活性炭吸附、解吸联合光催化降解装置18(好)

Chemical Engineering Science58(2003)929–934/locate/cesA rapid treatment of formaldehyde in a highly tight room using aphotocatalytic reactor combined with a continuous adsorption anddesorption apparatusFumihide Shiraishi∗,Shunsuke Yamaguchi,Yusuke OhbuchiDepartment of Biochemical Engineering and Science,Faculty of Computer Science and Systems Engineering,Kyushu Institute of Technology,Iizuka820-8502,JapanAbstractA novel air-puriÿcation system,consisting of the photocatalytic reactor with a parallel array of blacklight blue uorescent lamps and the continuous adsorption and desorption apparatus with a cylindrical ceramic-paper honeycomb rotor retaining activated carbon or zeolite ÿne particles,was constructed and the performance of this system was investigated for treatment of gaseous HCHO at a concentration level of ppbv(¡1mg m−3)in a10m3highly tight closed room.With the zeolite rotor,it was very di cult to desorb the adsorbed HCHO by exposing the rotor to heated air and then to photocatalytically decompose the desorbed HCHO.In contrast,the activated carbon rotor provided an excellent performance.With this rotor,the indoor HCHO concentration was reduced to the neighborhood of the WHO guideline(0:1mg m−3)in10min and to an almost zero value in90min.Needless to say,this surprisingly high performance is owing to the cooperative work by the activated carbon rotor to adsorb the indoor HCHO and the photocatalytic reactor to rapidly decompose the HCHO desorbed by heating the rotor.This system o ers several other advantages.The adsorption rotor can be used semi-permanently because it is continuously regenerated.In addition,the HCHO adsorbed on the activated carbon rotor is readily released at a desorption temperature of120◦C.Under such a low temperature condition,little loss in the photocatalytic activity was caused.?2003Elsevier Science Ltd.All rights reserved.Keywords:Photocatalytic decomposition of formaldehyde;Continuous adsorption and desorption;Sick-building syndrome;Environment; Photochemistry;Reaction engineering1.IntroductionIn the indoor environment,HCHO exists at a con-centration level of ppbv(usually below300ppbv; 1ppmv=1000ppbv=1:25mg m−3)(Miyazaki,1996; Godish,1998),which makes the photocatalytic decomposi-tion much harder.Actually,if one starts the photocatalytic decomposition with a high concentration,for example, above10ppmv,HCHO is initially decomposed without problems,but when the HCHO concentration drops below 1ppmv,the decomposition is rapidly slowed down and then almost terminated on the way(Toyoda,2000).Two possible reasons are considerable for this unexpected time-transient behavior of HCHO.One is owing to an insuf-ÿcient UV-light intensity per unit surface area(Fukinbara,∗Corresponding author.Tel.:+81-948-29-7827;fax:+81-948-29-7801.E-mail address:fumi@bse.kyutech.ac.jp(F.Shiraishi).Shiraishi,&Nakano,2001).Various kinds of active species that are produced on the TiO2surface under UV-irradiation are greatly responsible for the expression of the photo-catalytic activity.However,these are very unstable,being quickly formed and then immediately decomposed.There-fore,the densities of the active species on the surface of TiO2under continuous UV-irradiation would deÿnitely de-pend on the light intensity.However,since the HCHO con-centration in the indoor environment is on the order of ppbv, the probability that the HCHO molecules encounter the ac-tive species becomes rather low even when they reach the TiO2surface,thereby reducing the occurrence of the reac-tion.The other reason is owing to the presence of large ÿlm-di usional resistance(Fukinbara&Shiraishi,2001). In general,in a gas–solid catalytic reaction,it is common knowledge that theÿlm-di usional resistance is negligible and the pore-di usional resistance alone should be taken into consideration.This may be valid for the case where the re-actant concentration is very high,as carried out in industrial0009-2509/03/$-see front matter?2003Elsevier Science Ltd.All rights reserved. doi:10.1016/S0009-2509(02)00630-9930 F.Shiraishi et al./Chemical Engineering Science58(2003)929–934production,but this is not always valid for the photocatalytic decomposition of harmful substances contained at very low concentrations in the indoor air.Indeed,in the photocatalytic decomposition of HCHO at a concentration below1ppmv, theÿlm-di usional resistance becomes marked because of a small di erence between the reactant concentrations in the bulk uid and on the TiO2surface(Toyoda,2000),which results in a remarkable decrease in the decomposition rate. In the pastÿve years,the authors have intensively exam-ined the relationshipbetween the rate of the p hotocatalytic decomposition and the structure of the photocatalytic reactor (Obuchi,Sakamoto,Nakano,&Shiraishi,1999;Shiraishi, Toyoda,Fukinbara,Obuchi,&Nakano,1999;Xu&Shi-raishi,1999;Fukinbara&Shiraishi,2001;Wang,Shiraishi, &Nakano,2002a,b)and found that a parallel array of nine 6-W blacklight blue uorescent lamps,each of which is ÿxed in a Pyrex glass tube whose inside surface is coated with a transparent thinÿlm of TiO2,is very e ective to de-compose HCHO at a ppbv concentration level toward a zero value(Shiraishi et al.,2001;Wang&Shiraishi,2002;Wang et al.,2002b).Since the distance between the light source and photocatalyst in the reactor is only3:5mm,the photocat-alytic surface is exposed to the UV light of a high intensity. In addition,the indoor air containing HCHO,introduced by rotation of the electric fanÿxed just over the parallel array of light sources,passes through the inside of the glass tubes at a very high linear velocity,which makes theÿlm-di usional resistance negligible.When this reactor was used to treat the 1:0m3air containing HCHO at an initial concentration of 0:375mg m−3,the HCHO concentration became below the WHO guideline(80ppbv,or0:1mg m−3)in20min and then reduced to an almost zero value in one hour(Shiraishi et al.,2001).To commercialize this photocatalytic reactor as an air cleaner for spacious houses,however,the treatment capacity must be further increased.In the present work,therefore,the authors constructed a novel air-puriÿcation system,consisting of the photocat-alytic reactor and the continuous adsorption and desorption apparatus equipped with a cylindrical ceramic-paper hon-eycomb rotor retaining activated carbon or zeoliteÿne particles,and investigated the performance of this novel air-puriÿcation system for the treatment of indoor HCHO. In this system,dilute HCHO in the air is once adsorbed on the rotor rotating at a low speed,while a section of the rotor is continuously exposed to heated air to desorb the adsorbed HCHO;as a result,the adsorbent is continu-ously regenerated.Since the desorption is carried out into a loop,HCHO is increasingly concentrated.In parallel,the desorbed HCHO is photocatalytically decomposed in the loop.2.ExperimentalFig.1shows a schematic of the air-puriÿcation system used in the present work.The photocatalytic reactor has a parallel array of nine6-W blacklight blue uorescent lamps(wavelength of UV;300–400nm),each of which isÿxed in a Pyrex glass tube(28mm in internal diame-ter and230mm long)whose inside surface is coated with a transparent thinÿlm of TiO2(total area of the catalytic surface,0:182m2).The coating method is described else-where(Matsuo,Takeshita,&Nakano,1990;Wang et al., 2002a).The distance between the light source and TiO2ÿlm in the reactor is3:5mm and the photocatalytic surface is exposed to the UV light of a high intensity(15 W cm−2 at350nm).By rotation of the electric fanÿxed just over the parallel array of light sources,the indoor air containing HCHO is introduced into the rectangular reactor(215L×215W×295H mm3)at a ow rate of3:0m3min−1andFig.1.A schematic of an air-puriÿcation system consisting of the photocatalytic reactor with a parallel array of nine blacklight blue uorescent lamps and the continuous adsorption and desorption apparatus with a ceramic-paper honeycomb rotor retaining activated carbon or zeoliteÿne particles.F.Shiraishi et al./Chemical Engineering Science58(2003)929–934931allowed to pass through the inside of the glass tubes at a very high linear velocity(12:7m s−1).A cylindrical ceramic-paper rotor(300mm in diameter and50mm in thickness)consists of honeycomb laminates (corrugation pitch,3mm),on which zeolite(ZSM-5)or ac-tivated carbon(coconut husk)ÿne particles are loaded.The indoor air was supplied to5/6of the surface area of the cylindrical rotor to adsorb HCHO,while heated air was to 1/6of the surface area to desorb the HCHO adsorbed on the rotor.All the experiments were performed in a10m3highly tight room,unless otherwise noted.The adsorption and des-orption experiment was carried out by connecting the ad-sorption and desorption apparatus circularly with a0:09m3 small box,as shown in Fig.1.Since the heated air was re-circulated through a section of the rotor belonging to the loop,the HCHO desorbed from the rotor was increasingly concentrated in the loopuntil the system came to equilib-rium.The simultaneous operation of the adsorption and des-orption apparatus and the photocatalytic reactor was carried out by placing the photocatalytic reactor in the small box. The indoor air was su ciently mixed with two electric funs, placed at two distant positions in the highly tight room.A certain amount of the indoor air was bubbled into a distilled water to collect HCHO and its concentration was measured by the AHMT method.3.Results and discussionparison of adsorption performancesThe air containing HCHO at an initial concentration of 0:625mg m−3was treated using air-puriÿcation system equipped with either zeolite or activated carbon rotor.The desorption temperature for both the rotors was set at180◦C because at the temperature less than180◦C,it was unable to su ciently desorb the HCHOÿrmly adsorbed on zeo-lite.With the zeolite rotor,the HCHO concentration in the highly tight room was rapidly decreased to0:313mg m−3 in10min,but the decomposition was slowed down and then stopped around at0:125mg m−3.There was little di erence in the HCHO concentration whether or not the photocatalytic reactor was operated.Such a variation in the HCHO concentration was re ected by the time-transient behavior of the HCHO concentration in the small box. That is,the HCHO concentration wasÿrst increased up to3:13mg m−3after10min and then decreased slowly. From the result of other experiment,it was found that the HCHO released from the zeolite rotor is photocatalytically undegradable.This is considered due to the occurrence of some chemical change in HCHO molecules,for example, the polymerization of HCHO.With the activated carbon rotor,a similar behavior in the HCHO concentration was ob-served in the highly tight room when the photocatalytic re-actor was switched-o .When the photocatalytic reactor was operated,on the other hand,the HCHO concentration was rapidly decreased to the WHO guideline and subsequently reduced toward a zero value(Fig.2).The HCHO concen-tration in the small box was decreased in the same manner as the zeolite rotor when the photocatalytic reactor was switched-o ,while it reached a maximum of2:38mg m−3 after10min and then decreased rapidly under operation of the photocatalytic reactor.The experimental results ob-tained here clearly indicate that the activated carbon rotor must be used in the adsorption and desorption apparatus.3.2.Performance of each apparatus for removal of HCHO from indoor airFigs.2(a)and(b)represent the time courses of the HCHO concentration in the highly tight room and small box, respectively,when the air containing HCHO at an initial concentration of0.56–0:59mg m−3was treated by the photocatalytic reactor alone,the adsorption and desorp-tion apparatus alone,and both of these apparatuses.In the treatment by the photocatalytic reactor alone,the HCHO concentration was decreased gradually but steadily.How-ever,this treatment system is not practical because it takes a long time to reach the WHO guideline.In the treatment by the adsorption and desorption apparatus alone,on the other hand,the HCHO concentration was decreased to the neighborhood of the WHO guideline in15min,but subsequent change was quickly slowed down andÿnally stopped.In parallel,the HCHO concentration in the small box indicated a maximum of3:2mg m−3at15min and then remained almost unchanged.This is probably because the HCHO concentration reached an adsorption equilibrium state among the highly tight room,the small box,and the activated carbon rotor.In principle,one cannot expect that the HCHO concentration is further decreased by use of the adsorption and desorption apparatus alone.When this apparatus was combined with the photocatalytic reactor, on the other hand,the HCHO concentration was initially decreased quickly and then steadily decreased toward a zero value.The initial stage of the decrease in the indoor HCHO is obviously owing to the adsorption of HCHO on the activated carbon rotor.Taking into consideration that the HCHO concentration in the small box passed through a maximum of2:5mg m−3at5min and then decreased at a relatively high speed,the last stage of the decrease in the indoor HCHO is owing to the photocatalytic decomposition of HCHO successively released from the activated carbon rotor.3.3.E ect of temperatureIn the above experiments,the desorption temperature was set at180◦C.To economize on electric power,however,the desorption temperature should be lowered.Also,operation at such a high temperature should be avoided because un-known substances liberated from the activated carbon rotor932 F.Shiraishi et al./Chemical Engineering Science 58(2003)929–934H C H O c o n c . [m g m -3]Time [min]H C H O c o n c . [m g m -3]Time [min]Fig.2.Time courses of HCHO concentration when air containing HCHO was treated by a photocatalytic reactor alone,an adsorption and desorption apparatus alone,and both of these apparatuses;(a)10m 3highly tight room,and (b)0:09m 3small box.H C H O c o n c . [m g m -3]Time [min]050100150H C H O c o n c . [m g m -3]Time [min]Fig.3.E ect of desorption temperature on time courses of HCHO concentration when air containing HCHO was treated with an adsorption and desorption apparatus alone;(a)10m 3highly tight room,and (b)0:09m 3small box.were strongly adsorbed on the glass surface,which led to a marked loss in the photocatalytic activity.Although the photocatalytic activity was recovered by washing the glass tube with a dilute hydrochloric acid solution,this certainly becomes a serious problem in practical application.There-fore,the same experiments were conducted at lower tem-peratures,120◦C and 150◦C.Figs.3(a)and (b)represent the e ect of desorption tem-perature on the time courses of the HCHO concentration in the highly tight room and small box,respectively,when the air containing HCHO was treated with the adsorption and desorption apparatus alone.The most rapid decrease in the HCHO concentration in the highly tight room was found at 180◦C.Also,the highest concentration of the HCHO des-orbed into the small box was obtained at this temperature,indicating that the higher the desorption temperature,the more largely the HCHO adsorbed on the activated carbon rotor is desorbed.From a standpoint of practical application,however,it can be considered that there is no remarkable di erence in the variation of the HCHO concentration in the range from 120◦C to 180◦C.Figs.4(a)and (b)represent the e ect of desorption tem-perature on the time courses of the HCHO concentration in the highly tight room and small box,respectively,when the air containing HCHO was treated with the photocatalytic re-actor combined with the adsorption and desorption apparatus that were operated at the same desorption temperature as in Fig.4.Unexpectedly,the lower the desorption temperature,F.Shiraishi et al./Chemical Engineering Science 58(2003)929–934933H C H O c o n c . [m g m -3]Time [min]H C H O c o n c . [m g m -3]Time [min]Fig.4.E ect of desorption temperature on time courses of HCHO concentration when air containing HCHO was treated with a photocatalytic reactor combined with an adsorption and desorption apparatus;(a)10m 3highly tight room,and (b)0:09m 3small box.the more rapidly the HCHO concentration in the highly tight room was decreased.This is probably due to the degree of deactivation in TiO 2.That is,the desorption at 180◦C pro-vides the highest HCHO concentration in the small box,but reduces the photocatalytic activity more rapidly as described above,which results in a slower decomposition of HCHO.At every temperature,the HCHO concentration in the small box was lowered owing to the photocatalytic reaction.The HCHO concentrations in the small box were decreased more rapidly at 120◦C and 150◦C than at 180◦C,which caused more rapid decreases in the HCHO concentration in the highly tight room.Taking into consideration the energy sav-ing,operation should be made at 120◦C.At this temperature,little loss in the photocatalytic activity was caused.3.4.Concentration of an intermediateIn the photocatalytic decomposition of HCHO,there is a possibility that formic acid is formed as an intermediate.To clarify the extent of this formation,HCHO was pho-tocatalytically decomposed in a 0:06m −3container.First,the decomposition experiment was carried out at an indoor HCHO concentration level,but no formic acid was detected.Therefore,the HCHO concentration was increased upto 1130mg m −3(2700times of the HCHO concentration in the actual indoor environment).As a result,formic acid of 0:425mg m −3at maximum was detected.From this result,it is estimated that the present photocatalytic reactor system decomposes HCHO quickly to CO 2via formic acid.4.ConclusionsIn the present work,a novel air-puriÿcation system,con-sisting of the photocatalytic reactor with a parallel array ofnine blacklight blue uorescent lamps and the continuous adsorption and desorption apparatus with a ceramic-paper honeycomb rotor retaining activated carbon or zeolite ÿne particles,was constructed and the performance of this sys-tem was investigated by treating HCHO artiÿcially gener-ated at a concentration level of ppbv (¡1mg m −3)in a 10m 3highly tight room.As a result,it was found that the HCHO concentration can be decreased to the neighborhood of the WHO guideline in 10min and to an almost zero value in 1h.As far as the authors know,this is the ÿrst fruits in which dilute HCHO contained in such a large amount of air was decomposed to an almost zero value in such a short time.Needless to say,this surprisingly high performance is based on the cooperative work by the activated carbon ro-tor to rapidly adsorb the indoor HCHO and the photocat-alytic reactor to e ciently decompose HCHO in the small box.The air-puriÿcation system provides several other ad-vantages.The adsorbent can be used over a long period of time because it is continuously regenerated by exposing the rotor to heated air.Also,the use of the activated carbon rotor makes the system more practical because the HCHO adsorbed is readily desorbed at 120◦C,which contributes greatly to energy saving.At such a low temperature,the photocatalytic activity was satisfactorily stable.ReferencesFukinbara,S.,&Shiraishi,F.(2001).Characteristics of the photocatalytic reactor with an annular array of glass tubes surrounding a light source:2.Kinetic analysis.CELSS Journal ,13,11–23.Fukinbara,S.,Shiraishi,F.,&Nakano,K.(2001).Characteristics of the photocatalytic reactor with an annular array of glass tubes surrounding a light source:1.Selection of a light source and photocatalyst support.CELSS Journal ,13,1–10.Godish,T.(1998).Sick buildings deÿnition,diagnosis and mitigation .Florida:CRC Press.934 F.Shiraishi et al./Chemical Engineering Science58(2003)929–934Matsuo,K.,Takeshita,T.,&Nakano,K.(1990).Formation of thinÿlms by the treatment of amorphous titania with H2O2.Journal of Crystal Growth,99,621–624.Miyazaki,T.(1996).Formaldehyde concentration in a private home. 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