Process Diagnostics

28 www.aiche.org/cep May 2007 CEP

Flow maldistribution in process units such as packedcolumns can severely reduce process efficiency andoperability. To address this issue, computer-aided

tomography (CAT) using high-energy gamma rays canprovide information about the distribution of liquids,vapors and solids inside a vessel (1).

CAT-scans generate a cross-sectional density profile of acolumn or riser duct at a fixed elevation. This profile isdeveloped by taking several different angular measure-ments between a source and a detector from many differentpoints on the circumference (1). Gamma-ray tomographytakes advantage of the penetrating properties of high-ener-gy gamma radiation to detect internal process densities ofreactors, absorbers and distillation columns. Recentdecades have seen rapid growth in process tomography (2).

This article describes the use of gamma-ray CAT-scan-ning for measuring liquid distribution in industrial packed-bed columns, as well as for troubleshooting such equipment.

Gamma CAT-scan technologyMany different radioisotopes can be used as gamma-ray

sources. Proper source selection is critical to the successor failure of gamma scans. When selecting a source, keepthe following guidelines in mind:

Photon energy. The larger or more dense the bed, themore energetic the source should be. Source energy shouldbe adapted to the column diameter, process density andholdup in the bed for good transmission and accuracy.

Activity. Sufficient source activity is required to pene-trate the vessel, refractory or packing, and process fluids.

When source activity is low, the radiation detected is low,and levels of background radiation can be significant (lowsignal-to-noise ratio). Longer counting times are necessaryfor smaller sources, since the accuracy of the counting-ratemeasurement depends on the counting time. To minimizecounting time, the highest possible source activity must beselected. On the other hand, sources with high activity canreduce resolution and pose personnel exposure problems.

Detectors. Scintillation counters with a solid inorganiccrystal are superior to gas-filled detectors (i.e., Geiger-Mueller counters) in terms of stability and sensitivity. Anintermediate-sized crystal (such as NaI) will detect scatteredradiation in addition to the pulse measured from the originalmono-energetic gamma-ray source. Scattered radiation caninterfere with achieving an accurate result if its effects arenot recognized and managed effectively by using the properequipment, procedures and equipment settings. As crystalsize increases, the scatter component becomes less signifi-cant (3). Crystal size must be tailored to the job.

Scan lines. In the first gamma-ray tomography experi-ment (4), a 5-millicurie cobalt-60 source and a Geiger-Mueller detector were used. It involved 18 discrete meas-urements, which were shaped in a fan pattern. The densitydistribution was calculated graphically and analyticallybased on these results. For the analytical method, the datawere reconstructed by assuming a fourth-order polynomialdensity function and determining its coefficients by theleast-squares method.

That approach can be revised for packed towers. Unlikea reactor riser pipe, the gamma absorption and density

Gamma CAT-scan technology makes possible the collection of much more vital online process information than conventional diagnostic techniques.

Simon X. XuShaw Stone & WebsterWilliam MixonTracerco

Diagnosing Maldistribution in

Towers

CEP May 2007 www.aiche.org/cep 29

effects of the packing must be considered, since both willvary with installation and operation. Secondly, more scansare required for the towers, since most towers are larger indiameter, and, hence, cross-sectional area. The third andmost important consideration in industrial towers is todevise a system that is both functional and transportable— the system must be flexible, adapting to the individuali-ty of a column’s design and construction, as well as com-pact so it can be easily moved from one location to anoth-er in a timely manner. It must provide reproducible posi-tioning for both the source and detector on several scans.

Dry scan. For applications that involve common com-mercial packings and clean processes, the bed densitiesand gamma absorption coefficients can be determined inthe lab. Otherwise, a “dry scan” of the tower is needed toaccount for the packing. This dry scan should be donewhen the tower is not operating, either before startup orafter shutdown.

Data reconstruction. A fourth-order polynomial densityfunction contains 15 coefficients that must be determinedby regression of the scan data. The more data points (scanpaths), the greater the statistical accuracy of the regresseddensity profile. However, the more data points taken, themore time is needed to perform the data acquisition, whichcan be a problem if the column is unstable.

CAT-scan for packed columnsThe “new generation” of packed column may exhibit

higher capacity, better efficiency and lower pressure dropthan a trayed column of the same size. However, this isbased on the premise of good vapor/liquid distribution inthe packed bed.

In industrial distillation columns, common causes ofliquid maldistribution include: design, manufacturing orinstallation defects in distributors or packing beds; damageor plugging; and process disturbances. The challenge fortroubleshooting engineers is diagnosing the maldistribu-tion problems inside the beds.

Since the 1980s, initial liquid distribution hasreceived increasingly more attention, and many guide-lines and test facilities for the design and assessment ofliquid distributors have been developed (5). However,understanding the liquid distribution within industrialpacked columns is still nothing more than mathematicalspeculation based on laboratory tests (6, 7).

In packed towers, liquid maldistribution can be clas-sified as micro-maldistribution (random) or macro-maldistribution (non-random). Micro-maldistribution ischaracterized by slight and random differences in flowsfrom the drip points across the distributor, or from “ran-dom wandering” of liquid in the packing channels. This

type of maldistribu-tion can be offset bylateral mixing of theliquid traffic in thepacked bed, whichcounterbalances anyill effect on overallefficiency (8).However, the impactof large-scalemacro-maldistribu-tion is much moresevere (5).Malfunctioning liq-uid distributors andfouling/plugging in packing beds are two common prob-lems that cause macro-maldistribution.

In an industrial setting, CAT-scans detect macro-, butnot micro-, maldistribution. It is not practical or reason-able to expect a fourth-order polynomial to provide a high-resolution image. Should micro-maldistribution become amajor concern, more-complicated models for data collec-tion and reconstruction will be needed.

Data collected from 20 to 200 scan lines are generallysufficient to construct a representative tomography of mostindustrial packed towers. Figure 1 shows a fan-beamscheme of three fan rotations, with nine scan paths perrotation (a 3 × 9 pattern). The object is to optimize theplacement of the scan paths to produce the best image.Figure 2 shows two different conventions of presentingCAT-scan results.

Demonstrating the techniqueTo evaluate the accuracy of CAT-scan techniques in

identifying liquid maldistribution, a series of scans wasperformed on a 3-ft-dia. (914 mm) Plexiglass tower sec-

� Figure 2. CAT-scan results can be presented as a 3D contour map ora 3D surface map.

3D Contour 3D Surface

Den

sity

, lb

/ ft3

Density, lb / ft3

0

5

10

15

20

Nor

th

North

14–20 10–14 6–10 4–6

X

Y

Y

–0.9

0

0.9

–0.90

0.9–0.9

0

0.9

–0.9 0 0.9

� Figure 1. The 3 × 9 scheme consistsof three fan rotations with nine scanpaths per rotation.

*

*

*

* Source Detector

30 www.aiche.org/cep May 2007 CEP

tion with a 5-ft (1,524 mm) random packing bed, at theKoch-Glitsch Research Center. A ladder pipe distributorspread water to the top of the bed (with no countercur-rent vapor flow), and the liquid was collected in anannular collector or a chord collector beneath the packedbed (Figure 3). Liquid flowrates were measured for eachsection or annulus using the bucket-and-stopwatchmethod. By sealing some of the distributor holes, annu-lar and chord liquid maldistribution patterns were simu-lated at liquid flowrates ranging from 5 to 20 gpm/ft2

(12–49 (m3/h)/m2). The CAT-scan elevation was 6 in.(152 mm) above the bottom bed support. A 9 × 9 scanpattern was utilized.

Figures 4–7 show CAT-scan results of the overall beddensity for simulated operations involving:

• good distribution• chord maldistribution (more liquid flowing to one side)

Process Diagnostics

� Figure 3. Chord collectors and annular collectors are used todetermine liquid distribution.

1 2 3 4 5 6 7 8 912

34

56

78

910

1112

1314

1516

1718

Chord Collector Annular Collector

Nor

th

� Figure 4. CAT-scan for the good liquid distribution case.

Den

sity

, lb

/ ft3

Nor

th

Density, lb / ft311–22 10–11 9–10 <9

0

0.9

0

–0.6

4

8

12

16

20

0.90

–0.9

� Figure 6. CAT-scan for the center annular maldistribution case.

Density, lb / ft314–20 10–14 6–10 <6

Den

sity

, lb

/ ft3

North0

0.6

0

5

10

15

20

–0.9

–0.90.9

0

� Figure 7. CAT-scan for the outer annular maldistribution case.

Den

sity

, lb

/ ft3

North

Density, lb / ft312–14 11–12 8–11 7–8 <7

0.9

0

–0.6

0

4

8

12

16

20

0.90

–0.9

� Figure 5. CAT-scan for the chordal maldistribution case.

North

Density, lb / ft313–18 6–13 <6

Den

sity

, lb

/ ft3

0

5

10

15

20

–0.9

0.9

0

0.9

0

–0.6

CEP May 2007 www.aiche.org/cep 31

• center annular maldistribution (more liquid flowingtoward the center area)

• outer annular maldistribution (more liquid flowingtoward the outer area).

From a qualitative standpoint, the bed densities asmeasured by the CAT-scans reflect the actual maldistribu-tion patterns very well.

Quantitative CAT-scan results, however, present moreof a challenge. As noted earlier, a dry scan is necessary tomeasure packing density and distribution in order to elimi-nate the packing contribution from the overall bed density.

Figure 8 presents the results of a dry CAT-scan of the test bed. Bed density varied over the range of 8–12 lb/ft3

(130–195 kg/m3) across the tower area, although the aver-age bed density was close to the packing bulk density of 10.6 lb/ft3 (170 kg/m3). Random packings can have localbed-density variances due to installation and loading pro-cedures, but weighting tests of beds that were packed dif-ferently did not show significant density differences.Further investigation found that the test column variancewas due to different orientations of the packing pieces onthe scan paths.

This indicates that a packing’s bulk density is not theonly factor that must be considered when investigatingmaldistribution within a packed bed. A dry scan is neededto account for packing orientation and external influencessuch as stiffening rings and conduits, which can also affectthe scan results.

The volumetric fraction of liquid in the total flow forany specific area can be obtained by integrating the liq-uid density distribution, which is the difference betweenthe operating-bed density distribution and the dry-beddensity distribution.

Figures 9 and 10 compare the liquid distributionsobtained by the bucket-and-stopwatch method and theCAT-scan. The distributions are in good agreement for the

“good distribution” and the “chordal maldistribution”cases. Observations during the test indicated that part of theannular collector was flooded and liquid overflowed fromone annulus to another. This precluded a quantitative com-parison of the liquid distributions from the CAT-scan andthe collector for the annular maldistribution tests. However,there is little reason to doubt the possibility of measuringthe annular maldistribution quantitatively with the CAT-scan approach, based on the good qualitative images forthe annular maldistribution tests (Figures 6 and 7) and theexcellent quantitative agreement for the tests of good distri-bution and chordal maldistribution (Figures 9 and 10).

Troubleshooting a refinery vacuum towerThe fractionation efficiency of a lube vacuum tower’s

top fractionation packed bed decreased significantly, affect-ing the yield and quality of the top sidestream product.This loss was attributed to liquid maldistribution resulting

� Figure 8. CAT-scan for the dry packing bed.

Density, lb/ft310–12 9–10

Den

sity

, lb

/ft3

North

–0.9

0

0.6

X

0

5

10

15

20

0.90

–0.9

� Figure 9. Liquid flows determined by CAT-scan and by thechord collector were in good agreement during good-distributionoperation.

30.0

25.0

20.0

15.0

10.0

5.0

0.00 2 4 6 8 10 12 14 16 18 20

Liqu

id F

low

rate

, gpm

/ft2

Collector Area Number (#18 = West Side)

Measured byBucket-and-Stopwatch

Technique

Calculated fromCAT-Scan Results

� Figure 10. Liquid flows determined by CAT-scan and by thechord collector were in good agreement during chordal maldistri-bution operation.

4.0

3.0

2.0

1.0

0.0

Liqu

id F

low

rate

, gpm

/ft2

0 2 4 6 8 10 12 14 16 18 20Collector Area Number (#18 = West Side)

9.0

8.0

7.06.0

5.0

Measured byBucket-and-Stopwatch

Technique

Calculated fromCAT-Scan Results

32 www.aiche.org/cep May 2007 CEP

from corrosion products plugging the liquid pre-distributorthat fed the gravity-flow distributor above the bed. Thepotential financial consequences of lost fractionation weresignificant. The refinery staff were able to diagnose themost likely cause of the tower problem based on thermalprofiles, analytical data and tower inspection history. Toquantify the location and magnitude of liquid distribution,the gamma CAT-scan diagnostic technique was employed.

A grid scan was performed on the tower (Figure 11) toprovide an initial look at the distribution in the two bedsof packing (the top fractionation bed (F1) and the toppumparound bed (TPA) above it). A grid scan consists offour equal scans, one through each quadrant of the tower.

For each scan, adetector and aradioactive sourceare lowered simulta-neously along thetower and gamma-ray intensity meas-urements are takenat specific eleva-tions. Under idealconditions and uni-form liquid load-ings, each scan plotwill closely overlaythe next. Non-uni-form liquid loadings

or mechanical items such as manways can cause devia-tions between the scan plots (1).

The grid scan data are shown in Figure 12. The dataindicate that both beds were in place and were experienc-ing severe liquid maldistribution. The top (TPA) bedappeared to exhibit liquid biasing to the south side of thetower accompanied by a liquid deficiency on the west sideof the tower. The F1 bed appeared to be holding up a sig-nificant amount of liquid in the top section of packing andexhibited severe liquid maldistribution in the top half ofthe packing. The maldistribution appeared to be lesssevere lower in the bed, indicating that the structuredpacking was redistributing the liquid.

A close look at the gamma absorption for the chimneytray between the beds indicated that liquid was overflow-ing the risers during the south and east scans. Liquidappeared to be approximately even with the top of the ris-ers on the north scan and two to three inches below the topof the risers during the west scan. The high liquid levelcould have been caused by plugged drip tubes or areduced draw rate from the collector.

A CAT-scan was performed on each bed of packingat the elevations noted in Figure 12 in order to quantifythe liquid maldistribution. Figure 13 represents theCAT-scan performed on bed F1 located below the vacu-um gas oil (VGO) Draw. Two dry spots were identified,one in the northeast quadrant and the other on the westside of the tower.

With knowledge of the maldistribution patterns, a teamof refinery staff, local engineering/design contractor person-nel and corporate engineers developed a novel, first-of-a-

Process Diagnostics

� Figure 11. The scan line orientationsfor the grid scan of the lube vacuumtower.

Gamma SourceRadiation Detector

N

� Figure 12. Grid scan data for the lube vacuum tower.

123456789

101112131415 16 171819202122

100 1,000 10,000

Tan Line

CAT-Scan Elevation

Manway

Platform

VTPA/VGO Draw

Top Bed

Bed F

1

Ring 194'6"Risers Hats 194'1"

Top of Risers 192'7"Pan #5 191'9"Bottom of Stump 190'4"Distributor 190'3"Drip Tube Distributor

CAT-Scan Elevation

Ring

Riser Hats 180'5–5/8"

South ScanNorth Scan East Scan

West Scan

� Figure 13. The CAT-scan of the top fractionation bed (F1) identified two dry spots.

Manway

PumparoundReturn

Draw

Density, lb / ft3

15–16

14–15

13–14

12–13

11–12

10–11

CEP May 2007 www.aiche.org/cep 33

kind online fix that involved a pump-back reflux circuit offthe top pumparound. CAT-scanning was used to determinethe orientation of the new liquid distribution system, focus-ing on the low-liquid-density areas of the tower cross-sec-tion. After startup of the new circuit, 65–70% of the lost topsidestream yield was recovered, exceeding expectations (9).

Identifing the cause of liquid maldistributionin a revamped distillation tower

A petrochemical plant performed a major revamp of oneof its most critical distillation towers. This tower was over150 ft tall, over 20 ft in diameter, and contained severallarge beds of structured packing. The scope of the revampincluded removing all of the old distributors and packedbeds and replacing them with a more-efficient design.

Upon startup, the tower was unusually unstable. Theoperations staff could not increase the feed rate whilemaintaining the desired product quality. Both the overheadand bottoms products were off-specification, even whenthe tower was stable and operating below design rates.

As a first diagnostic step, samples were taken at eachpacked bed. Analysis of these samples indicated that all ofthe beds were performing poorly, but the source of the prob-lem could not be determined. One possibility was maldistri-bution of vapor and/or liquid through all the beds. If the dis-tribution was good, then the packing was falling well short

of its design efficiency. Another possibility was thatmechanical damage might have occurred during startup.

To help identify the cause of the problem, a grid scan ofthe tower was performed. The grid scan would reveal anymechanical damage and provide information on the qualityof the liquid/vapor distribution through each of the beds.

The results of the grid scan showed that all distributors,collectors and packed beds were in place with no evidenceof mechanical damage. It also showed slight liquid maldis-tribution in all of the beds, which did not appear severeenough to cause the very poor efficiency of the tower. Thescan line orientations and grid scan results from the feedbed are illustrated in Figure 14.

Because a grid scan is comprised of only four scanlines, it does not give a picture of the entire cross-sectionalarea of the tower. The grid scan would identify liquidmaldistribution only if it had an asymmetric pattern (e.g.,liquid channeling down one side and vapor channeling upanother side).

Liquid maldistribution was still strongly suspected. Toobtain a more-detailed distribution profile, a CAT-scanwas performed near the top of the bed.

The CAT-scan results (Figure 15) showed that a largeamount of liquid was channeling down the center of thebed. This confirmed that a liquid maldistribution problem,rather than poor packing performance, was the cause ofthe efficiency reduction in the tower.

With the information provided by the scans, plant per-sonnel decided to shut down the tower and inspect the liq-uid distributors. Upon inspection, an error in distributor

� Figure 14. Grid scan for the revamped packed column.

56'54'52'50'48'46'44'42'40'38'36'34'32'30'28' 26'24'22'20'18'16'14'12'10'

6'8'

4'2'

CAT-Scan Elevation

100 1,000 10,000

Manway

ManwayRing

Platform

Distributor

Distributor

Collector

Collector

Feed

Scan Line OrientationRing

Ring

Feed Bed

N

SourceDetector

Southwest Scan ChordSoutheast Scan ChordClear Vapor Bar

Nouthwest Scan ChordNoutheast Scan Chord

� Figure 15. The CAT-scan shows more liquid channeling downthe center of the revamped packed bed.

Manway Draw

Feed

Average Liquid Flux, %153%–165%141%–153%129%–141%

106%–118%94%–106%82%– 94%

71%–82%59%–71%47%–59%

118%–129%

34 www.aiche.org/cep May 2007 CEP

installation was discov-ered. The problem wascorrected, and the towerwas restarted withoutincident. The tower waspushed to maximumdesign rates, and thedesired separation effi-ciency of the new pack-ing was achieved.

CAT-scanning forFCC risers

Figure 16 depicts atypical refinery fluid catalytic cracking (FCC)riser. A gamma scan ofthe riser combined witha CAT-scan can be usedto evaluate the perform-

ance of the injection system. The riser scan, performed byplacing the gamma source and detector across the riserdiameter, provides a catalyst density profile along the riserheight. The CAT-scan produces a density distributionacross the riser cross-section at a given elevation.

Optimizing the steam injection rate. The conversionefficiency of gas oil into gasoline in an FCC riser was

lower than expected. Based on operating experience anddesign specifications, engineers were mainly concernedwith the fluidization regimes of the catalyst particles in thefeed zone. To improve the atomization of the feed oil, ameasured amount of steam is usually injected with it. Theengineers were also interested in the effect of the steaminjection rate on the fluidization regimes.

Conventional measurements by the thermocouplesinstalled at the feed zone did not provide sufficient infor-mation for analyzing the fluidization process or catalystdistribution inside the riser. CAT-scans were performed tostudy the effect of steam injection on fluidization.

The vertical (or riser) scan gave the catalyst densityprofile along the height of the riser at high and lowsteam rates (Figure 17). It appeared that the overall cata-lyst density in the riser feed zone at a scan elevation of13–18-ft (4.0–5.5-m) was lower at the higher steam rate.The scan also indicated that the catalyst traveled upwardabout 8 ft (2.4 m) from the feed nozzle before the feedwas completely vaporized.

A CAT-scan was then performed above the feed noz-zles’ elevation of 12 ft (3.7 m) to map the catalyst distri-bution across the riser at the two steam rates (Figure 18).There was a high-density zone in the center of the riser atboth operating conditions, meaning that the catalyst in thecenter of the riser was not evenly fluidized at the feedzone. However, the core of high-density area did becomesmaller with the higher steam rate. Based on the tests, theoperators were able to optimize the steam rates to improvethe riser’s performance.

Diagnosing feed nozzle plugging. Poor product yield ofan FCC unit can be caused by plugged or coked riser feednozzles, which are critical for efficient catalyst/hydrocar-bon mixing and fluidizing.

A riser feed nozzle arrangement consisted of six nozzles

Process Diagnostics

� Figure 18. CAT-scans at low and high steam rates indicate thatcatalyst in the center area of the riser is not fluidized well,although the dense core was smaller at the high steam rates.

Density, lb / ft3

10–12 8–10 6–8 4–6 2–4 0–2

CAT–Scan at Low Stream Rate CAT–Scan at High Stream Rate

� Figure 16. A fluid catalyticcracking (FCC) riser.

To Cyclones

VaporizationZone

RefractoryLiner

Feed Injection

Zone

Feedstock (Liquid)

Feedstock (Liquid)

Feed InjectionNozzles

RiserWall

Catalyst (Solid)

ReactionZone

� Figure 17. A riser scan shows changes in the catalyst densityprofile with changing steam rate.

24'

22'

20'

18'

16'

14'

12'

10'

8'

6'

CAT-Scan Location

LowSteam

HighSteam

Density, lb/ft3

6'–9" 1.7 1.8

9'–0" 1.4 2.1

10'–10" 2.4 2.7

12'–6" 4.7 4.2

13'–9" 7.1 5.7

15'–3" 10 8.3

Platform

1,000 10,000

5,000 lb/h Steam 15,000 lb/h Steam

Scan Line Orientation

Elevation

CEP May 2007 www.aiche.org/cep 35

spaced evenly at 60° apart. The operations staff had con-cerns about the integrity and functionality of the nozzles. Ariser scan performed while all six nozzles were operatingshowed the riser density vs. elevation, and enabled a CAT-scan elevation to be selected based on analysis of the hydro-carbon/catalyst mixing zone. A CAT-scan of the riser ap-proximately 3 ft above the feed nozzles in the catalyst/feedacceleration zone was then performed (Figure 19).

The hydrocarbon feed to one of the nozzles (Nozzle #3)was then turned off to simulate a plugged or coked nozzle.Another vertical scan of the riser and a repeat CAT-scan atthe same elevation were performed (Figure 20).

The riser scans showed no appreciable difference in theacceleration density profile between all six nozzles operat-ing and only five operating. The CAT-scan of the riserwith Feed Nozzle #3 closed appeared as expected, with anarea of increased density in the vicinity of the closed noz-zle due to the loss of hydrocarbon transport and reaction.

Closing remarks

The CAT-scan has proven to be a practical tool forinvestigating flow distributions of vapor/liquid andvapor/solid systems inside process vessels.

CAT-scans have been used in packed bed and misteliminator applications with column diameters rangingfrom 2 ft to 35 ft (0.6–10.7 m), and in fluidized catalystbeds in reactors ranging from 0.7 ft to 6 ft (0.2–1.8 m).

Application of gamma CAT-scan technology in packeddistillation columns and FCC risers has made it possibleand practical to collect much more vital online processinformation than just using conventional techniques forprocess diagnosis.

� Figure 20. CAT-scan with Feed Nozzle #3 closed.

1 6

5

43

2

Density, lb / ft3

25–30

20–25

15–20

10–15

5–10

Nozzle #3Closed

� Figure 19. CAT-scan of the riser with all six feed nozzles open.

1 6

5

43

2

Density, lb / ft3

25–30

20–25

15–20

10–15

5–10

Literature Cited

1. Bowman, J. D., “Troubleshoot Packed Towers with Radio-isotopes,” Chem. Eng. Progress, 89 (9), pp. 34–41 (Sept. 1993).

2. Scott, D. M., and R. A. Williams, eds., “Frontiers in Industrial Pro-cess Tomography,” Engineering Foundation, New York, NY (1995).

3. Price, W. J., “Nuclear Radiation Detection,” McGraw-Hill, NewYork, NY (1964).

4. Bartholomew, R. N., and R. W. Casagrande, “Measuring SolidsConcentration in Fluidized Systems by Gamma-Ray Absorption,”Ind. Eng. Chem., 49 (3), pp. 428–431 (Mar. 1957).

5. Killat, G. R., and T. D. Rey, “Properly Assess Maldistribution inPacked Towers,” Chem. Eng. Progress, 92 (5), pp. 69–73 (May 1996).

6. Klemas, L., and J. A. Bonilla, “Accurately Assess Packed-ColumnEfficiency,” Chem. Eng. Progress, 91 (7), pp. 27–44 (July 1995).

7. Stikkelman, R. M., “Gas and Liquid Maldistributions in PackedColumns,” Academisch Boeken Centrum, Delft, The Netherlands(1989).

8. Kister, H., “Distillation Design,” McGraw-Hill, New York, NY(1992).

9. Xu, S. X., et al., “Troubleshooting Industrial Packed Columns byGamma-Ray Tomography,” presented at CE Expo’99, Houston,TX (Jun. 9, 1999).

SIMON (XIAOMIN) XU is currently a separations specialist with Shaw Stone& Webster (1430 Enclave Parkway, Houston, TX 77077; Phone: (281) 368-3292; Email: simon.xu@shawgrp.com). Previously he spent ten years withTru-Tec Div. of Koch Engineering (then Tru-Tec, Quest TruTec and Tracerco)in Texas and nine years with the Univ. of Petroleum in Beijing, primarilyinvolved in R&D and troubleshooting of distillation units. He earned BSand PhD degrees in oil refining and chemical engineering from the Univ.of Petroleum China. He is a member of AIChE.

WILLIAM MIXON is the Eastern Regional Manager for Tracerco (8181 GSRIRd., Baton Rouge, LA 70820; Phone: (225) 761-0621; Fax: (225) 767-2637; E-mail: william.mixon@tracerco.com). After receiving a BS inchemical engineering from Louisiana State Univ., he joined Tru-TecServices (now Tracerco) as a project engineer in the Southeast Regionaloffice, and later was named manager of that office. In his current positionas Eastern Regional Manager, his territory now covers the SoutheastRegional Office in Baton Rouge, LA, and the Northeast Regional Office inNewark, DE.

AcknowledgementsThe authors wish to thank Mike Flenniken of Quest TruTec LP for hisassistance in preparing the FCC case studies and Koch-Glitsch, Inc. for theuse of the test column. They also appreciate the guidance provided by C. Conforti, T. Marut and J. Dusseault of ExxonMobil for the field test.

CEP