CHINA PARTICUOLOGY Vol. 4, Nos 3-4, 167-177, 2006GAS-SOLIDS FLOW BEHAVIOR WITH A GAS VELOCITYCLOSE TO ZEROH. Zhang and J.-×,zhuDepartment of Chemical Engineering, University of Westem Ontario, London, Ontario, Canada N6A 5B9, Canadahor to whorce should be addressed. E-mail: zhu@uwo.caAbstract In a 9. 3 m high and 0. 10 m i.d. gas-solids downflow fluidized bed(downer), the radial and axial distribu-tions of the local solids holdups and particle velocities along the downer column were measured with the superficial gaselocity set to zero. a unique gas-solids flow structure was found in the downer system with zero gas velocity, which iscompletely different from that under conditions with higher gas velocities, in terms of its radial and axial flow structures aswell as its micro flow structure. The gas-solids flow pattem under zero gas velocity conditions, together with that undelow gas velocity conditions, can be considered as a special regime which differs from that under higher gas velocityconditions. According to the hydrodynamic properties of the two regimes they can be named the"dense annulus" regimefor the flow pattem under zero or low gas velocity conditions and the dense core" regime for that under higher gas ve-locity conditionsKeywords downer reactor, gas-solids cocurrent downflow, fluidized bed, hydrodynamics, flow development, particlevelocity, solids holdup, solids flux, fiow regime1 Introductionflow systems including the cocurrent downflowinggas-solids system. This diagram provided useful considThe cocurrent downflow circulating fluidizederations in process and equipment design. This work was(downer), as a novel gas-solids contactor, possescontinued by other researchers such as Kwauk(1963,advantages over the conventional upflow riser of a circu- 1964& 1992), Zenz and Othmer(1960 )and Wallis(1969)lating fiuidized bed, as stated by many researchers (Yang Kwauk(1963, 1964& 1992) provided refined derivations ofet al., 1991; Bai et al., 1991; Wang et al., 1992; Zhu et al., equations for each of the vertical-moving flow systems. All1995: Wei& Zhu, 1996; Zhang et al., 1999; Zhang& Zhu, these formed the earliest framework of fluidization regimes2000). For instance, it would be an ideal candidate for which has been of significance in understanding fluid-solidprocesses where extremely short but uniform contact times vertical flow systems in a sense of generalization On thebetween gas and solids are required(Zhu et al., 1995), other hand, all of the above work was one-dimensionsuch as the processes of fluidized catalytic cracking(FCC) based without counting the radial distribution of the vari-and residual oil fluidized catalytic cracking(RFCC). Fur- ables. Yet many of their theoretical statements still needthermore, superficial gas velocity provided to the downer verifications in practices. However, few experimental inreactor can vary in a theoretically unlimited range, from 0 vestigations up to date have been conducted on the hy(even from a negative value( Luo et al., 2001) )to a value drodynamic behavior with zero or close to zero superficialas high as needed, because both the gas phase and the gas velocity in the downer reactor. Zhang et al. (1998)solids phase flow concurrently in the direction of gravity employed a technique to largely increase solids holdup inand there is no minimum superficial gas velocity required their small diameter(0.0127 m i d )downer column soto suspend the solids phase. When the superficial gas to calibrate a fiber optic probe for solids concentrationvelocity is set to a very low value or even a negative value throughout a wide range of solids holdup they found that(i. e, with gas upflow), a higher bed density could be by establishing a backpressure at the bottom of the downerreached(Zhang et al., 1998: Luo et al., 2001). This could which ensures a stagnant gas phase in the downer column.overcome the shortcoming of low gas-solids suspension they reached a high solids holdup of about 0.2. Furtherdensity associated with rapid solids acceleration in most increasing the solids flux caused the flow regime to sud-downers operated with a downward gas flow. This char- denly skip to a moving bed with a solids holdup of 0.56acteristic with low gas velocities makes the downer reactor Luo et al. (2001)and Liu et al.(2001)carried out somealso favorable for some processes in which a long reaction studies on the hydrodynamics with low or zero or eventime is needed for the gas phase and/or a high solids/gas negative(upwards) superficial gas velocity in a 0.025 m idratio is crucial. Studies on zero gas velocity conditions, on downewere measured alotthe other hand will contribute to the fundamentals of the the中國煤化 es and solids holdupsgas-solids two-phase flow researches.CN MH Gxial levels, howeverLapidus and Elgin(1957)derived, from their theoretical were not measured, given a small diameter of their downernalyses, a flow regime diagram for generalized fiuidiza- Also, at zero superficial gas velocity, the highest solidstion systems. It covered all the possible vertical-moving circulation rate they reached was only 19.5 kg-m"s".ACHINA PARTICUOLOGY Vol 4. Nos 3-4. 2006parently, much higher solids circulation rates needticles to be present at any position of the downertested so as to configure the picture of the hydrodycross-sectionbehavior in a zero Ug downer system. ThereforeA local solids holdup measurement only provides a grossdynamic studies on the flow structure in a larger diameter time-average solids concentration. However, a local parti-downer, with 2-dimensional data measurements and cle velocity measurement, beside giving average particleanalyses, as well as a much wider range of solids circula- velocities for downflowing particles and upflowing particlestion rate, are highly desirablealso records the numbers of particles moving in both direcIn this study, local solids holdups and local particle ve- tions in front of the probe tip within the sampling timelocities at various axial and radial locations were measured However, these sample numbers do not represent thealong with the pressure gradients, under operating condi- contributions of the downflowing and upflowing particles totions of different solids circulation rates with zero superfi- the local gross solids concentration, since particles withcial gas velocity to characterize the flow behavior in the higher velocity contribute less to the local gross solidsdowner reactor of zero gas velocityconcentration due to their shorter residence times. Takingboth the particle velocity and the sample nun2. A Unique Gas-Solids Flow Structure consideration, the relative contributions of the downflowingwith Zero Superficial Gas Velocityand upflowing particles to the local gross solids concentration can be calculated by the following equationsWhen superficial gas velocity is set to zero or close tozero(Ug=0 or a 0), the gas-solids flow structure in adowner reactor could be characterized by a different pat-tern than that with a superficial gas velocity far from zeroThe difference comes from the effect of the gas phasemomentum on the motion of the solids phase. Because ofthe complexity of the gas-solids flow in a downer reactor, itis not conceivable to predict, from known data for condind and nu are the numbers of downflowing andtions of non-zero superficial gas velocities, the flow struc-particle velocity data, collected by the particleture under conditions of zero superficial gas velocitylocity probe within the sampling time: Esd and esu are theIn this study, some unique phenomena of the flow pat- local solids holdups of the downflowing and upflowing partern were observed with zero Ug. One of the most re- ticles respectively e is the local gross solids holdup Vpdmarkable ones is that a number of particles were detected and vpu are the local average particle velocities of theto have negative velocities under zero Ug conditions al-downflowing and upflowing particles repectivelythough the number is only a small fraction of the total the net local solids flux, Fs, for a zero Ug downer systemnumber of particles detected at each of the local positions is the difference between the local solids flux of down-ile mosis not quite surpnsing because the falling particles exert The s, rticles, Fsd, and that of upflowing particles, Futhe particles flow downwards under zero Ug conditionsthere is a small portion of the particles flow upwards. thisFs= Fsd-Fau Vpd Esd P,-vpu &su Ap.( 3)mean particle velocity, Vpm, can thus be derived:drag on the gas phase and generate a positive pressuregradient in the axial direction(i.e, the farther from thedowner entrance, the more compressed the gas phase). Therefore the net local solids flux and the local mean parThis pressure gradient would push the gas to flow upwards ticle velocity can be calculated by using equations (1)through some vulnerable" spots where the local solids through(4)holdup happens to be lower than nearby positions. Thussome troughs may form along the axial direction in which 3. Experimental Apparatusacross the cross-section of the downer, the gas outside the The experiments were conducted in the downer columntroughs has to flow downwards at the same mass flow rate of a riser-downer system as shown in Fig. 1. Solids fromto keep the mass balance. The upflowing gas in the the storage tank were carried up by the riser air through thetroughs could gain a velocity that is much higher than the 15.10 m tall riser(0. 10 m i d ) to the riser primary cyclonedownflowing gas outside the troughs provided that the total installed at the top of the downer, where solids werecross-sectional area of the troughs is much smaller than separated from the air at an efficiency of greater than 99%the rest. Therefore particles inside the troughs could beTheentrained up as long as the slip velocity is less than the the s中國煤化工 vas further cleaned bybefore being finallyupflowing gas velocity. It is worth pointing out that the strippeCN GSe filter At the downertroughs could be unstable in terms of their sizes and their top, solids were redistributed by a solids distributor locatedpositions given the turbulent flow pattern in the downer. below the dipleg of the riser primary cyclone. The solidsThis characteristic makes it possible for the upflowing par- distributor had a 0.20 m i.d. fluidized bed from which parti-Zhang Zhu: Gas-Solids Flow Behavior with a Gas Velocity Close to Zerocles fell down into the downer through 31 vertically posi- ing)and a logic circuit to take or discard each single piecetioned(with triangular pitch) brass tubes(10.7 mm i.d., of data and finally calculate the particle velocity. The detail12.7 mm o.d. and 0. 36 m long ). In order to prevent the of the particle velocity probe have been presented bym disthe air in the riser primary cyclone from flowing down into manufacturer and was verified before the experimentsthe downer, a minimum of 150 mm bed height was kept for using a rotating disk device(Zhu et al., 2001). the pres-all the time by partially plugging the top of each of the 31 sure drops alone the downer column were measured by abrass tubes From the downer entrance, the solids traveled series of differential pressure transducersdown through the 9. 3 m long downer column of 0.1 m di- Experiments were conducted under three different operameter. After that, the solids slid down through a quick ating conditions(three solids circulation rates: 49, 100 andinertial separator(which was built to separate solids from 202 kg m"'s with a superficial gas velocity of zero ). Thedowner air for other operating conditions with non-zero highest solids circulation rate(202 kg-m-s")was deter-downer superficial gas velocities)and then drained to the mined by the capacity of the downer solids distributor. Forstorage tank. the solids circulation rate was controlled by each condition, local solids holdups and local particle ve-the solids valve at the riser entrance and was measured by locities were measured at eleven radial positions(dRdiverting the collected solids into the measuring vessel for 0.000, 0.158, 0.382, 0.498, 0.590, 0.670, 0.741,0.806a given period of time0.866, 0.922 and 0.975)on eight axial levels(H=0.020512,1.198,2112,4.398,6227,8056and9155mDownerowner distn4. Results and Discussions/01mid.30m4.1 Radial solids flow structureFull radial solids holdup profiles, taken under three oferating conditions, at eight different axial levels are shownin Fig. 2. Near the entrance of the downer reactor, adis-Tertiary cyclonetributor effect is clearly seen, where the solids holdupfluctuates up and down in the radial direction At a axiallocation about 0. 5 m away from the top entrance a moredefinite trend is established where the solids holdup isDowner solids distributorrelatively uniform in a core region of r/R<0.6-0.8 but in-creases monotonically with the radius in a annular regionMeasuring vessalof r/R>0.6-0.8 with the densest peak right at the wall. Further down the column, the radial solids holdup profile be-comes more and more flat in the core while its magnitudeSolids focontrol valvedecreases and the radial profile finally approaches a uni-form distribution with a constant value in the axial directionIn the annulus, however, with increasing distance from thedowner entrance, the peak of the radial solids holdup pro-↑file first decreases to a minimum value and then increasesFig. 1 Schematic diagram of the riser/downer circulating fluidized bed until a constant value is reached. From Fig. 2, it can alsobe noticed that the transition of solids holdup from the coreregion to the annular region becomes smoother as theThe particulate materials used for this study were Fcc distance from the downer entrance increases, particularlycatalyst particles (Sauter-mean diameter=67 um, particle when Gs is high. The typical developed radial profile of solidsdensity=1490 m, bulk density =849 kg-m). The holdup is distinguished by a flat core region(r/R<0.6-0.8)local solids holdup was measured using an optical fiber and an annular region in which the profile increasessolids concentration probe which had a 3.8 mm od.(/R>0.6-0.8 ) rapidly towards the wallstainless steel probe tip, containing approximately 8 000 The results obtained by Zhang et al.(1999)shows thatemitting and receiving quartz fibers, each 15 um in di- the peak of the radial solids holdup profile, under a superameter. The details of the solids concentration probe have ficial gas velocity equal to or higher than 3. 7 m-s, movesbeen presented elsewhere(Zhang et al., 1998 ). The con- from中國煤化工 d its magnitude de-centration probe was precisely calibrated using a novel creaseperficial gas velocity iscalibration procedure, as described by Zhang et al. ( 1998). high)C N MH Gthe downer entranceThe local particle velocity was measured using an optical Unlike these results, under zero Ug conditions, the peak offiber particle velocity probe. This particle velocity probe the radial solids holdup profile remains present at the wallhad five fibers(two for light emitting, three for light receiv- throughout the whole downer and the magnitude of the170CHINA PARTICUOLOGY Vol 4, Nos 3-4, 2006u,/mG /kg-"-s0.020m0.020m::I0.512mn:;:·2112m02112m;;I iIII··身:44I1:;1000206646.227m:;8.056m8.056m60;···"::r880:::·……0,··000206100206000206100206100206ormallzed radial distance from the downer center, n/RNormalized radial distance from the downer center, rRFig. 2 Radial profiles of the gross solids holdup along the downer Fig 3 Radial profiles of the downflowing and upflowing particleunder different solids circulation rates with zero superficial gasvelocities along the downer under different solids circulationpeak in the bottom section of the downer is still comparable the particle velocity increases, either quicker or slowerto that in the top section thus the radial solids holdup depending upon the solids circulation rate, with the radialdistribution is less uniform than that under conditions of position. The typical developed profile of Vpu is also in thenon-zero superficial gas velocitysame pattern except that the increase in the annulus be-Figure 3 shows all the radial profiles of particle velocity, comes almost invisible when the solids circulation rate isfor both particles moving upwards and downwards, taken equal to or over 100 kg m2s-1under the same three operating conditions, at eight differ- It should be pointed out that the upwards particle veloci-ent axial levels. Near the entrance of the downer reactor, a ties detected by the probe were not individual ones occur.distributor effect"is seen again where the downwards ring among the positive particle velocities. They appearedarticle velocity, Vpd, and the upwards particle velocity, Vpu, in" cluster pattern"in the data stream. This indicated thefluctuate up and down in the radial direction. At a location existence of the unstable troughs in which particles areabout 0.5 m away from the top entrance, a distinct trend entrained up by up-flowing gas. Observations through thehows up where both ved and vpu slowly decrease with the transparent wall of the downer column also comfirmed thatradius. With increasing distance from the entrance, the this kind of troughs did exist.radial profiles of both Vpd and vpu decrease in the core Compared to the data obtained by Zhang and Zhuregion but increase in the annular region and finally their (2000 ) the developed profiles of vpd with zero Ug in-highest velocities shift from the center to the wall. It is no- creases monotonically in the large annular region(aboutticeable that despite the position shifts of their highest r /Ruith a Ug of 3. 7 m-s, dropvalue from the center to the wall, the upwards particle ve-dow中國煤化工 peak around a radiallocity always exhibits a more uniform radial distribution posiCNMHGevident that the distinc-than the downwards particle velocity. The typical devel- tion betweerU ieyIuI anu the annular region, inoped profile of Vpd is characterized by a relatively flat core view of the profiles of vpd and vpu, is vague with zero Ug(about r/R<0.6)and a relatively nonuniform annulus where conditions, especially at a higher Gs. The higher the Ug isZhang Zhu: Gas-Solids Flow Behavior with a Gas Velocity Close to Zerothe more defined boundary is seen between the two reCompared to the data obtained under non-zero superficial gas velocities by Zhang and zhu(2000), the developedThe local solids fluxes of downflowing and upflowing profiles of Fsd under zero superficial gas velocity increaseparticles, Fsd and Fsu, calculated through Eq (1),(2)and monotonically in the large annular region (/RP0.6)(3), are presented in Fig 4. The local net solids fiux, Fs, is whereas those with non-zero superficial gas velocitiesrepresented by the vertical distance between the above have a sharp drop adjacent to the wall(r/R06-0.95)x bromeintegrating Fs over the The effects of solids circulation rate Gs, on the radialcross-section at all measured levels, it was found that the distributions of s, vpd and Vpu, as well as Fsd and Fsd cancalculated solids circulation rates were all in good agree- also be seen from Figs. 2 through 4. An increase in Gs hasment with the set Gs, with the maximum deviation of 4.5%. a significant effect on the radial distribution of solids holdupThe radial profiles of Fsd resemble the radial profiles of under zero Ug conditions. It significantly increases the localsolids holdup which is dominated by the contribution of solids holdup in the wall region and also extends the rangedownflowing particles. The typical developed radial profile of this region, but only increases the local solids holdup inof solids flux is also distinguished, like the typical devel- the core region to a smaller extent(in terms of the absoluteoped profile of solids holdup, by a flat core region value of increase). This is similar to the effect on those(r/R<0.6-0.8)and an annular region in which the profile under an Ug of 3.7 m s, as described in details by Zhangrises (r/RP0.6-0.8)rapidly towards the wall. The devel- and Zhu(2000 ) The same change in Gs increases theoped radial profile of Fsu also has a similar trend despite downwards particle velocity in the core region and slightlythe fact that Fsu is always much lower than Fsd across the lowers it in the wall region, making the radial distribution ofradial direction. From the figure it can be seen that the Vpd more uniform. Similar effect on the radial distribution ofradial distribution of the local net solids flux, fs, is more vpu can also be seen. Furthermore, it is quite noticeableuniform than those of Fsd and Fsu although there still exists that, with increasing Gs, the local upwards particle velocitya peak at the wallshifts from higher than the downwards particle velocity tolower than it. This is possibly because of the increasedresistance from the denser downflowing particles. Theinfluence of increasing Gs on the shape of radial profiles ofdownards and upwards solids fluxes is not obvious. This isUpwards0020m00m測due to the opposite effects of increasing Gs on the radialdistributions of solids holdup and particle velocity. However,an increasing Gs do largely elevate the radial profile of Fsd0.512m200while it merely raises the radial profile of Fsu slightlyFigure 5a shows the radial profiles of the gross solidsholdup(measured) and the solids holdup of the upflowingparticles(calculated from Eqs. (1 )and(2)at the axial levelof 9. 155 m from the downer entrance, where a stablegas-solids flow has been fully established. Both the grosssolids holdup and the solids holdup of the upflowing parti-2bi:=-8-u2112m200cles increase with increasing radial distance from the cen-ter for all three solids circulation rates tested the radialprofiles of the relative contribution of the upflowing parti.cles to the local gross solids concentration are plotted inFig. 5b. All the three profiles under different Gs have fairlyuniform distributions within a large region of r/R<0. 8 and a6227msharp increase with radial distance outside this region Thisshows that more particles flow up through the troughs neathe wall than through those in the center. This is probablybecause the wall region provides a stable environment forthe troughs to transfer solids upwards, whereas the higherturbulence in the center region imposes larger resistanceto thincrease in G. not only increases the中國煤化工 s the holdup of the up-Normalized radial distance from the downer center, r/RtribCNMHthe local gross solidsFig 4 Radial profiles of the local solids fluxes of downflowing and concentration becomes smaller as Gs increases, becauseplowing particles, along the downer under different solids the increased overall solids flux is likely to reduce thecirculation rates with zero superficial gas veltential for solids upflowCHINA PARTICUOLOGY Vol 4, Nos 3-4, 20068::104+m;wards solids fluxes(LOD d; LOD), under three different49100202solids circulation rates For the radial solids holdup profilesthe core region develops much faster than the annularregion and LOD in both regions are affected by thechange in Gs. An increase in Gs from 49 to 202 kg-m sextends LODc in the core region from 1.2 to 6.0 m. On the0.0other hand, LOD in the annular region is extendedproximately from 4. 4 to 8.0 m by the same change in GsA△△△AAFigure 3 shows that a stable radial distribution of the000204060810adia istana upwards particle velocity is established much sooner thanFig 5 (a)Fully developed radial profiles of the local gross solids gion, the radial profile of veu develops as fast as that in theholdup and the solids holdup of the upflowing particles under core region. Moreover, higher Gs encourages an earlierdifferent solids circulation rates with zeroicial gas ve- establishment of the stable radial profile for vou. when glocity, at H=9. 155 m( b Relative contributions of the upflowparticles to the local gross solids holdup under different solids is increased from 49 to 100 or 202 kg- m s, LOD iscirculation rates with zero superficial gas velocity, in the fully shortened from 4.4 to about 1.2 m. )On the other handdeveloped zone(H=9. 155 m).LODd seems not affected by Gs and a stable radial distri-bution is achieved at 6.0 and 8.0 m for the core and theIt needs attention that the solids holdup of the upflowing annular regions respectively. Compared to that under conparticles, esu, is in fact the average holdup of the upflowing ditions of non-zero gas velocity(Zhang and Zhu, 2000)particles within a small volume which contains the volume LODvd is much shorter under zero Ug conditions, due to theof the troughs in it. Because the upflowing particles are uniform distributions of downwards particle velocityinside the troughs instead of uniformly distributed in this throughout the whole length of downersmall volume the actual holdup of these upflowing partiThe developments of downwards and upwards solids fluxcles inside the troughs is much higher than Esu. Likewise, profiles for the three different Gs can be seen in Fig 4.Allthe actual holdup of the downflowing particles outside the the radial profiles of downwards solids flux have a LODd oftroughs should be slightly higher than esd )On the other 6m in the core region and 8 m in the annular region.Thehand, the actual holdup of the upflowing particles inside the development of the radial profile of upwards solids flux in theoughs must be smaller than the actual holdup of the core region is infiluenced by Gs. As Gs is increased from 49downflowing particles outside the troughs due to the shear to 100 and then to 202 kg m- s, LODu in the core region isbetween the steams of upflowing particles and the bulk first shortened from 4. 4 to 2.0 m, and then extended todownflowing particles Therefore, the volume fraction of the 6.0 m. At the same time, LODhu in the annular region is ex-troughs at any position of the downer cross-section must tended from 4. 4 m to 6.0 m and then to 8.0 mbe higher than the relative contribution of the upflowingIn summary, although LODc, LODd and LOD areparticles to the local gross solids concentration (Esu/Es)fluenced by Gs, the longest one of these three, seemsIn summary, the gas-solids flow structure under zero Ug remain constant in both the core and the annular regions,conditions is quite unique compared to that under condi- as Gs changes. As a resultthe "comprehensive"length ofions with U/>3.7 m-s". In the fully developed region, it is radial flow development, LOD, stays unaffected by the Gs,characterized by a non-uniform radial distribution of solids being about 6.0 m in the core region and 8.0 m in the anholdup, with the highest value at the wall, and a relatively nular regionuniform radial distribution of downwards and upwards par- 4.3 Axial solids flow structureticle velocity, again with the highest values right at the wallFigure 6 illustrates the axial profiles of the gross4. 2 The development of the radial solids flow cross-sectional average solids holdup, E,, and thestructurecross-sectional average solids holdup of the downflowingIn order to conduct comparisons upon the same base, particles, en, under different solids circulation rates. FcZhang et al.(1999)performed their analyses based on a each condition, the difference between the two profilesso adopted in this study when the development of radial upflowing& e cross-sectional average solids holdup of thens. For the same reason, a dividing of r /R=0.775 isrepresentsE., which constitutes only a smallflow structure is discussed, although the actual core region frac中國煤化工 he average holdup ofcould be smaller or larger than r /R=0.775the dCNMH Gapidl in the top sec-Figures 2 to 4 also provide evidences of the lengths of tion oruximum value and thenradial flow development, in terms of the radial profiles of it quickly decreases to reach a constant value The axialsolids holdup( LODc), downwards and upwards particle position of the peak and the starting point of the constantZhang Zhu: Gas-Solids Flow Behavior with a Gas Velocity Close to Zero173tions, because the cross-sectional gas velocity is zeroG,kgms14910020particles entering the downer entrance from the distributor0.08ownwards口o令tubes with a certain initial velocity are subjected to a40.06de-acceleration caused by the"stagnant gas(althoughthere exist unstable troughs where small amount of gasflows upwards, the net gas flow is zero). At the same time,the streams of solids coming down from the distributortubes disperse, which leads to higher upwards drag fromthe gas and- intensifies the de-acceleration effect. This00010 process is manifested as a decreasing o and an inDistance from the downer top, H/mHowever, a dispersed solids suspension isFig6 Axial profiles of the cross-sectional average solids holdup and not a stable state in a gas-solids flow system when thethe cross-sectional average solidslup of the downflowing solids concentration is not too low. The solids particles tendparticles under different solids circulation rates with zero su-perficial gas velocityto form clusters until a stable structure is attained. Inducedby the solids aggregation, Ve starts to increase after aEn section are affected by the solids circulation rate whenturning point and leaves a valley behind. A constant Vo isGs varies between 49 to 100 kg. s. However, theyremain unchanged for the two higher solids circulationreached further down the column when this de-accelerarates. Also, the solids circulation rate affects the magnitudetion-re-acceleration process is accomplished. Accordingly,of the three different cross-sectional average solids hold- ad turns around at the peak point and decreases untilps (5,, d and f)throughout the downer column and reaching a constant value. the above- mentioned solidsdetermines the height of the peaks. A higher G, leads to aggregation process was proven by the variations of solidshigher F,, Ed and Eu as well as higher peaksholdup values alalong with measurement timesFigure 7 also shows that a higher Gs slows down theFigure 7 shows the axial profiles of the cross-sectional particle acceleration while a slightIncreasein Ug(toverage particle velocity for particles moving downwards, 3.7 m-s-l)as indicated by the data of Zhang and ZhuVed, and upwards, V, respectively, under different solids(2000 ), does not significantly affect the length of accelera-circulation rates. It can be seen that the cross-sectionaltion zone(LOA)rapidly in the top section of the downer and reaches aminimum value and then it quickly increases until a constant value is attained these variations are consistent withthose of the a. as described above. Also, the effects ofGs on the axial position of the valley point, the starting point 3. Bof the constant Vpd section, as well as the relative mag-G,kms14910020anitude of the valley point are similar to the effects on those aof the Esd. However, the value of constant Vpe has only a惠E500G,ms149100202small increase with increasing Gs. The similar V valuesfor different Gs are reasonable considering that particlesare only accelerated by the gravity. A slightly higher Vpdfor higher Gs is likely due to the increased slip velocity withreduced drag at higher E,Figures 6 and 7 show the solids flow structure in theDistance from the downer top, Himlowner with zero Ug and suggest that quite a difference in Fig. 7 Axial profiles of the cross-sectional average particle velocity forsolids flow pattern exist between zero and non-zero Ugconditions. Under non-zero Ug conditions, particles areds circulation rates with zero superficial gas velocityR2中國媒化工 sure gradientgravity in the second acceleration section until the upwards TheCNMHGes under different zerodrag from the gas counter-balances the gravity and a con- Ug conditions are shown in Fig. 8. It can be seen that thestant velocity is reached(Wang et al., 1992; Zhang et al. pressure gradient first increases at the entrance region and1999; Zhang& Zhu, 2000). However, under zero Ug condi- then drops down until a constant value is reached at an174CHINA PARTICUOLOGY Vol 4. Nos. 3-4, 2006axial level of about 1.5 to 2 m. these trends contrast set to zero or negative values, as mentioned earlier. Theirsharply with the results under non-zero Ug conditions pressure gradient profiles with zero Ug conditions show a(Zhang et al., 1999), where the pressure gradient starting similar trend to the one in this study with Gs ofwith a high negative value becomes positive quickly and 49 kgm"s". Because the highest Gs tested in their studthen gradually approaches a constant value. However, they was very low(19.5 kg m"s"), also because the axialare in good agreement with the axial profiles of the solids resolution of the pressure gradient measurement was notholdup and particle velocity as shown in Figs. 6 and 7.high enough, no peak was noticeable close to the en-In Fig. 8, the values of the first 4 data points represent the tranceaverage pressure gradients within axial sections of 0-0.492 m, 4.5 General discussion0492-1177m;1.177-2091mand2.091-3005m.Thuscorresponding to the de-acceleration section(0-1.2 m)ofthe solids flow under conditions of 100 and 202 kg- m-2 s-1 of solids holdup under a zero Ug condition is characterized(see Figs. 6 and 7), the first 2 data points of pressure gra- by a flat core region(rR0),until amainly due to the change in solids holdup, so that the large radial mass transfer balance is reached. On the other hand,initial negative values and the monotonic increase in the this denser wall region would be unstable since the axialaxial pressure gradient profile associated with non-zero pressure gradient could not"support the particles in thegas velocity operation are not found herewall region. These particles would thus move down at aLuo et al.(2001)measured the pressure gradient in a higher velocity, leading to a positive radial gradient of par-0.025 m i.d. downer column with superficial gas velocity ticle velocity(dVp/dr>0). In turn a shear stress would begenerated by the radial gradient of particle velocity and theG,kgm2s149100202shear would eventually counter-balance the extra weight ofthe solids(see Fig 9)in the fully developed zone6, Pg 2m/dHdr=2x/dr dH+ 2r/dHr -2x(r dr ) dHtrdrHE中國煤化工4.060Axial distance from downer top H/mNMHGFig 8 Axial profiles of the pressure gradient under different solidscirculation rates with zero superficial gas velocitydP1 d(rr, )dP 1 d(rudVpm/drdh r dr dHthang& Zhu: Gas-Solids Flow Behavior with a Gas Velocity Close to Zero175where EsA is the density of the isolated unit; r is the shear tributions of solids holdup and particle velocity understress on the side surface of the unit; Vpm is the local mean non-zero Uo conditions can also be explained, consideringparticle velocity calculated from Eq (4): u is the viscosity of the possible influences brought up by the apparent gasthe gas-solids mixture, which is a function of the solids flow in the downer. Typical radial profiles of Vpm, &s in thefully developed zone in the downer under non-zero gasIntegrating Eq (6)we have:velocity operation are shown in Fig 9. At a given Gs,as-r dP 2pg b ardrsuming that downer fluidization air is introduced into thedr 2udH( downer and hence the U, is increased from zero to a lowvalue it can be expected that a high negative radial gra-dient of gas velocity(dVg/dr