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Enhancement of Energy Yield for Ozone Production viaPacked-Bed ReactorsHsin Liang Chen a , How Ming Lee a & Moo Been Chang aa Graduate Institute of Environmental Engineering , National Central University , Chung-Li,TaiwanPublished online: 18 Aug 2006.

To cite this article: Hsin Liang Chen , How Ming Lee & Moo Been Chang (2006) Enhancement of Energy Yield for OzoneProduction via Packed-Bed Reactors, Ozone: Science & Engineering: The Journal of the International Ozone Association, 28:2,111-118, DOI: 10.1080/01919510600559393

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Ozone: Science and Engineering, 28: 111–118

Copyright # 2006 International Ozone Association

ISSN: 0191-9512 print / 1547–6545 online

DOI: 10.1080/01919510600559393

Enhancement of Energy Yield for Ozone Production viaPacked-Bed Reactors

Hsin Liang Chen, How Ming Lee, and Moo Been Chang

Graduate Institute of Environmental Engineering, National Central University, Chung-Li, Taiwan

The present work aims to enhance the energy yield ofozone production via packed-bed reactors. It has beenexperimentally demonstrated that ozone concentration andcorresponding energy yield achieved by packed-bed reactorsare significantly higher than that achieved by DBD only.The so-called packed-bed reactor is constructed by packinggranular dielectric pellets within a DBD reactor. Two kindsof dielectric materials including glass beads and Al2O3 pel-lets are tested. Experimental results indicate that an ozonegenerator packed with Al2O3 pellets results in a higherozone production compared with one packed with glassbeads. The maximum ozone production takes place whenAl2O3 pellets with diameter of 2 mm are packed. The maxi-mum ozone concentration, ozone production rate, andenergy yield achieved in this study are 61 gO3/m

3, 3.7gO3/hr, and 173 gO3/kWh, respectively. The highest ozoneconcentration and energy yield achieved with the packed-bed reactor are about 8 and 12 times high as those withDBD reactor, respectively. Although the packed-bed reac-tors have a shortcoming of high temperature, it can besolved by adding a cooling system and the ozone generationcan be improved thereof. As a result, the packed-bed reactoris a promising and state-of-the-art technology for ozonegeneration based on this study.

Keywords Ozone, Ozone Generation, Dielectric Barrier Dis-charge, Glass Beads, Al2O3 Pellets, Packed-BedReactor

INTRODUCTION

With an oxidation potential second only to fluorine,ozone is a strong oxidizing agent that has been applied inmany areas such as water disinfection, flue gas treatment,medical application, food industry, indoor air cleaning,and so forth.

Ozone can be generated through electrolysis, UV irra-diation, or electrical discharges. The last measure is themost widely used and effective method to produce ozone.Among various plasma technologies used for ozone gen-eration, DBD (dielectric barrier discharge) has been themainstream technology so far.

For most plasma-type ozone generators, the maximumozone concentration and energy yield cannot be attainedat the same time in the same operation condition(Samaranayake et al., 2001). Therefore, a choice mustbe made between ozone concentration and energy yieldin most cases. In the present work, it is true as well for thepacked-bed reactors mentioned in the following section.

At present, the major bottleneck encountered by DBD isthat the energy yield is still too low compared to the value of400 gO3/kWh, which was theoretically estimated by

Eliasson et al. (1987) based on the assumptions of neglect-ing energy losses to ions and keeping relative oxygen atom

concentration [O]/[O2] in amicrodischarge under 10–4. Evenif the estimated energy yield can be achieved, it is merelyone-third of the theoretical ozone formation efficiency

derived from heat of formation, corresponding to 1,220gO3/kWh. In recent years many attempts and efforts havebeen made with an eye to improving the energy yield of

ozone generation. Nomoto et al. (1995) proposed a silent-surface hybrid discharge ozonizer of two separated dis-

charge spaces. A silent discharge and a surface dischargecan take place simultaneously in this configuration. At lowapplied voltages, only the surface discharge can occur. As

the applied voltage is above a critical value, both surfacedischarge and silent discharge occur at the same time. They

indicate that energy yield reached by using the hybrid dis-charge ozonizer is higher than that by surface discharge orsilent discharge only. Cieplak et al. (2000) reported a plane

rotating electrode ozone generator that can make the dis-charge more uniform. The ozone generation efficiency of

the one with a rotation electrode is 15% higher than thatwithout a rotation electrode. Samaranayake et al. (2000,

Received 05/27/2004; Accepted 07/17/2005Address correspondence to Moo Been Chang, Graduate

Institute of Environmental Engineering, National CentralUniversity, Chung-Li 320, Taiwan. E-mail: mbchang@ncuen.ncu.edu.tw

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2001) applied pulse power to generate ozone using oxygenand air as the feeding gas, respectively. According to theirexperimental results, extremely short duration pulsed vol-tage can reduce heating of the gas and discharge reactor,which is beneficial for ozone generation. However, thepractical energy yield obtained using DBD is about 100 to270 gO3/kWh with pure oxygen and up to 100 gO3/kWhwith dry air so far (as listed in Table 1). Besides, parts of theenergy yields listed in Table 1 are calculated using thedeposited power, which is exclusive of energy consumedby other elements in the electrical circuit. Considering thetotal power, these energy yields will be even smaller.

In addition to the DBD described here, the packed-bedplasma reactor is used for ozone synthesis as well. Thepacked-bed type reactor, however, has been applied mainlyto the removal of gaseous pollutants so far (Jogan et al.,1993; Mizuno et al., 1992, 1993; Yamamoto et al., 1992,1996; Yamamoto and Jang, 1999). The so-called packed-bed rector is constructed by packing granular dielectricmaterials (such as glass beads, Al2O3 or BaTiO3 pelletstypically) within the discharge region of a DBD reactor.This configuration achieves a higher electron density andenergy because of the enhancement of electric field insidethe void between dielectric pellets, which is favorable forabating gaseous pollutants. Chang et al. (2000) have devel-oped a simplified one-dimensional, parallel-plate model in adielectric, packed-bed plasma reactor for evaluating theeffect of electron density and energy. They indicated that

the electric field augmentation inside the void, caused bythe ferroelectric pellets, can be expressed as

E ¼ Ex"p"g

where E = the electric field inside the void betweenpacking pellets,

Ex=the electric field in the gas phase,ep=the dielectric constant of packing pellets,eg=the dielectric constant of backgroundgases.

Based on the formula, one can evaluate the electron den-sity and energy. However, the packed-bed reactor is sel-dom used for ozone production. Only a few papers havebeen published on the topic to date (Jodzis, 2003; Moonand Geum, 1998; Murphy and Morrow, 2001; Schmidt-Szalowski and Borucka, 1989; Schmidt-Szalowski et al.,1990), and their energy yields are almost less than 100gO3/kWh (see Table 1). For all that their energy yields arelower than some researches using DBD, the most impor-tant result revealed in their studies is that packed-bedreactors actually result in higher ozone concentrationand energy yield.

According to the foregoing literature review, it seemsfeasible to enhance the energy yield via packed-bed reac-tors. Therefore, a packed-bed reactor is proposed inthis study. Two kinds of granular dielectric materials

TABLE 1. Comparisons Between Experimental Results Obtained from Different Researchers and this Study

Gas C R Z Ref.

Oxygen 5.8 0.7 *274 Nomoto et al. (1995)Oxygen 3.0 0.3 *80 Cieplak et al. (2000)Oxygen 16.6 3.5 *202 Samaranayake et al. (2000)Dry Air 12.5 1.1 *122 Samaranayake (2001)Oxygen 300 — — Kitayama and Kuzumoto (1997)Dry Air 63.0 — — Kitayama and Kuzumoto (1999)Oxygen 48.0 — *185 Garamoon et al. (2002)Oxygen 43.5 3.6 162 Chang and Wu (1997)Oxygen — — *121 Ohe et al. (1999)Oxygen 33.0 16.0 — Diaz et al. (1999)Oxygen 57 — *91 Schmidt-Szalowski, and Borucka (1989)Oxygen 130 — *83.3 Schmidt-Szalowski et al. (1990)Dry Air 0.1 — — Moon and Geum (1998)Oxygen 13.8 2.8 *60–70 Murphy and Morrow (2001)N2/O2 Mixtures 260 22.5 *134 Jodzis (2003)Oxygen 61 3.7 173 This Study

Remarks:

1. Energy yield labeled with an asterisk means that it was determined by deposited power which was smaller than the total power consumed during

discharges.

2. The symbols of C, R and Z in first row represent ozone concentration in gO3/m3, ozone production rate in gO3/h and energy yield in gO3/kWh,

respectively.

3. The data shown in this table do not take the operational conditions (such as temperature, gas flow rate, purity and so on) into account. Anyone

who is interested in any specific data should refer to the original reference.

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involving glass beads and Al2O3 pellets are tested.Besides, the effects of the applied voltage as well as thesize of the dielectric pellets on ozone generation have beenexperimentally investigated in this study.

EXPERIMENTAL APPARATUS AND PROCEDURE

Experimental Apparatus

The experimental setup is schematically shown inFigure 1. It consists of four parts, i.e., gas flow system,ozone generator, ozone monitor and high-voltage powersupply. The details are described as follows.

Gas Flow System

Oxygen used for ozone generation was supplied from agas cylinder having a purity of 99.9%. The oxygen gasflow was separated into two channels. One was intro-duced into the ozonizer to generate ozone while theother was led into the ozone monitor for 10 seconds topurge the cell every 10 minutes. During purging, the gasflow passing through ozonzier was rejected until the pur-ging process was completed. Gas flow rates were regu-lated with mass flow controller (Teledyne, HFC 202, LosAngeles, USA).

Ozone Generator

A DBD reactor and a packed-bed reactor were usedin this study for comparison purpose. The former wasconstructed by placing a stainless steel rod with adiameter of 2 mm as the grounded inner electrodewithin a glass tube. The inner diameter and thicknessof the glass tube are 26 and 2 mm, respectively. Theannular gap space formed between the outer surface ofthe inner electrode and the inner surface of the glasstube is 12 mm. Copper thin-foil wrapped around theglass tube served as the outer electrode and the effec-tive discharge length was fixed at 15 cm. The packed-bed reactor was formed by packing granular dielectricmaterial within the DBD reactor. Two kinds of gran-ular dielectric materials including glass beads andAl2O3 pellets were tested.

Ozone Monitor

The ozone concentration was measured with an ozonemonitor (Sorbios, Model OMH, 100.2/EG-200, Berlin,Germany). It utilizes the ozone’s absorption spectrumcurve peaked at wavelength of 254 nm in ultravioletregion to measure ozone concentration by detecting itsabsorption of the light beam energy emanating from amercury lamp having an intensive bright spectral line at253.7 nm. The ozone concentration measured by theozone monitor ranges from hundreds of ppm to severalpercent.

High-Voltage Power Supply

The high-voltage power supply was composed of apower meter (Chen Hwa Co., Model 2100, Taipei,Taiwan) and a high-voltage transformer (Jui-HsiangPTY Co. Ltd., Taiwan). The applied voltage and fre-quency were controlled by the power meter and onecould read the power (P) and the power factor (PF)from the power meter real time. The power here repre-sents the total power dissipated by the whole systemduring operation instead of deposited power.

Experimental Procedure

In this study, two types of reactor were used. One is theDBD reactor, the other is a packed-bed one that is formedby packing dielectric pellets within the original reactor.Two kinds of dielectric pellets including Al2O3 pellets andglass beads were selected. Three different sizes of Al2O3

pellets with diameters of 2, 5 and 10 mm are tested. Glassbeads of three different diameters (2, 3 and 5 mm) wereused. Ozone generated in the discharge gap and the refer-ence gas (oxygen with the flow rate of 1 L/min in thisstudy) were fed into the ozone monitor in turns for mea-surement. The reactor and the ozone monitor were con-nected by a Teflon tube with a length of 2 m. It generallyrequires 15 to 20 minutes for the ozone concentration toreach steady state depending on the operation condition.The ozone concentrations reported in this study were thevalues taken at steady states. The energy yield (Z) of ozonegeneration in g/kWh was determined with

� ¼ C�Q

P� 60

where C is concentration of ozone in g/m3, Q and P arethe gas flow rate (L/min) and the total power (W) mea-sured with power meter, respectively. All experimentaltests were carried out with oxygen flow rate of 1 L/minand applied frequency of 60 Hz. The effective voltageranged from 15 to 21 kV.

RESULTS AND DISCUSSION

Figure 2 (a) and (b) show the ozone concentration and theenergy yield of the DBD reactor operated with different

3

1

4

ON

OFF87

0

4

1

5

2

9

6

3

2

5

6

3 3

0

1 1

2 2

Vent

1: Power Meter2: High Voltage Transformer3: Oxygen Cylinder4: Mass Flow Controller5: Ozonizer6: Ozone Monitor

FIGURE 1. The schematic experimental setup.

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applied voltages. From the results of Figure 2a, it can beconcludedthatozoneconcentration increaseswith increasingapplied voltage. However, the energy yield decreases slightlyas the applied voltage increases, as shown in Figure 2b. Theexperimental result can be explained from two perspectives.First, the growth rate of the total power with increasingapplied voltage is larger than that of ozone concentration.When Q is fixed, the ratio of C to P slightly decreases withincreasing applied voltage. According to the formula used todetermine energy yield as described in the Experimental pro-cedure, Z will have the same trend as the ratio of C to P. Inaddition, higher temperature caused by higher applied vol-tage accelerates decomposition of ozone, which is anotherreasonwhy theenergyyieldobtainedbyDBDreactor slightlydecreases with increasing applied voltage. As described pre-viously the maximum ozone concentration and energy yieldcannot be obtained under the same operation condition. Itmust rely on the demand to make a compromise betweenozone concentration and energy yield. The followingpacked-bed reactors are all consistent with the result.

Dependences of ozone concentration on the appliedvoltage for the packed-bed reactors packing with differ-ent sizes of glass beads and Al2O3 pellets are shown inFigure 3a and 3b, respectively. Compared with the resultspresented in Figure 2a, it can be seen that the ozoneconcentrations obtained by the packed-bed reactors aresubstantially higher than that achieved with DBD. Theresult agrees with the previous works (Jodzis, 2003; Moonand Geum, 1998; Schmidt-Szalowski and Borucka, 1989;Schmidt-Szalowski et al., 1990). One of the advantagesfor the packed-bed type reactors is that numerous contactpoints exist between the dielectric pellets. Because of theshort distances between those pellets, the electric fieldnear those contact points will be greatly enhanced. Thehigher electric field will impart higher energy to the elec-trons. As a result, more atomic oxygen radicals, the pre-cursor for ozone formation, will be generated. Hence, theozone concentrations are promoted.

Figure 4 shows the reactions scheme for ozone for-mation and destruction. Major reactions taken intoconsideration for ozone formation and decomposition

in pure oxygen are listed as follows (Langlais et al.,1991):

Reaction1 : Oð3PÞ þO2 þM! O3 þM

0

3

6

9

12 15 18 21

Voltage (kV)

Ozo

ne C

once

ntra

tion

(gO

3/m

3)

0

3

6

9

12

15

18

12 15 18 21

Voltage (kV)

Ene

rgy

Yie

ld (

gO3/k

Wh)

(a) (b)

FIGURE 2. Effects of applied voltage on (a) ozone concentration and (b) energy yield of the DBD reactor (frequency = 60 Hz; oxygen flow rate =1 L/min).

0

10

20

30

40

50

12 15 18 21

Voltage (kV)

Ozo

ne C

once

ntra

tion

(gO

3/m

3)

2 mm

3 mm

5 mm

0

10

20

30

40

50

60

70

12 15 18 21

Voltage (kV)

Ozo

ne C

once

ntra

tion

(gO

3/m

3 )

2 mm

5 mm

10 mm

(a)

(b)

FIGURE 3. Relationships between ozone generation and appliedvoltage of packed-bed reactor packed with different sizes of

(a) glass beads and (b) Al2O3 pellets (frequency = 60 Hz; oxygen

flow rate = 1 L/min).

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and

Reaction 2: Oð1DÞ þO2 ! O3

However, O(1D) can also decompose ozone molecule viathe following reactions:

Reaction 3: Oð1DÞ þO3 ! 2O2

Reaction 4: Oð1DÞ þO3 ! O2 þOþO

It seems favorable for ozone production to quench O(1D)by the following reaction:

Reaction 5: Oð1DÞ þWall! Oð3PÞ

Packing dielectric pellets within the DBD reactoroffers more solid surface area to deactivate O(1D)through Reaction 5, which is beneficial for ozone forma-tion in that Reaction 3 and 4 can be inhibited. Besides,the large surface area of the packing pellets can also playthe role as third body for Reaction 1.

Schmidt-Szalowski and Borucka (1989) indicated thatthe diameter of the dielectric pellets made from silica didnot affect ozone concentration, because the number ofactive particles reaching the solid surface did not increaseas the diameter of the silica grains was decreased.Nevertheless, it is obvious that both kinds of dielectricpellets tested in this study have their own optimum diam-eters. The packed-bed type reactors have a feature ofsurface discharge occurring near the contact pointsobserved in this study. As is well known, both the totalsurface area of the dielectric pellets and the number ofcontact points between dielectric pellets increase withdecreasing diameter of the packing material. The prob-ability of colliding with micro-discharges for oxygen

molecules can be enhanced by decreasing pellet sizes.The amount of atomic oxygen will increase in this way.However, when the mole fraction of [O] is greater than10–4, the efficiency of ozone generation decreases due tothe fact that recombination of O becomes considerable(Eliasson et al., 1987). Furthermore, electrons can alsodecompose ozone, which limits the acceptable dischargestrength (Langlais et al., 1991). Eventually, the changingof dielectric pellets sizes cause the void volume betweenpellets and residence time to change, which may beanother cause for the existence of optimum diameter.We infer these as the possible reasons why both glassbeads and Al2O3 pellets have their own optimumdiameters.

Figures 5a and 5b show the energy yields achievedat various applied voltages for glass beads and Al2O3

pellets, respectively, with different diameters. When thediameters of the two types of pellets are kept the same,it can be seen that the reactor packing Al2O3 pelletsresults in a higher ozone concentration and energyyield compared with that packing glass beads. Thisresult can be interpreted from the viewpoint of O(1D)quenching and three-body reaction for ozone forma-tion. Because Al2O3 is a porous material, it provideshigher porous surface area than glass (nonporous). Asa result, the reactor packed with Al2O3 result in betterperformance than the one packed with glass beads dueto higher probability of occurrence of Reaction 1 andReaction 5.

In addition to the reasons described in the previoussections, the surface reactions between gaseous speciesand surface of the dielectric pellets may contribute tothe improvement of ozone generation in packed-bedreactors. However, this perspective needs furtherstudy.

+ e

+ O3

+ O3

+ O(3P)

O2

O(1D)

+ e

+ wall O3

+ O2 + M

+ O2

O-

O(3P)

reactions beneficialfor ozone formation

reactions unfavorablefor ozone formation

FIGURE 4. Reaction scheme for ozone formation and destruction.

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Experimental results obtained by different types ofreactors in this study are listed in Table 2. The max-imum ozone concentration and energy yield both occurwhen DBD reactor is packed with Al2O3 pellets withdiameter of 2 mm. They are approximate 8 and 12times the values obtained by DBD. The lowest energyyield of packed-bed reactor is 84 g/kWh, which is stillabout 4 times higher than that of DBD. Enhancing theozone concentration and energy yield via packed-bedreactors is undoubtedly feasible. Compared to theDBD, it seems that the packed-bed reactors are notsuitable for operating at high applied voltages. Figure

6a and 6b show the ratio of Cn/C15 and Zn/Z15 atdifferent voltages, where C and Z are the ozone con-centration and energy yield and the subscript standsfor the voltage in kV. It is apparent that the values ofC21/C15 and Z21/Z15 of DBD are larger than those ofthe others. But C18/C15 and Z18/Z15 of packed-bedreactors packed with Al2O3 pellets and glass beadswith diameter of 5 mm are higher than those ofDBD. It can be concluded that the efficiency of pro-moting ozone concentration via voltage decreases asthe applied voltage increases. Moreover, Zn/Z15 almostdecreases with increasing voltage under the operationconditions of this study. But it needs to be emphasizedthat Zn/Z15 is possible to increase with increasing vol-tage if one lowers the applied voltage. We infer thatthese phenomena are caused by the fact that removingheat from the packed-bed reactors is more difficultthan that from DBD reactor, in particular for a reactorwithout a cooling system.

The packed-bed reactor, however, has a shortcoming.Its temperature is higher than that of the DBD reactor by20 to 50�C, depending on the operation condition. Asmentioned previously, high temperature would accelerateozone decomposition rate, resulting in lower ozone gen-eration. Fortunately, this problem can be solved by add-ing a cooling system. Besides, for industrial ozonegenerators, the cooling system is an essential part.Therefore, the packed-bed reactor is a promising andstate-of-the-art technology for ozone generation basedon this study.

Table 1 lists the experimental results obtained fromdifferent studies and this investigation. The energy yieldmarked with an asterisk indicates that it was determinedby deposited power that was always smaller than thetotal power. In this study, we use the total power todetermine energy yield. With respect to energy yield,the packed-bed reactor constructed and developed inthis study is very competitive compared with otherstudies.

CONCLUSIONS

The effect of packed-bed reactors packed with differ-ent sizes of glass beads and Al2O3 pellets on ozone con-centration and energy yield has been experimentallystudied and the following conclusions were obtained.

0

20

40

60

80

100

12 15 18 21

Voltage (kV)

Ene

rgy

Yie

ld (

gO3/k

Wh)

2 mm

3 mm

5 mm

0

30

60

90

120

150

180

12 15 18 21

Voltage (kV)

Ene

rgy

Yie

ld (

gO3/

kWh)

2 mm

5 mm

10 mm

(a)

(b)

FIGURE 5. Energy yield versus the applied voltage for different

sizes of (a) glass beads and (b) Al2O3 pellets (frequency = 60 Hz;

oxygen flow rate = 1 L/min).

TABLE 2. Comparisons of Experimental Results of this Study Using Different Types of Reactors

Reactor type DBD DBD Reactor Packed with Glass Beads DBD Reactor Packed with Al2O3 Pellets

pellets size (mm) — 2 3 5 2 5 10Max. C 8 38 44 36 61 55 49Max. R 0.5 2.3 2.6 2.2 3.7 3.3 2.9Max. Z 14 96 92 84 173 134 118

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1. Under the same operating conditions, it has beenexperimentally demonstrated that the ozone concen-tration and corresponding energy yield achieved bypacked-bed reactor are better than that by DBD. Thehighest ozone concentration and energy yield obtainedby using the packed-bed reactor are about 8 and 12times as high as those by the DBD reactor,respectively.

2. Packing Al2O3 pellets results in a higher ozoneproduction compared with packing glass beads. In

addition, the maximum ozone production takesplace when the reactor was packed with Al2O3 pel-lets with diameter of 2 mm.

3. The maximum ozone concentration, ozone pro-duction rate, and energy yield achieved in thisstudy are 61 gO3/m

3, 3.7 gO3/h, and 173 gO3/kWh,respectively.

4. The packed-bed reactor has a shortcoming of hightemperature. As is well known, ozone decomposi-tion rate increases with increasing temperature.

Al2O3 (2mm)

Al2O3 (5mm)

Glass (2mm)

Glass (5mm)

Al2O3 (2mm)

Al2O3 (5mm)

Glass (2mm)

Glass (5mm)

FIGURE 6. Ratio of (a) Cn/C15 and (b) Zn/Z15 versus voltage (subscript represents voltage in kV).

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However, the problem can be solved by adding acooling system.

ACKNOWLEDGMENTS

The authors would like to express their appreciationfor the funding provided by the National Science Councilof the Republic of China under grant numbers of NSC-91-2622-E-008-005-CC3.

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