Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 602–612

Some industrial applications of gamma-ray tomography

Dinesh V. Kalaga a, Anand V. Kulkarni a, Rajesh Acharya b, Umesh Kumar b, Gurusharan Singh b,Jyeshtharaj B. Joshi a,*a Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400019, Indiab Isotope in Industry Division, Bhabha Atomic Research Center, Mumbai 400085, India

A R T I C L E I N F O

Article history:

Received 6 February 2009

Received in revised form 25 May 2009

Accepted 25 May 2009

Keywords:

Tomography

Gamma-ray

Packed column

Scanning

Image reconstruction

A B S T R A C T

Various tomographic techniques such as electrical capacitance, electrical resistivity, MRI, X-ray/gamma-

ray tomography have been reviewed with special emphasis on applicability for the industrial set-ups are

discussed. It was found that gamma-ray tomography is most suitable technique which could be used for

industrial set-ups. Gamma-ray tomography was also applied for variety of industrial equipment, such as

paddle bed dryer, packed distillation column, to explore the possibility of their use for diagnosis of

unidentified problems. From the preset work it is recommended to undertake tomographic scanning of

critical equipments periodically to enhance the efficiency and reduce possible downtime.

� 2009 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.

Contents lists available at ScienceDirect

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journal homepage: www.elsev ier .com/ locate / j t i ce

1. Introduction

Tomography is an important tool for accessing the structuraldetails of any object, specifically, if any interstitial part of the objectis inaccessible. Such as any part of human body or internal flowstructures in any reactor or separation equipment in industry.Computerized Tomography (CT) scanning is a standard diagnostictool for medical imaging, however for industrial application is stillunder development stage. This is specifically because equipmentsize can be too large also there may be some internals likeimpellers, packings (structure as well as random packing), catalyst,insulation (inside or outside of the equipment) etc. Safety issuesare even more stringent while applying a specific technique inindustrial equipment. Hence much of work is needed to use suchtechnique in industry.

Computerized tomography consists of two steps (a) the object isfirst scanned with respect to some property like gamma/X-rayabsorption, capacitance, electrical resistivity, magnetic resonanceetc. (b) the scanned property is then reconstructed by means ofsuitable reconstruction technique. Similarly tomography has twomajor objectives (a) to reconstruct the geometrical details of theunderlying object and (b) to reconstruct the property values of theunderlying object.

Tomography is normally non-destructive technique and henceis of immense use for industrial columns/reactors to detect the

* Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614.

E-mail address: jbj@udct.org (J.B. Joshi).

1876-1070/$ – see front matter � 2009 Published by Elsevier B.V. on behalf of Taiwan

doi:10.1016/j.jtice.2009.05.012

phase distribution or any kind of non-uniformity inside theunderlying object. The present work is focused on the industrialapplications of such technique.

2. Previous literature

Scanning of an object can be based on various properties. In caseof electrical capacitance tomography (ECT) (or electrical impe-dance tomography (EIT)) difference in electrical permittivity ofmaterial is measured by placing the sensors around the peripheryof the object. Hence this technique is suitable for electrically non-conducting materials. In case of electrical resistance tomography(ERT) difference in electrical conductivity is measured by thesensors placed along the periphery of the object as shown in Fig. 1.Hence this technique is used for electrically conducting material.Both ECT and ERT methods are fast and can be potentially appliedas an on-line tool for control purpose for a process. Further thesemethods are more suitable for imaging of transient processes. Thesensors used are robust and cheap (Bolton et al., 1999; Reineckeand Mewes, 1997; Vijayan et al., 2007; Wang and Yin, 2001).However, while applied for industrial cases these poses problemssince safety issues of the underlying process may not permitpassage for electric field through the equipment and contents ofthe equipments.

Magnetic Resonance Imaging (MRI) is the most advanceddiagnostic tool, which can produce almost real-time 3-D images ofthe underlying object as shown in Fig. 2. When the nuclei ofhydrogen atom, single proton, spinning randomly are caughtsuddenly in a strong magnetic field, they tend to line up in the

Institute of Chemical Engineers.

Nomenclature

g the property of the object in the pixel

Io incident intensity of gamma-radiation

I subsident intensity of gamma-radiation

L distance between source and detector (m)

N number of pixels in a ray

Pa measured value of the property of the object

Greek symbols

a mass energy absorption coefficient (m2/kg)

e hold-up

m linear attenuation coefficient (m�1)

r density of the medium (kg/m3)

Subscripts

f measurement in flow condition

g gas phase

i respective medium

ij index of the pixel

L liquid phase

m mean

TP two phase condition

v vessel wall

z zero measurement

Fig. 2. Schematic of MRI technique.

D.V. Kalaga et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 602–612 603

direction of magnetic axis. If the protons are then hit with a short,precisely tuned burst of radio frequency (RF), they will momenta-rily flip around. While returning to their original orientation, theyemit a radio signal (echo). The intensity of this emission reflects thenumber of protons in a particular slice of matter. Multi-echo-MRI,samples the NMR-signal in time for every pixel in echo-train to getthe real-time image. The advanced MRI technique can give imageshaving special resolution of 400 mm and temporal resolution of theorder of 1 ms. The particle velocity up to 1 m/s can be measuredwith an accuracy of 2% (Muller et al., 2008). However MRItechniques has some serious limitations (a) the imaging objectshould contain an atom like hydrogen, carbon and (b) there is alsolimitation for the maximum size of the object, which could be incentimeters, since the magnet should be even higher than theobject itself (Muller et al., 2008). Hence, even if MRI is mostadvanced and sophisticated tool it is most costly as well. The above

Fig. 1. Schematic of capacitance measurement technique.

discussion itself suggests that MRI cannot be used on industrialset-up as on today.

The X-ray and gamma-ray technique is most widely exploredfor several equipment as well as it appears to be the oldesttechnique used for imaging (Bartholomew and Casagrande, 1957;Grohse, 1955). X-ray and gamma-ray both work on similarprinciple. The difference lies only in the source of radiation.Whenever X-ray/gamma-ray passes through a medium someradiation is absorbed by the medium and less is attenuated at thereceiving end. The radiation attenuated depends upon the densityof the medium and the distance between the source and detector.Usually this technique is used for acquiring the steady-stateimages of the object.

Above discussion summarizes the advantages and limitations ofvarious techniques. Thus while applying for industrial cases thelimitations may arise with respect to the sensor to be used.Electrical resistivity probes, capacitance measurement, or mea-surement of magnetic resonance would be much more cumber-some to adapt for industrial columns/reactors. The X-rays are alsomore suitable for laboratory purpose. Gamma-ray source isrelatively easily available, easy to handle and use with propersafety precautions and hence are more suitable to use on industrialset-ups. Hence, in the present work gamma-ray tomography wasused to detect the phase separation, phase distribution and othernon-uniformities in various unit operations.

Table 1 highlights the work on tomography applied for variousequipments with specific objective by using a specific technique,by the individual researcher. In view of applicability of gamma-rayfor industrial columns/reactors the developments in the past fewyears are summarized here in detail. Probably the first attempt ofapplication of tomography was by Bartholomew and Casagrande(1957) where gamma-ray tomography was used to detect the solidphase distribution in a fluid catalytic cracking (FCC) unit. Gamma-rays were used for scanning the riser of FCC. Source was 60Co of5 mCi strength. The solid distribution was non-uniform and therewas a high density region on one side of column cross-section.There onwards several attempts can be seen in literature onvarious unit operations and reactors. The maximum vesseldiameter covered is 0.7 m by Azzi et al. (1991). Tomographywas applied for industrial FCC unit to detect the solid distribution.Radial asymmetry was evident from the tomograms. Shollenbergeret al. (1997), Kumar et al. (1997), Parasuveer and Joshi (1999),Marchot et al. (1999), Yin et al. (2002) showed considerable effectof type of distributor used in the respective case. The effect of

Table 1Summery of previous literature on tomography.

Reference Set-up Objective Scanning and reconstruction

methodology

Conclusion

Reinecke and

Mewes (1997)

Trickle bed, capacitance

tomography. Column diameter

0.12 m and height 2 m. Packing

material was ceramic spheres of 10,

5 and 3 mm hydraulic diameter.

Tomography of regular and irregular

packing for maldistribution.

16 peripherally mounted sensors.

Reconstruction by backprojection

method.

Gas rich and liquid rich regions in a pulse flow regime have been identified. Overall

liquid hold up was found to be independent of flow regime.

Bolton et al. (1999) Capacitance tomography to

immiscible liquid–liquid system.

Three different column diameters

(0.1 m, 0.15 m, and 0.6 m) were

used. Liquid–liquid system was

kerosene–water.

To develop non-invasive technique

for large diameter columns. To

develop a system for on-line

monitoring of phase distribution

this could be used for control

purpose.

8–12 measurement electrodes were

used. Reconstruction was done by

linear back-projection method.

The application of electrical capacitance tomography to liquid–liquid phase

contacting in the bench-scale stirred tank and flow contactor has demonstrated

accurate measurement of the volume fraction of the dispersed phase in, and on-line

visualization of the distribution of the contents. Much development is required to

provide quantitative distribution results. The results pave the way for the

development of ECT as an on-line monitoring and control tool, with the potential to

operate liquid–liquid countercurrent contacting processes closer to the flooding

point, with the obvious benefit of increased throughput, and under improved

control. In addition, ECT can identify cases of poor distributor performance due to

poor design or becoming partially blocked as a result of corrosion.

Wang and Yin (2001) Miscible liquid–liquid mixing by

using ERT. Vessel diameter was

0.19 m. Scanning was done at 0.11 m

from bottom.

To study the unsteady dynamics of

liquid–liquid mixing by estimating

the concentration gradients.

16, square electrodes placed flush on

the periphery (area 1 cm2).

Reconstruction was done by back-

projection and multi-step inverse

solution algorithms.

Multi-step inverse solution algorithm gives more accurate data than back-

projection method. Angular velocities were obtained by using auto-correlation.

Vijayan et al. (2007) Bubble column. Column diameter

0.24 m, height 2.75 m, ERT.

Effect of sparger geometry on flow

pattern transition in bubble column

by using ERT.

Void fraction characteristics were found to vary with sparger geometries. Effects of

total pressure and superficial gas velocity using different sparger designs were

observed. The hold-up measured from ERT was found to be in good agreement with

those measured by pressure transmitters. Three different flow regimes (discrete

bubbly flow, cluster bubbly flow and churn-turbulent flow) have been identified

based on void fraction properties and the wall pressure fluctuations. The spectral

analysis of ERT measurements yields the quantitative information, such as a

characteristic time and a characteristic frequency of void fraction waves, which are

closely related to flow structure in the prevailing regime.

Heibel et al. (2001) MRI in monolith film flow reactor. 25

cells per square inch (cpsi) monolith

with square channels of a hydraulic

diameter of 0.00411 m and a length

of 0.5 m. Three different distributors

were used.

To detect maldistribution in

monolith reactor. To find the

appropriate distributor and location

of distributor.

The accumulation of liquid in the corners of the square channels due to capillary

forces is confirmed. An arc-shaped gas–liquid interface was determined. An

appropriate choice of the distributor gives uniform distribution of the liquid across

the monolith. This is in good agreement with a cold-flow liquid distribution test.

Higher resolution channel-scale measurements reveal that, on a more microscopic

level, significant liquid distribution differences occur over the corners of the

individual channels. These variations show a systematic behavior, independent of

the liquid velocity, and are therefore believed to be a result of specific geometries of

monolith and the distributor. The average liquid saturation (ratio of liquid and void

area) is in very good agreement with CFD predictions. Imposed maldistribution

confirmed the expected trends in the deviation of liquid saturation. The gas–liquid

mass transfer is rather insensitive to the channel-scale distribution differences. The

effect on the residence time distribution was found to be pronounced. Especially,

the high local velocities in the corners of higher liquid saturation shift the break-

through time to lower values and result in an overall broadening of the residence

time distribution, which is not desirable for the reactor performance.

Muller et al. (2008) MRI of fluidized bed reactor. Column

diameter 0.05 m. Particle size was

0.0005–0.0012 m and density 900–

1050 kg/m3.

To develop a tool to get almost

instantaneous image of fluidized bed

to visualize the phenomena like

bubbling, jetting, falling particle

films. 3-D particle velocity around

an air jet was obtained.

A tool has been developed to obtain detail particle motions in a fluidized bed. The

images show various phenomena as bubble formation, bubble rise, formation of jet,

particle velocities near jet, slugging bed, gulf-streaming and particle dispersion.

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Toye et al. (1998) X-ray tomography in packed

column. Column diameter 0.6 m,

height 2 m, cascade min-ring 1A

random type packing.

Detection of liquid phase

maldistribution.

Fan beam (408) scanning with

aperture 1 mm thick.

The measured value of the bed void fraction is equal to the value provided by the

manufacturer. The analysis of the axial profile of void fraction shows that the ‘end

effect’ may be neglected. Analysis of the radial profile evidences the existence of a

non-negligible ‘wall effect’. The dependence of the liquid hold-up on the liquid

superficial velocity can be expressed in terms of a power law. The fitted value of the

exponent equal to 0.65 is in the range of exponent values found in correlations of

the literature.

Marchot et al. (1999) X-ray tomography in packed

column. Column diameter 0.6 m,

packed height was 2 m, random

packing of CMR1A Glitsch, cascade

mini-ring packing

To study the maldistribution in

packed column.

Fan beam (408) scanning with

apertures of 1 mm thickness.

Reconstruction by linear filtered

back-projection, with pixel grid of

512 � 512 and 1024 � 1024 with

resolution of 1 mm.

Liquid maldistribution in packed beds appears at two scales: bed scale and particle

scale. At the bed scale, the maldistribution, was found to be caused by the liquid

distributor and by the column wall. The wall effect was reasonably described by

diffusion like equation with appropriate boundary conditions. This description

introduces the radial dispersion coefficient D, which is a purely geometrical

parameter. At the particle scale, for a given operating conditions and for each

packing an equilibrium flow establishes at sufficient packing depth. This

distribution, which is not uniform, is stable in time. It may be characterized by a

non-zero maldistribution factor. A statistical model was developed which, at least

semi quantitatively was found to explain most of the experimental observations. X-

ray computed tomography was used to access the scales smaller than the particle

and leads to the determination of l. An Ergun-like equation was applied at this

scale which was found to allow computing the local liquid superficial velocities

from the local liquid holdups obtained by tomography. The radial dispersion

coefficient and maldistribution factor which are subsequently evaluated on

tomographic images, seem in fair agreement with previously published results.

Grassler and

Wirth (2000)

Gas–solid circulating fluidized bed

by using X-ray tomography. Riser

diameter 0.19 m. Downer diameter

was 0.15 m. Solid phase was glass

beads of 70 mm and superficial gas

velocity of 2–7 m/s. The experiments

were performed at four locations for

both riser and downer.

Detection of solid distribution in

riser. Potential and limitation of X-

ray tomography were investigated

for detection of solid phase

distribution.

Fan beam (358) having linear

detector array of 1024 elements.

Reconstruction was done by ART.

X-ray computed tomography does not influence the flow structure. Moreover, this

system is applicable even at higher temperatures or when electric charging caused

by flowing particles occurs. After calibrating the X-ray tomography system, it is

possible to measure average solid hold-up up to 20 vol% and a minimum spatial

resolution of 0.2 mm. Testing the tomographic system with well-defined objects

showed the results to be reliable within an error range of 5%. Experiments in riser

show a parabolic solid concentration profile with maximum concentration at wall

and minimum at the center. The cross-sectional mean solid concentration

decreases as riser height increases. At the bottom of riser, the maximum solid

concentration reaches a concentration near to minimum fluidization. The shape of

the solid concentration distribution is similar for all elevations of the riser even at

different superficial gas velocities and different circulating mass fluxes. In case of

downer, it was observed that near the distributor (at the top) most of solid is

concentrated at the center, homogeneous distribution was observed as one moves

downstream. This indicates a strong influence of the gas–solids distributor on the

flow patterns inside a downer.

Schimt et al. (2001) X-ray tomography for counter

current air–water packed column.

Column diameter 0.15 m. Packing

type was random packing of metal

ballast rings.

Liquid distribution and hold-up

values obtained at three different

elevations.

Fan beam scanning with linear array

of 125 detectors. The aperture for

scans was 0.3 mm � 0.3 mm and

0.2 mm � 0.2 mm.

The local liquid hold-up values were much useful for determination of hydraulic

behavior and mass transfer characteristics.

Schmit et al. (2004) X-ray tomography of packed column

with structured packing of Mellapak

of 500 Y. Column diameter 0.15 m.

Packed height was 0.61 m.

To measure the flow distribution

and validate it against simulations.

Fan beam scanning with linear array

of 125 detectors. The aperture for

scans was 0.3 mm � 0.3 mm.

Ring like artifacts were observed in the CT images. Accumulation of liquid in the

region above the joint between the two packing elements was also observed, in

spite of operation below the loading point for this packing. The liquid hold-up was

measured from the transmission data obtained during the CT scans and compared

with the traditional measurement technique of liquid holdup.

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Table 1 (Continued )

Reference Set-up Objective Scanning and reconstruction

methodology

Conclusion

Van der Merwe

et al. (2007)

X-ray tomography of trickle bed

reactor. Column diameter 0.4 m,

Packing is spherical porous g-

alumina catalyst of diameter 25 mm.

Flow is cocurrent downward.

Flow distribution and stability in a

trickle bed.

The radiographic images indicate that the liquid hold-up stabilizes shortly after the

end of the start-up procedure for all pre-wetting modes except non-pre-wetted

beds. These flow non-uniformities persist at high liquid and gas flow rates.

Although the bed-averaged liquid hold-up and general flow type (sometimes

referred to as rivulets for Levec pre-wetted beds and films for Kan and Super beds)

are completely reproducible, the exact location of the liquid in the bed appears to

have a stochastic nature. Local hold-up changes were observed (without changing

the bed-averaged values) in the Levec mode, but not in the KanLiq, Kan-Gas or

Super modes. These changes are sudden but with a low frequency, occur at both

high and low liquid and gas flow rates and do not appear to alter the general flow

type. Apart from these small perturbations in this one mode the overall flow

structure remains stable for several hours, suggesting that the different modes of

operation will prevail for longer operating times. These observations form the basis

of an ongoing computed tomography study of trickle flow in the various pre-

wetting modes.

Boden et al. (2008) X-ray tomography in stirred tank.

Cylindrical autoclave with

hemispherical bottom. Reactor

diameter 0.08 m. Gas inducing type

impeller with six blades

3D Gas hold up distribution in

stirred tank with high accuracy

which would be used for CFD.

Fan beam tomography with

transform based reconstruction.

3D images showing the gas phase distribution were obtained. The high accuracy in

time-average phase fraction distribution is achieved by proper and practical

correction measures for the problems of beam hardening and radiation scattering.

The values of gas fraction are better than 3.5% for a single voxel size of 200 mm.

Bartholomew and

Casagrande (1957)

Gamma-ray tomography of riser of

Fluid catalytic cracker. Column

diameter was 0.52 m

To map the solid concentration

profile in the riser.

Scanning was done by fan beam

method. Reconstruction was done

by algebraic method.

The non-uniform flow distribution was found. The high density region was seen

near the wall on one side where as gas-rich region on the opposite end of the

scanned cross-section was observed.

Azzi et al. (1991) Gamma-ray tomography applied to

Circulating fluidized bed.

Experiments were performed in

three different set-ups, (a) a stand-

pipe of FCC unit (id 0.609 m), (b)

riser of FCC (id 0.19 m) and (c)

industrial FCC (id 0.7 m) at 4 m from

feed point.

To map the solid concentration

profiles in the FCC units.

Parallel beam scanning. In three

different directions was used with 7

locations per direction. Source was137Cs of 57 mCi. Detector was

collimated with 30 mm diameter

opening.

The tomographic method was developed for detecting the solid concentration

profile in the riser with minimum number of measurements. Even if number of

scans is limited the resolution of the image is sufficient to diagnostic purpose.

Hosseini-Ashrafi and

Tuzun (1993)

Gamma-ray tomography applied to

granular flow in cylindrical and

conical vessels. Maximum diameter

of object is 0.1 m.

To develop Gamma-ray Tomography

set-up for granular flow in a hopper

of diameter 0.096 m and 2 m high

and also in a conical discharge of

same hopper.

Parallel beam scanning was used.

The source and detector were moved

every 1.58 till 1808. This forms a grid

of 155 by 155 mm square grid.

Source was 153Gd with half life of

240 days. Collimators of aperture

width of 1 mm and 2 mm

Reconstruction was done by filtered

back-projection.

Tomographic scanning was done for mom-sized and binary mixture of two

different particle sizes. The resolution of 1–2 mm is sufficient for the present case.

The voidage profile data generated in the model silo are used to identify the planes

of maximum and minimum voidage as well as that of the propagation velocity of

the voidage maximum as a function of the time of discharge. These results agree

well with the expected initial flow transients and the subsequent flow regime

transitions in the cylindrical and conical sections of the model silo.

Shollenberger

et al. (1997)

Gamma-ray tomography for bubble

column. Column diameter was

0.19 m and 0.48 m and height of

1.8 m and 3 m respectively. A bubble

cap sparger was used for column

diameter of 0.19 m and ring type

sparger for 0.48 m diameter column.

For 0.19 m column diameter,

scanning was done at 0.57 m from

distributor. For 0.48 m column

diameter, scanning was done at

0.96 m from distributor.

Gamma-ray densitometer for

detection of radial hold-up profiles

for industrial size reactors.

Gamma-ray source 5 Ci 137Cs. Fan

beam scanning. Aperture for source

collimator was 3.175 mm and that

for detector was 6.35 mm.

Reconstruction was done by Abel

inversion.

Little difference was observed in average gas hold up values obtained by gamma-

ray tomography and differential pressure measurement. This may be due to

difficulty in measurement of accurate gas hold-up near wall. The gas hold-up

profile for large diameter column is flatter as compared to small diameter column,

at the same gas flow rate. Also for large diameter column the wall hold-up is non-

zero, this was attributed to different type of sparger used.

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Kumar et al. (1997) Gamma-ray tomography was

applied to bubble columns. Five

different column diameters were

used (0.102, 0.14, 0.19, 0.26, and

0.3 m). Gas phase was air and liquid

phase was deionized water and 50%

iso-propanol–water mixture. Ratio

of static liquid height to diameter in

all cases was 5. Five different sieve

plate distributors were used. Also

simple cone and a single bubble cap

riser were also employed.

Measurements were made at five

different axial locations.

To study the effect of column

diameter, superficial gas velocity

distributor design and liquid phase

properties on gas hold-up profile in

bubble column.

Gamma-ray source was 137Cs of

100 mCi encapsulated in a lead block

so that it provides a fan beam

subtending and angle of 408 in

horizontal plane. Source was

collimated with a central slit of

dimensions 0.2 m � 0.1 m � 0.1 m.

Detectors were collimated with a

hole of 5 mm � 10 mm. Image was

reconstructed by convolution-

backprojection, ART and

estimation–maximization (E–M)

algorithm. E–M reconstruction was

found most appropriate and hence

was used for bubble column

experiments.

The observation that effect of column diameter above 0.15 m on gas hold-up is

negligible has been validated. The power law exponent of 2 to 2.5 for the radial

hold-up distribution is reasonable. As column diameter and superficial gas velocity

increases, the gas hold-up profile changes from flatter distribution to a parabolic

distribution. The gas hold-up profile changes with axial location, however the

entrance length required for characterizing the fully developed flow could not be

estimated. The type of distributor shows effect only for low superficial gas velocity.

In churn-turbulent regime negligible effect of distributor found. Significant effect of

liquid phase property (surface tension) was found on magnitude and shape of gas

hold-up profile. The liquid with low surface tension produces very fine bubbles,

hence large values of gas hold-up and flatter hold-up profiles were obtained.

Parasuveer and

Joshi (1999)

Gamma-ray tomography of bubble

column. Bubble column diameter

(D) was 0.38 m, height was 3.2 m.

Five different sieve plate spargers

were used. Hole diameter ranges

0.8–87 mm, % free area ranges

0.135–5.4. Tomography was done at

three axial locations (0.259D, 3D and

5D). Gas–liquid system was air–

water.

To study the effect of sparger design

and height of dispersion on gas hold-

up profile.

Scanning was done by both fan beam

and parallel beam. Fan beam was

used till dimensionless radial

distance (r/R) of 0.7 there onwards

parallel beam was used for 0.8, 0.9,

and 0.95 r/R. Source was 137Cs of

67.5 mCi. Source was collimated

with a slit of

35 mm � 8 mm � 30 mm. Fan beam

angle was 308. Detector was

collimated with a vertical slit with

6 mm � 18 mm � 30 mm. 1D hold-

up profile was estimated by using

Abel inversion technique.

For multipoint sparger, the gas hold-up profiles are relatively flat at the bottom and

become steeper with an increase in height of dispersion. For single point sparger,

(single hole with diameter 25 mm), gas hold-up profile is very steep at bottom and

becomes flatter as axial distance increases. Consequently, centerline hold-up

decreases and wall hold-up increases. For single point sparger, as hole diameter

increases (from 25 mm to 87 mm) centerline hold-up was practically constant and

wall hold-up increases. For air–water, the shape of gas hold-up profile remains

nearly constant beyond HD/D of 5 for multipoint sparger. Absence of any axial hold-

up profile for multipoint spargers and for single point sparger axial hold-up profile

exists till HD/D of 3. For multipoint sparger, the gas hold-up profiles are flat at low

values of HD/D and are steeper as height of dispersion increases.

Wang et al. (2001) Gamma-ray tomography in

randomly packed column. Column

diameter 0.6 m. Packing was Pall

ring of diameter 16, 25 and 38 mm.

Liquid distribution in packed

column

Fan beam scanning with detector

opening of 50 mm. 12 � 7 scan lines

were used per plane. Reconstruction

by Fourier Transform method and

also based on least square.

The spatial porosity distribution in random packed columns is not uniform. There

are always some pockets in the packed beds, where the porosity is higher than the

average value. The size of the pockets is generally one to three diameters of the

packings. For the circumferentially averaged radial porosity distribution, the

porosity in the wall region tends to be higher than that in the bulk region. The

experimental results also show that the porosity variation can be described by a

normal distribution function in the bulk region of the packed bed.

Boyer and

Fanget (2002)

Gamma-ray tomography for large

diameter trickle bed reactor. Column

diameter 0.6 m.

To find the gas–liquid distribution in

trickle bed for various distributor

configurations

Fan beam scanning was used. High accuracy gamma-ray tomography tool was developed for large diameter

columns.

Yin et al. (2002) Gamma-ray tomography of

randomly packed column with pall

rings. Column diameter 0.6 m,

Distributor was ladder type with 6

branches and 31 drip points. In

second distributor 15 holes along

periphery were plugged. The third

was single pipe distributor located at

axis.

To study the effect of sparger design

on liquid phase maldistribution.

Fan beam scanning. 12 � 7 lines

scanning. Measurements were done

at two different axial locations.

Liquid distribution was found to be non-uniform and strongly dependent upon the

type of distributor. The experimental liquid phase distribution agreed with the CFD

predictions (CFX4.2). To achieve a better distribution of liquid inside a column, the

liquid distributor should be designed to supply liquid on the top of the packings as

uniformly as possible.

D.V

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60

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26

07

Table 1 (Continued )

Reference Set-up Objective Scanning and reconstruction

methodology

Conclusion

Roy et al. (2004) Gamma-ray tomography in

structured packed column. Column

diameter 0.3 m, height 1.52 m,

packing was Norton type.

To find the extent of uniformity in

liquid distribution and its effect on

superficial gas and liquid velocity.

Fan beam (408) scanning with

aperture of 2 mm in vertical plane

was used. Source was 137Cs of

70 mCi strength. Detector collimator

has 2mm width. 9 detectors were

used. The projection was at every

0.28. Scanning was done at three

axial levels 1.5, 2.5, and 3.5 D.

Reconstruction was done by

maximization algorithm.

The solid fraction as determined by CT scan was about 2.5%, which was close to the

value of 3% as determined by water displacement method. The liquid saturation

increases with increase in the superficial liquid velocity. Moreover, the liquid

saturation increases as the liquid phase moves downward. The liquid distribution

was found to be fairly uniform. Liquid distribution was better at the bottom of the

bed, compared to the upper section. The effect of gas velocity, was in general, found

to be very small at the conditions used in the study.

Jin et al. (2005) Gamma-ray tomography in a high

pressure bubble column. Column

diameter 0.3 m, height 6.6 m.

Superficial gas velocity up to 0.4 m/s,

operating pressure 1 MPa.

Distributor was a four nozzle having

diameter of 20 mm. Gas–liquid

system was air–water and air–acetic

acid. Sodium oleate was used to

reduce surface tension of water.

The effects of superficial gas

velocity, liquid surface tension,

liquid viscosity, and system pressure

on the axial gas hold-up have been

investigated in this study.

Four differential pressure sensors

were placed at 0.25, 0.75, 1.25, 1.75,

and 2.25 m above distributor.

Gamma-ray source was 100 mCi

Cs137.

The experimental results of the pressure and the gamma-ray measurements in this

study were found in a very good agreement. The axial holdup was found to vary

considerably along the height of the column. The axial gas hold-up increases with a

decrease in the liquid surface tension and liquid viscosity and an increase in the

system pressure. According to the axial distribution of gas holdup, the

accumulation part of foam in the upper section of the column was obtained, and the

gas hold-up in foaming zone linearly increases as the axial height increases. The

foaming height was found to be dependent on the gas and liquid properties and the

operating condition.

Schubert et al. (2008) Gamma-ray tomography for trickle

bed reactor. Column diameter

0.09 m. Packing material was an

alumina catalyst and glass beads

with diameter of 4 mm.

To present a comparative analysis of

the liquid flow dynamics for two

different initial liquid distributions

and two different types of reactor

configurations. Thus, the

hydrodynamic behavior of a glass

bead packing was compared to a

porous Al2O3 catalyst particle

packing using inlet flow from a

commercial spray nozzle (uniform

initial liquid distribution) and inlet

flow from a central point source

(strongly non-uniform initial liquid

distribution), respectively.

Gamma-ray source was Cs137 with

160 GBq activity. Focal spot

diameter of source was 4 mm.

Collimated source beam has height

of 2 mm and an angle of 468.Distance between source and

detector was 512 mm. Total 320

detectors with active area of 2 mm

width and 8 mm height. Area of

cross-section of 2.6 mm by 2.7 mm.

Distance between source and the

column axis was 350 mm.

Reconstruction was done by ART.

High resolution gamma-ray tomography was used for trickle bed reactor. With the

experimental study, the maldistribution of the dynamic liquid hold-up for a spray

nozzle producing a uniform initial distribution and a point source distributor

producing a central liquid stream in both glass bead bed and porous catalyst bed

could be seen. For the glass bed well developed liquid channels and totally dry

regions in the cross-sections at every plane downstream from the spray nozzle

were identified. In the catalyst packing cross-sectional dynamic liquid saturation

distribution has been found nearly uniform which is attributed to the boosting

effect of the porosity. Liquid spreading from a point source was clearly observed in

the glass packing, while in the catalyst packing after an entrance region with a

length-to-diameter-ratio of about one, dynamic liquid saturation was almost

uniformly distributed over the cross section. The very different hydrodynamic

behavior of the non-porous glass beads and real catalyst particles with porous

Al2O3 support suggest that a bed of glass beads is not representative for catalytic

packings and should not be used for the study of hydrodynamics in such columns.

Tortora et al. (2008) Electrical impedence tomography

(EIT) and gamma-ray tomography

for circulating fluidized bed. Riser

diameter 0.14 m and height 5.77 m.

Solid phase was catalyst particles.

EIT was verified with gamma-ray

tomography. Verification of

accuracy of correlations for radial

voidage profiles using the two

techniques.

Fan beam tomography was used.

Source was 100 mCi 137Cs. 8

Detectors were used and 7 views

were used. Reconstruction was done

by Abel inversion. In EIT 16

electrodes were used along the

periphery.

Radial voidage profiles were compared with literature correlation and good

agreement was found. Radial solid flux was obtained from radial voidage profiles

and found that accuracy of correlation strongly depends on the measurement

accuracy at the center of riser. EIT is less expensive more safe and faster than GDT

and hence recommended in place of GDT.

D.V

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Fig. 4. Schematic of parallel beam method of scanning.

D.V. Kalaga et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 602–612 609

distributor was found to be pronounced. Tortora et al. (2008)compared the ECT and gamma-ray tomography technique andfound that ECT is much less expensive, safer and faster thangamma-ray tomography. However ECT cannot be used forindustrial units for the above mentioned reasons. Schmit et al.

(2004) has successfully demonstrated the maximum resolution byusing X-ray tomography in a packed bed. The high resolutionimages show the liquid accumulation regions in the packingmaterial. Further Boden et al. (2008) could get 3D images by usingX-ray tomography, in which the single voxel size was as low as200 mm. In other cases the resolution of few millimeters iscommon (Azzi et al., 1991; Roy et al., 2004; Wang et al., 2001; Yinet al., 2002). As long as industrial set-ups are concern very fewattempts can be seen in literature (Azzi et al., 1991; Bartholomewand Casagrande, 1957), both are applied for FCC. The abovediscussion suggest that tomography is required to be applied inindustry to identify the real world problems whose causes could befar different though result may be simple like product degradation,loss in yield of the desired product etc.

3. Experimental set-up

The gamma-ray absorption technique was used in severalindustrial set-ups with two major objectives. In the first case,objective was to detect the solid level in a paddle bed dryer, fromwhich quantity of solid could be estimated and hence the residencetime of solid could be estimated. Hence measurements wereundertaken at three different locations along the axis of the dryer,with a separation of 1 m. The source and the detector were suitablyplaced so that both could be vertically moved in steps of 0.05 m.The set-up is shown in Fig. 3. The gamma-ray source used in all thecases was 137Cs of 1 mCi strength. The source was collimated witha slit of 3 mm by 50 mm. The scintillation detector wasuncollimated in all the cases. The dwell time in all the caseswas 20 s and ten readings were taken for each scan and averagevalue was used for estimation of line integral of the hold-up values.

In second set of experiments, gamma-ray tomography wasemployed for three different packed bed distillation columns. Infirst case column was packed with random packing and columndiameter was 0.5 m. In other two cases column was packed withstructured packing and column diameters were 0.8 and 1.6 mrespectively. The objective was to detect the non-uniformity indistribution. The reflux distributor was placed at the top. In all thecases, scanning was conducted away from the distributor since themaldistribution is likely to occur away from distributor. Scanningwas done by parallel beam scanning in four directions as shown inFig. 4. The scanning was conducted at the interval of 0.01 m.

Fig. 3. Schematic of paddle bed dryer.

4. Data analysis

The gamma-ray absorption of monoenergetic source in anymedium follows the Lamberts–Beers law given by the followingequation:

Io ¼ Ie�mL (1)

where, Io is the incident intensity of radiation, I is the subsidentintensity of radiation, m is the linear attenuation coefficient, whichdepends on the physical and chemical state of the medium and canbe expressed as ar. Where r is the density of the material and a isthe mass energy absorption coefficient. The intensity of gamma-ray propagating in a different homogeneous absorbing medium inseries with different density can be given by following equation:

Io ¼ Ie�P

iriaiLi (2)

where i is the medium like vessel wall, insulation, any otherinternal, etc. The measurement of Io can be avoided by means ofzero reading. This means if radiation intensity is measured for anempty vessel consisting insulation, vessel wall for which radiationintensity can be given by following equation:

Iz ¼ Ie�ðrairaairLairþrvavLvþrinsulationainsulationLinsulationÞ (3)

Fig. 5. Variation of intensity of gamma-radiation at various axial locations in a

paddle bed dryer.

Fig. 6. Solid level at various axial location in the paddle bed dryer.Fig. 7. Intensity of gamma-radiation in a packed distillation column during dry

condition.

D.V. Kalaga et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 602–612610

Similarly radiation intensity measured during actual flow condi-tion then Eq. (3) can be rewritten as:

If ¼ Ie�ðrgagLgþrvavLvþrinsulationainsulationLinsulationþrfaf Lf Þ (4)

taking ratio of Eqs. (3) and (4)

Iz

If¼ eðafrf LfþagrgLg�aairrairLairÞ (5)

however

Lair ¼ Lf þ Lg (6)

substituting Eq. (6) in Eq. (5)

Iz

If¼ eðafrf�agrgÞLfþðagrg�aairrairÞLf (7)

and the mean density along the path can be given as

rm ¼ rf

Lf

Lair(8)

Fig. 8. Liquid hold-up distribution in the packed distillation column having column diam

color in this figure legend, the reader is referred to the web version of the article.)

eliminating Lf from Eqs. (7) and (8) and considering aair rair isapproximately equal to ag rg and solving for rm

rm ¼1

rf Lairln

Iz

If

� �(9)

The Eq. (9) forms the basis of measurement and the respectivevalues of hold-up can be easily estimated. The values of linearattenuation coefficient as well as mass energy absorptioncoefficient for different material and for sources of differentenergy are reported in the literature (Tsoulfanidis, 1983).Otherwise they can be easily measured.

The estimation of linear attenuation coefficient or mass energyabsorption can be eliminated by making the measurements in avessel filled with gas, then with liquid/solid and then in theoperating condition. Under these circumstances Eq. (1) can beapplied for each case and then after rearrangement hold-up can beestimated by using following equation:

e ¼ lnðITP=ILÞlnðIg=ILÞ

(10)

eter 0.5 m and packed with random packing. (For interpretation of the references to

Fig. 9. Liquid hold-up distribution in the packed distillation column having diameter 0.8 m and packed with structured packing. (For interpretation of the references to color

in this figure legend, the reader is referred to the web version of the article.)

D.V. Kalaga et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 602–612 611

However, in order to use Eq. (10) one needs to take measurement,when column is filled with only gas, then with only liquid and then inactual flow condition. In industrial set-up it is practically difficult tofill the column with only liquid (operating fluid) and hence in thepresent work hold-up values were estimated using Eq. (9), since themass energy absorption coefficient can either be obtained fromliterature or from experiments.

The reconstruction of image from the individual scans was doneby algebraic reconstruction technique (ART). The ART can bedescribed by the following equation (Hesselink, 1989):

gkþ1ij ¼ PaP

gkij

gkij (11)

where, Pa is the measured value of the hold-up, g is the hold-up inthe ijth pixel and k is the iteration number.

5. Results and discussion

Fig. 5 shows the count difference (radiation intensity) withrespect to vertical displacement in the paddle bed dryer at three

Fig. 10. Liquid hold-up distribution in packed distillation column having diameter 1.6 m

this figure legend, the reader is referred to the web version of the article.)

different axial locations as described in the previous section. Thepresence of solid level can be easily seen by sudden difference inthe counts. The measured solid level is shown in Fig. 6. It can beseen from Fig. 6 that solid level is high at the entrance anddecreases towards the discharge point of the dryer.

Fig. 7 shows the count rate in blank readings (in absence ofliquid and gas in a packed distillation column) in both thedirections. A sudden drop in the counts can be clearly seen, whichindicates the presence of highly dense matter in the gamma-raypath. Similarly the count rate increases at the center and it is lowtowards wall. This observation may be because of scatteringoccurred due to dense region, since detector was uncollimated.Hence primarily it was concluded that some solid material isrigidly accumulated in the central region of column. Thereconstructed image is shown in Fig. 8. It shows very less ornegligible hold-up of liquid in the central region, whereas most ofthe liquid is channeled near the periphery of column. In order toidentify the problem the column was shut-down, the packingswere removed and it was found that a lot of residue had struck tothe packing material in the similar region. After suitable cleaning

and packed with structured packing. (For interpretation of the references to color in

D.V. Kalaga et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 602–612612

and repacking, distillation efficiency was improved by 5%. In caseof packed column of diameter 0.8 m, with structured packing thehold-up profile is shown in Fig. 9. The average liquid hold-up was3% and maximum liquid hold-up was 36%, concentrated in verysmall portion which can be seen in Fig. 9. Further, it can be seenthat there is void space in nearly central region. Lot of channelingcan be seen, this must be because scanning is 4 m away from thesparger. Further no much discontinuity in count rate was observedduring blank reading. Hence it is unlikely that any residue hasstruck as observed in the previous case. In the third case, of columndiameter of 1.6 m, the hold-up profile is shown in Fig. 10. Theaverage hold-up was 3.5%. The maximum liquid hold-up was 15%.

6. Conclusion

Gamma-ray tomography is most suitable technique to diagnosethe various kinds of non-uniformities in industrial equipment.Gamma-ray technique was successfully applied for detecting thesolid level measurement in a paddle bed dryer. Further gamma-raytomography was successfully applied for various industrialcolumns covering range of column diameters. Industrial problemin a packed distillation column like chocking or corrosion ofpacking material was successfully identified by using gamma-raytomography. It is worth mentioning that distillation efficiency wasimproved after installation of new packing. Thus periodic scanningof critical equipment by using gamma-ray tomography canimprove the efficiency and also reduce the possible down timeof the plant for unidentified problem.

Acknowledgements

K.S. Dinesh kumar would like to acknowledge the financialsupport from UGC in the form of fellowship. A.V. Kulkarni wouldlike to acknowledge the financial support from BRNS in the form offellowship.

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