Adsorption Dynamics Of Phenolic Wastewater In Structured Fixed Bed

With the high-speed development of society, the increasing demands of environmentare required by people. More and more attentation has been paid on the the treatment oforganic wastewaters containing phenolic compounds. The removal of phenolic compoundsis of prime importance due to their widely resource, toxicity and carcinogenicity.Adsorption method, solvent extraction method, chemical method, membrance separationmethod and bioanalysis as useful methods have been widely applied to deal with wastewater.Adsorption by activated carbon has been recognized as one of the most used methods forremoving emissions of phenolic compounds from wastewater. Fixed bed adsorber is onekind of efficient arrangements for conducting adsorption operation. However, conventionalfixed bed adsorber filled with granular activated carbons offers disadvantage of lowadsorption efficiency, high mass/heat transfer resistance and low bed utilization. Therefore,the development and application of a novel material, which can improve adsorption rate andenhance mass/heat transfer, have been recognized as a cutting-edge research topic. In thispaper, the microfibrous composite was prepared and characterized by using SEM, N2adsorption, and mercury intrusion method. The adsorption kinetics of phenol andp-nitrophenol （PNP） in stirred batch was investigated and the adsorption dynamicsparameters were obtained. The adsorption dynamics in fixed bed filled with individualgranular activated carbons and structured fixed bed filled with microfibrous composite weresimulated by models.First, the microfibrous entrapped activated carbon composites were fabricated throughthe wet lay-up papermaking process and sintering process. The properties of coconut shellactivated carbon （CSAC） and coal based activated carbon （CBAC） were tested by nitrogenadsorption method and mercury intrusion method. The microstructure of microfibrouscomposites before and after sintering under the optimized condition was measured by usingSEM. The results show that the activated carbon particles were well entrapped into a threedimensional network of stainless steel fibers with cellulose as binder, and the junctures of stainless steel fibers were sintered together to form a sinter-locked three dimensionalnetwork with large voidages. The apparent desity of coconut shell activated carbon and coalbased activated carbon are718.6kg/m³and852.4kg/m³, respectively. The BET surfacearea of coconut shell activated carbon is larger than that of coal based activated carbon. Thelarger surface area with more adsorption location is good for adsorption. For these twokinds of carbon, the micropore volume is three-forth of total pore volume, which isaccordance with the necessary happened conditon of surface diffusion.Second, the adsorption equilibriums of phenol and PNP on activated carbon wereinvestigated. The results indicate that the adsorption equilibrium of phenol on activatedcarbon could be described by by Langmuir model, and the adsorption equilibrium of PNP onactivated carbon could be well described by Freundlich model. The Langmuir model valuesfor phenol and the Freundlich equilibrium values for PNP are used afterwards as input datafor the computer simulations of stirred batch adsorption kinetics and fixed bed adsorptiondynamics.Thirdly, the adsorption kinetics of phenol and PNP on CBAC and CSAC in stirred batchadsorber were also investigated. The effects of initial concentration of phenol/PNP andactivated carbon doages on concentration decay curves were investigated. The homogeneoussurface diffusion model （HSDM） based on external mass transfer and intraparticle surfacediffusion was used to describe the adsorption kinetics for phenol and PNP in stirred batchadsorber. The confidence interval analysis of every parameters and the effect of surfaceadsorption coverage on the surface diffusivity were investigated. The confidence intervals ofkfand Dswere also analyzed, when kinetic parameters calculated through single experimentand several experiments together. The adsorption control process was obtained by Biotnumber which represents the ratio of the rate of transport across the liquid film to the rate ofintraparticle mass transfer. The results show that the decay curves with the different initialsolution concentrations and adsorbent dosages can be well represented with identical constantfilm mass transfer coefficient and surface diffusivity. Confidence intervals calculated fromsingle decay curve are larger, and the more decay curves are involved, the smaller confidence interval is obtained, which means that more precise estimates of the parameters from moredecay curves. The values of the film mass transfer coefficient and surface diffusivity are in theorder of magnitude10^-5and10^-12, respectively. The decay curves had bad fitting results withadsorbed phase concentration-dependent surface diffusivity, proving that the surfacediffusivity should be considered as a constant. According to the Biot number, the adsorptionprocess of phenol on coal based activated carbon is controlled by surface diffusion. Theadsorption process of phenol on coconut shell activated carbon, and that of PNP on two kindsof activated carbon are controlled by both surface diffusion and film mass transfer. The valuesof Biot number also proved that the caculated kintic parameter is suitable for applying intofixed bed adsorption system.Then, the adsorption dynamics was measured under different inlet concentrations andflow rates, and simulated by fixed bed model. The adsorption dynamics experiments ofphenol and PNP in fixed bed were carried out under the conditions of different flow ratesand inlet concentrations. The fixed bed model considering external/internal mass transferresistances as well as axial dispersion with non-linear isotherm was utilized to predict thebreakthrough curves of phenol and PNP adsorption in fixed bed. The three dimension mapand mass transfer zone of phenol and PNP adsorption in fixed bed was obtained. And thethe controlling mass transfer mechanism is obtained through sensitivity analysis. The resultsshow that the predicted breakthrough curves fit well with the experimental breakthroughcurves, indicating that the proposed model for fixed bed is suitable for describing theadsorption dynamics of phenol and PNP in fixed bed. And the increasing flow rate and inletconcentration resulted in faster movement of mass transfer zone and smaller length of masstransfer. The sensitivity analysis and Biot number both confirm that intraparticle diffusionand film mass transfer are the controlling mass transfer mechanism in fixed bed adsorptionsystem. And the model is sensitive to both k_f and D_s, but insensitive to D_L.Finally, adsorption dynamics of phenol and PNP in the structured fixed bed filled withgranular activated carbon in the inlet of bed and microfibrous entrapped activated carboncomposites in the outlet of bed at various flow rates and inlet concentrations were studied. The adsorption dynamics phenol and PNP in the structured fixed bed were measured underdifferent bed height ratio, inlet concentrations and flow rates. The length of unused bed（LUB） theory was used to analyze the breakthrough curves of phenol and PNP in thestructured fixed bed. The mathematical model involving surface diffusion, film masstransfer and axial dispersion was proposed to simulate the adsorption dynamics of phenoland PNP in the structured fixed bed. The three dimension map and mass transfer zone ofphenol and PNP adsorption in structured fixed bed was obtained. The mass transferresistances of phenol and PNP in structured fixed bed was analyzed by by using dynamicparameters. All the results show that the breakthrough curve of PNP in the structured fixedbed is steeper than that of the individual activated carbon fixed bed under the same bedheight. The length of unused bed of the structured fixed bed decreased, comparing with thatof the individual activated carbon fixed bed. The predicted breakthrough curves of phenoland PNP in structured fixed bed fit well with the experimental breakthrough curves. Theresults also show that the increasing flow rate and inlet concentration leads to fastermovement of mass transfer zone and smaller length of mass transfer. The mass transferresistances of phenol and PNP adsorption in structured fixed bed was also investigated,indicating that the intraparticle diffusion resistance, film mass transfer resistance, and theresistance attributed to axial dispersion in microfibrous entrapped activated carbon layer areall smaller than that of activated carbon particles layer.