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Particuology 8 (2010) 623–630

Contents lists available at ScienceDirect

Particuology

journa l homepage: www.e lsev ier .com/ locate /par t ic

EPT study of particle motion for different riser exit geometries

hian Wen Chana, Anke Bremsb, Shiva Mahmoudia, Jan Baeyensa, Jonathan Sevillea,∗,avid Parkerc, Thomas Leadbeaterc, Joseph Gargiuli c

University of Warwick, School of Engineering, Coventry, CV4 7 AL, UKKatholieke Universiteit Leuven, Department of Chemical Engineering, de Croylaan, 46, 3001, Leuven, BelgiumUniversity of Birmingham, School of Physics and Astronomy, Birmingham, B22 T15, UK

r t i c l e i n f o

rticle history:eceived 28 July 2010ccepted 15 August 2010

a b s t r a c t

Laboratory and industrial risers are equipped with exits of many different layouts, and numerous publi-cations discuss the influence of riser exit geometry on local and overall solids hydrodynamics in the riser.

eywords:FBiserxit geometriesositron Emission Particle Tracking

The present paper reviews literature findings—mostly based upon indirect experimental techniques andoften somewhat contradictory. Direct measurement of particle velocity and particle occupancy near andin the riser exit provide a better indication of the effect of riser exit geometry. Positron Emission ParticleTracking (PEPT) was used in this work for the first time to investigate the exit region of the riser. An abruptor sharp exit causes particles to be knocked out of the gas flow, so forming a recirculation or reflux regionin the upper part of the riser. This is much less pronounced with a curved or gradual exit.

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. Introduction

Laboratory and industrial risers are equipped with exits of manyifferent layouts, some of which are illustrated in Fig. 1. Numer-us publications discuss the influence of riser exit geometry onocal and overall solids hydrodynamics in the riser. Previous work,ften based on indirect experimental techniques such as record-ng pressure drop profiles or residence time distributions obtainedy tracer response, shows considerable contradiction. Direct mea-urement of particle velocities and particle occupancy near and inhe riser exit provide a better indication of the effect of exit geom-try. Positron Emission Particle Tracking (PEPT) was used for therst time in this work to investigate the exit region of the riser, andesults are reported in this paper. Experiments are still ongoing,ostly for abrupt riser exits of larger and/or restricted diameter.

. Literature survey

.1. General observations

The essential findings of previous workers are summarised inable 1 below. The riser exit geometry can affect the particle flow,ossibly leading to a densification and/or recycling of the solids. Ineneral, a smooth exit is recognised as having a negligible effect,

∗ Corresponding author.E-mail address: J.P.K.Seville@warwick.ac.uk (J. Seville).

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674-2001/$ – see front matter © 2010 Chinese Society of Particuology and Institute of Process Eoi:10.1016/j.partic.2010.08.006

of Particuology and Institute of Process Engineering, Chinese Academy ofSciences. Published by Elsevier B.V. All rights reserved.

hereas an abrupt exit promotes both a densification (increasedolids hold-up) in the top zone of the riser and increased recyclingf the solids flow to the downward flowing annulus. The result-ng solids hold-up profiles throughout a riser equipped with anbrupt exit have been illustrated by Mei, Shadle, Yue, and Monazam2007) for a 0.3 m I.D. riser, 15.5 m high. Their findings are concep-ually represented in Fig. 2. The increased hold-up fraction (1 − ε)t the bottom of the riser with increasing solids circulation flux Gs due to the presence of a bottom turbulent fluidized bed (Chan,eville, Parker, & Baeyens, 2010). The height-dependent increasef (1 − ε) near the riser exit clearly indicates some degree of flowensification near the abrupt riser exit.

.2. Quantitative predictions

.2.1. Empirical equations using the reflection coefficientThe empirical approaches define either a reflux (or reflection)

oefficient or a reflux ratio, the former expressing either the effectf the exit geometries on the particle slip factor when comparedo a smooth exit or the length of influence in comparison with theiser height, and the latter being the ratio of the downward solidsow to the imposed circulation flux (G).

The extent of the influence of the riser exit down the riser col-

mn can be predicted by the following equation (Harris, Davidson,Thorpe, 2003):

= Length of influenceRiser height

, (1)

ngineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

624 C.W. Chan et al. / Particuolo

Nomenclature

Ar Archimedes number, Ar = gd3p�g(�p − �g)/�2

gA, Ac Cross-sectional area of the riser and core, respec-

tively, m2

D, De Diameter of riser and of riser exit, respectively, mdp Diameter of particles, mFrR Froude number at the exit, FrR = v2

p,top/gR

G Solids circulation flux, kg/(m2 s)g Gravitational acceleration, 9.81 m/s2

H Height of riser, mHe Height above riser exit in T-blinded exit geometry,

mhi Axial height along the riser, mkm Reflux ratio, as defined by Eq. (10)NDe De/DNHe (H − He)/HN˛ (˛ − 45◦)/45◦

NG G/(�pU)NGt G/(�pUt)R Radius of riser, mRin Centre-line radius of the curvature at the inlet of the

exit bend, mRout Centre-line radius of the curvature at the outlet of

the exit bend, mt Time, st̄ Arithmetic mean experimental residence time of

solids, sU Superficial air velocity through the riser, m/sUt Terminal velocity of particle, m/svp Velocity of particle, m/sv∗

p Velocity of particle in fully-developed riser flow, m/svp,up Upward velocity of particle, m/svp,down Downward velocity of particle, m/sX, Y, Z Cartesian coordinates, mm˝ Ratio of the length of downward influence of the

exit and the riser height�g Viscosity of air, kg/(m s)� Reflux or reflection coefficient�p Density of solids, kg/m3

w

˝

a

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ϕ

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ϕabrupt Slip factor in abrupt riser exitϕsmooth Slip factor in smooth exit

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=(

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nd

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in and Rout are the respective centre-line radii of the curvature athe inlet and outlet of the exit bend. The equation should howevere used with caution, since the effect of some competing groups,

−0.08 0.23

.g. (dp/D) , (�pUD/�g) , is very important and can lead toalues of ˝ in excess of 1.

Other researchers have introduced an exit-dedicated slip factor,s also used for fully developed riser flow (Chan et al., 2010). Thelip factor for an abrupt exit is related to the slip factor for a smooth

ebT

gy 8 (2010) 623–630

xit by the following equation:

abrupt = ϕsmooth(1 + �), (4)

here � is the reflux coefficient (Brereton & Grace, 1994; Gupta &erruti, 2000). ϕabrupt is the experimentally observed slip factor inhe abrupt riser exit, whereas ϕsmooth is the equivalent slip factorith a smooth exit configuration.

The reflection coefficient, �, was first predicted by Senior (1992),nd further developed by different authors, as summarized inable 2.

Gupta and Berruti (2000) have derived distinct sets of empiri-al equations for Group A and for Group B powders. They proposehat the exit geometry plays a greater role in Group B than Group

particles are smaller and/or lighter, so that they follow the gastreamlines closely and most of the solids are able to leave withhe gas through the riser exit, regardless of restrictions in the arear the abruptness of the exit geometry. Group B particles are largernd/or denser, and so do not follow the gas streamlines very closely,esulting in easier separation from the gas stream via impact in thexit. The increased solids reflection becomes more prevalent withncreasing U. Gupta and Berruti’s correlations for � are given inable 3.

Eq. (5) is simple in its use, provided Ac can be accurately pre-icted, whereas Eqs. ((6)–(9)) are very sensitive to the values of theontributing parameters, and predictions may differ by orders ofagnitude.

.2.2. Reflux ratioA different way to express the influence of the riser exit configu-

ation on the riser solid flux profiles uses a “reflux ratio” approach.he reflux ratio, km, is defined as:

m = solids downward flowexternal circulation rate

. (10)

Unfortunately, the work by Van der Meer, Thorpe, and Davidson2000) provides values for km only at 0.14 m below the riser exit.he values of 0.11–2.72 for different riser exit geometries indicateither a negligible or very profound reflux action as a function ofhe exit geometry, but only at this fixed position below the exit.xtrapolation to other positions inside the riser is hence impossible.

.2.3. Computational approachesInitial work by De Wilde, Marin, and Heynderickx (2003) pre-

icted that a smaller outlet surface area will result in moreronounced solids downflow in the riser. The dilute core of theiser thereby shifts from the centre towards the side of the out-et. An increase in the riser outlet surface area significantly reduceshe effective depth of influence. Similar findings were presented by

u, Wang, Jiang, Xu, and Xiao (2009), Gnanapragasam and Reddy2008) and Benyahia, Arastoopour, Knowlton, and Massah (2000).

CFD modelling of the riser exit geometries has been presentedy Hussain, Ani, and Darus (2004): results illustrate the effectsf the different riser exits on the particle velocity contours, withignificant effects predicted in the upper region of the riser only.

A pressure balance model was proposed by Kim, Kim, and Lee2002) for a riser with an abrupt exit. Reflux coefficients have beenorrelated with pertinent dimensionless numbers and exit geom-try.

. Experimental set-up

Positron Emission Particle Tracking (PEPT) was applied in thexperimental set-up of Fig. 4 at ambient conditions to study theehaviour of a single solid tracer particle in a bed of similar material.his technique requires that a particle of the bed material be made

C.W. Chan et al. / Particuology 8 (2010) 623–630 625

Table 1Literature survey on the reported effects of the riser exit geometry.

Authors Riser I.D. and height (m) Findings

Rhodes, Zhou, Hirama, and Cheng(1991)

0.15 m I.D., 6.2 m high The solids residence time distribution shows a limited impact of theabrupt riser exit only

Senior (1992) 0.152 m I.D., 9.3 m high Experiments in a riser with smooth exit show no exit-impact at G = 20to 60 kg/(m2 s)

Martin et al. (1992) 0.5 m I.D., 30 m high An abrupt riser exit leads to an increasing solids hold-up in bothlaboratory and industrial FCC risers.

Zheng and Zhang (1994) 0.1 m I.D., 5.25 m high For a blind T-geometry, as illustrated in Fig. 3, a cavity will formbetween the exit level and the projected roof. Beyond a certain He, theadditional height increment has very little or no effect on the solidsreflection: a cushion of particles is formed in the projected blind end,acting as a barrier for additional particles. The effective He is restrictedto the distance from the bottom of the cushion to the top of the exitpipe.

Zhou et al. (1994a, 1994b); Zhou,Grace, Lim, and Brereton (1995)

0.146 × 0.146 m2, 9.14 m high An abrupt exit results in an asymmetrical lateral distribution of thevoidage at the top, being denser close to the riser wall opposite theexit. The axial velocity profile is also asymmetric with the maximumupward velocity shifted to the exit side. The falling wall layer is thickeron the opposite side of the exit.

Brereton and Grace (1994); Zheng andZhang (1994, 1995); Pugsley et al.(1997)

0.152 m I.D., 9.3 m high For a riser of sufficient diameter and when the particle terminalvelocity is sufficiently large, the particles that are reflected by theabrupt exit may reflux along the riser wall to the base of the column,affecting the entire riser hydrodynamics. For a smaller riser diameter,the refluxing particle layer can only attain a certain maximumthickness before the particles are re-entrained upward by the shearingaction of the gas phase (Pugsley et al., 1997).

Grace (1996) 0.076 m I.D., 6 m high For a smooth exit, the particles follow gas streamlines closely, withoutflow disturbance. In an abrupt exit configuration, the top of the riseracts like a gas-solid separator: due to the inertia of moving solids, theparticles hit the top wall of the vessel and flow down near the wall ofthe vessel, resulting in a densification around the riser exit area.

Horio (1997); Werther and Hirschberg(1997); Lim et al. (1995)

These authors concluded that the exit design can affect the densityprofile over several metres in the upper region of a riser of largerdiameter.

Smolders and Baeyens (1998, 2000,2001)

0.1 m I.D., 6.5 m high An abrupt riser exit has a limited effect on the pressure gradient andsolids residence time distribution near the exit, and over the wholeheight of the riser.

Johnson, Vrager, and Leckner, (1999) 0.12 × 0.7 m2, 8.5 m high An increase in the net solid flux is seen by narrowing the exit duct ofthe riser. No differences in the solids flow were noticed whenswitching between smooth, abrupt and T-exit.

Glicksman et al. (1993); Van der Meeret al. (2000); Harris et al. (2003)

0.14 m I.D. and 0.14 × 0.14 m2, 5.1 mand 4.5 m high

The particles can accumulate in the horizontal exit duct leading intothe cyclone (dune formation). If sufficient solids accumulate at theriser exit duct, some particles may slip back periodically from thehorizontal duct into the riser where they will move downwards alongthe wall.

Reddy and Nay (2001) 0.1 × 0.1 m2, 5.25 m high Densification occurs for about 2 m in length in the top region of theriser fitted with abrupt exit.

Lackermeier and Werther (2002) 0.3 × 1.0 m2, 8 m high The abrupt and T-exit appear not to affect the residence time of solids;neither do they appear to affect the voidage profile along the riser,even around its exit region.

Yan et al. (2003) 0.076 m I.D., 10 m high Experiments in a FCC-riser with smooth exit demonstrate no exiteffect at low G (100 kg/(m2 s)). At higher G (up to 550 kg/m2 s) anasymmetric solids flow profile develops near the wall opposite the exitin the top section (1.26 m) of the riser.

Van de Velden, Baeyens, Doughan,et al. (2007)

3.8 × 3.8 m2, 18 m high Pressure gradients in a CFB combustor with abrupt rectangular exitshow no densification near the exit.

Van de Velden, Baeyens, and Smolders(2007); Van de Velden et al. (2008)

0.1 m I.D., 6.5 m high The solids residence time in the riser is not affected by the abrupt riserexit.

Mei et al. (2007) 0.3 m I.D., 15.5 m high Study solids concentration profiles in a riser with an abrupt exit.Except at low gas velocity and solid flux, the apparent solid hold-up atthe top exit region was higher than in the middle section of the riser.The local particle velocity showed a downward flow near the wall atthe top of the riser: the abrupt geometry reflected the solids andcaused solid particles to travel downward along the wall. At a locationbelow but near the top of the riser, the particles were observed flowingupward near the wall, re-establishing the DRU operating mode.

Castilho and Cremasco (2009) 0.082 m I.D., 2.42 m high The abrupt riser exit causes an increase in solids concentration in thetop zone of the riser, with high values of solids hold-up near the riserwall.

626 C.W. Chan et al. / Particuology 8 (2010) 623–630

Fig. 1. Illustration of various riser exit geometries: (1) L-elbow, (2) T-elbow, (3) tapered axial exit, (4) sharp 135◦ elbow, (5) shaved 45◦ elbow, (6) extended 45◦ elbow, (7)round elbow and (8) smooth elbow.

Fig. 2. Average axial solids hold-up (1 − ε) versus the axial position in the riser (hi)for a smooth exit (1) and an abrupt exit (2) at equal values of U and G. Increasing Gshifts the profile to a lower voidage (3).

Fig. 3. Blind T-exit with projected height (He), exit diameter (De) and exit angle (˛)(Zheng & Zhang, 1994).

Table 2Equations for predicting the reflection coefficient.

Literature Equations

Senior (1992)a � =

{(G/vp,up) − �p (1 − ε)

}{

(G/vp,down) − �p (1 − ε)} , (5)

where vp,up = U(A/Ac) − Ut.The equation is very sensitive to valuesof ε.

Hannes (1996) (1 − �) = KAexit

A

G/�p

(U − Ut), (6)

where K = 180◦ or 80◦ for circular orsquare exits, respectively.This equation is only valid within anarrow (U, G) range.

Harris et al. (2003) � = 1 + −1.05Fr0.51R

(0.3 + FrR)0.51, (7)

0

itwth1bfD2eY

tnu

t

TE

N

where FrR = (v2p,top/gR).

a vp,down is the particle downward velocity in the annulus which is generally.5–1.5 m/s (Harris et al., 2003) but was assumed to be 1.1 m/s by Senior (1992).

nto a positron emitter. The positrons emitted by the radioactiveracer annihilate with electrons to produce back-to-back �-rayshich are then detected by two large position-sensitive detec-

ors. The tracer position is located by triangulation. The techniqueas been developed at the University of Birmingham (Parker et al.,997) and used in numerous research applications, as summarizedy Seville, Ingram, and Parker (2005) and recently reported uponor CFB studies by Van de Velden et al. (Van de Velden, Baeyens,oughan, & McMurdo, 2007; Van de Velden, Baeyens, & Smolders,007; Van de Velden, Baeyens, Seville, & Fan, 2008) and by Chant al. (Chan, Seville, Fan, & Baeyens, 2009a, 2009b; Chan, Seville,ang, & Baeyens, 2009; Chan et al., 2010).

Since previous research as represented in Table 1 has found thathe impact of the riser geometry, even for abrupt exits, is less pro-

ounced in risers of larger I.D., a CFB with a 0.05 m I.D. riser wassed.

A blower provides compressed air at ambient temperature tohe L-valve. The particle positions were determined in real time

able 3mpirical equations to predict reflection coefficient, �, by Gupta and Berruti (2000).

Powder group Equation

Group A �A = 22.5N1.49G N−1.36

Gt N−1.32He N4.06

˛ (8)Group B �B = 0.00091N−2.43

G N2.02Gt N−1.54

De N−2.8He N˛ (9)

De = De/D, NHe = (H − He)/H, N˛ = (˛ − 45◦)/45◦ , NG = G/(�pU), NGt = G/(�pUt).

C.W. Chan et al. / Particuolo

F(

(Ypavr

tv

1

wesvastst(ntv

4

4

rpwa

Gme

(nnnd

ecpsifv

t(l

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irexit, albeit limited in length to about 0.2 m. The same penetration

ig. 4. Experimental set-up: (1) CFB-riser, (2) detectors, (3) high-efficiency cyclone,4) downcomer and L-valve, (5) vent to filter and atmosphere, and (6) tracer location.

∼1 location every 4–10 ms). The list of consecutive locations (X,and Z co-ordinates) obtained was then used to determine the

osition of the tracer. The positioning of the �-ray cameras is suchs to view the riser exit only. The U and G operating values werearied between 3.7 and 6.2 m/s and between 86 and 225 kg/(m2 s),espectively.

The bulk bed material was round sand (0–300 �m), average par-icle size ∼150 �m, and density of 2300 kg/m3 with a transportelocity, UTR, of about 2.5 m/s.

An ion exchange resin bead of ∼300 �m was labelled with8F. The initial radio-activity was about 1.2 mCi. Geiger countersere used to check whether the particles were circulating and to

stimate the solid circulation rate from the tracer velocity in thetandpipe/downcomer so that it could be adjusted to the desiredalue. The 0.05 m I.D. riser was fitted with 3 exit geometries, i.e.brupt (sharp) exit, a round exit (radius of curvature = D) and amooth exit (radius of curvature = 2D). The transport velocity ofhe bulk bed material was ∼2.5 m/s. The operating velocity andolids circulation flux thus determine the mode of riser opera-ion, being either in core-annulus or dense riser upflow modeChan et al., 2010). Solids circulation rates are given within aarrow range of measured values, since even small pressure fluc-uations along the riser make it difficult to maintain G at a constantalue.

. Experimental results

.1. Recorded tracer tracks and velocities

Fig. 5 below illustrates some of the particle motion trajecto-

ies obtained in the different exit geometries, with the associatedarticle velocity vectors. A wide range of particle velocities is seen,ith upward and downward moving particles observed around the

brupt and round exits.

ddHr

gy 8 (2010) 623–630 627

The experimental conditions were maintained at constant U andduring each experiment, which lasted about 30 min, providingultiple passes of the tracer in the exit geometry during successive

xperiments.At the tested U and G values, the particle motion in the sharp

abrupt) exit geometry is subject to significant refluxing, which sig-ificantly contributes to the overall residence time of the particleear the exit, with refluxing particles considerably reducing theet upward particle velocity. The effect of U and G will be furtheriscussed.

The same refluxing occurs in the round exit, but to a lesserxtent, and with a more limited effect on the net upward parti-le velocities. The smooth exit has virtually no impact although thearticles accelerate in the bend, possibly due to centrifugal den-ification of the bulk solids near the outside radius of the bendtself. There is some degree of deceleration just after the bend (duneormation) followed by a re-acceleration to the average particleelocities.

For an unambiguous treatment, both particle velocities andotal residence time were determined, the latter from y = 0 mmthe start of the bend) to x = 250 mm (the bend end, i.e. ∼0.5 m inength).

The cumulative distribution of all experimental results is pre-ented in Fig. 6, with the average residence time in the bend, t̄,aken as arithmetic mean of all data obtained. The average resi-ence time t̄ was 0.11 s, 0.89 s and 4.02 s for respectively smooth,ound and sharp exits.

Fig. 6 illustrates the spread of residence times, with some parti-les moving unhindered through the exit, whereas other particlesre decelerated by centrifugal force towards the outer bend radius,r caught in a decelerated zone immediately after the bend (ashown by the reduced velocity vectors in Fig. 5), which in someiterature is described as dune flow.

The experiments were thereafter repeated for the abrupt riserxit, at different U and G. Again results are expressed as the averageolids residence time in the specific section of the riser exit. Resultsre illustrated in Fig. 7. Clearly, the effect of the abrupt riser is lessarked with increasing U and/or G.

.2. Discussion

.2.1. Particle velocities and exit residence timeThe particle velocities in the zone of fully developed riser flow,

∗p, depend upon the operating mode (either core-annulus flow CAFr dense riser upflow DRU) were determined by Chan et al. (2010),nd are given in Table 4.

If it is assumed that the measured net particle velocity is theatio of the travelled distance being covered (∼0.5 m) to the aver-ge residence time, the experimental results lead to net particleelocities of respectively 4.55, 0.56 and 0.13 m/s for the cases ofmooth, round or abrupt exit geometry for the experimental con-itions of Fig. 5. There is a local acceleration for the smooth exit,ut a considerable refluxing for both other exits.

There is hardly any difference in the solids velocities in the fullyeveloped riser flow in the case of the smooth exit, where the effectf local deceleration after the bend is counteracted by the localcceleration in the bend itself. No reflux of particles is seen.

The difference in velocities becomes more marked when mov-ng from a round to an abrupt exit. Fig. 3 already illustrated theeflux action, which is very significant in the case of the abrupt

epth into the riser is also noticed for the round exit. This limitedepth of impact confirms previous statements by Pugsley, Lapointe,ischberg, and Werther (1997) that for a small riser diameter, the

efluxing particle layer can only attain a certain maximum thick-

628 C.W. Chan et al. / Particuology 8 (2010) 623–630

Fig. 5. Particle trajectories and velocity vectors in three different riser exit geometries: (a) sharp exit, at U = 5.4 m/s, G = 85–90 kg/(m2 s); (b) round exit, at U = 3.7 m/s,G = 110–115 kg/(m2 s); (c) smooth exit, at U = 5.3 m/s, G = 150–155 kg/(m2 s).

Table 4Characteristics of the solid flow in the fully-developed riser.

Exit configuration Fully developed riser flow

U (m/s) G (kg/(m2 s)) Flow regime ε v∗p (m/s)

Abrupt 5.3 85–90 CAF 0.985 2.6Round 3.7 110–115 CAF 0.981 2.7Smooth 5.3 150–155 DRU 0.984 4.3

C.W. Chan et al. / Particuology 8 (2010) 623–630 629

Fig. 6. Cumulative residence time distribution of all tracer recordings v

FU

na

iGr

4

c

iio

br

tao

lt

s(v

oias

5

bsru

twt

ig. 7. Arithmetic average residence time of solids in the abrupt riser exit at differentand G.

ess before the particles are re-entrained upward by the shearingction of the gas phase.

When studying the abrupt riser exit at different U and/or G, its clear that the exit effect becomes less significant as U and/or

increase. In these cases, the shearing action of the gas streameduces the downflow depth of the refluxing particles.

.2.2. Comparison of the findings with previous literature resultsThe main conclusions of this preliminary investigation can be

ompared with previous literature findings, as tabulated below.

Present findings Literature comparison

Abrupt exitPronounced impact at

low UAlso stressed by Martin et al. (1992), Grace(1996), Castilho and Cremasco (2009)

Limited impact athigher U and/or G

Confirms findings by Smolders and Baeyens(1998, 2000, 2001), Van de Velden, Baeyens,Doughan, et al. (2007), Van de Velden, Baeyens,and Smolders (2007), Van de Velden et al.(2008), Lackermeier and Werther (2002)

Limited penetrationdepth of reflux insmall I.D. exit

Confirmed by Pugsley et al. (1997), Mei et al.(2007)

A larger penetration depth was measured byBrereton and Grace (1994), Zheng and Zhang(1994, 1995), Horio (1997), Werther andHirschberg (1997), Lim, Zhu, and Grace (1995),Reddy and Nay (2001)

Round exit

More limited impact

than for the abruptexit

No specific findings reported in literature

Possible dune flowafter the bend

Confirms findings of Glicksman, Hyre, andWestphalen (1993), Van der Meer et al. (2000),Harris et al. (2003)

rnaft

ersus time, where (a) smooth exit, and (b) round and abrupt exit.

Present findings Literature comparison

Smooth exitNegligible impact Confirmed by all previous literature findings,

e.g. Senior (1992), Grace (1996), Yan,Päressinen, and Zhu (2003)

Asymmetric velocityprofile (densificationat outer radius ofbend)

Confirmed by Yan et al. (2003)

As far as the comparison with quantitative predictions of Table 2s concerned, the reader is cautioned, especially in view of the find-ngs reported in Fig. 7, where U and G are important at low valuesnly, that Eq. (2) over-predicts the zone of reflux.

The experiments moreover prove that the slip factor in a smoothend is equal to the slip factor in the fully developed riser flowegime, and thus close to 1.3 (Chan et al., 2010).

For the round and abrupt exits, the particle velocity differs fromhe velocity in fully developed riser flow only at low values of Und/or G. With increasing U and/or G, these exits lose their impactn the solids flow.

At high U and/or G, Eq. (4) can hence be applied with � ∼0.1. Forower U and G, where CFB-risers would not commonly be operated,he reflection coefficient can reach values close to 10.

The reflection coefficient is a strong function of U and G, and thistrong dependency is not represented by Eqs. (6), (7) and (9). Eq.5) is – despite its simple form – difficult to use due to the unknownalues of vp,up and vp,down.

Unless operating at lower values of U and/or G, the influencef the riser exit is proven less significant than previously quotedn literature. Additional PEPT experiments are however ongoing tossess the influence of the diameter and possibly restricting theize of the riser exit.

. Conclusions

Previous studies of the influence of different riser exits haveeen often based upon indirect experimental techniques, e.g. pres-ure drop profiles or the residence time distribution by traceresponse. Positron Emission Particle Tracking (PEPT) has here beensed for the first time to investigate the exit region of the riser.

Whereas there is hardly any influence on the solids motion inhe case of the smooth exit, the impact becomes more noticeablehen moving from a round to an abrupt exit, although the pene-

ration depth of the densification zone into the riser is limited: the

efluxing particle layer can only attain a certain maximum thick-ess before the particles are re-entrained upward by the shearingction of the gas phase. When studying the abrupt riser exit at dif-erent air velocities, U and/or solids circulation fluxes, G, it is clearhat the exit effect becomes less significant as U and/or G increase.

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The experiments prove that the slip factor in a smooth bend isqual to the slip factor in the fully developed riser flow regime,nd thus close to 1.3 (Chan et al., 2010). For the round and abruptxits, the particle velocity differs from the velocity in fully devel-ped riser flow only at low values of U and/or G. With increasingand/or G, these exits lose their impact on the solids flow. Liter-

ture equations to determine the axial zone of influence, and theeflection coefficient should be used with caution because (i) forhe riser and exit geometries tested, the axial penetration (reflux)s limited to 0.2 m only, and (ii) the reflection coefficient is a strongunction of U and G, and this strong dependency is not representedy the literature equations. Unless operating at lower values of Und/or G, the influence of the riser exit is proven less significanthan previously quoted in literature.

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