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1、Chemical Engineering and Processing 48 (2009) 396407Contents lists available at ScienceDirectChemical Engineering and Processing:Process Intensificationjournal homepage: multi-staged radially cross-flow fluidizeRDepartment 16 IndiaarticleArticleRAAKIon-eFluidizeMassMathematicalWy coximatisinuid/r1a

2、pplication.iseperiodictinuous operation may be achieved in staged-column, with waterflowing upward through the sieves on stages and fluidized resinparticles flowing across on the stage and then to the subsequentstage through a downspout. The physical configuration of the con-tinuous ion-exchange col

3、umn is apparently similar to that of adisrofthedemowthewConcentrflobouridizationundersolidmadeion-ephosphatto be treated in the column. Commercially available resins wereused as ion-exchangers. Salts of nitrate and phosphate have con-siderably large solubility in water and consequently pose challeng

4、efrom the perspective of treating waste water. One of the commonexamples of the wastewater containing binary anions of nitrate and0255-2doi:tillation column, with liquid and vapor flowing counter cur-ently. Such ion-exchange columns are in operation in a numbercommercial applications 13.In our recen

5、t study we have experimentally demonstrated thatstaged column may be operated continuously within the pre-termined flooding and loading limits, with solid resin particlesving smoothly under fluidized state across the stage and theater to be treated flowing upward through the sieve holdingresins 4. T

6、ransfer of resins to the subsequent downstageas rendered through a downspout fitted near the column walls.ation measurements made for varying water and solidwrate, number of stages, and stage height (depth of the fluidizeded) showed approximately 80% removal of the dissolved salts. Insubsequent stud

7、y on hydrodynamics we have shown that flu-Corresponding author. Tel.: +91 512 2597629; fax: +91 512 2590104.E-mail address: nishithiitk.ac.in (N. Verma).phosphate is the effluent from aquaculture industry 7. Water per-colating through the agricultural lands also contains appreciablequantitiesofnitra

8、teandphosphateions8.Theexperimentcarriedout under varying conditions of water and resin flowrate, numberof stages and stage height showed more than 90% removal of thetwo ions. Approximately 40% increase in the mass transfer rate wasobtained in comparison to that obtained in the previously devel-oped

9、 (conventional) column operated under identical conditions.The immediate effect of enhancement in the mass transfer rate isrealizedintermsofproportionatereductioninthenumberofstagesof the new column. Alternatively, the extent of removal of dissolvedaqueous solutes is relatively higher in the new col

10、umn operatedwith the same number of stages. The enhanced mass transfer rateis attributed due to uniform mixing between the solid and liq-uid phases and improved residence time distribution (RTD). In thisstudy we have qualitatively discussed the hydrodynamic aspectsrelated to the continuous operation

11、 of the column, which includeRTD of the two phases and loading/flooding criteria. The detailed701/$ see front matter 2008 Elsevier B.V. All rights reserved.10.1016/j.cep.2008.05.005upesh Verma, Gaurav Srivastava, Nishith Vermaof Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208

12、0infohistory:eceived 29 March 2008ccepted 15 May 2008vailable online 28 May 2008eywords:xchangerd bedtransfermodelater pollutionabstractA novel multi-staged radiallsolved solutes in water. Apprconventional staged-columnapproximately 40%. The enhanceidized resin particles flowingThe experimental data

13、 demonsoperating conditions of liq. IntroductionIon-exchange is a common unit operation in water purificationRemoval of water dissolved solutes (anions or cations)usually carried out in a fixed bed packed with the solid ion-xchangeresins.Theoperationisnotcontinuousasthebedrequiresregeneration follow

14、ing saturation with the solutes. Con-d bed ion-exchange columnross-flow fluidized bed ion-exchanger is developed for controlling dis-ely 40% improvement in mass transfer rate in comparison to that indemonstrated. Number of stages in the present column is also fewer byd mass transfer rate is attribut

15、ed due to uniform mixing between flu-radial direction on a stage and water flowing upward through bed-voids.trate more than 90% removal of dissolved solutes in water under varyingesin flowrates, number of stages, and stage heights. 2008 Elsevier B.V. All rights reserved.of solid resin particles by w

16、ater on a stage is essentiallycross-flow conditions, although the overall flow of liquid andmay be considered to be counter-current 5.This study is concerned with the experimental measurementspertaining to mass transfer aspects in the newly designedxchange staged column 6. The water containing nitra

17、te ande ions was used as a model contaminants laden liquidR. Verma et al. / Chemical Engineering and Processing 48 (2009) 396407 397description of the hydrodynamic study is reported elsewhere 9.A mathematical model is developed in this study to predict soluteconcentration during the stage wise conti

18、nuous operation of theion-exchange column and is shown to explain the data reasonablywell. The model analysis is based on the non-equilibrium approachand takes into account the finite mass transfer rate of the solutefrom the bulk liquid phase into the pores of the resin matrix.Prior to conducting ex

19、periment in the column under continu-ous operation, experimental breakthrough curves were obtainedto determine the effective pore diffusivities of the nitrate andphosphate ions in the resin matrix. Separate concentration mea-surements were made for the single-stage, with resin and waterflowing count

20、er currently. From the data an empirical correla-tion was developed for calculating mass transfer coefficient forthe present case of fluidized resin particles flowing in the radialdirection on the stage and the water flowing upward through thevoids between the particles. Equilibrium isotherms for ni

21、trate andphosphate ions vs. the resins were determined from the batchequilibrium data. The pore diffusivity, particlefluid mass trans-fer coefficient, and equilibrium isotherm, all were required in thedevelopment of the model as described in the subsequent sectionof the paper.2. Geometrical configur

22、ationFig. 1 is the schematic of the three-stage fluidized bed columndesigned and fabricated in the invention 6. The Perspex made col-umn (500mm H100mm ID) consists of stages (65mm height perstage) assembled together with flange joints. A brass made meshwith openings smaller than the particle size is

23、 fitted onto an alu-minium ring sandwiched between every pair of adjoining flanges.The solid particles (ion-exchange resins) are fed from the top, whilewaterispumpedfromthebottom.Thefluidizedsolidresinparticlesmove across on the stage to the next stage through a downspout (ordowncomer), as water flo

24、ws upward through the mesh openings.Fig. 1. Schematic of three-stage radial-flow fluidized bed column.There are two types of downspouts in the column. These are fittedalternately on successive stages: one is at the center of the columnand the other is around the column walls. We will call the one fi

25、ttedat the center of the stage as the central downspout and the otherfitted around the outer periphery of the stage as the circumferen-ater radial flow fluidization.Fig. 2. Schematic of solidw398 R. Verma et al. / Chemical Engineering and Processing 48 (2009) 396407w patttialinclesdospout.thetrpermi

26、tsheight.v3.columngrrcflorthealleadjacenttheviceonthesefloracrheightsvesolidctheinics,andflostheviebleladencalFig. 3. (a) Schematic of a three-stage conventional solidliquid column; (b) flodownspout. Fluidized solids on each stage flow across radiallyward or outward to the next downspout. Depth of s

27、olid parti-on the stage is controlled by adjusting the height of the centralwnspout tube or that of the collar of the circumferential down-In principal, the required bed height is determined frommass transfer consideration and should be equal to the massansfer zone (MTZ). Counter current operation o

28、f solidwater flowachieving the required mass transfer rate within a smallIn the existing arrangement, the resin bed heights could bearied between 3 and 20mm.Mechanistic steps in the continuous operation of theFig. 2 is the pictorial representation of flow conditions pro-essively existing on the stag

29、e, beginning with a fixed bed ofesin particles, with water flowing upward, and then graduallyhanging over to the continuous operation with water and resinwing counter-currently at constant flowrates. To begin with, theesin particles may fill up the bed only up to a certain fraction ofdowncomer heigh

30、t (hhd). As the flowrate of water is gradu-y increased up to the minimum fluidization condition, the bedxpands up to the downcomer height, hd. As resins pour from theupper stage through the downspout, difference betweenbed-depths from the center to the periphery of the stage orversa, develops depend

31、ing upon the type of the downspoutthat stage (central or circumferential). This results in the flow offluidized resin particles on the stage, and subsequently over thetageheight,hdthroughthedownspouttothenextlowerstage.Thexperiments revealed that for given downcomer height and waterwrate, there is a

32、 certain range of resin flowrates over which theesin bed completely fluidizes and resin particles move smoothlyoss the stage to the next stage due to difference in the bedacross the stage, while water flows upward through theoids between the solid resins. Thus, the state of fluidization onvery stage

33、 is cross-fluidization, although the overall flow of theion-eereccentricwithtparticlesnificantimmerineftwerinacrriallarrthelotheofouterns in the conventional column; and (c) flow patterns in the new column.and water phase is in the counter current direction. Suitablehoice of liquid and solid flowrat

34、e ensures smooth operation ofcolumn without loading and flooding. As pointed out earlierthe text the measurement and analysis related to hydrodynam-including pressure-drop distributions between various stagesdownspouts, loading/flooding of the stage at relatively largerwrate, and the RTD of two phas

35、es, are discussed in our separatetudy 9.It would be, however, judicious to compare the configuration ofpresentdesigntothatoftheconventionaldesign(Fig.3a)withaw to emphasizing on the improved RTD in the former, responsi-for the enhanced mass transfer rate between the contaminantswater and solid resin

36、 particles. Fig. 3b describes the typi-flow patterns existing on stage of the conventional solidliquidxchanger column having downspout fitted near the periph-y of the column. In such design, location of the downspout beingthe flow of solid particles across the stage is non-uniform,significant fracti

37、on of the solids short-circuiting from one sideo the other side of the stage. In other words, fraction of the solidhas longer paths than the remaining particles, with sig-regions of solids under nearly stagnant conditions. Thediate impact of the mal-distribution is on the non-uniformesidence time di

38、stribution of the solid particles causing reductionthe overall mass transfer rate. The present design overcomes thisfect. The configuration of the two types of downspout betweeno successive stages in the column, one around the outer periph-y of the stage and the other at the center of the stage, res

39、ultsthe identical path-length of each fluidized solid particle movingoss the stage to the successive stage in the column. Fig. 3c picto-y describes the movement of fluidized resin particles in such anangement,whereresinparticlesflowintheradialdirectionfromcircumferentialdownspoutintothecentraldownsp

40、out.Thefol-wing four significant improvements are realized with respect toconventional design: (1) smooth fluidization and movementthe particles over the stage, (2) wider range of operation with-flooding and loading (the limit being approximately 1.5 timesR. Verma et al. / Chemical Engineering and P

41、rocessing 48 (2009) 396407 399wider), (3) less pressure-drop (30% smaller), and most importantly,and (4) small number of stages (40% fewer) due to higher masstransfer rate between solid and liquid phases.4. Mathematical model for the ion-exchange columnThemodelanalysisforthepredictionofsoluteconcent

42、rationsinthematIfarconcentrwrittqHerumn,DuringthetNwherrcentrconcentrcentroNsywisoCthewherthemktheallcolumn:quidmosrinsituationmassinrtaltransfercoefficient,wecarriedoutseparateexperimentonasingle-stage fluidized bed operated under varying conditions of resin andwater flowrates, particle size, and t

43、he bed porosity. Based on thedata, a working correlation was developed for calculating masstransfercoefficientviathedimensionlessquantity,Sherwoodnum-ber (Sh), as a function of particles Reynolds number (Re), Galileonumber (Ga), bed porosity (), and the slip ratio (ratio of veloc-itytran(LDF)arfusip

44、articlerThemationbtion,ditionsandphatMerDphatwtialfinalthespondingThe(0.0wobservsolutbalance:qwherandcontactconcentraqlibriumrarspondingandadsorbionsinincrion-exchangeroperatedundersteady-stateincludesessentiallyerialbalanceateachstage.Considerthenthstageinthecolumn.concentrations of the solid phase

45、 leaving and entering the stagee qnand qn1, respectively, and the corresponding aqueous phaseations are Cn+1and Cn, the solute mass balance may been asn qn1=QlQs(Cn+1 Cn) (1)e Qland Qsare the flow rates of water and resin in the col-respectively, and are assumed to be constant over each stage.contac

46、t between the two phases on the stage, molar flux ofsolute transferred from water to the resin may be expressed inerms of solute concentrations as follows:= kf(Cn Cn,i) = ks(qn,i qn) (2)e kfand ksare the mass transfer coefficients in the liquid andesin phases, respectively, and Cn,iand qn,iare the s

47、olutes con-ation at the liquidsolid interface. If Cnbe the liquid phaseation of the solute in equilibrium with qn, solid phase con-ation, the molar flux can also be expressed in terms of theverall liquid phase mass transfer coefficient as follows:= KF(Cn Cn) (3)It also follows that an appropriate is

48、otherm for the solidliquidstem is required for determining Cnin terms of qn. For theaterresin system under the present investigation, the Freundlichtherm was found to be suitable and used in the following form:n=parenleftBigqnkparenrightBignprime(4)Knowingwater(kf)andresin(ks)sidemasstransfercoeffic

49、ients,overall mass transfer coefficient KFmay be calculated as1KF=1kf+1mks(6)e m is the slope of the isotherm and obtained by rearrangingFreundlich equilibrium isotherm equation as=qiCi kC(1nprime)/nprime)i(7)and nprimeare the isotherm parameters.Finally, an overall component balance is required to completeset of equations required for solving solute concentrations atstages and predicting the extent of solutes removal from then q0=QlQs(Cn+1 C1) (8)One clarification is in order