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ABSTRACTThe performance of the miniaturized photocatalytic air purifier including a continuous adsorp-tion/desorption unit with a zeolite particles-loaded honeycomb rotor was investigated in the photocat-alytic purification of 1m3 air containing toluene at concentrations of 3–11mgm−3 (about 1–3 ppmv).While operating the continuous adsorption/desorption unit only, and for desorption temperatures con-trolled within the range of 90–120 and 130–160 ◦C, the unit took approximately 10min to reduce thetoluene concentration in a 1m3 room, to a value of almost zero. Almost the same time courses of tolueneconcentrations were obtained when the photocatalytic reactor was switched on. 57950
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This result clearly showsthat the rapid decrease in the toluene concentration in the 1m3 room is mainly due to the adsorption oftoluene onto the zeolite rotor. When the reactor was switched off, the concentration of toluene desorbedinto the 0.022m3 reactor box increased rapidly and then levelled off in 10min. On the other hand whenthe reactor was switched on, the toluene concentration in the reactor box increased rapidly, passingthrough a maximum in 10–150min, and then decreasing to a value near zero, leading to the tolueneconcentration in the 1m3 room becoming negligible. The apparent rates of decomposition determinedfrom the decreasing toluene concentration in the reactor box were 10-fold smaller than the intrinsicrate constant in the Langmuir–Hinshelwood equation, which would suggest that the desorption processof toluene from the rotor is the rate-limiting. All the decomposition experiments were performed usingthe same zeolite rotor and photocatalytic reactor over a six month period; and the runs were repeatedmore than one hundred times.
Nevertheless, there was no distinct decrease in decomposition activity ofthe photocatalyst or any significant loss in ability for the zeolite rotor to adsorb toluene could be seen,indicating that these materials can perform constantly over a long period of time. However, as the rotorwill sooner or later become saturated with toluene, it may become necessary to introduce a regenerationprocedure in order to periodically remove toluene from the rotor. 1. IntroductionThe photocatalytic treatment of volatile organic compounds(VOCs) has several advantages that are not provided by other treat-ment methods. For example, the decomposition reaction can easilyoccur at room temperature without the addition of other chemicalreagents and the reactants may be decomposed into carbon dioxideand other minerals (Maira et al., 2003; Xu and Shiraishi, 1999).
It isthus reasonable to apply the photocatalytic treatment process to theindoor environment (Obee and Brown, 1995; Shiraishi et al., 2004).Toluene is one of the typical indoor VOCs that threaten hu-man health. Therefore, many reports have been published on the treatment of this compound using photocatalysts (Barraud et al.,2005; Bouzaza and Laplanche, 2002; Bouzaza et al., 2006; Chen andZhang, 2008; Coronado and Soria, 2007; Demeestere et al., 2008;Guo et al., 2008; Inaba et al., 2006; Nakajima et al., 2005; Sanoet al., 2004; Sekiguchi et al., 2008; Tomasic et al., 2008; Tsoukleris etal., 2007; Young et al., 2008; Yu and Lee, 2007; Zou et al., 2006a,b).Several research groups found that intermediates accumulated dur-ing the photocatalytic decomposition of toluene quickly deactivatetitanium dioxide (Blount and Falconer, 2001, 2002; Marci et al.,2003). In the decomposition of other VOCs on the other hand, therate of decomposition of gaseous toluene in the environment wouldbe markedly lowered by the film-diffusional resistance being in theneighborhood of photocatalyst surface, because the toluene concen-tration is around 1 ppmv (parts per million in volume). For example,in the decomposition of HCHO, a remarkable decrease in the rate ofdecomposition was observed when its concentration dropped be-low 1 ppmv (Shiraishi et al., 2005a). This problem can be overcome by the use of a photocatalytic reactor with a parallel array of ninelight sources (6Wblacklight fluorescent lamps) inpidually insertedinto glass tubes whose inside surfaces are coated with a transparentthin film of titanium dioxide (Shiraishi et al., 2005a,b). This photo-catalytic reactor provides the following major advantages: (1) thephotocatalytic reaction takes place in the absence of diffusional re-sistance since the air treated passes through the annulus betweenthe photocatalyst tube and light source at a linear velocity above11ms−1; (2) the photocatalytic reaction occurs under a condition ofhigh UV intensity per unit surface area since the distance betweenthe photocatalyst surface and light source is only 5mm; (3) the UVlight that permeates through a glass tube acts on the titanium diox-ide film on neighboring glass tubes (Shiraishi et al., 2005b; Wang etal., 2002), so that the decomposition rate is further increased. Thus,HCHO at a concentration level of ppbv in 1m3 air was decomposedto the WHO guideline (80 ppbv or 100mgm−3) within 20–60min(Shiraishi et al., 2005b, 2009).Nevertheless, the performance of the reactor is not suffi-cient to purify a large amount of indoor air within a satisfactorytime. Therefore, we subsequently developed an air purifier (size;50 cm×50 cm×125 cm) where this photocatalytic reactor was com-bined with a continuous adsorption/desorption unit (Shiraishi etal., 2003, 2007). As a result of experimental treatment of HCHO in10m3 air, we found that in this modified air purifier, the continuousadsorption/desorption unit can lower the HCHO concentration tobelow the environmental guideline (80 ppb) within the first 10minand then the photocatalytic reactor quickly decomposes the HCHOreleased from the reactor unit to a near zero concentration in about180min (Shiraishi et al., 2007).Although the size of the air purifier may restrict practicalapplication, if it could be miniaturized sufficiently without changingthe principles of the treatment, application could become extendedto air purification in small spaces, such as in cars. Therefore, thispresent work miniaturizes the air purifier and elucidates perfor-mance through the treatment of air containing toluene at a lowconcentration level.2. Experimental2.1. MaterialsThe coating solution of anatase titanium dioxide (0.5–20 minsize) was a product of Sundecor Co., Ltd. (Fukuoka, Japan). Toluenewas purchased from Wako pure Chemical Industries, Ltd. (Tokyo,Japan). Nine 6W blacklight blue fluorescent lamps (FL6BLB-A),210mm long and 15.5mm in diameter were a product of ToshibaCo., Ltd. (Tokyo, Japan); this light source irradiates UV light witha wavelength distribution between 300–400 nm and a maximumintensity of 10.3 Wcm−2 at 350 nm.2.2. Experimental apparatus and procedureThe coating solution (Nakano, 2005) was applied to the insidesurfaces of Pyrex glass tubes (28mm in inside diameter, 210mmlong, and 2mm thick), followed by heating at 100 ◦C for 1 hr. Thesame operation was repeated five times. A more detailed account ofthis procedure can be found elsewhere (Shiraishi et al., 2009).Fig. 1 shows a schematic of the air purifier, which consists of acontinuous adsorption/desorption unit and a photocatalytic reactor.The continuous adsorption/desorption unit has a rotary adsorbent-loaded rotor in a honeycomb structure, two electric fans for adsorp-tion and desorption, and an electric heater for desorption of toluene.The rotor (120mm in diameter, 50mm thick, and 5.64×10−4 m3 inapparent volume) consists of a roll of corrugated ceramic paper(3mm in corrugation pitch and 1.7mm in height) on which about 63 g of zeolite particles (XSM-5) with a pore size of 6Å are deposited.The rotor was slowly spun at a constant rate of 1/12 rpm. The air con-taminated with toluene was continuously supplied into three quar-ters of the cross-sectional area of the rotor at a flow rate of 0.27m3min−1 in order to adsorb toluene. At the same time, the air beingcirculated at 0.09m3 min−1 through the rotor and the reactor boxwith a volume of 0.022m3 was instantaneously heated and suppliedinto one quarter of the cross-sectional area of the rotor in order todesorb toluene. Toluene desorbed in the loop was subsequently de-composed in the photocatalytic reactor fixed in the reactor box.