Efeitos Da Altura Do Sparger e a Direcao Dos Furos

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Effects of Sparger Height and Orifice Orientation on SolidsDispersion in a Slurry Bubble Column

BIMAL GANDH I, ANANDPRAKQSH*and MAURICE A. BERGOUGNOU

Department of Chemical and Biochemical Engineering, Th e Universityof Western Ontario, London, ON N6A 5B9,Canada

The effects of gas distributor height and the orientation of its orifices are investigated on solids dispersion and gasholdup profiles in a three-phase slurry bubble column. The height of the distributor was varied to cover locationsfromnear column bottom to above the settled solids bed height. The orifice orientations were changed from upward facing t odownwards facing directions. The measurements were conducted in a Plexiglas column of0.15 m D and 2.5 m height.The gas phase was oil-free compressed air while tap water was usedas liquid phase. Glass beads with an average particlediameter of 35 pm and density of2450 kg/m3 constituted the solid phase. The settled bed height was about0.4 m whichprovided an averageslurry concentration of about15 (v/v) when all solids were dispersed.Both axial and column averagephase holdups were measured. Effects of sparger location, gas jets form ation and liquid circulation pattern s on gasholdups and solids dispersion are analyzed. Empirical correlations are developed to relate sparger location to solids dis-persion as a h ncti on of gas velocity. Optimum sparger height and orifice orientation is proposed based on the measure-ment of this study.

On a etudie les effets de la hauteur du distributeur de gaz et de Iorientation de ses orifices sur les profils de disper-sion des solides et de retention de gaz dans une colonne a bulles en suspension triphasique. On a fait varier la hauteurdu distributeur afin de couvrir les differents emplacements, en partant du fond d e la colonne jusqua la hauteur d e lit de

solides sedimentes. Les orientations dorifices ont ete modifiees de la direction vers le haut a la position v ers le bas.Les mesures ont ete menees dans une colonne en plexiglass de0,15 m d e diametre interieur et de2,5 m de hauteur. Laphase gazeuse etait de Iair comprime sans huile tandis que de Ieau du robinet etait utilisee comme phase liquide. Desbilles de verre ayant un diametre de particule moyen de5 pm et de m asse volumique egale a2450 kg/m3 constituaientla phase solide. La hauteur de lit fixee etait denviron0,4 m e qui donnait une concentration moyenne de boues denv iron15 en volume lorsque tous les solides etaient disperses. Les retentions de phase axiales et moyenne pour la colonneont toutes deux etaient mesurees. Les effets d e la position de Iaerateur, de la formation de jets d e gaz et des profils d ecirculation des liquides sur les retentions de gaz et la dispersion de solides sont analyses. Des correlations empiriques sontetablies pour relier la position de Iaerateura la dispersion de solides en fonction de la vitesse de gaz. On propose unehauteur daerateur et une orientation des orifices optimales a partir des mesures de cette etude.

Keywords: sluny bubble column, sparger height, orifice orientation, solids dispersion.

he slurry bubble column reactor isan important multi-

T hase particulate system used for a number of processesin chemical, petrochemical and biochemical industries(Deckwer, 1985; Fan, 1989; Dudukovic and Devanathan,1992). The advantages offered by slurry bubble columnsinclude: high liquid (slurry) phase con tent for reactions totake place, reasonable interphase mass transfer rates at lowenergy input, high selectivity and conversion per pass,excellent heat transfer properties and easy temperaturecon-trol (isothermal operation), and online catalyst addition andwithdrawal. Also, there is a low maintenance requirementdue to simplicity in con struction and absenceof any movingparts. Some of the drawbacks of slurry bubble columnsinclude: considerable backmixing in both the continuousliquid (sluny ) phase and the dispersed gas phase, low volu-

metric catalyst loading, bubble coalescence , and difficultiesin scaling up. The productivity of catalytic slurry bubblecolumn reactors could be improved by increasing catalystloading. This can, however, lead to regions of po or mixingand ma ss transfer, especially in the distributor region. Th edistributor design therefore, is expected to play an importantrole in proper design and operation of slurry bubblecolumns.The placement of the gas distributor system is alsocritical foran effic ient and trouble Free operation of the slurry

Author to whom correspondence may be addressed. E-mail address:[email protected]

bubble column reactor. The placement of a sparger with

downward facing orifices too close to the column ba se canlead to eventual erosion of the base place. Moreover,forhigher solids corrcentration systems, there may be startupproblems due to solids plug formation (Gandh i, 19 97).

There is currently a lack of information on the hydro-dynamic behavior of slurry bubble columns with varyingsparger heights which can lead to optimum sp arger location.This study investigates the effects o f sparge r height, orificeorientations and superficial gas ve locities on hydrod ynamicsof a slurry bubble column. The hydrodynamic parametersinvestigated are solids dispersion and gas h oldup.

Experimental

Experiments were conducted in a Plexiglas colum n whichhad an inner diameter of0.15 m and a total height of2.5 m.The columnwas designed with four sections for easy con-struction and flexibility (Figure 1). The gas phase was oil-freecompressed air. Filtered air passed through a sonic nozzleand entered the colum n through a gas distributor at the bottomof the column. The sonic nozzle provided the advantageofa controlled air flow whichis independent of downstreampressure (which would fluctuate during experimentalruns).The air flow rate was varied by adjusting the pressureupstream of the sonic nozzle with a pressure regulator. Thesuperficial gas velocity was varied between 0.05d s nd0.28 m/s based on ambient conditions. Air exited the column

THE CANADIAN JOURNALOF CHEMICA L ENGINEERING, VOLUME77, APRIL, 1999 383


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Section 4(0.30m)

Section t 3(0.50m)

Section # 2(1.20m)

Section W1(0.50m)

/

rl.D. = 0.15m

CyclonicSeparator

iI I

E Sluny Sampling Taps(9.53mm)

..1 I A i r

Pressure Taps(6.35mm) Drain

Sonic Nozzle

Figure 1 etailsof experimentalsetup.

top via a fume ho od. Prior to exiting in the fume hood, theair passed through a cyclonic separator and bag filter toremove any fine particulates which may have been entrained.

Tap water was used as the liquid phase for both two phase(G-L) and three phase (G-L-S) systems. Since the system

was operated in batch mode, the static slurry height wasmaintained at 1.5 m above the bottom of the column. Glassbeads with an average diameter of 35 pm and particle den-sity of 2452 kg/m3 constituted the solid phase. The settledsolids bed height was about 0.395 m giving an average slurryconcentration of about 15% (v h ) solids.

The gas was d istributed at the column bottom through afour arm sparger with orifices facing downwards or upwards.The distance between the orifice and the column bottom(base plate) could be adjusted with a special arrangementshown in Figure 2a. It consisted of a Plexiglas ring, with itsinner diameter flush with the gas inlet pipe. The spargercould be moved up or dow n with respect to the ring and anO-ring prevented any leakage of slurry. The following pro-

cedure was followed for a djusting sparger height: first thescrews on the Plexiglas ring w ere loosened, then the spargerheight adjusted by either raising or lowering the sparger andfinally the screwson the Plex iglas ring were tightened. Theheight of the orifices from the ba se plate was varied between0.015 to 0.45 m. Figure 2a shows the sparger with down-ward facing holes. For upward facing holes, the spargerarms were turned up by 180 . Eacharm of the sparger hadfive orifices of 1.5 mm diameter. The orifices were spacedas shown in Figu re 2b, based on the criterion of uniform dis-tribution of gas across column cross-section.

Average and axial gas holdups were measured by thepressure profile technique using manometers located at

S p a i e rAdjustable)eight 550Column Wall

PlexiglassRing

___-________--------------crew0-nng

Air from sonic nozzle

Figure2a djustableheight sparger designdetails.

Sparger Arm

Column Wall /

0

O IjI

n

Orifice(diameter = 1 5 mm)

Figure2b - op view of sparger with orificespacing.

approximately 0.05, 0.25, 0.45, 0.75 and1.15 m, above thebase plate. To avoid plugging of pressure ports by fine solidparticles, a U-tube manometer system with air backflushingwas used. Each rotameter allowed a small amount of air toenter the column, thereby preventing liquid and/or solidsfrom entering the lines. To minimize any errors due to fric-tional pressure drop in the back flushinglines, the length oftubingfrom the tee splitter to the column wall was minim ized

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and kept constantfor all pressure taps. The pressure gradientcan be related to the manometer pressure differential by:

Column Wall

w I165 mm ;

P = p,p( ). . . . . . . .A 2

. . . . . . . . . . . . . . . . . . . . (1)

Where p,is the density of the manometer fluid,Ay is themanometer pressure d ifferential andAz is the height differ-ence of the pressure taps. However, for the three-phase flu-

idized system, pressure gradient is also defined as:

. . . . . . . . 2)

where pd is the dispe rsion density. Since water was used asthe manom eter fluid, a relationship between dispersion den-sity and pressure profile can be o btained:

Pd = -Ow *y ). . . . . . . . . . . . . . . . . . . . . . . . . (3)For bubble column systems (G-L), the gas holdup

between two adjacent pressure taps can be directly correlatedfrom the ratio of pressure differential to the height differ-ence of the pressure taps as:

. . . . . . . . . . . . . . . . . . . . . . . . . . . .g = +(Ay ) 4)

For slurry systems, Equation(2) can be rearranged togive:

or.

Pd - PSI& =- . . . . . . . . . . . . . . . . . . . . . . . . . .Pg PSI

Finally, for low pressure operations the g as density(p,) issmall compared to slurry density(p,,), therefore:

Thus, the gas holdup in a d ifferential section of the col-

umn Az), ould be calculated from Equations3) and (6) ifthe slurry density in this section was alsoknown. Therefore,slurry samples were taken along the co lumn height and slurrydensity was determine d by the pycnometric technique. Theset of slurry samples withdrawn along the column heightgave an axial profile of the slurry density. From this profileand Equations 3) and (6 ) , he axial gas holdup profilein thecolumn was determ ined. The averag e gas holdup in the col-umn was calculated from the pressure difference betweenthe top and the bo ttom pressure taps and the a verage slurrydensity obtained from total solids dispersion. Since back-flushing introduced a small amoun t of air into the system,tests were performed to m easure the e ffect of back-flushing

.-t

- . J9 53 mm

Slurry Sample

Figure 3 -Details of slurry sampling probe design and operation.

on average gas holdups. Measurements were made for agas-liquid system with air back -flushin g and with back-flushing turned off. The effects of back-flushing were foundto be less than1%.

Local slurry concentrations were obtained from slurrysamples withdrawn with a specially designed samplingprobe shown in Figure3. The sampling probe was designedso as to avoid entrainmentof gas bubbles and prevent solidsfrom settling within its shaft. Five sampling ports wereavailable along the axial column heightto collect slurry

samples. They were located at0.05, 0.25, 0.65, 1.05 and1.45 m respectively, above the base of the column. Eachsampling probe consisted ofan outer sleeve and a pistonrod. Figure 3 shows the probe assembly and direction offlow of slurry sample. As can be seen in Figure3, when thepiston rod was pulled outward the slurry flowed out of thenozzle pointing downwards. After collection of the slurrysample the piston was pushed back into the sleeve to stopthe flow of slurry. A brush attach ed at the tip of the pistoncleaned the sleeve when the piston was pushed back.Samp les sizes of75 mL to 100mL were withdrawn and thesolid fraction in slurry sampley,) was obtained by the pycno-metric technique. Average solids dispersion density wasdetermined by measuring static solid heights prior to and

during experime ntalruns.Results and discussion

Measurements were made for solids dispersion and gasholdups for varying sparger heights. The spa rger was abov ethe initial bed of solids when positioned at0.45 m and0.40 m from the bottom, and within the bedfor other spargerpositions. When air was sparged into the column someofthe settled solid got dispersed, causing a reduction in theset-tled bed height. Figure4 ives the settled solid bed height a sa function of superficial gas velocity for varying spargerpositions for the downward facing orifices.A relativelyeven interface between the solid bed and slurry could be

observed for different conditions, allowing visual measure-ments of the settled bed heights. For a given sparger posi-tion, the settled bed height decreased with increasing gasvelocity due to the dispersion of the solids. The total amo untof solids dispersed had two components; the solids dis-persed from above the sparger and those dispersedfrombelow it. Dispersion of solids from above the sparger wasalways complete and dependent only on the sparger posi-tion. This is illustrated in Figure4, where the heightof theundisturbed solid bed is alway s below the sparger positionat any given velocity. When the sparger was positioned at0.45 m, there was practicallyno dispersionof solids at thesuperficial gas v elocity of0.05 m / s . The distance between

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Spargar I 0.40mA Smrasr at 0.35m 1 o

0.8

PHi5UI 0.6P5u

-6C. 0.4PLL

0.2

0 0

0.15 -/ ------v

,

0.05

0 00

0 00 0.05 0.10 0 15 0.20 0.25 0.30SuperficialGas Velocity (m/s)

Figure 4 ettled solid bed height for various sparger positionsand superficial gas velocities (downward facing orifices).

the sparger and the solid bed was about 0.065 m, indicatingthat any effect of bubbles generated turbulence and liquidrecirculation extended to a lower distance. Som e reductionin the settled solid bed height was, however, observed at thesame gas velocity when the sp arger was positioned at 0.40m.The distance between the sparger and the defluidized bedwas about 0.035 m, indicating that the effect of the gas bubblesand the turbulence induced by it extended to about 0.035 m.The length to which the so lid bed height was reduced for thesuperficial gas veloc ity of0.05 m / s remained substantiallyconstant (at about 0.035 m) fo r all other sp arger positions.As the gas velocity was increased further, the dispersion ofsolids increased and the solids bed height decre ased. It can

also be seen in Figu re 4 that the se ttled solid bed heigh t linesare almost parallel to each other for different spargerheights. This implies that the amount by which the settledsolid bed height is reduced from below the sparger is mainlya function of the gas velocity and relatively independent ofthe sparger position. However, when the sparger height isdecreased, there is higher dispersion of s olids from the bedleading to a maximum slurry concentration of 15% (v/v)when all solids were dispersed. Therefore, it may be con-cluded that for the downward facing orifices thereis no sig-nificant effect of slurry concentrationon solids dispersionup to the highest slurry concentration (15% v/v) used in thisstudy.

With increasing gas velocity through sparger orifice,

there is a tran sition from the bubbling regime to gas jettingregime. The transition from uniform bubbling to bubble coa-lescence and gas jetting h as been reported in the literature(Leibsan et al., 1956; Ozawa andMori, 1983; Rabiger andVogelpohl, 1983 ). For a superficial gas velocity of0.05 m / sthe orifice Reynolds number was about 2500, indicatingoperation in the bubble coalesce nce regime (Leibsan et al.,1956) between uniform bubbling and gas jetting. For thesuperficial gas velocity of 0.1 m / s , the orifice Reynoldsnumber was ab out 6000, indicating transition to gas jettingregime (Leibsan et al., 1956).For the downward pointingorifices, the solids dispersion below the sparg er is aided bythe momentum of gas jets. The dispersion of solids would

hI I I I

0.0 0 1 0.2 0.3 0 .4 0.5

Sparger Height from Base Plate (m)

Figure 5 -Fractional solid dispersion with varying sparger heightfor downward facing orifices.

increase with increasing gas velocity, due to increasing gasjet penetration and upward solids entrainm ent by the risinggas bubbles. The kinetic energy of the gas jets (1/2p,V2would increase with increasing gas velocity. The gas bubglesformed from the penetrating gas jets, create an upwardmomentum of the suspension.In turn he suspension travelsup the column center and back down at the column walls,creating circulation patterns. If there is eno ugh kinetic energyin this recirculation, solids may also be entrained and dis-persed: T he length of the gas jets and the recirculation patternin the liquid both increase withan increase in gas superficialvelocity. This can account for the increased dispersion (smallerundispersed bed) with the gas veloc ity seen in Figure4.

Local slurry samples were taken along the column withsampling probes #2 to 5 (w herever possible). The averageconcentration of the dispersed solids could be calculatedfrom these local slurry concentrations. Figure 5, shows theamount of dispersed solids in the slurry, (represented as afraction of the maximum solid concen tration), as a functionof the gas supe rficial velocity, at different sparger location s.Nearly com plete solids dispersion could be achieved up to asparger height of about 0.1 m at the highest gas velocities(>0.2 d s ) . For sparger heights above0.1 m, complete dis-persion of solids couldnot be achieved. Figure5 can be usedfor a quick estimation of sparger height for uniform disper-sion of solids at a given velocity and vice versa.

The distance between the sparger and the undisturbed

solid bed (Ldis),could be related to the modified Froudenumber based on the orifice velocity and orifice diameter.Figure 6 presents the distance of the settled solid bed fromthe spargeras a function of the modified Froude number forvarious sparger positions. Data for theruns with gas super-ficial velocity varying from0.10 m / s to about 0.25 m / s , andwith sparger positions ranging from 0.45m to 0.10 m abovethe bottom, have been included in this figure. It is seen fromFigure 6, that LdiS is relatively independentof the spargerlocation (as indicated by the 95% confidence interval),except for the sparger position of 0.45 m from the bottom.However, L , for the sparger location at the 0.45 mcon-verges with those for the other sparger positions at high

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0.14

-E 0.12

p 0.10B

; .085k

n_

r

g 0.06v )

6

852 0.02

0 04

Sprger at 0.35mA S p r g e r at 0.25mv Sprger at O.15m+ S p r g a r at 0.10m

0.00 I I I I I I I I200 400 600 800 1000 1200 1400 1600 1800

Modified Froude Number K O )

Figure 6 elationship between modified Froude number anddistance between the sparger and the settled solid bed for thedownward facing orifices.

Froude number(or high g as velocity). At low su perficial gasvelocities, there was very little dispersion of solids for thissparger height since the gas jets cou ld not reach the bed andsolids were dispersed mainly by turbulence in the regionabove the bed.

There is very little informationin the literature on g as jetpenetration length in gas-liquid or gas-liquiholid flu-idized bed systems. However, extensive workhas been donein gassolid fluidized beds and several correlations of jetpenetration length have been proposed (Zenz1968; Meny,1975;Wen et al., 1982; Yates et al., 1988; Benjelloun et al.,1995).The correlations proposed by Yates et al.(1988)andBenjelloun et al. (1995) for vertical downward jets weretested against the data of this stud y.

Yates et al., (1988):

Benjelloun et al.,(1995):

Figure7 comparesL , for the g iven gas-liquid-solidsys-tem, with the ga s je t lengths,L . for vertical downward jetsin ga sso lid systems predictebby the above correlations. Itcan be seen thatLjetpredicted by the above correlations, areabout half of the d ispersed solids height below the sparger.It may be noted thatLdjs is a cu mulative effect of dispersionby the g as jets and dispersion of solids by liquid recircula-tion and bubble turbulence. It was pointed out earlier, thatbubble formation would extend its influence up to about0.035 m of bed height. If this value is subtracted fromL,,the predicted values ofL.etbecome close (within10 ) toexperimental values.For &e dataof this study, a correlation

0.02

0.00 I I I I I I I I200 400 600 800 1000 1200 1400 1600 1800

Modified Froude Number (Fr;)

Figure 7 - omparison o distance between sparger and settledsolids bed height with gas jet penetration length predictions by lit-erature correlations for ga ss ol id systems.

was also developed to predictLdi,?based on a modifiedFroude number using orifice velocity(FrJ. This correla-tion can be directly used to estimate dispersed solid bedheight below the sparger:

. . . . . . . . . . . . . . . . . . . . . ., = 0.0082(FJ,)0.335 (9)In order to separate the effects of gas jet penetration and

liquid recirculation patterns, additional experiments wereconducted with the sparger orifices facing upwards. Un likethe flat bed surface observed for the downward facingsparger, the settled solid bed surface wa s dish-shaped whenthe orifices were facing upwards. This indica ted that solidsdispersion was aided by liquid (slurry) recirculation flow.Since the bed surface was not flat, it was not possible tomake visual measurements of the heightof the solid bed.The total amount of solid dispersed was estimated from theconcentration of the slurry samples withdrawn at variousaxial positions along the colum n. For these calculations, thecolumn was divided into five sections. These sections werechosen so that each slurry sampling probe location wasroughly in the middle of the resp ective section (except thetop section). Thus, it was assumed that the a verage solid con-centration of each section was that m easured by the samplingprobe. The average gas holdup in each section was calculatedby using the taps closest toor at the top and bottom of thesection (taps at 0.05 m and 0.15 m for section 1, taps at0.15 m and0.45 m for section2, taps at 0.45 m and 0.95 mfor section3, and 0.95 m and 1.15 m for section4). It wasassumed that the gas holdup for section5 was the sameas insection4. Thus the total amoun t of solid dispersed could becalculated as:

where subscripti denotes a section. Since the system wasoperated at 15 (v/v) solids, the total amount of solidsadded ( M J was known. F rom the initial settled height of the

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Sprpar at 0.25mA sp r p a r at 0.15m

Sprper at 0.10m+ 8 p r p ~ a I O . O S mSparpar at 0.OE.m

..-.-.-.-

0.05 -

0.00

A0.15

AA

0.10

w

I I I I

total amount of solids(HSi)he ave rage voidageof the solidbed was calculated:

. . . . . . . . . . . . . . . . . . . . . . . . .

The amou nt of solids remaining in the bed wa sM, - M d )and the height of the so lids remaining in the bed could thusbe estimated as:

. . . . . . . . . . . . . . . . . . . . . . . . (12)M i - Md )P sE sb A c

Hbed =

The expanded height of dispersion was estimated bytaking the average of three readings over an interval of about10 min. The maximu m deviation between the three readingswas found to be less than 2%. In order to find the errorsassociated with this estimation procedure, the solids bedheight was calculated from the slurry samples collected forthe downward facing sparger. The deviation between theestimated and the measured solid bed heights for the down-ward facing sparger was found to be lessthan 5 for spargerpositions up to 0.35 m from the bottom.This error generallyincreased to abo ut 10% for higher sparger positions, due toerrors associated with measurement of samples with lowsolids concentrations.

Figure 8 shows the estimated height of the settled solidbed, estimated using Equation (12), for the upward facingsparger as a function of the superficial gas velocity andsparger location.As in the case of the downward facingsparger, there is an increased dispersionof solids from thebed with increasing gas velocity. Lowering the sparger alsoenhanced dispersion, as expected. An increase in the gassuperficial velocity implies a larger power input perunitvolume of slurry which cau ses stronger circulation patternsto develop in the column. As the kinetic energy in this recir-culation grows, more solids are e ntrained and dispersed. The

1 o

0.8

f0.6

u

80.4

0.2

0.0

VQ 0.05mlavp.o.11 mls

A Vg 0 . lSWavp=o.21 mls+ V ~ 0 3 7 m l s

95 % Confidence nterval

-

-

-

1 I I I

0.0 0.1 0.2 0.3 0.4 0.5

Sparger Height From Base Plate (m)

Figure9 - ractionalsolid dispersionwith varying spargerheightfor upward facing orifices.

fact that there was dispersionof solids from below thesparger for the upwards pointing orifices, indicates that liquidcirculation also plays a role in solids dispersion. It wasobserved that there wasno dispersion of solids when thesparger was at0.45 m, thus ind icating that the e ffect of theliquid circulation and turbulence did not e xtend to0.065 mbelow the sparger. It can be observed from Figure8 thatmaximum dispersion of settled bed height was less than0.06m for all cases. For the sparger positions below0.10 m,the gradient of decrease of the solid bed height with theincreasing superficial gas velocity became smaller. Thiscould be attributed largely to a reduced recirculation rate

due to inc rease in slurry concentration. The solids dispersiondata as a b c t i o n of the sparger position for the upward facingorifices are presented in Figure9. Nearly complete dispersionof solids could be ach ieved only at the lowest sparger location(0.015 m from the bottom). However, even at this positionsmall pockets of solids were observe d to remain at thecol-umn bottom around the comers.

Figure 10 compares the settled solid bed heights for thedownward facing and upward facing spargers. It can be seenthat at any given superficial gas velocity and sparger posi-tion, the amount of dispersion from the solid bed is less inthe case of the upward facing spargerthan the downwardfacing sparger. This indicates that the effect of the liquid cir-culation pattern extends to a much smaller distance than thegas jet effects. Almost no dispersion was noted in the caseof the upward facing sparger at the lowest velocity(0.05 d s ) , implying that the liquid circulation developed atthis velocity is too weak to en train and disperse any solids.On the other hand, the downward facing sparger showedsome solid dispersion even a tthe lowest gas velocity whenthe sparger orifices are not jetting. This might be due to theturbulence developed by the bubbles forming at the orificeof the sparger. This effect extended to a lengthof about0.03 m to 0.04 m. As the gas superficial velocity wasincreased,LdjS increased for both the upward facing and thedownward facing spargers. At the highest gas velocity

388 THE CANADIAN JOURNALOF CHEMICAL ENGINEERING, VOLUME77, APRIL, 1999


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0.50

0.45

0.40

0.35

r 0.3081

0.25j .20v )

0.15

0.10

0.05

0.00

6p rgo ra tO . tO m m Sprgera t0. tOmA 8parg.r at 0.15 m A sparperat O.tSm

I Sprger a t O.25mv Sp.rg.rn1025m

0.00 0.05 0.10 0.15 0.20 0.25 0.30Superkial gar velocity W S )

Figure 10 - omparison of settled solid bed height for upwardand downward facing orifices (filled symbols for downward facing

and hollow symbols for upward facing orifices).

(0.275 m / s Ldiswas about 0.10 m for the downward facingsparger, while it was on ly about 0.04 m for the upward fac-ing sparger.

At any given superficial gas velocity the relative positionof the sparger from the column bottom determines theamount of solid dispersion that can be achieved.A regres-sion analysis basedon the fraction of the solids dispersedwfraC)and the superficial gas velocity yielded the following

expression for the position of the downw ard facing sparger:

dsp, ,,= 0.472 - .4721 yfiac 0.2737 Vg(R2= 0.97)

For the upward facing sparger, the relation between thesparger position, fractional dispersion and the superficialgas velocity had the form:

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . * (13)

= 0.4158- .38551 yifiuc+ 0.0607 Vg R2 0.97)(14)

The fractional solids dispersionis a measure of the amountof dispersion actually obtained relative to the maximumamount of dispersion that can be achieved, and is given as:

dYP,UP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . (15)SWfiac =-Wmax

The gas introduced at the sparger contains both kineticand potential (buoyant) power. The buoyant power istrans-ferred to theslurry (from the gas)as the gas moves upwardand expands. Moreover, it can be reasonably assumed thatthe gas ex pansion takes place un der isothermal conditions(Lamont, 1958). Thus, the potential power of the incominggas can be exp ressed as:

Wpot= PoQo In[2 = PoQoIn[ r dg]. . . (16)

0.18

0.16

0.14

7 0.12-0.10

0 .08

50.06

0.04

0.02

0.00

o .-------d _ _ _ _ _ _ _V n

0.0 0.2 0.4 0.6 0 8 1.0 1.2 1 .4

Axial Dislanca alona Column(rnl

Figure 11 olids concentration profiles for upward and down-ward facing orifices at superficial gas velocity of0.05 m/s (filled

symbols for downward facing and hollow symbols for upward fac-ing orifices).

The kinetic power in the inco ming gas is related to the gasvelocity at the orifice and can be calculated as:

(17), , -t i 2 1, = -(p,Qo)Vo 2 . . . . . . . . . . . . . . . .2 2

For su perficial gas velocities between0.05 and 0.25 m/s,the total power input with the gas(WpolentiULgWkinelic,g)ranged from about 12 to 160W. The kinetic power accountedfor only about 0.2% of the total power at a superficial veloc-ity of 0.05 m / s , but went up to 45% at the highest supe rficialgas velocityof 0.25 m / s . It has been reported (Abramovich,1963; Lehrer, 1968) that only a small fractionof the jetkinetic energy < 10%) is transmitted out of the jet re gion.This implies that the kinetic power transmitted out of the jetregion to maintain the bulk motion of the slurry was lessthan 3% of the total input power. This indicates that thepotential energy due to gas flow is responsible for maintain-ing the bulk slurry motion, while the kinetic energy in thegas is dissipated within the jet region without significantcontribution to the bulk motion.

The effect of the sparg er orientation on the dispersion ofsolids from the settled bed c an be related to this differencein kinetic and buoyant pow er utilization. When the sp argerorifices are pointing downwards, the gas jets impinge directlyon the settled bed of so lids and the k inetic energy of the gasis utilized to disperse solids from the bed. T he gas jet wouldentrain the surrounding fluid in its boundary and expand.Eventually, the buoyancy force would take over, turningthegas column upwards from which gas bubbles will breakaway. The rising gas bubbles would provide the upward lift(in their wake) to the surrounding fluid. Therefore, thedownward gas injection provides good agitation for solidsdispersion. In the case of the upward poin ting sparge r ori-fices, the kinetic power in the gas is dissipated above thesparger without contributing to solid agitation and disper-sion below the sparger. Any d ispersionof the solids is due

THE CANADIAN JOURNALOF CHEMICAL ENGINEERING, VOLUME77, APRIL, 1999 389


8/9


9/9

HbedHdHsiLdi,y

Ljelm

= acceleration d ue to gravity,(m/s2)= height of settled solid bed, (m )= height of dispersion, (m)= height of initial settled bed of solids, (m)= distance between sparger and rem aining settled solid

= jet penetration length, (m)= mass flowrate of gas, (kg/s)= mass of slurry sample, (kg)= total mass of solids in column, (kg)= mass of solids dispersed in column, (kg)= pressure at the bottom of column, (Pa)= pressure at top of the column, (Pa)= gas volumetric flowrate, (m3/s)= superficial gas velocity, (m/s)= orifice gas velocity( d s )= potential (buoyant) power in incoming gas as defined

by Equation 1 6), (W)= kinetic power in incoming gas as defined by Equation

= height of section in Equation(S), (m)= static pressure increment along the column, (Pa)= difference in U-tube manom eter reading, (m)= a small increment in vertical distance, (m)

bed, (m)

(1 7), (W)

Greek letters

= volume fraction or holdup= porosity of settled solid bed= dispersion density, (kg/m3)= density of manometer fluid, (kg/m3)= density of water, (kg/m3)= density, (kg/m3)= fractional solid dispersion as defined in Equation1 5)= achievable maximum solid fraction in suspension, (v/v)= solid fraction in slurry sample, (v/v)= average solid fraction in slurry, (v/v)

Subscripts

d = dispersion

= gasi = section= liquid

p = particleS = solidsb = settled bedsl = slurry

References

Abramovich, G. N., Theory of Turbulent Jets,M.I.T. Press,Cambridge, MA (1963).

Benjelloun, F., R. Liegeois, and J. Vanderschuren, FluidizationVIII, C. Laguerie andJ. F. Large, Eds., Engineering Foundation,New York, NY, pp. 239-246 (1995).

Deckwer, W. D., Bubble Column Reactors, John Wiley andSons, New York, NY (1985).

Dudukovic, M. P. and N. Devanathan, Bubble Column Reactors:Some Recent Developments, NATO-AS1 Symposium Series,

Kluwer Publishing(1992).Fan, L.-S., Gas-Liquid-Solid Fluidization, Butterworths, Boston,MA 1989).

Gandhi, B. C., Hydrodynamic Studies in a Slurry BubbleColumn, M.E.Sc Thesis, The Universityof Western Ontario,London, ON Canada 1 997).

Lamont, A. G. W., Agitation in Pachuca Tanks, Can.J. Chem.Eng. 36, 1 5 3 4 6 0 1 958).

Lehrer, L. H., Agitation of Liquids, Ind. Eng. Chem. ProcessDes. Dev. 7,226-239 (1968).

Leibsan, I. E. G. Holcomb, A.G Cacoso, and J. J. Jamic, RateofFlow and Mechanics of Bubble Formation from SingleSubmerged Orifices, AIChE J.2,296-306 1 956).

Merry, J. M. D., Penetration of Vertical Jets into Fluidized Beds,

Ozawa, Y. and K. Mori, Characteristics of Jetting ObservedinGas Injection into Liquid, Trans. I.S.I.J.23, 764 (1983).Rabiger, N. and A. Vogelpohl, Calculation of Bubble Size in the

Bubble and Jet Regimes for Stagnant and Flowing NewtonianLiquids, Ger. Chem . Eng.6 17S-182 1983).

Wen, C. Y., N. R. Deale, and N. H. Chen, A Studyof Jets in aThree-Dimensional Gas Fluidized Bed, Powder Technol.31,175-1 84 I 982).

Yates, J. G., S. S. Cobbinah, D. J. Cheesam andS. P. Jordon,Particle Attrition in Fluidized Beds Containing O pposing Jets,AlChE Symposium Series No.28 1, U87U, 13-19 1 988).

Zenz, F. A., Bubble Formation and Grid Design,I. Chem. E.Symposium Series No. 30, (Instn Chem. Engrs, London),136-139, (1968).

AIChE J. 21,507-510 (1975).

Manuscript received June17, 1998;revised manuscript receivedJanuary 8, 1999; accepted for publication January29, 1999.

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Efeitos Da Altura Do Sparger e a Direcao Dos Furos

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