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Nuclear Instruments and Methods in Physics Research A 542 (2005) 219–225

www.elsevier.com/locate/nima

Quantitative measurement of segregation phenomena in abinary-mixture fluidized bed by neutron radiography

Hisashi Umekawaa,�, Shuji Furuia, Yoshihiro Oshimaa, Masashi Okuraa,Mamoru Ozawaa, Nobuyuki Takenakab

aDepartment of Mechanical Engineering, Kansai University, Yamate-cho 3-3-35, Suita, Osaka 564-8680, JapanbKobe University, Rokodai-cho 1-1, Nada-ku, Kobe 657-8501, Japan

Available online 16 February 2005

Abstract

Owing to a wide variety in fluidization properties of particles, segregation phenomena are hardly avoided in

industrial fluidized-bed facilities. For the stability and controllability in the operation of such fluidized-beds,

understanding of the relevant mechanism of particulate segregation is indispensable. Many factors, such as the

difference in particle size, density, shape, and also arrangement of distributors, can be pointed out as the dominant

causes for the segregation. Thus, any marking of particles for quantitative measurement may severely influence the

segregation characteristic to be measured. But neutron radiography can be used for the quantitative measurement

without disturbing the segregation characteristic. For estimating the segregation phenomena, evaluations of the particle

fraction and the void fraction are required. For this purpose, two types of radiographies, i.e., X-ray and neutron

radiographies were used in this investigation. Experimental results demonstrated the high performance of this method

in investigating of segregation phenomena.

r 2005 Elsevier B.V. All rights reserved.

PACS: 47.90.+a

Keywords: Fluidized-bed; Segregation; Neutrona radiogrpaphy; X-ray radiography

1. Introduction

Particulate systems such as fluidized-bed com-bustors or reactors are composed of a varietyof particles in various sizes/shapes, and thus

e front matter r 2005 Elsevier B.V. All rights reserve

ma.2005.01.103

ng author. Tel.: +816 6368 0804;

0804.

ss: umekawa@kansai-u.ac.jp (H. Umekawa).

the segregation may occur within specific operat-ing ranges of the system. In the fluidized-bedcombustors, this segregation harms the efficientcombustion/reaction and heat transfer and thusthe harmful segregation is probably avoidedfor achieving high performance of the system.In a chemical unit operation, on the other hand,the segregation is effectively used, e.g., fora particle classification. Consequently, sufficient

d.

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Nomenclature

Dp particle diameterG gainH bed heightJG fluidized gas velocityJGmf minimum fluidization velocityO offsetS gray level of visualization imaget timeX volume fraction of flotsamx horizontal coordinate

y vertical coordinateGreek letters

d thicknesse void fractionm mass attenuation coefficientr densitySuffixes

f flotsam (floating particle)j jetsam (sinking particle)0 mono-components bed materialw test section

Flotsam

100

Exit port

65

350

Jetsam

H. Umekawa et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 219–225220

understanding of the particulate segregation/mixingmechanism is quite important in designing thesesystems. The segregation characteristic depends onmany factors, e.g., the density ratio betweenparticles, size ratio, shape of particles, fluidizinggas velocity, distribution of fluidization gas andmaterial packing ratio [1,2]. To examine the effectsof these factors on the segregation, variousapproaches have been conducted, such as the directobservation of two-dimensional bed, rapid shut-offmethod, RI tracers, X-ray photography so far [3–6].However, sufficient understanding of the segrega-tion suffers from the complexity of the phenomena.In particular, the quantitative measurement of thedynamic characteristics of the segregation processhas not been succeeded so far.

To estimate the segregation characteristics, theparticle fraction and/or the void fraction areessential quantities to be evaluated. In this presentinvestigation, two kinds of radiography, neutronand X-ray radiographies were applied for thispurpose. Results confirmed that the high avail-ability of this method in the quantitative measure-ment of the dynamics characteristics of thesegregation. The calibration method for the effectof the scattered neutron becomes significant by thesegregation phenomena was also described in thispaper.

Distributorplate Injection port

Fig. 1. Test section.

2. Experiment

In this investigation, a rectangular test sectionwith dimensions of 100mm width, 65mm depth

and 350mm height was used (Fig. 1). This testsection was made of aluminum plates, which arerelatively transparent for neutron and X-raybeams. The fluidization-gas distributor was asintered plate of synthetic resin, and was locatedat the bottom of the test section. The segregationfactor focused in this paper was the size differenceof particles. Thus, the two types of particles whichhad different diameters were used as fluidized-bedmaterials. These particles were sieved from thesame silica sand (99.7% SiO2 and density 2500 kg/m3). The larger particle that sinks is referred to as‘‘jetsam’’ (mean diameter dPj ¼ 404, 610 mm). Thesmaller one that floats to the top of bed is referred

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H. Umekawa et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 219–225 221

to as ‘‘flotsam’’ (dPf ¼ 143, 176 mm). The minimumfluidization velocities (JGmf) of these particles were0.192m/s (for 404 mm particle), 0.379m/s (610 mm),0.033m/s (143 mm) and 0.040m/s (176 mm), respec-tively.

The bed height of jetsam particle (Hj) was set at100/150mm, and the flotsams were contaminatedat 20wt.% relative to the jetsam. To distinguishthe flotsam, flotsam particles were colored bycontamination of the same kinds of sand thin-coated by gadolinium nitrate (immersed in aqu-eous solution of 2wt.% Gd(NH3)3 and thendried). Gadolinium has a very high attenuationcoefficient for neutrons, so that the sufficientcontrast between flotsam and jetsam is obtainedwithout any influence on the segregation charac-teristics. This characteristic of neutron radiogra-phy is very attractive in the experiment ofsegregation.

In this experiment, at first, the fluidized bed ofbinary-component was fully mixed at high volu-metric gas flux, and then the volumetric gas fluxwas suddenly turned down to a certain predeter-mined value by switching the air supply line. Then,the segregation process started. The real-timeimage was visualized by using the radiographytechnique.

Visualization image was recorded at 30 frame/sin 8-bit image. Neutron radiography was con-ducted by using JRR-3M facility (JAERI). Inaddition, X-ray radiography was also used forestimating the void fraction distribution. In thepresent experiment, the industrial X-ray inspectionsystem FI-30 (Shimadzu Co. Ltd.) was used.

3. Image-processing procedure

A gray level, S, of a radiography image isexpressed by

Sðx; y; tÞ ¼ Gðx; yÞ exp½�rwmmwdw� ð1� �ðx; y; tÞÞrsmmsdS� þ Oðx; yÞ, ð1Þ

where the void fraction, e, is the averaged valuealong the beam path, and attenuations by envir-onment and fluidization-gas are neglected.

In this investigation, the bed material isclassified into two components, i.e., jetsam andflotsam, thus Eq. (1) is rewritten as Eq. (2) byintroducing the flotsam fraction Xf, i.e., volumefraction of the flotsam relative to the total volumeof both the jetsam and flotsam

Sðx; y; tÞ ¼ Gðx; yÞ exp½�rwmmwdw� dsð1� ð�ðx; y; tÞÞfX tðx; y; tÞ

rsfmmsf þ ð1� X tðx; y; tÞÞrsjmmsjg�

þ Oðx; yÞ. ð2Þ

The gray level of the test section without bedmaterial Sw and the gray levels of the packed-bedof mono-component system S0i (i ¼ j or f) isexpressed by

Swðx; yÞ ¼ Gðx; yÞ exp½�rwmmwdw� þ Oðx; yÞ, (3)

S0iðx; yÞ ¼ Gðx; yÞ exp½�rwmmwdw� ð1� �0iÞrsimmsids� þ Oðx; yÞ. ð4Þ

In the first stage of image processing, the offsetimage was subtracted from the original image tocancel the offset value (represented by S0). Thenthe shading correction that compensates for thenon-uniformity of neutron/X-ray beams wascarried out by the logarithmic subtracting of thetest section image without bed materials from theimage of the fluidized-bed. Finally, the flotsamfraction is given by a function of the visualizedimages and void fractions as follows:

X f ðx; y; tÞ ¼

log S0ðx;y;tÞ�log S0wðx;yÞ

1��ðx;y;tÞ �log S0

0j�log S0wðx;yÞ

1��0j

log S00f�log S0

wðx;yÞ1��0f

�log S0

0j�log S0wðx;yÞ

1��0j

,

(5)

where the gray level of the packed-bed of mono-component system S0(x,y) is considered almostuniform in the bed, and thus the space-averagedvalues is used in this paper. The void fraction ofmono-component e0i was determined in the addi-tional calibration experiment.Thus two variables, Xf(x,y,t) and e(x,y,t),

remain unknown in the neutron radiography. Onthe contrary, the difference between the jetsam andthe flotsam is hardly detected by X-ray radio-graphy, while only the void fraction, e(x,y,t),remains unknown in the X-ray radiography.

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Consequently, the void fraction evaluated byusing X-ray radiography is substituted togetherwith other values from the neutron radiographyinto Eq. (5), so that the flotsam fraction isevaluated.

4. Results and discussion

4.1. Visualization of segregation phenomena

Fig. 2 is an example of the visualization image ofneutron radiography, and also Fig. 3 is a result ofX-ray radiography under the same experimentalcondition. (The dimension of the visualization areais different in both methods, and the visualizationarea of X-ray radiography corresponds to theupper part of the fluidized-bed.) In these figures, t

means the elapsed time after the abrupt decrease inthe fluidization-gas flow.

In Fig. 2, the flotsam and the jetsam are clearlydistinguished. A dark region corresponds to theflotsam particles and a light zone is the jetsam. Thefirst photo, t ¼ 0 s; represents well-mixed state.After decreasing the fluidization-gas, the flotsamstarts to concentrate successively at the top of thebed. The bubble is also observed as white zone. Asmentioned in former section, the separation

Fig. 2. Visualization image of neutron radiography (DPj ¼ 610mm

Fig. 3. Visualization Image of X-ray radiography (DPj ¼ 610mm

between the influences of the flotsam fractionand the void fraction is required.In the case of X-ray radiography (Fig. 3), the

difference of both particles is hardly detected, butvoid fraction distribution is clearly observed. Withthe progress of the segregation phenomena, thehigh void fraction region becomes to be limited inthe upper region of the fluidized bed. At the finalstage, the bubble formation is observed only in theflotsam rich region.For the calculation of Eq. (5), the simultaneous

measurement with neutron and X-ray radiogra-phies is, in principle, required, while beingimpossible. Thus, the measurements by X-rayand neutron radiographies were conducted indifferent runs but keeping the same operatingcondition. Therefore the reproducibility of thephenomena becomes of prime importance in thisexperiment.As the individual bubble movement has a wide

dispersion, a certain spatio-temporal averaginghas to be introduced. As shown in Fig. 2, thesegregation is observed along the vertical direc-tion, and the shape of the boundary is nearlyhorizontal. Thus, the segregation phenomenaare considered one-dimensional. In addition, theresidence time of the bubble in the whole bed wastypically 5–15 s. Providing a narrow horizontal

DPf ¼ 176mm Hj ¼ 100mm JG ¼ 0:73 m=s ¼ b0:329 m=s).

DPf ¼ 176mm Hj ¼ 100mm JG ¼ 0:73 m=s ¼ b0:329 m=s).

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0.4 0.5 0.60.4

0.5

0.6

+1.1%

-5.7%

Void fraction (1st)

Voi

d fr

actio

n(2n

d)

JG = 0.36->0.16m/s

DPj = 404 µm

DPf = 143 µm

Hj = 100 mm

Fig 5. Reproducibility of the experiment.

0 50 100 1500

0.1

0.2

0.3

0.4

0.5

Ave

rage

of

Xf -

Superficial gas velocity0.119m/s 0.180m/s0.146m/s 0.197m/s0.163m/spacking ratio

Hj =150 mm DPj = 404 µm DPf = 143 µm

t s

H. Umekawa et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 219–225 223

frame of reference, e.g., 0.5mm, this correspondsto about 0.1 s of residence time for the respectiveframe. Then the spatio-temporal averaging wasconducted along the horizontal direction with thewidth of 0.5mm and over 1 s, respectively.

The example of the void fraction data obtainedby the spatio-temporal averaging of X-ray radio-graphy image is shown in Fig. 4, where thedistance from the top of the fluidizing bed, z, is aparameter. At the initial stage of the segregation,void fractions drastically change. The void frac-tion is unevenly distributed along the verticaldirection even after a long elapsed time. Fig. 5 isthe comparison of the two experiments under sameexperimental condition with Fig. 4. The deviationof both data sets is quite small, and this confirmsthe high reproducibility in the present experiment.

As mentioned before, the visualization area ofthe X-ray radiography is narrow compared withthat of the neutron radiography. The void fractionat the bottom of view area of X-ray radiographywas used as the void fraction of the outside theview area of X-ray radiography. As there existonly small difference in the data at z ¼ 60 and80mm in Fig. 4, this hypothesis may be accep-table.

Fig. 6 is a comparison between the measuredvalue and the actual flotsam fraction (broken line).The measured value was the mean value of thewhole test section, and the actual value wascalculated from the weight fraction of the compo-nents. The comparison indicates that, the accuracy

0 50 100 150

0.5

0.6 z = 5mm

10 mm

20 mm

40 mm60 mm

80 mm

JG = 0.36->0.16m/s

t s

Voi

d fr

actio

n

DPj = 404 µm

DPf = 143 µm

Hj = 143 mm

Fig. 4. Void fraction behavior (X-ray radiography).

Fig. 6. Evaluation error.

of this method at this stage is very low. Next, thecalibration method for this deviation will bediscussed.

4.2. Influence of the scattering

In the previous experiment, the fluidized bedwas, in principle, composed of a mono-compo-nent, and the thin-coated particles were distributedalmost uniformly in the bed without the segrega-tion. In the quantitative measurement of the voidfraction in such fluidized-bed, the neutron scatter-ing caused negligible error [7]. In the present

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0 0.2 0.4 0.6 0.8 1400

600

800

1000

1200

X

Gra

y le

vel (

in 1

4bit

scal

e)

Scattering levelby test section's wall

Fig. 8. Relationship between neutron scattering and flotsam

fraction.

0 0.5 1 1.50

0.5

1

1.5

Xf (calculated by packing ratio)

Xf (

calc

ulat

ed b

y im

age

proc

essi

ng)

without calibrationcalibrated data

+0.07

-0.07

Fig. 9. Calibration of neutron scattering effect.

H. Umekawa et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 219–225224

experiment, on the other hand, coated particlesconcentrate in a particular area, i.e., the particleswith different attenuation properties segregate inthe bed. This harms severely the accuracy of themeasurement.

Fig. 7 typically shows this scattering effect. Thegray level around the test section is considered asthe distribution of scattered beam. In this figure,raw image is superimposed on the inside area ofthe fluidized bed for indicating the particledistribution. Fig. 7(a) shows the distribution ofgray level caused by the neutron scatteringradiated from the mono-component jetsam-parti-cle bed. By adding colored flotsam particles on thejetsam bed, the intensity of the scattering abovethe test section is drastically decreased as shown inFig. 7(b).

Fig. 8 represents the relationship between thegray level and the flotsam fraction, which wasobtained by the calibration test by using a cooled-CCD camera. In this calibration test, phantompieces were used. These phantom pieces packedwith silica sands had the same dimension with thetest section along the beam direction, while Xf waspredetermined by the weight fraction. On the sidewalls of the phantom faced to the beam source andthe converter, cadmium plates were attached forcalibration. The gray level at the cadmium platefaced to the converter side was considered as thereal offset value, and the difference between thisvalue and the gray level at the cadmium plate facedto the beam source can be considered as thescattered component caused by the fluidized-bedmaterial.

Fig. 7. Distribution of neutron scattering around the particle

bed. (a) J-sam, (b) J-sam+F-sam.

The influence of the scatter is treated in thesimilar manner to the offset correction, so that arough discussion becomes possible. The calibra-tion results obtained by this correction are shownin Fig. 9. By this calibration the measurement datacoincided with the actual value within 70.07 of Xf

in Fig. 9.The present calibration experiment with the

phantom was conducted under the static bedcondition, but the effect of scatter will depend onthe amount of the fluidized-bed material itself.Then, the void fraction will also have an influence

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0 50 100 1500

0.1

0.2

0.3

0.4

0.5

Ave

rage

of

Xf -

Superficial gas velocity0.119m/s 0.180m/s0.146m/s 0.197m/s0.163m/spacking ratio

DPf = 143 µmDPj = 404 µmHj = 150 mm

t s

Fig. 10. Calibration results.

H. Umekawa et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 219–225 225

on the deviation shown in Fig. 9. Spatio-temporalaveraging, however, may reduce the effect ofscattering, and also small difference in the voidfraction between the static and fluidizing statesmay contribute to the reduction, so that theinfluence of the void fraction is ignored in thispaper.

Fig. 10 presents the calibrated results of Fig. 6,and the measured value shows a good agreementwith the actual value. Detailed discussion aboutthe segregation phenomena based on the measuredvalue with this method is found elsewhere [8].

5. Conclusion

Neutron radiography using thin-coated particleswith high attenuation materials has high advan-tage in the visualization of segregation phenom-ena. This, advantage, however, brings aboutsignificant scattering effect in the quantitative

measurement of the segregation phenomena, whilesuch effect causes negligible error in the quantita-tive visualization of mono-particle bed free fromsegregation. Calibration using a well-identifiedphantom reduced the scattering error to anacceptable level.

Acknowledgments

This investigation was carried out in the frame-work of the Inter-University Program for the JointUse of JAERI Facilities, and also partiallysupported by the Kansai University ResearchGrants: ‘‘Grant-in-Aid for Encouragement ofScientists, 2004’’

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