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Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247

www.elsevier.com/locate/nima

Visualization of the hygroscopic water distribution in anadsorbent bed by neutron radiography

Hitoshi Asano�, Takehiko Nakajima, Nobuyuki Takenaka, Terushige Fujii

Department of Mechanical Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

Available online 3 February 2005

Abstract

It is important for the design of an adsorption heat exchanger used in an adsorption refrigerator to know the

hygroscopic water distribution in the adsorbent bed with cooling or heating, because the adsorption/dehydration

process is a complicated phenomenon with heat conduction, vapor diffusion, and vapor flows in the particle bed, and it

is a dominant factor on the performance. A neutron radiography method was applied to measure the hygroscopic water

distribution in an adsorption bed. In this study, a transient phenomenon in a dehydration process was investigated.

Silica gel particles whose component is 99.8% silicone dioxide and diameter is around 0.5mm are used as an adsorbent,

and they are packed in an aluminum cylinder with a 36mm inner diameter and 100mm depth. From the radiographs,

the transient characteristics of the hygroscopic water distributions were clearly visualized via some image processing

methods.

r 2005 Elsevier B.V. All rights reserved.

PACS: 87.59.B

Keywords: Adsorbent bed; Silica gel; Hygroscopic water distribution; Dehydrate process; Neutron radiography

1. Introduction

For the improvement in the efficiency of energyutilization, a combined heat and power supplyingsystem called a co-generation system has attractedinterest. In this system, heat can be supplied froma power engine driven by a primary energy, such asnatural gas, petroleum, LPG, wood chips, and so

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

ma.2005.01.107

ng author. Tel.:/fax: +81 78 803 6122.

ss: asano@mech.kobe-u.ac.jp (H. Asano).

on. In order to obtain a high efficiency of energyutilization by this system, it is necessary to recoverthe exhausted heat. On the other hand, there is aproblem with the energy demand in summer inJapan. A large electrical power supply is requiredfor air conditioning by a compression-type refrig-erator. This fact makes the installation of co-generation system difficult. So, it is desired toincrease the heat demand and decrease the powerdemand in summer. An adsorption refrigeratingsystem is one of the efficient equipments for such

d.

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0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

Tested particle

Type-A

Type-B

Relative humidity [%]

Coe

ffic

ient

of

hygr

osco

pic

adso

rptio

n [%

]

25 °C, atmospheric pressure

Fig. 2. Adsorption isotherm of the silica gel.

H. Asano et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247242

requirement, because this system can be driven notby electricity but by heat. A schematic figure of itsoperation is shown in Fig. 1. Refrigerant vaporfrom an evaporator at a low pressure must becarried to a higher-pressure condenser in order toconstruct a refrigerating cycle. In a compression-type refrigerator, a large power is needed to drivethe mechanical compressor. An adsorption refrig-erating cycle can carry vapor by utilizing theadsorption/dehydration process.

Silica gel and water are widely used as acombination of adsorbent and refrigerant. Anadsorption isotherm of silica gel at a temperatureof 25 1C and atmospheric pressure is shown inFig. 2. The adsorbed water amount is decided bythe particle temperature and vapor pressure, and itincreases with increase in vapor pressure. Thisfigure shows the adsorption characteristics inhumid air. Temperature means the particle tem-perature, and the relative humidity shows thepartial pressure of the vapor. If silica gel is placedin an atmosphere of vapor, it can adsorb water at ahigher temperature than the saturation tempera-ture.

Adsorption process.

Cooing water

EvaporatorAdsorbent heat exchanger

Chilled water

Vapor

Dehydration process.

Hot water

Condenser Adsorbent heat exchanger

Coolingwater

Vapor

Expansionvalve

(a)

(b)

Fig. 1. Adsorption refrigerator. (a) Dehydration process.

(b) Adsorption process.

The operation of an adsorption refrigerator isexplained in Fig. 1. At first, low-pressure vaporfrom an evaporator is adsorbed on the adsorbent.The adsorbent should be cooled to adsorb thewater and to remove the latent heat in the phasechange. After the adsorption process, the heatexchanger is connected to a condenser by valveswitching. Then, the vapor is regenerated by aheating of the adsorbent. The heating rate is forthe temperature rise of the adsorbent and thelatent heat in the vaporization. Generally, thissystem has a low coefficient of performance due toits batch operation and the heat capacity of theadsorption heat exchanger. In order to improvethe performance, it is important to optimize theheat exchanger to minimize the amount of drivingheat. The adsorption/dehydration process is acomplex phenomena consisting of a thermalconduction, vapor diffusion, and a vapor flow inthe adsorbent bed. Especially, the spatial relationof the temperature and hygroscopic water ondistribution on the heating surface is important forthe transient characteristics. However, there is nomeasured data on the hygroscopic water distribu-tion.

Neutron radiography method is very effectivefor visualizing the water distribution in a solidmaterial. For example, some experiments, such ashygroscopic water in a rock [1], drying process ofconcrete [2], have been reported.

In this study, in order to confirm the effective-ness of the neutron radiography method for themeasurements of the transient distribution of

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Cooling water Vapor

14

Irradiation area

H. Asano et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247 243

hygroscopic water in the adsorbent bed, thedehydrating process of silica gel particles wasvisualized, and the two-dimensional distributionsof the dehydrated water amounts were measuredvia some image processing methods.

1. Test capsule 3. AC power supply 2. Silicon rubber heater 4. Condenser

23

Fig. 4. Experimental setup for dehydration test.

φ 40

Heater

80

120

100

φ 36

2. Experimental apparatus

Silica gel particles shown in Fig. 3 were used asan adsorbent. The composition is over 99% silica-dioxide. Water is mainly adsorbed into themicrocavities by a physical adsorption. Theparticles have been developed for the adsorptionrefrigerator by Fuji Silysia Chemical Ltd. Thediameters of the particles of 95.1% were in therange of 0.25–0.71mm. The adsorption perfor-mance has already been shown in Fig. 2.

A schematic diagram of the experimental setupis shown in Fig. 4. Silica gel particles were packedin an aluminum cylindrical capsule. The detail ofthe capsule is shown in Fig. 5. The inner diameterand wall thickness are 36 and 2mm, respectively.The capsule was heated by a silicon rubber heaterset on the outside wall with a constant walltemperature. The wall temperature, monitored bya thermocouple on the surface of the capsule, wasmaintained at a constant value of 110 1C by anon–off controller. The capsule was covered by arock wool thermal insulator. Regenerated vaporwas condensed in a double tube condenser 4, andthe condensed water was corrected in a beaker.

Visualization experiments were conducted byutilizing the JRR-3m thermal neutron radiogra-phy facility at JAERI (Japan Atomic Energy

Fig. 3. Appearance of the silica gel particle.

(unit : mm)

Fig. 5. Aluminum test capsule.

Research Institute). Neutron beam was irradiatedhorizontally in the range of the dotted line inFig. 4. Radiographs were photographed by acooled charged-coupled-devise (C-CCD) still cam-era, whose view field is 1000� 1018 pixels, andbrightness has 16384(14 bit) intensity levels. An

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Fig. 6. Original visualized image.

H. Asano et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247244

original image of the neutron radiograph is shownin Fig. 6. Two aluminum rectangular containerswith the same configuration were also visualized inorder to estimate the attenuation ratio of the silicagel particles. The upper capsule is empty, and thelower one is filled with dry silica gel particles.

The variation of the wall temperature and theexposure timing are shown in Fig. 7. The walltemperature reached the set value of 110 1C at10min after the start of the heating, and it waskept at the set temperature.

3. Image processing

Assuming that the brightness of a visualizedimage is proportional to the beam intensity on ascintillation converter and neglecting the attenua-tion term due to the vapor in the adsorption bed,the brightness of the silica gel bed Ssg(x, y) isexpressed as

Ssg x; yð Þ ¼ Gðx; yÞ exp½�rwmm;wtwðx; yÞ

� rsgf1� �ðx; yÞgtcfmm;sg

þ mðx; yÞmm;H2Og þ Osgðx; yÞ ð1Þ

where r and mm are the density and massattenuation coefficient, respectively, and these arethe physical properties of the objects. The functionof t is thickness along a neutron beam. Thesubscripts of w, sg, H2O, and c mean the wall,silica gel, water, and internal of the capsule,respectively. �(x, y) means the average packing

factor of the silica gel along the neutron beam atcoordinate (x, y). m(x, y) means the massadsorption ratio of the water and is expressed asthe following function:

m ¼MH2O

Msg¼

rH2O� tH2O

rsgð1� �Þtc. (2)

G(x, y) in Eq. (1) is the gain and depends on theposition due to a non-flatness of the initial beamintensity and of the sensitivity in a imaging system.Osg(x, y) is an offset value in the brightness. Theoffset value is caused by neutron scatterings in anobject, optical scatterings in a neutron camera,and the dark current of an imaging system. Sincethe main factor of the neutron attenuation throughwater is scattering, the offset value must depend onthe water distribution. In this study, a dark currentwas used as the offset value. Therefore, theprocessed results show qualitative ones. In orderto measure quantitatively, a two-dimensionaldistribution of the offset value under each condi-tion is necessary. Using the image of the initialcondition S0(x, y), a dehydrated water distributioncan be obtained as a difference by the followingequation:

rH2Oðx; yÞDtH2Oðx; yÞ

¼1

mH2Oðx; yÞ

Ssgðx; yÞ � Osg

Ssg;0ðx; yÞ � Osg;0

� �. ð3Þ

The left side shows the dehydrated water amountfrom the start of the heating.

4. Experimental results and discussion

4.1. Attenuation coefficient of silica gel particles

The attenuation coefficient of the silica particles,mm;sgg; was estimated from the test rectangularcapsules seen in Fig. 6. Non-flatness of thebrightness due to a shading effect was correctedby the brightness distribution of an image withoutan object. The value of mm;sgg can be calculatedfrom the average brightness of an empty capsule,Sempty, and filled with a dry silica particles, Sfilled,

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Fig. 8. 2D distribution of the dehydration amount obtained from radiographs via some image processing.

0 20 40 60 80 100 12020406080

100120

Tw

all [

°C]

exposure timing

time [min]

Fig. 7. Variation of the wall temperature of the test capsule measured by a thermocouple at the surface.

H. Asano et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247 245

by the following equation:

mm;sg ¼1

rsgð1� �Þtcln Sempty � O

� ��Sfilled � Oð Þ

� �.

(4)

Using rsgg of 1.170 g/cm3 and � of 0.63 from aproduct’s information, the attenuation coefficientof the silica gel was 0.121 cm2/g. The value is quite

lower than that of water of 3.42 cm2/g. In this case,it can be said that the neutron scattering effect issmall.

4.2. Dehydrated water distribution

Measured results of the dehydrated waterdistributions are shown in an arbitrary gray scalewith the original image at the initial condition in

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H. Asano et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247246

Fig. 8. The wall temperature change in theexperiment is shown in Fig. 7. These images showthe difference between each condition as well asthe initial condition. White part shows zero, and itbecomes darker with the increase of the dehy-drated water amounts. From the figures until30min after, Fig. 8 (b)–(f), it can be seen that thedehydrated water increased with time. Especially,the value might be higher at the top of theadsorbent bed and near the inner wall. The reasonof the former part might be on a lower partialpressure of the vapor than that inside of the bed.On the other hand, at the position closer to thesidewall, the dehydrated water amount was higherdue to the higher heating rate. However, at 40minafter the start a unique pattern was observed.

A large white area appeared near the top of theheater. This image states that the silica gel in thisregion adsorbed higher amount of the water thanthat at the initial condition in spite of the heating.

0 20 40 60 8020406080

100120

Tw

all[˚

C]

time [min]

0 20 40 60 800

0.02

0.04

0.06

0.08

0.10

time [min]

0 20 40 60 80

time [min]

∆mw

[g]

0

0.02

–0.02

0.04

0.06

0.08

∆mw

[g]

(a)

(b)

(c)

Fig. 9. Change of dehydration amount from the silica gel. (a) Variatio

the start. (c) Dehydration amount during interval of image acquisitio

This fact was caused by two reasons. One is theheating problem. Since this region is near or out ofthe heating area, the heating rate is not enough forthe dehydration. The other is the change of thecircumstance. Until 30min after the start, it isestimated that the relative humidity did not reach100% in the area. So, the particles could notadsorb vapor. However, since the relative humid-ity increased to 100% due to the regenerated vaporin the adsorption bed, the adsorb temperaturebecame higher than the particle temperature. Afterthe pattern was observed, the white area was keptuntil the end of the experiment, and the dehydra-tion in the bed was inhibited. Especially, it couldbe seen that the value decreased at the lower whiteregion.

The integrated dehydration amount from theinitial condition and the deferential amountbetween each exposure timing were calculated foreach layer indicated in the right figure, and these

0 5 mm11 mm16 mm21 mm27 mm32 mm38 mm43 mm48 mm54 mm

100 120

exposure timing

100 120

0 5 mm11 mm16 mm21 mm27 mm32 mm38 mm43 mm48 mm54 mm

100 120

0 mm

54 mm~~~~~~~~~~

~~~~~~~~~~

n of wall temperature. (b) Dehydration amount integrated from

n.

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H. Asano et al. / Nuclear Instruments and Methods in Physics Research A 542 (2005) 241–247 247

were plotted in Figs. 9(b) and (c), respectively. Itcan be seen most of the water was regenerateduntil 30min after the start, and the adsorptionprocess occurred in the region higher than 43mmfrom the bottom and at 50min after the start.

5. Conclusions

In order to evaluate the effectiveness of theneutron radiography to the measurements of thehygroscopic water in an adsorbent bed, the silicagel particles in an aluminum cylindrical capsulewere visualized by the thermal neutron radio-graphy. The results are summarized as follows:

1.

The tested silica gel had the low neutronattenuation coefficient of 0.121 cm 2/g.

2.

Distributions of the dehydration amount ofwater could be clearly obtained by some imageprocessing.

3.

In spite of the dehydration process, the unevenheating and change in the vapor pressure causedthe adsorption process partially.

As a future subject, it is necessary for thequantitative measurements to evaluate the scatter-ing effect of the neutron in the adsorbent bed.

References

[1] F.C. de Beer, et al., Appl. Radiat. Isotopes 61 (4) (2004)

487.

[2] F.C. de Beer, et al., Appl. Radiat. Isotopes 61 (4) (2004)

609.