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. Fig. 6 shows the results obtained in thesame experiments as in Fig. 5 except the desorption temperaturecontrolled in the range of 130–160 ◦C. Again, in all the purificationexperiments, the toluene concentrations in the closed room arelowered to near zero within 10–15min. The time courses of tolueneconcentrations in the closed room are not remarkably dependent onFig. 4. Relationship between equilibrium concentrations of toluene in a reactor box(CSeq) and equilibrium concentration of toluene adsorbed on a rotor (CReq)fortworanges of desorption temperature. The values of CSeq were determined from Figs. 2and 3.the desorption temperature and are also almost the same as thosein Figs. 2 and 3. Therefore, it is obvious that the rapid decrease inthe toluene concentration within 10–15min is mainly due to theadsorption of toluene onto the zeolite rotor. It should be noted thatthe toluene concentration becomes finally almost zero as a result ofthe photocatalytic decomposition.The toluene concentrations in the reactor box increase quickly,reaching their respective maximums in 10–15min and finallydecreasing to nearly zero values as a result of the photocatalyticdecomposition. Obviously, this decrease can be attributed to thephotocatalytic decomposition of toluene.
The maximum concentra-tion of toluene is higher when a larger amount of toluene is initiallyvolatilized in the closed room. Thus, it takes a longer time until thetoluene concentration decreases to a nearly zero value. Moreover,when the desorption temperature is higher, the maximum tolueneconcentration becomes higher and it takes a longer time untiltoluene reaches a nearly zero concentration. When a larger amountof toluene is desorbed at a higher temperature, its concentration inthe reactor box becomes higher, so that it is considered that tolueneover the photocatalyst surface is increased to a higher concentrationand decomposed at a higher rate.The apparent rates of decomposition of toluene in the reactor boxwere determined from the decreasing part of experimental data inFigs. 5 and 6. Fig. 7 shows a plot of these values against the estimatedtoluene concentrations on the rotor, CReq. The high desorption tem-perature provides a high apparent rate of decomposition since thetoluene concentration in the reactor box is high. For both desorptiontemperatures, the apparent rate of decomposition tends to becomeconstant when CReq becomes larger than a specific value. This mayimply that the desorption process of toluene from the rotor is a rate-limiting step. Exact evaluation of the air purifier will be given by asystem analysis using differential mass balance equations for the airpurifier in a subsequent paper.3.3. Effect of the number of reaction tube unitsTo further save electrical energy, we investigated how the per-formance of the air purifier is changed when the number of reactiontube units is reduced as shown in Fig. 8. The experimental result ob-tained when the initial toluene concentration was set at 3.4mgm−3is shown in Fig. 9 , where the time courses of toluene concentrationsin the reactor box are compared between nine and four reactiontube units in the photocatalytic reactors. The experimental data for the time courses of toluene concentrations in the 1m3 room are notshown because they are almost the same as the experimental datafor different numbers of reaction tube units. With the nine reactiontube units, themaximumconcentration of toluene is lower than withthe four reaction tube units because the rate of decomposition is sohigh that the decomposition is completed in a shorter time.With thefour reaction tube units, on the other hand, the toluene concentrationdecreases more slowly because of a lower rate of decomposition.However, it should be noted that toluene is again decomposed toa near zero concentration. The differing performance is mainly dueto the present photocatalytic reactor decomposing toluene underthe conditions of negligible film-diffusional resistance and high UVintensity per unit photocatalyst surface area (Shiraishi et al., 2005b).Thus, it is reasonable to reduce the number of reaction tube unitsaccording to the requirements. 3.4. Comparison of treatments of air containing toluene using an airpurifier and a photocatalytic reactorThe photocatalytic reactor was placed in the 1m3 room andoperated to directly decompose toluene at various initial concentra-tions. Fig. 10 shows the time courses of toluene concentrations inthese decomposition experiments. The toluene concentrations aredecreased to almost zero within 40–110min for initial concentra-tions of 1.3–6.4mgm−3. However, this required time is longer thanthat of HCHO decomposition (Shiraishi et al., 2005b). This may bedue to the fact that toluene has more bonds in one molecule thanHCHO and the decomposition of intermediates such as benzaldehydecompetes with that of toluene (Blount and Falconer, 2001).The initial rates of decomposition v (mgm−3 min−1) deter-mined from the experimental data in Fig. 10 were applied to theLangmuir–Hinshelwood type expression v = kKHC/(1+KHC)toob-tain the rate constant k (mgm−3 min−1) and adsorption equilibriumconstant KH (m3 mg−1). As a result, these were determined to bek = 1.563mgm−3 min−1 and KH = 0.0266m3 mg−1. All the apparentrates of decomposition of toluene in Fig. 7 are 10-fold smaller thanthe value of k, indicating that the ability of photocatalyst to decom-pose toluene is not sufficiently utilized. At the desorption temper-ature in the range of 90–120 ◦C, the toluene concentration in the reactor box in the decomposition of toluene in 1m3 air using the airpurifier decreased more slowly than did the toluene concentrationin the direct decomposition of toluene in 1m3 air.