FLOW DYNAMICS OF GAS-SOLID FLUIDIZED BEDS WITH EVAPORATIVE LIQUID INJECTION

CHINA PARTICUOLOGY Vol 4 No. 1. 1-8, 2006FLOW DYNAMICS OF GAS-SOLID FLUIDIZED BEDS WITHEVAPORATIVE LIQUID INJECTIONBing Du, W. Warsito and Liang-Shih FanDepartment of Chemical and Biomolecular Engineering, The Ohio State University140 West 19th Avenue, Columbus OH 43210, USAAuthor to whom correspondence should be addressed. E-mail: fan 1@osu. eduAbstract The electrical capacitance tomography(ECT) with neural network multi-criteria image reconstructiontechnique(NN-MOIRT) is developed for real time imaging of a gas-solid fluidized bed using FCC particles with evapora-ive liquid injection. Some aspects of the fundamental characteristics of the gas-solid flow with evaporative liquid injection.including real time and time averaged cross-sectional solids concentration distributions, the cross-sectional solids con-centration fluctuations and the quasi-3D flow structures are studied. a two-region model and a direct image calculationare proposed to describe the dynamic behavior in both the bubble/void phase and the emulsion phase based on thetomographic images. Comparisons are made between the fundamental behaviors of the gas-solid flows with and withoutevaporative liquid injection for various gas velocities ranging from bubbling to turbulent fluidization regimes. Significantdifferences are observed in the behavior of the gas-solid flow with the evaporative liquid injection compared to the fluid-ized bed without liquid injectionKeywords gas-solid fluidized bed, evaporative liquid injection, electrical capacitance tomography, real time imaging1 Introductionwith increasing liquid addition. Wright and Raper8Injections of evaporative liquids into fluidized solid parti- a gas-solid fluidized bed At low liquid loadings, the addicles are routinely practiced in industrial processes involv- tion of the liquid increases the bed voidage, resulting in aning gas-solid fluidization systems such as fluid catalytic increase in the minimum fluidization velocity. As the liquidcracking, polymerization, and plastic coating(Fan et al., loading increases, however, the voids between particles2001). In the FCC riser system, heavy oil is injected into become filled by the liquid and hence, both the bed voithe system to evaporate rapidly by contact with the hot dage and the minimum fluidization velocity decrease. Theanalyst particles. Simultaneously, thermal and catalytic bed pressure drop increases slightly with the addition of acracking reactions take place. During a polymerization small amount of liquid due to capillary and viscous forcesprocess, a condensing mode of operation involving injec- that are generated from the liquid and reduction in the voidtion of an evaporative liquid(monomer or inert liquid hy- space between particles. With the addition of non-volatiledrocarbon )with a boiling point lower than the operating liquids to a gas-solid fluidized bed, McLaughlin and rho-temperature is utilized to increase reactant conversion. a des(2001)observed that the flow behavior changes fromsudden expansion of the evaporated liquid would result in Group b to Group a and eventually to Group C. The BAsignificant effects on bed fluidization properties such as and AC boundaries are determined based on the ratiolocal fluidization velocity and solid agglomeration or clus- between the interparticle liquid bridge forces and the fluidtering. However, fundamental understanding of thedrag force. Nagahashi et al.(2003)reported that fluidize-of evaporated liquid injection on bed flow behavior,tion can be enhanced with the addition of small quantitiescially the dynamic flow structure, is still inadequate toof liquid to a gas-solid fluidized bed with large particlesoptimum reactor designwhen some conditions are satisfiedA number of studies have been conducted to investigate Compared to non-evaporative liquid injection, the gas-the effect of non-evaporative liquid injection on a gas-solid solid fluidized bed with evaporative liquid injection exhibitsfluidization system. With the addition of small quantities of more complex flow behavior. Both the effects of liquidliquid, the gas-solid fluidized bed can be immobilized or including liquid bridging and liquid droplet, and the effectsdefluidized due to bridging or enhanced interparticle forces, of liquid vapor, including liquid evaporation and interfacialwhereas fluidization, especially for fine particle fluidized heat transfer, need to be considered. Skouby(1998)eds, can be enhanced due to the reduction of electrostatic measured the catalvst mass fiux. catalyst concentrationforces(Jiang et al., 1996; Al-Adel et al., 2002). Seville and and中国煤化工 r with liquid nitrogenClift(1984)found that the addition of small quantities of a injecCNMHGed to show afferentnon-evaporative liquid affected the fluidization characteris- penet.wt al. ( 1998)simulatedtics of Group B particles moving through Group A to Group the FCC riser reactors based on the governing conserva-C. The interparticle cohesive forces increase with the in- tion equations for mass, momentum, and enthalpy togethercrease of the loading of non-evaporated liquid. Accordingly, with phenomenological models such as the interfacial dragbed voidage and the minimum fluidization velocity increase model, the interfacial heat transfer model, the dropletCHINA PARTICUOLOGY Vol 4 No. 1. 2006evaporation model and the lumped integral reaction model. catalysts with a mean diameter of 60 um and a particleNewton(1998)applied the X-ray imaging technique to density of 1400 kg m are employed as the fluidized parti-study the flow patterns in fluidized bed reactors with direct cles. The electrical capacitance tomography(ECT)systemliquid injection used for FCC process and gas phase poly- comprises a capacitance sensor array, electronics for dataethylene high productivity technology. Zhu et al. (2000) acquisition, and computer system for measurement controlstudied the microstructures of evaporative liquid spray jets image reconstruction and display. The sensor array com-in dilute gas-liquid flows using the argon ion laser sheet prises a twin-plane measuring sensor with 12 electrodesand the CCD camera. The jet penetration length or the for each plane and two guard sensor planes located belowevaporation length is determined by temperature meas- and above the measuring sensor planes(show in Fig. 1)urements using fast response thermocouples. The evapo- Measurements are conducted at levels 10 and 15 cmration length can be significantly shortened with the pres- above the gas distributor/injection nozzle for planes 1 andence of particles in the bed compared to that in parti- 2, respectively. A new image reconstruction algorithmcle- free flows. Zhu et al.(2002) developed a parametric basedon neural network multi-criteria optimization imagemodel to describe the mixing characteristics for the liquid reconstruction technique(NN-MOIRT)developed by theevaporation in dilute gas-solid suspension flows. Fan et authors is used to produce cross-sectional images of theal.(2001)investigated the effect of evaporative liquid in- multiphase flow within the sensor from the capacitancejection on a dense gas-solid fluidized bed using non-po- values the algorithm has shown its accuracy, consistencyrous glass beads. The study indicates that the minimum and robustness to noises( Warsito & Fan, 2001a). Thisfluidization velocity decreases with the liquid injection into technique has been successfully applied in gas-solid bub-a gas-solid fluidized bed due to the additional gas flow bling, turbulent and circulating fluidized beds(Du et algenerated from liquid evaporation. Compared to a dry 2002, 2003& 2004). In the present work with liquid nitro.fluidized bed, the pressure drop in a fluidized bed with gen injection to a gas-solid fluidized bed, the multiphaseliquid injection is lower, resulting from the reduced sus- system is composed of the three components of gas, liquidpended bed weight due to attachment of the wet particles nitrogen and FCC particles. However, since the relativeon the wall. Fluidizing gas velocity has an insignificant permittivities of the gas and the liquid nitrogen are veryeffect on the jet penetration length while it shortens the near to each other compared to those of the solids partiradial dispersed distance of the evaporative liquid in a cles, the three-phase system can be regarded dielectricaldense-phase fluidized bed. Electrical capacitanceas a two-phase systemgraphy is also used to visualize both the solid and thflow structures in a dense phase fluidized bed with liquidnitrogen injection. The tomographic image indicates a dis-tribution of reduced bubble sizes compared to a dry bed.Leclere et al. (2001)investigated the droplet vaporizationin a fluidized bed with liquid injection. They developedmodel to estimate the critical bed temperature below whichagglomeration would occur. House et al. (2004)applied theX-ray scanner to study the interaction between the dropletsand the particles. They found that the initial particle/liquidmixing is rather poor in the spray jet.This study is intended to examine the flow dynamics of agas-solid fluidized bed by using the real time electricalcapacitance tomography developed by this research groupData acquisitonreconstruction(Warsito& Fan, 2001a& b). FCC particles with an evapo-systemrative liquid (nitrogen)injection are used in the experimentsalong with a wide range of gas velocities covering theFig. 1 Experimental set-up and the ECT systembubbling and the turbulent fluidization regimes. Fluidization To determine the bubble/void phase and the emulsionbehaviors with and without liquid nitrogen injection arephase in a gas-solid fiuidized bed, a two-region model andcompareda direct image calculation are proposed. Based on the2. Experimental Studiessolidsce near the wall region toThe experimental apparatus shown in Fig. 1 includes a be th中国煤化工 of mixed emulsionfluidized bed column of 10.16 cm id with a porous plate phaseCNMHGby assuming that thegas distributor. The jet nozzle is located at the center of the solidsin tne ouDDiervora phase is negligible asdistributor and has a 0.8 mm orifice with a 300 fan-angle to compared to that in the emulsion phase, the bubble/voidprovide a flat, 2-D jet. Air and liquid nitrogen are used as phase holdup can be calculated from the overall solidsthe fluidizing gas and evaporative liquid, respectively. FCc concentration expressed asDu, Warsito Fan: Flow Dynamics of Gas-Solid Fluidized Beds6=1-6(1)for gas or bubbles and 1 (dark red)for a packedfigure shows the changes of cross-sectional so(2) Centration distributions with a time interval of 0.02 s. thebright red area found in the near-wall region correspondswhere &s is the overall solids concentration obtained di- to the emulsion phase with almost no bubble. the arearectly from the ECT measurement; a and are the emul- indicated by light red to light blue surrounding the bubblession and bubble/void phase fractions, respectively; es and corresponds to the region with a mixed bubble/void phaseCbs,respectively, are the solids holdups in the emulsion and and an emulsion phase. Some regions with a dark redthe bubble/void phases. The bubble/void phase fraction color( high solids concentration ) to be considered as solidand the solids holdups in each phase can also be calcu- clusters or cakes, are observed, particularly, in the gaslated directly from the quasi-3D image. Details of the de- solid system with liquid injection. Black areas within bub-termination of the bubble/void phase and the emulsion bles or surrounded by a dark red area(clusters orphase based on the ECT measurement are given in Du et are areas with 100% gas or 100% dense solidal.(2003)bed), respectively. It indicates that injection of liqugen promotes particle agglomeration due to enhanced3. Results and Discussioninterparticle forces by liquid bridging where a large amount3. 1 Dynamic behavior in the bedof the liquid is adsorbed by the porous FCC particles andremains un-evaporated. It is observed that the bubble sizeFigure 2 shows the real time tomographic images of a represented by the blue area in Fig. 2 decreases with thegas-solid fluidized bed in plane 1(lower image)and plane injection of liquid nitrogen, which is consistent with the2(upper image)with and without liquid injection. The color results reported by Fan et al.(2001)using the Group Bbar shows the relative solids concentration as 0(dark blue) particlesFig 2 Real time tomographic images of gas-solid fluidized bedFigures 3(a)and 3(b)show the time averaged cross. theH中国煤化工9msNMHGas in rig. 2. Averaging is carried outsectional distributions of solids concentrations in a over 500 image frames taken in 10 s. The time averagedgas-solid fluidized bed for various gas velocities without cross-sectional solids concentration decreases with inand with liquid injection respectively. The color map has creasing gas velocity. The cross-sectional average solidsCHINA PARTICUOLOGY Vol 4. No. 1, 2006the gas-solid fluidized bed without liquid injection. A transi-tion from the bubbling regime to the turbulent regimewhere large bubbles are broken up into small bubbles orvoids, is observed at a gas velocity of 0. 49 m.s. Dark redregions indicating high concentration of solids at the corehich is attributed to the formation of bubble wakare seen at low gas velocity With the injection of evapotive liquid, the bubble sizes and the bubble numbers arereduced at low gas velocity and the bubble/void flow isseparated to discrete flow at high gas velocity, as the in-terparticle cohesive forces exerted by the liquid bind theparticles together. Compared to the dry bed, more dark redareas corresponding to particle aggregates are observedFig 3 Time averaged cross-sectional solids concentration distribu- at the core region at low gas velocity, which can be attrib-ions of gas-solid fluidized bed with and without liquid injec. uted to both the effects of bubble wake and enhancedinterparticle forces by liquid bridging. As the gas velocityincreases, the violent movement of gas and solids flowconcentration in the gas-solid fluidization with liquid injec- enhances the evaporation of the liquid Particle aggregatestion is slightly higher than that of a dry bed, especially at by liquid bridging at the central region thus break up intothe center of the bed. The distributions of the solids con- either small ones or particle clusters with relatively lowcentrations are radially symmetrical for high gas velocities, solids concentration as shown by the red color in Fig. 4especially for the bed without liquid injection. It is observed The small particle aggregates then move to the near wallthat the solids concentration distributions in the annular region, resulting in the small dark red areas observed inregion, including the red area and yellow area(high solids the near wall region at high gas velocityconcentration areas)in Fig 3 are radially symmetricalexcept for very low gas velocities such as 0.19 m-s foroth modes and 0.39 m-s for the liquid injection modeWhen there is no liquid injection, the asymmetric distribu-tions of the solids concentration at lower gas velocities aredue to the spiral motion of bubbles in the bubbling regimeand the dynamic behavior of bubble breakage and coalescence during the transition from the bubbling to theturbulent regimes. With the breakage of large bubbles intosmall voids in the turbulent regime, the time averagedcross-sectional solids concentration exhibits a radiallysymmetric distribution. However, with liquid injection to thegas-solid fluidized bed, particle aggregation due to en-hanced interparticle forces by liquid bridging also contributes to the asymmetric solids concentration distributioneven in the turbulent regime at high gas velocitiesFigure 4 shows the quasi-3D flow structures of the gas-solid fluidized bed with and without liquid injection obtainedby stacking 200 frames of tomographic images at the bed Fig 4 Quasi-3D flow structures of the gas-solid fluidized bed with andlevel of 10 cm above the distributor/injection nozzle. Threethout liquid injectionsgas velocities covering the bubbling regime, transitionegime and turbulent regime are shown in Fig. 4. For each The variations of time averaged cross-sectional solidsgas velocity, the images from two sliced sections repre- concentrations with gas velocity in a gas-solid fluidized bedsenting solids concentration distribution in the X-Z and Y-z with and without liquid injection are shown in Fig. 5, exhib-planes are shown in Fig 4 along with the quais-3-D bubble itingflow. The direction Z represents the time variation. The Co中国煤化工 out liquid injection.tecolor map has the same significance as in Fig. 2. The solCNMHGtive liquid injection isbubbling flow structure indicated by discrete bubbles rising higher. the solids concentration difference between thesespirally and rocking between the sides of column wall at two modes is larger in the bubbling regime as compared tolow gas velocity in the bubbling regime, and the turbulent the turbulent regime. With the injection of the evaporativebubbly flow indicated by continuous rising bubbles/voids at liquid to the fluidized bed, part of the liquid is evaporated toDu, Warsito Fan: Flow Dynamics of Gas-Solid Fluidized Bedsthe gas phase while the remaining liquid is adsorbed by theD=0.1morous FCC particles. Due to liquid bridging between theH:015mliquid injection)particles, the interparticle forces are enhanced and particle049(without liquid inection)aggregates are formed. The voidage between particles ar-.- 0.49 (with liquid injection)the expansion of the bed then decrease correspondingly,山·0.78( with liquid in ectionleading to a higher solids concentration in the bed. withincrease of gas velocity, the evaporation of liquid nitrogenand entrainment of the liquid droplets are enhanced, thusreducing the interparticle forces exerted by liquid bridgingto form particle aggregates, to result in less significanteffect on bed expansion and solids concentration at high00gas velocity in the turbulent regime1.00.75050025000025050075100RFig 6 Radial profiles of time-averaged solids concentrationH=015mFigure 7 shows that the standard deviation of the crosssectional solids concentration fluctuation in the gas-solidfluidized bed for both modes of operation with and withoutliquid injection, increases in the bubbling regime, peaks atthe transition velocity, Uc, and then decreases in the turbulent regime. The transition velocity for operation withoutliquid injection is in agreement with that obtained by thepressure fluctuation measurement and that calculated bythe correlation of Cai et aL. (1989). It is seen that the transition velocity for operation with liquid injection shifts to thelower gas velocity range, indicating that particle aggrega-tion due to liquid bridging can break up the larger bubblesFig. 5 Variations of time averaged cross-sectional solids concentra- to smaller voids/bubbles. that liquid injection yields higherith gas velocity.standard deviation of the cross-sectional solids concentraFigure 6 shows the radial profiles of the time-averagtion fluctuation for all the fluidization regimes than thatsolids concentration in the gas-solid fluidized bed with arwithout liquid injection, is attributed to the formation ofwithout liquid injection at different gas velocities, indicatingarticle aggregates by liquid bridgingthat increase of solids concentration with liquid injectionD=0.1moccurs mainly at the center region of the bed where r/R issmaller than 0.5. This region is within the scope of liquidinjection to the fluidized bed, showing that liquid injectionmainly affects the flow behavior at the center region of thegas-solid fluidized bed The solids concentration near the viwall region remains almost unvaried with liquid injection Inthe bubbling regime, solids concentration exhibits radiallyasymmetrical distribution, corresponding to the cross-sec-tonal ECT images as shown in Fig 3. The smallest solidsoncentration where bubbles are most concentrated doesnot occur at the very center of the bed, possibly due to thespiral movement of bubble flow in the bed With injection ofthe evaporative liquid, solids concentration at the centerregion increases greatly due to particle aggregation by Fig. 7 Standard deviation of cross-sectional solids concentrationliquid bridging, and at some radial positions it is equal tothat near the wall region. At higher gas velocities in thetransition and turbulent regimes, the solids concentration 3.2 Dynamic behavior in the bubble/void phaseprofile is radially symmetric for both modes of operation中国煤化工with and without liquid injection, Compared to the bubblingregime, the increase of the solids concentration at theCN MH Gic characteristics ofcenter region is moderate due to the enhancede bubble/void phase and the emulsion phase can beevaporation of the liquid and entrainment of the liquid quantified by the two methods mentioned above.Thevariation of the time-averaged cross-sectional bubble/voidCHINA PARTICUOLOGY Vol 4. No. 1, 2006phase fraction with gas velocity in a gas-solid fluidized bed voids(Cai et al., 1989). With the injection of an evaporativefor both modes of operation with and without liquid injec- liquid to the gas-solid fluidized bed, the standard deviationtion, as shown in Fig 8 indicates that the bubble/void exhibits a similar tendency but with higher values asphase fraction over the cross section increases linearly compared to that without liquid injection, though the transi-with gas velocity in the bubbling regime, and the rate of this tion velocity from the bubbling to the turbulent regimes isincrease is slightly reduced in the turbulent regime. At a reduced, which is consistent with the results for the stan-iven gas velocity, the bubble/void phase fraction de- dard deviation of the solids concentration fluctuation increases with liquid injection, especially at high gas veloci- Fig. 7. The formation of particle aggregates by the adsorp-ties in the transition and turbulent regimes, possibly due to tion of liquid by the porous particlesfacilitates bubblereduction of bubble size at low gas velocity. The large re- breakage, yielding a higher standard deviation and a lowerduction of bubble/void phase fraction with liquid injection in transition velocity with liquid injectionindicates that the separation of the Figure 10 shows the variation of the time averagedcontinuous bubble/void flow has a significant effect on the cross-sectional solids concentration in the bubble/voidvariation of the bubble/void phase fractionphase with the gas velocity. For both of the two modes ofoperation with and without liquid injection, the solids con-H=0.15mcentration in the bubble/void phase increases linearly withgas velocity in the bubbling regime, and the rate of in-crease then decreases in the turbulent regime. Solidsconcentration in the bubble/void phase is very low. Com-pared to the fluidized bed without liquid injection, the solidsconcentration in the bubble/void phase is lower in agas-solid fuidized bed with liquid injection at any given gasvelocity, this reduction being more remarkable at highergas velocities in the turbulent regime. The bubble/voidphase is located mostly at the center of the bed as shownby the blue area in Fig. 2, and so is the effective zone ofliquid injection as shown in Fig. 6. Due to enhanced interparticle forces by liquid bridging, the particles in the bub-Fig 8 Variation of cross-sectional bubble/void phase fraction with gas ble/void phase tend to aggregate and the aggregates aretransferred to the emulsion phase. Therefore, theconcentration in the bubble/void phase decreasesWithout liquid injectionD=0.1mBeWith liquid injectionH=0.15mspondingly. At the lower gas velocity in the bubbling regime,the solids concentration in the bubble/void phase is lower,and the effect of this decrease with liquid injection be-comes less significant. Inversely, the reduction effect in theturbulent regime is more pronounced due to the higher0.10solids concentration in the bubble/void phase, although theinterparticle forces may be weaker due to the evaporationof the liquidH=0.15mugFig 9 Standard deviation of cross-sectional bubble/void phase fraction fluctuationsFigure 9 shows the variation of the standard deviation ofthe cross-sectional bubble/void phase fraction fiuctuation中国煤化工ing in the bubbling regime, peaking at the transition velocityUc, and then decreasing in the turbulent regime, in aCNMH G 0810manner similar to that of cross-sectional solids concentra-tion with gas velocity in Fig. 7. The transition is character- Fig 10 Variation of time averaged solids concentration in bubble/voidized by the breakage of large bubbles into small bubbles orphase with gas velocityDu, Warsito& Fan: Flow Dynamics of Gas-Solid Fluidized BedsFrom the above discussion, particles aggregation due to decrease in the transition velocity from the bubbling to theliquid bridging with liquid injection tends to condense the turbulent regime, and the entire bed contracts with de-whole bed thus reducing its voidage. Fig. 11 shows that crease of bed voidage. Both the bubble/void fraction andliquid injection causes an increase in solids concentration the solids concentration in the bubble/void phase decreasein the emulsion phase. The time averaged cross-sectional with liquid injection, while the solids concentration in theolds concentrations in the emulsion phase with and emulsion phase increases with liquid injectwithout liquid injection decrease in the bubbling regime andlevel off in the turbulent regime. The increase of solids Acknowledgementconcentration in the emulsion phase with liquid injection isThe support of the National Science Foundation on the devel-caused by two factors: particle aggregation in the emulsion opment of the ECT used in this study is gratefully acknowledgedphase by liquid bridging and particle transport frombubble/void phase to the emulsion phase. Fig 11 shows Nomenclaturethat increase of solids concentration in the emulsion phaseis less pronounced in the turbulent regime, although more Ddiameter of the bedparticles are transferred from the bubbling/void phase. Thismeasurement position in the axial direction, mmay be caused by weakened particle aggregation in thestandard deviation of bubble/void phase fraction fluc-emulsion phase in the turbulent regime due to enhancedstandard deviation of solids concentration fluctuatioevaporation of the liquidradial position in the bed, mradius of the bed, mwithout bquid injectD=0.1mH=0.15mRuusuperficial gas velocity, mstransition velocity from the bubbling to turbulent re-Greek letterssolids concentration in the bubble/void phase.66.4solids concentration in the emulsion phasesolids concentrationbubble/void phase fractionemulsion phase fractionReferencesU/ms-tAl-Adel, M. F, Saville, D. A.& Sundaresan, S(2002). The efof static electrification on gas-solids flows in vertical risersig. 11 Variation of time averaged solids concentration in the emulEng.Chem.Res,41,6224-6234sion phase with gas velocityCai, P, Chen, S P, Jin, Y, Yu, Z.Q.& Wang, Z W. (1989). 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