EFFECTS OF GAS TYPE AND TEMPERATURE ON FINE PARTICLE FLUIDIZATION

CHINA PARTICUOLOGY Vol 4. Nos 3-4 114-121, 2006EFFECTS OF GAS TYPE AND TEMPERATURE ON FINEPARTICLE FLUIDIZATIONChunbao xu'. and J -xZhuDepartment of Chemical Engineering, Lakehead University, Thunder Bay Ontario, P78 5E1, CanadaTechnology Research Centre, Department of Chemical&tem Ontario, London, ontario, N6AAuthor to whom correspondence should be addressed. E-mail.Abstract The influence of gas type(helium and argon) and bed temperature(77-473 K)on the fluidization behav-iour of Geldart groups C and A particles was investigated. For both types of particles tested, i. e, Al O3(4.8 um) and glassbeads(39 um), the fluidization quality in different gases shows the following priority sequence: Ar He. In the samegaseous atmosphere the particles when fluidized at an elevated temperature usually show larger bed voidages, higherbed pressure drops, and a lower umt for the group A powder, all indicating an enhancement in fluidization quality Possibleechanisms governing the operations of gas type and temperature in influencing the fluidization behaviours of fine particles have been discussed with respect to the changes in both gas properties and interparticle forces(on the basis of theLondon-van der Waals theory). Gas viscosity (varying significantly with gas-type and temperature)proves to be the keyparameter that influences the bed pressure drops and Umm in fluidization of fine particles, while the interparticle forces(also varying with gas-type and temperature)may play an important role in fine-particle fluidization by affecting the ex-pansion behaviour of the particle-bedKeywords fluidization, fine particles, gas type, temperature, interparticle forces1. Introductionpossible changes in particle properties with increasing bedtemperature, it is obviously questionable to apply the re-The type of fluidizing gas may significantly affect the sults obtained at ambient temperatures to the gas-solidfluidization quality of fine particles. It has been found that fluidization units working at elevated temperatures. a lim-the fluidization behaviour of Geldart group A particles is ited number of studies on the influence of temperature onstrongly dependent on the type of fluidizing gas(Geldart& gas-solid fluidization have resulted in controversial findingsAbrahamsen, 1978; Piepers et a., 1984;Xie, 1997). In the (Yates, 1996). For instance, Ushiki(1995). when investi-work of Geldart and Abrahamsen (1978), it has beengating the fluidization behaviour of ultra-fine particles ofdemonstrated that the bubbling and bed expansion be- SiO2(7 nm)and Mgo(13 nm)at elevated temperatures uphaviours of group A particles of alumina and glass beads to 800C, observed larger sizes of agglomerates formed atgreatly differ with the type and pressure of the fluidizing a higher temperature due to the enhanced interparticlegas. It has been observed that a group A powder may be- forces. In contrast, Morooka et al. (1988)found that thehave more"A-like"when using different types of fluidizinggas. According to Piepers et al. (1984), physically ad- apparent minimum fluidization velocity and the size of ag-sorbed gases may enhance the cohesion of particles, re- glomerates in the bed of SiNa particles both decreasedsulting in an increased elastic modulus of a fluidized bed with increasing the temperature from ambient to 700Cand a growth in the bed expansion. The cohesion in. More recently, Lettieri et al. (2000, 2001)reported on thecreases further when the gas pressure is raised due to thefluidization behaviour of a wide range of group A FCCincrease of gas adsorption with pressure(Piepers et al., catalyst particles, doped with various contents of potas1984: Xie, 1997). This increase in gas adsorption may in-sium acetate, at a temperature ranging from ambient up tofluence the van der Waals interparticle forces in the bed 650C. Their work suggests that the influence of tempera-through varying either the Lifshitz-van der Waals coeff- ture on gas-solid fluidization depends on the properties ofcient of the particle system or the separatio distance be. primary particles used. For fresh FCC catalyst particles, anWaals forces has also been demonstrated theoretically by perature, whereas for the potassium-doped particles, theCottaar and Rietema(1986)and Xie(1997)using the interparticle forces were found greatly increased withHamaker theorytemperature due to the changes in particle properties,As seen from the above, a great deal of research on causing a transition behaviour from Geldart group a touidization has been carried out at ambient temperatures. grouHowever, most of the industrial processes associated with中国煤化工 particles, approximatethe fine powder fluidization are operated at temperatures 30CNMH Ged as group C(cohe-far beyond ambient temperatures(Kunii& Levenspiel, sive)pHHuI y w y uup a particles, group1991: Chaouki et al., 1986; Kato et al., 1989). Because of particles are generally troublesome in handling(suchthe significant variations of the gas properties and the fluidization)due to the strong interparticle forces (Baerns,Xu& Zhu: Effects of Gas Type and Temperature on Fine Particle Fluidization1151966: Chaouki et al. 1985: Pacek& Nienow, 1990: Ushiki periments it needs to be noted here that a small-diameter1995: Horio et alOn the other hand, because of and shallow fluidized bed is employed in this work, al-the high surface-area to volume ratio and other special though much wider and deeper beds are commonly usedcharacteristics, these group C particles are very attractive in other fluidization work. In the earlier work such as byin the chemical industry and other industries such as ad. Rowe et al. ( 1978), mostly much larger particles were usedvanced materials, food and pharmaceuticals. So far how- and the study was mainly on bubbling behaviors in the bedever, little attention has been paid on the influence of the where enough bed height must be provided to allow thetype of fluidizing gas and bed temperature on the fluidiza- development of bubble flow and the bubbles can be verytion behaviour of these particles, except for a work by large(100 mm) for large group B particles. However, theGeldart et al. (1984)demonstrating that the bed expansion present study aims at investigating the fluidization of groupby using different gases. Given the potentially extensive non-existent. Furthermore, small-diameter and"shallowapplications of group C particles, a thorough investigation are all relative, and if we compare the relative ratio of thesignificance. In this regard, a comprehensive study on flu. ems umh&into the influence of gas type and temperature on the flu- column diameter or the bed height to the particle diameteridization behaviour of these fine particles is thus of great our colurnd initial bed height are much larger in relativeidization of groups C and a particles with two types of inertgases, i.e., helium(He)and argon(Ar), at temperaturesranging from-196C to +200C, is carried out in the present work. Possible mechanisms governing the operationsof gas type and temperature in varying the fluidization be-haviour of fine particles have also been discusserespect to the changes in both gas properties and inteticle forces2. ExperimentalDmta AcqustonThe experimental setup used in this study, as depicted inig. 1, consists of a fluidized bed column made of quartzglass(42 mm i.d. and 700 mm tall), heating/cooling units, adata acquisition system, a differential pressure transducer,gas cylinders and flow controllers. High purity (99.999%)Cod asgases of helium(He) and argon(Ar) are used as the fluid-Fig. 1 Schematic diagram of the experimental apparatusizing gases, whose physical properties are given in Table 1The gas flow rate is controlled with a series of rotametersTable 1' Physical properties of the gases used(1and a digital mass flow controller(Fathom Technologiesd/×10mGR series). All the flowmeters are carefully calibrated withHeliuma Wet Test Meter(GCA/Precision Scientific) for each kind0.358"Data from CRC Handbook(2004):of gas used. a sintered quartz plate serves as the gas dis-tributor. The pressure drop across the whole particle bed is 1 Mean free path at 298.15 K and 1 bar:kTmeasured with a differential pressure transducer(Omega163 series). The pressure signals are collected by a In the tests, pressure drops across the entire bed andcomputer with LabVIEW DAQ program. A gravity convec- the bed height are measured simultaneously with de-tion oven(R International, 1300GM series)serves as creasing gas velocity(ug). The measured pressure dropsting unit,as and the loaded particles from room temperature up to are used to obtain the normalized pressure drop275.C. An advanttage of using this type of oven is that it [/( msg/S)), which is defined as the ratio of the meas-has a transparent window made of Pyrex glass, through ured pressure drop across the bed(AP)over the normalwhich the fluidization states can be clearly observed even pressure drop due to the solids weight(msg/S). The ob-under heated conditions. a cold bath with liquid nitrogen at served bed height is used to calculate the average be196 C or a saturated solution of dry ice in methanol at voldagcharacterize the bed expansion be-78C is used for the low temperature conditions. In each中国煤化工 Ascension. Both therun, the unexpanded bed height( Lo)is fixed at 80 mm, normalCNMHGVerage bed voidageabout two times of the inner diameter of the fluidized bed are pacolumn(D), and the particle bed is loosened with a flow of Generally, a good fluidization quality means a highergas at 2.0 cm. 'for 10-20 min prior to the fluidization ex- pressure drop with a fixed powder inventory and at theCHINA PARTICUOLOGY Vol 4, Nos 3-4, 2006same gas velocity(no or less channeling), a lower mini- Our experiments showed that to fluidize this group Cmum fluidization velocity (easily fluidizable )and a higher powder is very difficult because of its cohesive naturebed expansion ratio(better gas-solid contact). Hence, a Channeling was a severe problem occurring in the exbetter fluidization state should give a higher pressure drop periments using this powder with both gases, which pre-and/or a larger bed expansion ratiovented a completed fluidization of the entire bed. It isTwo types of particles, i.e., Al2O(4.8 um in average size) however clear that the fluidization behaviour of this groupand glass beads(39 um in average size ), pertaining to C powder varies greatly with the type of fluidizing gas.TheGeldart's group C and A, respectively, are employed in this use of argon leads to higher pressure drops and largerwork. Table 2 gives the key physical properties of these average bed voidages, especially at low gas velocities,two types of particles, The angle of repose and the bulk suggesting better fluidization qualitydensities(aerated bulk density Pa and tapped bulk density The fluidization behaviour of the group A particles of(PT-N). termined with a Hosokawa Powder Tester 39 um glass beads in these gases is depicted in Fig3The results of the bed pressure drop indicate the fluidization of the entire particle bed with both types of gas, theTable 2 Particles used in the experimentsnormalized pressure drop attaining unity once the gas ve-ParticlesDensity /kgR. AoR Geldart locity exceeds the minimum fluidization velocity(umt). It ish /. /deg. group also found that the fluidization behaviour of the group AAl2O34.8 3850 1610 790 2.04 52.6 c powder differs with the different types of gas used In argonhigher pressure drops and larger average bed voidages392500151013601.1133.5were observed at almost all gas velocities in the testingweighted mean diameter by laser diffraction(Malvern Mas- range. Therefore, the type of fluidizing gas also signifi-cantly influences the fluidization of group A particles, andne silica glass beadsthe fluidization quality of the particles displays a similarAnalyzed by a Hosokawa Powder Tester.priority as that for the group C particles, i.e., argon>helium3. ResultsGlass Beads (39 um)3.1 Influence of fluidizing gasFigure 2 shows the normalized bed pressure drop andaverage bed voidage as a function of the superficials velocity for Al2O (4.8 um)in different gases at 298 K. 3AA2O3(48m)Glass Beads(39 um)0560.75Superficial gas velocity /cm-s074Fig 3 Variations of (a)the normalized bed pressure-drop and (b)theaverage bed voidage with the gas type for the 39 um glass0.73中国煤化工0.1Superficial gas velocityCNMHGure on the fluidizationig. 4 illustrates the relaFig. 2 Variations of (a)the normalized bed pressure-drop and (b) the tionship between the average bed voidage and the in-situaverage bed voidage with the gas type for the 4.8 um al2O3particles at ambient temperature.superficial gas velocity of helium during the fluidization ofXu& Zhu: Effects of Gas Type and Temperature on Fine Particle Fluidization117077473KA2O3(48wm)Glass Beads (39 um)373K298Kin Helium-O-o-ooooOCO0o-o071001In-situ superficial gas velocity /cm-sperficial gas velocity /cm-sFig. 4 Average bed voidage against superficial gas velocity during Fig. 6 Normalized bed pressure-drop against superficial gasfluidization of the 4.8 um Al2O3 in helium at various tempera-during fluidization of the 39 um glass beads in heliumtures(77-473 K)ous temperatures(77-473 K)Glass Beads (39 um)Glass Beads( 39 um)in Helium7-373K473K0.55In-situ superficial gas velocity /cmFig 5 Average bed voidage against superficial gas velocity during Fig. 7 Effect of temperature on um for the 39 um glass beads in helium.fluidization of the 39 um glass beads in helium at varioustemperatures(77-473 K)the 4.8 um Al2O at various temperatures ranging from 77 shown in Fig 8, the interparticle forces (IPFs)increase withto 473 K. It shows that a higher temperature generally temperature. We thus believe that the observation of rela-leads to a larger bed voidage at the same in-situ superficial tively larger bed voidages at a higher temperature( Figs. 4velocity, although this trend is less significant compared to and 5)is due to the enhanced tensile strength of the partithat shown in Fig. 5 for the 39 um glass beads. Some cle bed hy the inrreacad ipEc differences have also beenreasons for the effect of temperature on bed voidage may obser中国煤化工 s versus the in-sitube considered. It has been commonly believed that the supCNMHder at various tem-voidage of a settled bed and the bed expansion during peratulfluidization are related to the tensile strength of the bed The results of the average bed voidages for the 39 ummaterials: a large bed voidage would be obtained upon in- glass beads fluidized in helium under various temperaturescreasing the tensile strength of the particle bed. As will be are shown in Fig. 5. For this group A powder, the tem-118CHINA PARTICUOLOGY Vol 4, Nos 3-4, 2006perature also has an influence on its bed expansion be- The relative importance between gas viscosity and gashaviour: a higher temperature results in a larger bed voi- density on the fluidization of fine particles may be exam-dage at a fixed superficial gas velocity, in particular for thelow gas velocities. Meanwhile, the pressure drop data inined by introducing the Reynolds numbfluidization of the 39 um glass beads under variousinto Eq (1),peratures are illustrated in Fig. 6. A complete fluidizationhe particles was achieved at any temperature within the1501-4)2F+1751-4)“2R2testing range once the superficial gas velocity exceeds Umf,the value of which is however dependent on the bed tem-perature. The dependency of Umt on the temperature is For fine particles with a small dp, the Reynolds number isshown separately in Fig. 7, from which the following con- generally much smaller compared with that for the largerdecreases with increasing size particles. As a result, the second term on thetemperature.right-hand side in Eq (3)is negligible, and the first term onht-hand side in Eq. 3 )is dominant, so4. Discussionequation can be approximated as4.1 Gas propertiesTheoretically speaking, fluidizationplace at Umfwhen the weight of a particle bed is sued by the dragforce due to an up-flowing gas stream.particle bed at1501-42a gas velocity at or lower than umf, the pressure drop(4P)62of the gas flow across the bed of a height L can be esti- Apparently, this equation does not contain gas density, andmated from the well-known Ergun equation(Ergun, 1952), it represents the viscous effects of the fluidizing gas onhich is expressed aspressure drop When plotting the gas viscosity (ag)againsttemperature(tin terms of the previous Eq (2)for the twoAP-150(1-6 2.1.75(1-6)Pu3.(1)gases, Fig 8 is obtained. As shown in the iure, the vis-dragcomities of both gases increase with increasing temperawhere ug is the superficial velocity of the fluidizing gas, dp ture, and argon is more viscous than helium at any a givenis the mean particle diameter, is the particle sphericity, temperature. Now, it becomes very clear that for fluidiza-Ag is the gas viscosity, p, is the gas density and e is the bed tion of fine particles (groups C and A), the gas viscosity isvoidage. This equation suggests that the viscosity and the dominant over the gas density in influencing the pressuredensity of fluidizing gas play important roles in the fluidize- drop and hence umf. The corresponding results on the in-tion of fine particlesfluence of bed temperature i.e., higher bed pressure dropsA higher pressure drop under the same gas velocity and smaller Umt were observed at elevated temperatures,suggests a better fluidization quality. For example, the bed as shown in Figs. 4-7, can be accounted for by the factexpansion of some group C powders was foundthat gas viscosity is increased with temperature. Also, it iscrease as the gas viscosity increases by using different100fluidizing gases(Geldart et al., 1984). According to Eq (1),the pressure drop in the fluidization of fine particles may be80enhanced by increasing either the gas viscosity Ag or thegas density P. However, the situation is not so simple forfluidizing fine particles at various temperatures. On the onehand, the gas viscosity increases with temperature by thefollowing correlation(CRC Handbook, 2004):267×1020where u is in the unit of (uPa.s), Mg is the molecularweight of the gas(in g-mor")and d is the diameter of gasmolecules(in m). Eq.(2)suggests that the fluidizationquality would be improved as the temperature increasesOn the other hand the gas density p decreases with in-creasing temperature, decreasing the pressure drop. Thus中国煤化工gas viscosity and gas density are competing factors in afCNMHG-8009001000fecting fluidization so that a simple conclusion that the fiuidization quality improves as gas viscosity increases can-Temperature /Knot be made easily8 Dependency of gas viscosity onXu& Zhu: Effects of Gas Type and Temperature on Fine Particle FluidizationTable 3 Dependency of minimum fluidization velocity (Um)on density ticulate materials is normally assumed as Zo= 4A=4x10and viscosity of the fluidizing gases for the 39 um glass m at the point of contact( Krupp Sperling, 1966). On theother hand however, quantifying the surface asperity ra-Fluidizing Gas Mg/g-mor1 A/kg-m#/uPa.s Umt/cm-s"dius fa proves to be controversial. According to the4.000.16419.9 0.35+0.02 well-accepted theory established by Molerus(1982)and1.63322.7 0.21*0.02 Seville et al. (2000), fa can be approximated by the radiusof the primary particlesconsistent with the experimental results, asprevious Figs. 2 and 3, that the fluidizations如in the It has been demonstrated by Xie(1997)that the attracis im- tion energy between two solid particles increases due toproved with higher pressure drops for both the groups gas adsorption, and the interparticle attraction forces be-and A powders, and that a smaller umt for the group a tween the two particles with gas adsorption are corre-powder was obtained by using more viscous gas (i.e, Ar), spondingly enhanced to a value ofas shown in Table 3IPFs Aa 1+4.2 Interparticle forces12(Az0Interparticle force is a significant factor in affecting fiu-The adsorption factor B in Eq. 8)is definedidization behaviours particularly for fine particles such asMgroups C and A powders. According to Piepers et al( 1984), physically adsorbed gases will increase the interparticle forces of powders, resulting in an increased elastic where 8 is the mass of gas molecules adsorbed per unitmass of the particle, d, and d2 are the diameters of particlemodulus of a fluidized bed and a growth in the bed expan. 1 and 2, respectively, Ms and Mg correspond to the mo-sion. Thus, in addition toroperties, the changes inlecular weight of the solid and the gas, Cog and css are theand their roles in varying the fluidization behaviour should London-van der Waals constants between gas-gas andalso be taken into accountsolid-solid. Assuming two particles are identical in diameter,Several types of interparticle forces, such as the van der i.e di=d2=dp, Eq (9)can be reduced toWaals force and the electrostatic force, may be simulta-neously present in a particle system. It has however beenB-An63 M. C9generally accepted that the van der Waals force is moresignificant than the electrostatic force and other types ofThe London-van der Waals constants, Cgg and Css,caninterparticle forces for most types of dry particle systems be obtained through( Gregory, 1969)(Baems, 1966: Visser, 1989). Therefore, in the following(11)discussion we assume the van der Waals force to be thedominant interparticle forces(IPFs). According to KruppMCss=ANoPs(12)(1967), the van der Waals force, which is assumed to bethe dominant interparticle forces(PFs), between two par- where s is the depth of the well of the energy curve be-ticles without gas adsorption or deformation can be esti-tween two molecules o is the separation at which the inmated asTable 4 Electrical and optical properties of the gases usedwhere ho is the Lifshitz-van der Waals constant and ZoA/K1022the distance between the two particles. Here R1.2 is de-342fined as100006510005172Lennard-Jones(12, 6)potential parameters from Hirschfelder et alwhere p1 and rp2 are the asperity radii for the two particlesAssuming fp1=p2=fa, and substituting this into Eqs. (5) Table 5 Physical constants of the gases and solids used in this workand (6)while replacing ho with the Hamaker constant11×10(Ao=A)(Krupp& Sperling, 1966; Krupp, 1967), one o for Ako00012中国煤化工00067×10-0 for glass/PFs=AHaCNMHGbeadsClJm°275×10- for Al2113×107 for glassThe distance between two particles in a settled bed of par-Data from Cottaar and Rietema(1986):"Estimated.CHINA PARTICUOLOGY Vol 4, Nos 3-4, 2006ter-molecular potential is equal to zero, No is the(a)Al, o, (4.8 um)Avogadro's constant(=6.022x102 mol)and A is thedensity of the solid particles. According to Hirschfelder et aland of for the two gases are given in Table 4. Hence, the a 10London-van der Waals constants, Con and Css for variousgases and solids used in this work can be tabulated in table 5In addition to gas adsorption, temperature may also in-fluence the interparticle forces through Hamaker constant5006007008009001000AH. According to Israelachvili (1992), AH is dependent on 9temperature through the relationshipb) Glass beads③39pm)3(13)103where k is Boltzmann's constant( =1.381x10'J-K-),T gis the absolute temperature &1 and n are the dielectricconstant and refractive index of the particulate materialsrespectively. The values are &1 =10.1 and n1 =1.753 forAl2O3, and 4=7.6 and n1=1.5 for glass beads. eo and noare the dielectric constant and refractive index of the me-dium where the particles are present; and the values of ao600700and no for the gases used in this work are also given in Fig 9 Effects of gas aderature/Kand temperature on interparticleforces(IPFs)the main electronic absorption frequency in the UV regiontypically around 3x10s-1.Hence, Eq (13 )can be written key parameter that influences the bed pressure drops andUmf in fluidization of fine particlesA=1036×10237541+2605×1019C1+Eo5 Conclusions(14)(1)For both powders tested, i.e., 4.8 um Al2O3(group c)qs. (8 -(14)can then be used to evaluate the effects of and 39 um glass beads (group A), the fluidization quality ingas adsorption and temperature on the interparticle forces. argon gas is better than that in helium in terms of bed ex-For the contacting systems in this work, i.e.,(Al2O3 -(Al2O3) pansion, bed pressure drops andand (glass beads) -glass beads), the relative interparticle(2)In fluidization of both powders in helium at temperaforces in relation to the grativational forces of the particals tures ranging from 77 to 473 K, an elevated temperaturein various gases as a function of temperature are shown in usually leads to enhancement in fluidization quality, suchFig 9. From the figure, on the one hand, it is obvious that as with larger bed expansion. For the 39 um glass beadsthe gas adsoption does significantly influence the IPFs, particles, an increased temperature results in a decreaseand the values of IPFs for each type of particle system at a in Umfsame temperature vary greatly with the type of fluidizing 3) The interparticle forces(varying with gas-type andgas. In both particle systems, the relative interparticle temperature) may play an important role in fine-particleforces are larger in argon than in helium, which is fluidization by affecting the expansion behavior of the paconsistent with the observations shown in Figs. 2 and 3, ticle-bed, while gas viscosity (varying significantly withwhere the bed expansion of both the 4. 8 um Al203 particles gas-type and temperature)is the key parameter that inand the 39 m glass beads is larger in argon than in helium fluences the principal behavior of fine particle fluidization(the larger IPFs and hence the increased elastic modulus i. e, the bed pressure drops and umfof a particle bed would result in a growth in the bed ex-pansion). Fig 8 also reveals that for both types of gases Acknowledgementsand both particle systems, the values of IPFs increaseslightly with temperature, which appears to be consistentThe authors are grateful to the Ontario Research and Dwith the observations that bed voidages generally increasethis study, throughearc中国煤化工zhith increasing temperature( Figs. 4 and 5). Therefore, theIPFs due to the van der waals forces may play an imporCNMHGtant role in fluidization of fine particles by affecting the bed Nonieiitlexpansion behavior of the particles On the other hand as AHHamaker constant, Jsuggested in the previous discussion, gas viscosity is theangle of repose, degXu Zhu: Effects of Gas Type and Temperature on Fine Particle FluidizationBparameter defined by Eq (9)influence of gas adsorption on interparticle forces in powders. J.London-van der Waals constant between subColloid Interface Sci. 109. 249-260Cdd中DhCRC handbook of chemistry and physics(84 ed. )(2004). 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E.& Rietema, K(1986). A theoretical study on the Manuscript received December 20, 2005 and accepted Apr/12,2006