therapid development of both the society and economy. The removal of pollutants from air is oneof the important environmental protection tasks. Volatile organic compounds (VOCs) are themajor air pollutants. There are much more techniques for treating VOCs including adsorption,absorption, condensation, catalytic combustion and thermal combustion. Adsorption is one ofthe domain technologies for VOCs removal. Conventional fixed bed adsorber with relativelylarger granular activated carbons is of low adsorption rate and high mass transfer resistance.How to prepare a novel material for the application in the conventional fixed bed which canincrease adsorption rate and decrease mass/heat transfer resistance is a cutting edge researchtopic. In this paper, the research progress on the preparation, characterization and applicationof microfibrous composite was reviewed. The preparation, characterization and application ofboth microfibrous entrapped activated carbon composite and silicalite1zeolite membrane onstainless steel microfibrous for VOCs purification were investigated. The adsorptiondynamics and mass transfer mechanism in the structured fixed bed were analyzed anddiscussed.First, the preparation process of microfibrous entrapped activated carbon composites wasstudied. Microfibrous entrapped activated carbon composites were prepared by wet layuppapermaking and sintering process from stainless steel fibers, coniferous wood fibers andactivated carbon particles (150200m). The effects of a variety of operation parameters onthe properties of microfibrous composites were investigated and the optimum processparameters were obtained. The experimental results showed that the carbon entrapment ratioof the microfibrous composites prepared with a ratio of13:6(W/W, carbon/fibers) is64.3%under the conditions of adding2L water, stirring for10min, and then sintering at1050C for20min with the sintering material mass of212g. The microstructure of microfibrouscomposites before and after sintering prepared under the optimized condition was measuredby means of SEM, the results showed that the junctures of stainless steel fibers are wellwelded together to form a sintered locked three dimensional network with large voidage, which entrap the micron size activated carbon particles very well. The pore size distributionsand specific surface areas of activated carbons before and after entrapment were measured bynitrogen adsorption method. The pore size distributions of the activated carbons before andafter entrapment are basically the same, and the specific surface areas are678and769m~2/gfor type111activated carbon,976and955m~2/g for type0960activated carbon respectively.Second, the preparation technique of zeolite membrane on microfibrous support wasinvestigated. The silicalite1zeolite membrane on stainless steel microfibrous support wasprepared by secondary growth from TEOS as Si source and TPAOH as a template. Theeffects of a variety of operation parameters on the properties of zeolite membrane wereinvestigated. The experimental results showed that it is feasible to prepare continuous anddense MFI zeolite membrane on the surface of stainless steel microfibrous by secondarygrowth method, and the thickness of the membrane is about2.3m. The specific surface areas,total pore volume and micropore volume of the membrane are93.24m~2/g,0.06545cm~3/g and0.03826cm~3/g, respectively. The membrane by treating support with anodic oxidation methodis more continuous and denser than that with calcinations. The surface of membrane is muchdenser and the size of crystal grains increased as increasing the crystallization temperature orcrystallization time. The effect of third growth based on secondary growth on the propertiesof zeolite membrane is not very clear.Then, the adsorption dynamics of the structured fixed bed with microfibrous compositeswere studied. The structured fixed bed was filled with granular activated carbons andmicrofibrous entrapped activated carbon composites in the inlet and outlet of the fixed bed,respectively. The adsorption dynamics of benzene, toluene, benzene/toluene andtoluene/p xylene binary mixtures in the structured fixed bed were measured, and comparedwith that in the conventional GAC fixed bed. The results showed that the breakthrough timeof benzene and toluene in the structured fixed bed increases about20min compared with thatin the conventional GAC fixed bed at relatively low breakthrough concentration. Thebreakthrough time of benzene and toluene in structured fixed bed decreases as decreasing beddepth or increasing gas flow rate and inlet concentration. The breakthrough curves of bothbenzene/toluene and toluene/p xylene binary mixtures in both structured fixed bed andconventional GAC fixed bed is of completive adsorption characteristics. Toluene is more easily adsorbed than benzene, and p xylene is more easily adsorbed than toluene. Thebreakthrough time of benzene, toluene and p xylene in the structured fixed bed clearlyprolongs than that in the conventional GAC fixed bed at the same conditions.Finally, the mass transfer mechanism in the structured fixed bed by adsorption dynamicsmodels was investigated. The length of unused bed and bed utilization for benzene andtoluene in both structured fixed bed and conventional GAC fixed bed were determined byLUB equation. The adsorption breakthrough curves were fitted according to three fixed bedadsorption models of Bohart Adams, Yoon Nelson and BDST models by the linear regressionanalysis. The results showed that the LUB value in the structured fixed bed decreases26%47%compared with that in the conventional GAC fixed bed, and bed utilizationincreases9%18%. Bohart Adams model is valid for the C/C0up to0.5, and largediscrepancies are found between the experimental and predicted values above this value forbenzene and toluene adsorption in the structured fixed bed. The adsorption rate constant k'and50%breakthrough timeτcan be obtained by Yoon Nelson model, the k 'value in thestructured fixed bed increases about50%compared with that in the conventional GAC fixedbed at the same conditions. The experimental breakthrough curves are very agreement withthose predicted by the Yoon Nelson model when the C/C0is above0.05for the adsorption ofbenzene and toluene in the structured fixed bed. The adsorption capacity N0and adsorptionrate constantk aof benzene and toluene in the structured fixed bed can be obtained by BDSTmodel.
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