Hydrodynamic Behavior of Three-Phase Airlift Loop Slurry Reactor

TSINGHUA SCIENCE AND TECHNOLOGYISSN 1007-0214 05/21pp17 - 24Volume 7，Number 1，February 2002Hydrodynamic Behavior of Three- Phase Airlift LoopSlurry ReactorRENFei (任飞), WANG Jinfu (王金福)*，WANG Tiefeng (王铁峰)，JIN Yong (金涌)Department of Chemical Engineering，Tsinghua University ，Beijing 100084， ChinaAbstract: A novel fiber optic probe system and a set of commercial ultrasonic Doppler velcimeters have beenused to study the hydrodynamic behavior of a three -phase airlift loop (TPAL ) slurry reactor. Experiments havebeen carried out in a loop reactor with 100 mm inner diameter and 2. 5 m height, in which air. tap water andsilica gel particles are used as the gas，liquid and solid phase，respectively. The radial profile of gas holdup ,bubble size， bubble rising velocity， liquid circulating velocity， and the influence of the main operatingconditions such as superficial gas velocity and solids concentration have been studied experimentally in theTP AL slurry reactor. The experimental results show that the bubble characteristics are different in various flowregimes and the radial profiles of some hydrodynamic parameters in the TPAL slurry reactor are more uniform .than those in traditional three phase reactors. The distribution of the bubble size and bubble rising velocity canbe described by a lognormal function. The influence of superficial gas velocity on the hydrodynamic parametersis more remarkable than that of the solids concentration.Key words: three phase airlift loop (TPAL) slurry reactor; hydrodynamics; optic fiber probe; ultr asonicDoppler velocimeterphases through both zones. The circulation resultsIntroductionin considerable mixing of all three phases, betterThree- phase slurry reactors have been widely usedmass transfer performance， even distribution ofin biological waste water treatment， fuel gassolids throughout the reactor， and efficientdesulphurization,Fischer-Tropsch synthesis ，suspension of solids in the liquid3].methanol synthesis， dimethyl ether production ，TPAL slurry reactors have many specificfermentation ( production of ethanol andadvantages including better interphase contact ，mammalian cells)， and hydro-treatment of heavyreduced backmixing， larger processing capability，petroleum residues processes[1 -5 Ihighergasholdup，more uniform and finerCompared with conventional slurry bubblebubbles，and easier heat transfer into or out of thecolumns，the three-phase airlift loop ( TPAL )reactor.These features exhibit that the TPALreactor is a new type of reactor. The loop principleslurry reactor has wide potential applications.of TPAL slurry reactors is based on a driving forceA knowledge of the hydrodynamic behavior inwhich is caused by a density difference in twothe TPAL slurry reactor, such as gas holdup,different reactor zones, i.e.，a denser zone of thebu中国煤化工8 velocity, and liquidliquid-solid phase (downer) and a dilute zone oficularly important in thethe gas-liquid-solid phase ( riser ). This densityd:THC N M H Grs. Several researchers，difference induces circulation of the liquid and solidhave studied these hydrodynamic behaviors inthree- phase bubble column slurry reactors14.5,but the research on hydrodynamics of TPAL slurryReceived: 2001-04-02 ; revised: 2001-05- 31reactors，especially on the local hydrodynamic* To wpondence should be addressedbehavior, is scarce. Perhaps the reason is the lack18Tsinghua Science and Technology, February 2002，7(1): 17- 24of effective measuring methods to study these localliquid separation room. Gas enters the bottom ofparameters.the reactor via an annular gas distributor with 36Wang et al. [6.7] developed a novel fiber opticpores of 1. 2 mm in diameter. The experimentalprobe system for studying the bubble behavior inapparatus is operated at room temperature andgas-liquid -solid three- phase circulating fluidizedatmospheric pressure, using air, tap water and 52-beds (TPCFBs)，and found that it could also beμm silica gel particles (density 1800 kg/cm*) as the .used for gas-liquid two- phase and gas-liquid-solidgas phase,liquid phasethree-phase systems. From the probe signal， werespectively.The gas superficialvelocity iscan get much useful information about the localmeasuredby a rotameter. The fiber optic probefluid parameters including gas holdup， bubbleconnected to the bed-wall is movable in the radialrising velocity， bubble size distribution， bubbledirection so that the bubble behavior in differentfrequency, and gas-liquid interfacial area.radialpositions can bemeasured，and theIn recent years， the ultrasound technique as an .ultrasonic probe is mounted on the bed wall at anintrusive measurement method has drawn muchangle to get the radial profile of the liquid velocityattention due to its attractive advantages compareddistribution.with conventionaltechniques[8-10].The mainadvantages include: (a) successful technique O1. 2 Fiber optic systemimage formation， (b ) applicability for opaqueThe novel fiber optic system， shown in Fig. 2,systems，and (c ) capability of measuring thewas developed by Wang et al. C6.7] for measuringspatiotemporal velocities. In this work，a set ofhe bubble behavior in multiphase flows. Thecommercial ultrasonic velocimeters DOP2000[8] wasprobe consists of two parallel optic-fibers withused to measure the liquid circulating velocity in a62.5 μm in diameter. The emitted light beam isTPAL slurry reactor.split through the splitter into two beams and then1 Experimentalsent into the fiber through the fiber coupler. Eachbeam is reflected at the end of the fiber. Theintensity of the reflected light when the probe tip1.1Experimental apparatusin the liquid is different from that when the probeThe experimental setup of the TPAL slurry reactortip in the bubble due to different refractions. Theused in this work is shown in Fig. 1. The wholepresence of particles in the system causes no signal↑Air ventresponse, which makes the probe systemapplicable to both gas-liquid-solid three- phase andSlury levelgas-liquid two-phase systems. The output lightdensity signals containing information of bubblebehavior are transformed into electrical signalsthrough a light detector, and amplified through anamplifier to output standard voltage signals. TheFiber optic probesignals are sampled by an analog- to- digital dataacquisition-card and the results are stored into aRotameterTo PCAir compressor 'DOP2000uFig.1 Diagram of the TPAL slurry reactor used inthis work中国煤化工ToPCreactor is made of Plexiglas with a height ofYHCNMHGMapproximately 2. 5 m, and consists of two verticalsections of a riser and a downer with an inner pipeFig.2 Hardware structure of the fiber optic probe fordiameter of 100 mm.The two sections aremeasuring bubblesconnectedhx就操0° U bend at the bottom of the1，laser source; 2, light splitter; 3，fiber coupler;4, light detector; 5， amplifier; 6，A/D transducer;reactor. At the top of the reactor, there is a gas-7，probeRENFei (任、飞) et al: Hydrodynamic Behavior of Three Phase ....!9data file ina PC. A specially written program hasbeen developed to control data acquisition ancEmissionPrincipalamplitieroscillatoranalysis.Wang et al.L6.7I introduced the data treatmentmethod in detail, and from the experimental dataReceptionSy nchrorizingwe can get gas holdup， bubble rising velocity,missiondemodulationbubble size distribution， and other usefulinformation.A/DI ow-passThe gas holdup is defined as the volume fractionconverterfilterof the gas phase in the reactor. The time fractionof the probe tip in a gas bubble is equal to the localgas holdup. Provided the acquisition frequency isHigh-passDopplerfrequencyconstant, the local gas holdup can be calculatedfromεg= Ng/N(1)where εg is gas holdup，Ng is the number of theacquisition points in the gas phase， and N is thetotal number of acquisition points.The bubble rising velocity can be determinedFig.3 The schematic block of the velocimeterfrom the signal correlation of the two probe fibers.Bubble measurement using the fiber optic probe isThe amplification of the echo signal is increasedbased on the refraction difference of the gas andaccording to the depth (time gain control or TGC)liquid phases. When the gas-liquid-solid three-n order to compensate for the attenuation of thephase mixture flows up concurrently, the outputwaves. After the amplification，the echo signal issignal contains bubble movement information. Thedemodulated, then filtered to isolate the Dopplerdownstream signal lags a little behind the upstreaminformation. A low pass filter suppresses artificialsignal due to the distance between the two tips.frequencies in the spectrum generated by theThe lag time can be determined by correlation, anddemodulation.then，with the distance between the two tips, theThe Doppler signal is then sampled andbubble rising velocity can be calculated.converted into digital form by a fast A/DThe bubble chord length is calculated byconverter.Thetimebetweenacquisitionsmultiplying the bubble rising velocity and thedetermines the axial spacing between sampleduration time of the probe signal of high level.volumes，while the delay between the emission andFrom the bubble chord length distribution，we canreception determines the distance of sampleuse certain mathematical treatment methods tovolume. The signal coming from the A/Dobtain the bubble size distribution.converter is stored and then filtered by a high passfilter，which eliminates the steady and quasi-1. 3 DOP2000 ultrasonic velocimetersteady components. Finally， the frequency of theA set of commercial ultrasonic velocimeters madeDoppler signal is estimated. The results may beby Signal Processing S. A.，Switzerland8]， hasused to calculate the velocity.been used to measure the radial profile of the liquid2 Results and Discussioncirculating velocity， which is based on the well-known Doppler effect. By careful analysis of theecho signal of the measured target, e.g. a particle2.1 Gas holdupor a bubble in the fluid, one may determine both: direntlv, aeeociated with the bubblethe location and the velocity of the target.siz中国煤化工eelocity. All factors thatThe schematic block of the velocimeter is shownaffMYHCN M H G bubble rising velocityn Fig. 3. The same transducer is used to bothmay attect gas holdup. The influence of thetransmit and receive the ultrasonic signals. Thsuperficial gas velocity and solids concentration onsignal coming from the principal oscillator providesthe cross- sectional average gas holdup is shown inthe trigger for the emitted signal at the pulseFig.4, where εg is gas holdup, Ug is superficial gasrepetition 力穷数据Y (PRF).velocity and C, is solids concentration. Gas holdupincreases with increasing superficial gas velocity ,20Tsinghua Science and Technology, February 2002，7(1): 17-24but when Ug increases to the vicinity of theconcentration increase to a highdegree,transition superficial gas velocity, the slope of theremarkable non-uniformity of the radial profile ofgas holdup increase becomes flat， as shown irthe gas holdup can be observed， as shown inFig.4 when Ug is equal to 0. 07 - 0. 14 m/s,Fig.5. Here r is radial coordinate and R is theindicating that the system enters the transitionradius of the reactor. From Fig. 5，it can be seenregime from the bubble dispersion regime. In somethat with the increase of superficial gas velocity ，three- phase systems， it may be found that gas .the gas holdups at all radial positions increaseholdup decreases with increasing superficial gasremarkably. Figure 6 shows the radial profile ofvelocity in the transition regime[1]. A bubblegas holdup for different solids concentrations,coalescence regime will appear with furtherwhere the superficial gas velocity is 0. 1415 m/s.increases of superficial gas velocity， and gasAt relatively high gas velocity， the gas holdup ofholdup increases rapidly with Ug， In general,near-wall region is much lower than that of centralaverage gas holdup decreases with increasing solidsregion. With the increase of solids concentration，concentration，and many researchers have drawnthe gas holdup decreases at all positions.similarconclusionsirvariousthree-phase0.35systems[1.41. Gandhi et al.[4] pointed out that thdecrease of gas holdup can be attributed to one ormore of the following factors: (a) increase ofbubble size at the gas distributor; (b) the decrease)f bubble break-up rate; or (c) the increase of0.1bubble coalescence rate. The decrease in bubblebreak-up rate seems to be a primary reason for a0.2 0.406 0.8fast drop in gas holdup due to the presence orIRsolids in heterogeneous regime. With the increase(a) C=0of superficial gas velocity， the rate of bubble0.3break- up increases faster than that of bubblecoalescencel4.0.20.400.0.32U, /(m.S)一■-0.0177●一0.0354- - U=0.1061一-0.0707一 v- 0.1061一A- U-=0.141.5- C:=2.012 %040.6(b) C=9.6960.160.080 0.00.08 0.12 D.16 0.20 0.24C orU /(m.5)Fig.5 The radial profile of gas holdup for differentsuperficial gas velocitiesFig.4 Influence of superficial gas velocity and solids0.30concentration on average gas holdupD.28Most research results in three-phase systems0.24show that the radial profile of the gas holdup is not02uniform; it is higher in central region and lower in中国煤化工thenear-wallregion.However，thisphenomenon is not true for the TPAL slurryMHCNMH G0.E223.62 %reactor in this work. Especially at the conditions of0.0ixlowsuperficialgas velocity and low solidsr/Rconcentrations，the radial profile of the gasholdup，measued in this work, is rather uniform.Fig.6 The radial profile of gas holdup for differentOnlywhe效猿gas velocity and solidssolids concentrationsRENFei (任、飞) et al: Hydrodynamic Behavior of Three Phase ....21increases later. Perhaps this profile is related with2.2 Bubble sizethe reactor structure， especially with the gasFigure 7 shows the bubble size distribution adistributor. The gas distributor used in this workdifferent superficial gas velocities， where db is theis a circline with pores， and bubbles generatedbubble diameter defined by the maximum verticalfrom the distributor are not released at the centralchord length. Figures 7(a) and 7(b) illustrate theor near-wall regions， which may affect the radial .results at the central region and near-wall region,profile of the bubble size.respectively.The bubble sizefollows thelognormal distribution， which is accordant withthe results in literature[1]. In general, the bubble4.6-size increases with the increase of gas velocity overUg/(m .5a wide range. This agrees with the reported4.4一一一0.0707一▲- 0.1061conclusions in three-phase systems，but Wang et。4.2al. gave a contrary trend in a three- phasecirculating fluidized bed-61. Perhaps the influence4.0>f gas velocity is different for different operatingconditions and experimental setups.0.20.4008r/R0.6u, /(m.s-)Fig.8 The radial profile of the Sauter mean diameter at一一一0017684- - 0.03537different superficial gas velocities一0.10610.1768Figures 9 and 10 show the influence of operatingconditions on the bubble size at different radialpositions. With the increase of gas velocity， the0.05bubble size will increase at all radial positions ，andd1mmthis result also differs from the result in thre(a)r/R= 0phase circulating fluidized bed-6. Fan'1l pointedout that the influence of gas velocity on the bubbleU/m ssize is associated with the flow regime. In the1- 0.01 76804一0.035 37bubble dispersion regime，bubbles are small, and-7- 0.1768their size increases with the gas velocity in a smalldegree，while in coalescence regime， bubbles arelarge，and the size increases remarkably with thegas velocity.0.0L(b)rIR=0.8rIRFig.7 Bubble size distribution at different superficialgas velocities3.6The radial profile of the Sauter mean diameter at中国煤化工different superficial gas velocities is shown inYHC N M H G014 0is-o18Fig.8，where dm is the bubble Sauter diameter.t, /1ms")The bubble size in the central region is larger thanthat in the near-wall region. However, the radialvariation of bubble size does not have a monotonicFig.9 Influence of superficial gas velocity on bubbletrend. Fr方数据central region to the near -wallsize at different radial positionsregion，the bubble size decreases first and then22Tsinghua Science and Technology, February 2002，7(1): 17- 24distribution in the radial direction. The radialprofile of bubble rising velocity at different10.5superficial gas velocities is shown in Fig. 12. The9.0rIRprofile of bubble rising velocity is not monotonic in昌the radial direction as reported in Ref. [6]. The7.5-average bubble rising velocity increases first and6.0then decreases along the radial direction. With theincrease of gas velocity， the bubble rising velocityincreases at all radial positions.025C./50.4Fig.10 .Influence of solids concentration on bubble名0.3-size at different radial positions0.017 680.070 74D. 1768The influence of the solids on bubble size ismainly attributed to two aspects. One is thebubble break-up effect due to the existence osolids which may lead to the decrease of bubble2 0.4 0.6 0.81.012 1.416 1 xsize with the increase of the solids concentration.(4./(m.S ')The other is that theincrease of solidsconcentration will result in the increase of apparentviscosity of liquid-solidpseudohomogeneousFig.11 Bubble rising velocity distribution at differentphase, and then the bubble size will increase withsuperficial gas velocitiessolids concentration. As shown in Fig. 10，whenthe solids concentration increases, the bubble sizedecreases slightly in the case of low solids0.7concentration and the bubble size becomes muchlarger in the case of high solids concentration..6-+-0.1415Because theparticles used in this work arerelatively fine and the effect of bubble break-up is0.s-not remarkable， the presence of solids mainlycauses the increase of apparent viscosity and leadsto the larger bubble size.D20.4 0.6riR2.3 Bubble rising velocityFig.12 The radial profile of bubble rising velocity atThe bubble rising velocity distribution in thedifferent superficial gas velocitiescentral region at different superficial gas velocitiesis shown in Fig. 11，where U is the bubble risingFigures 13 and 14 show theinfluence ofvelocity. It can be seen that like the distribution ofoperating conditions on bubble rising velocity atbubble size， the bubble rising velocity nearlydifferent radial positions. The increase of gasfollows the lognormal function distribution. Withvelocity will cause an increase of bubble risingthe increase of the gas velocity, the average bubblevelocity as shown in Fig. 13. Different solidsrising velocity also increases, and the range of theconcentrations will change the physical property ofvelocity distribution becomes wider. Increasing theliquid-solid pseudo homogeneous phase. Its densitygas velocity may cause more violent turbulence,an中国煤化工with the increase of thelead to a much frequent bubble break-up andsCMYHCN MH Gh density results in thecoalescence，and extend the distribution of bubblewuuuic uuoyanncy and leads to anrising velocity.increase of the bubble rising velocity; while a highIn three-phase systems， because of the bubbleapparent viscosity increases the drag force ofsize distribution and the violent turbulence， therebubbles and encumbers the bubble rising velocity.are differ万方 数据le rising velocities at differentFor two- factor control, the behavior of bubblepositions, and this leads to a bubble rising velocityrising velocity in our system is shown in Fig. 14.REN Fei (任~飞) et al: Hydrodynamic Behavior of Three- Phase ....23The bubble rising velocity decreases slightly at lowliquid circulating velocity at different superficialsolids concentration and increases remarkably atgas velocities under the condition of solids volumehigh .solidsconcentration，asthefraction 0. 0412，where UL.R is the liquid circulating .concentration increases.velocity. The velocity profile along the radialD.7pdirection is rather uniform except at the near-walfR_ 1-0region，where the velocity decreases remarkably.The figure also indicates that the flow is violently0.6turbulent.Figure 16 shows the influence of operating0.conditions on the cross-sectional average value ofliquid circulating velocity. Under the conditions of0:certain experimental equipment,the liquid02 0.04 0.06 0.08 0.10 0.12 0.14circulating velocity is only affected by the gas .Uus /(m.s7)velocity and solids concentration. The increase ofsuperficial gas velocity leads to an increase of gasholdup in the riser tube， which results in anFig.13 Influence of superficial gas velocity on bubbleincrease of the difference of apparent densitiesrising velocity at different radial positionsbetween the riser and downer which tends to theincrease of liquid circulating velocity. Under the0.80r/condition of low solids concentration， the liquid0.75一1一D.2circulating velocity keeps unchanged as the solids一-一0.2 0.70concentration varies. Only when the solidsconcentration increases to about 0. 03 volume0.65 Afraction will the liquid circulating velocity begin toS 0.60decrease. .0.55052025C 1/950.25 Iu, /Im.sFig. 14 Influence of solids concentration on bubbleE0D124E 0.20二 二100172700283S0.150.05662.4 Liquid circulating velocity0 0882The liquid circulating velocity is a very importantparameter in a TPAL slurry reactor. DOP2000, aC.1%commercial ultrasonic Doppler velocimeter， wasused to measure the liquid circulating velocity.Figure 15 demonstrates the radial profile ofFig.16Influence of operating conditions on liquidcirculating velocity0.300.25-3Conclusions0.20(1) With the increase of superficial gas velocity目0.15and the decrease of solids concentration， the gasS 0.10 ,U,/(m.sEholdup. increases， but the increase trend is0.0124 -一0.01770.051-0283 27-00424dif中国煤化工，regimes. The radial一0.0707PY;CNMHGoldupisusuallymore80unnorm than tnat in tnree- phase fluidized systems1/mmexcept for the case of much higher gas velocity orhigher solids concentration， in which the gasholdup near the wall region becomes much lowerFig. 15 The radial profile of liquid circulating velocitythan that in the central region.互市敵据nt superficial gas velocities(C,= 0.0412)(2) The statistical bubble sizeat certainTsinghua Science and Technology, February 2002，7(1): 17- 24positions nearly follows the distribution of a [3] Livingston A G, Zhang S F. Hydrodynamic behaviorlognormal function. The bubble Sauter diameterof three-phase (gas-liquid-solid) airlift reactors.Chem. Eng. Sci. ，1993， 48: 1641 - 1654.decreases along the radial direction in the central[4] Gandhi B， PrakashA， Bergougnou M Aregion and increases near the wall region. TheHydrodynamic behavior of slurry bubble column abubble size increases with increasing gas velocity.high solids concentrations. Powder Technology,The effect of solids concentration is also different1999，103: 80- 94.for different solids concentration ranges. As a[5] Rajamani Krishna， Jeroen W A de Swatr, Jurgresult the bubble size will decrease at first and thenEllenberger，et al. Gas holdup in slurry bubbleincrease when the solids concentration increases.columns: effect of column diameter and slurry(3) The local bubble rising velocity nearlyconcentrations. AIChEJ. ，1997，43: 311 - 315.follows the lognormal function distribution. The[6] Wang Tiefeng， Wang Jjinfu, Yang Weiguo, et al.Experimental study on bubble size distribution irradial profile of average bubble rising velocitythree- phase circulating fluidized beds. Journal ofpresents a trend of becoming high at an annularChemical Industry and Engineering， 2001， 52(3):region and low at the central and near -wall197 - 203. (in Chinese)regions. The bubble rising velocity decreases at[7] Wang Tiefeng， Wang Jinfu， Yang Weiguo, et al.first and increases later with the increase of solidsExperimental study of bubble rise velocity irconcentration.three- phase circulating fluidized beds. .J ournal(4) The radial profile of liquid circulatingChemical Engineering of Chinese U niverities, 2000,14(4): 334 - 339. (in Chinese )velocity is very uniform except in the near-wall[8] DOP2000 User's Manual. Switzerland: Signalregion. With the increase of gas velocity， ULRProcessing S A, September 2000.increases.. The solids concentration haslittle[9] Broring S, Fischer J, Korte T，Solinger S, Labbertinfluence on the liquid circulating velocity until itA. Flow structure of the dispersed gas. phase in realincreases to a rather high level.chemical reactors investigated by a new ultrasoundDoppler technique. Can. J. Chem. Eng. ，1991. 69:References1247- 1257.[1] Fan L S. Gas-Liquid Solid Fluidization Engineering.Ladner W P. Ultrasonic characterizations of slurriesNew York: Butterworths Publishers, 1989.in a bubble column reactor. Ind. Eng. Chem. Res. ,[2]Douek R s, Livingston A G, Johansson A C, et al.1999, 38: 2137- 2143.Hydrodynamics of an external-loop three- phase airlift(TPAL) reactor. Chem. Eng. Sci. ，1994， 493719 - 3737.中国煤化工MYHCNMHG