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Modeling and experimentation of a novel labyrinth bubble photoreactor for degradation of organic pollutant
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Modeling and experimentation of a novel labyrinth bubble photoreactor for degradation of organic pollutant Hao Xiao-gang ∗ , Li Hong-hui, Zhang Zhong-lin, Fan Cai-mai, Liu Shi-bin, Sun Yan-ping Department of Chemical Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan, Shanxi 030024, China
a b s t r a c t A novel labyrinth flow bubble photoreactor (LBPR) was designed and the model of multiple mixed flow reactors (MMFRs) in series was established to describe the behavior of the LBPR during batch recirculation system of low concentration organic pollutant degradation by photocatalysis. This model was developed by the mass balance combining the rate equation of the photocatalytic reaction. The influence of the structural parameters, such as the number of baffle plates, the space between the baffle plates and the height of liquid exit in the reactor, on the photocatalytic degradation of organic pollutant was mainly investigated. The relationships between the final conversion rate and the liquid rate, liquid volumes, light intensity and catalyst amounts were also predicted by this model. TiO2 was immobilized on quartz glass particles by sol–gel method and used as the photocatalysts; the degradation experiment of methyl orange (MO) was performed in a 2 l LBPR reactor to validate the above model. The experimental results showed that the removal rate of MO could reach 100% with 12 baffle plates and 4 cm height of liquid exit after 1.5 h treatment when the flow rate was 0.0111 m3 h−1 and the particle catalysts was 4.8 g l−1 . The prediction of the model is consistent with the experimental results. This research provides exploratory work for the design of a novel commercial scale photocatalytic reactor. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Labyrinth bubble photoreactor; Photocatalysis; Modeling; Structure; Water treatment
1.
Introduction
Heterogeneous photocatalysis is one of the advanced oxidation processes (AOP) that couples low energy UV light with semiconductor acting as photocatalyst (Yang et al., 2005). The appeal of this process technology is the prospect of complete mineralization of organic pollutants to environmentally harmless compounds (Kabir et al., 2006). Activation of the catalyst is achieved by electron–hole pair formation initiated through the absorption of an ultraviolet photon. The applied radiation may be artificial or solar and the process can be operated at ambient pressure and temperature (Brandi et al., 2002; Cheng et al., 2007; Sagawe et al., 2003). Moreover, TiO2 -based photocatalysis is more appealing than other conventional chemical oxidation methods because it is cheap, biologically and chemically inert, insoluble under most conditions, photostable, non-toxic, can be used for extended period without substantial loss of its activity, and more impor-
∗
tantly can even be activated by sunlight (Hao et al., 2003; Ray, 1999). In spite of the potential of this technology, development of a practical photocatalysis water treatment system has not yet been successfully achieved. A photocatalytic process must be constructed from three elements: a reactor, a light source, and a photocatalyst (Vaisman et al., 2005). Fine TiO2 powders present separation problems in slurry type reactors leading to complications and expenses. As a result, a number of efforts have been made to support TiO2 on fixed or mechanically manageable supports, which eliminates arduous step of separation (Fan et al., 2003; Haque et al., 2005; Kanki et al., 2005; Lee et al., 2003; Pozzo et al., 2006). When the catalyst is immobilized on particles, the effective design and scale-up of photocatalytic reactor becomes the key to the practical application of photocatalysis water treatment technology. Besides conventional reactor complications such as mixing, mass transfer, reaction kinetics, catalyst installation, etc., an addi-
Corresponding author. Tel.: +86 351 6018193; fax: +86 351 6018554. E-mail address: [email protected] (X.-g. Hao). Received 7 October 2008; Received in revised form 11 April 2009; Accepted 1 June 2009 0263-8762/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2009.06.002
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
Nomenclature acs CA0 Cin,t Cout,t C0 CN+1 Cm Ct H I k k N n Ql Qg r ri S Sg VR VRi VT X x xR xRt i T
catalytic surface area per unit volume (m2 m−3 ) initial concentration (mol m−3 ) inlet concentration in the tank (mol m−3 ) outlet concentration from the tank (mol m−3 ) inlet concentration to the reactor (mol m−3 ) outlet concentration of the reactor (mol m−3 ) catalyst loading or concentration (g m−3 ) liquid concentration at any time (mol m−3 ) depth of the liquid layer (m) light intensity (mW m−2 ) rate constant (m3 s−1 mW−1 ) rate constant (s−1 ) number of baffle plates reaction order liquid flow rate (m3 s−1 ) gas flow rate (m3 s−1 ) constant reaction rate (mol m3 s−1 ) cross-section area (m2 ) catalyst specific surface area (m2 g−1 ) whole volume of reaction area in reactor (m3 ) volume of each channel (m3 ) storage tank of volume (m3 ) dimensionless axial length conversion single pass conversion of the reactor total conversion of this system mean residence time of liquid in each channel (s) mean residence time in the tank (s)
tional engineering factor related to illumination of catalyst becomes relevant in a photocatalytic reactor (Ray, 1999). In the last few years, a large number of publications have appeared about the photocatalytic reactor configurations. Among these configurations, annular or cylindrical fluidized bed reactor with lamp(s) immersed within it is to be preferred (Couto et al., 2002; Kanki et al., 2005; Lim and Kim, 2004; Nam et al., 2002; Pozzo et al., 2006). However, the design and scale-up of this kind of reactor must consider the combination of the light source and cooling system, the structure is complicated and the efficient reaction volume is low (Hao et al., 2005). Therefore, the development of new photocatalytic reactors is still an important issue. Photocatalytic technologies are more appropriate for treatment and destruction of contaminants at dilute concentration, which constitute a very large volume of industrial wastewater (Lin and Valsaraj, 2003). The steady-state kinetic models presented in the literature usually ignore the influence of mass transfer (Biard et al., 2007); this is not a problem if the flow rate is sufficiently high to ensure a good mass transfer rate. Nevertheless, working with high flow rates is not usually possible, and then, the observed degradation rate is dependent on mass transfer rate. Consequently, the external mass transfer has a significant influence with those reactor configurations because of the loss of surface area upon immobilization of a photocatalyst on inert supports, especially for dilute wastewater treatment. In order to overcome some of these deficiencies inherent in conventional photocatalytic reactor, a labyrinth flow photore-
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actor with immobilized TiO2 bed was designed for removal of phenol and nonionic surfactant from water (Grzechulska and Morawski, 2003; Mozia et al., 2005). This kind of reactor is an innovative setup; however, the photocatalyst particles are fixed to the bottom of the reactor using polymer glue, such a manner that only single layer of catalyst is keep in the reactor and half of the particle surface is accessible for the UV light. The thickness of solution layer over the catalyst is thin (only 1 cm) and the photodegradation effectiveness is still low. In this paper, a novel labyrinth flow bubble photoreactor (LBPR) (Hao et al., 2008) was designed and a mathematical model was presented to describe the behavior of the LBPR during batch recirculation system of low concentration of organic pollutant degradation by photocatalysis. From the degradation experiment of methyl orange (MO) in LBPR with the light source (365 nm) outside it as an imitated solar light, possibility of water purification under solar light was proved. This new reactor allows for a much higher illuminated surface area per unit reactor volume and is flexible enough to be scaled-up for commercial scale application.
2.
LBPR system model
2.1.
Reactor description
Fig. 1 shows the schematic diagram of a labyrinth bubble photoreactor (LBPR). Detailed description of the reactor can be found in Hao et al. (2007, 2008). In brief, the reactor with rectangular-cross-section is divided by parallel baffle plates and the liquid labyrinth flow channels are built. The liquid flows along the space between the baffle plates in the labyrinth reaction area and contacts with the upflow gas bubbles from the gas distributor on the bottom of the reactor. The particle photocatalysts immobilized with TiO2 thin film are dispersed in the mixtures of liquid and gas bubbles. The top of the reactor is open to air and external light sources such as UV lamps or solar light can be used. Light energy or photons transferred into the flow channel are absorbed by TiO2 immobilized on support materials and a heterogeneous photocatalytic reaction takes place in the reactor. The fluidization of gas–liquid–solid ensures that all photocatalytic particles
Fig. 1 – Schematic diagram of labyrinth bubble photoreactor (LBPR).
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receive uniform illumination. This configuration meets the requirements of higher surface area-to-volume ratio, of long residence time for liquid, of better use of light, and of improvement of mass transfer conditions.
2.2.
LBPR model
The LBPR not only brings more contact of photocatalyst and organic pollutant but also enhances UV light penetration efficiently into the interior of photocatalyst bed through gas bubbles. The three phases of gas, liquid and solid in each channel divided by N equal-distance plates are mixed uniformly because of the air bubbling and an idealized mixed flow is assumed to exist in the channel. Therefore, the whole LBPR can be considered as a system of (N + 1) mixed flow reactors (MFR) connected in series. Though the concentration is uniform in each reactor, there is, nevertheless, a change in concentration as liquid moves from reactor to reactor. The behavior of a series of (N + 1) equal-size MFRs can be evaluated quantitatively with the mass balance combining the reaction kinetics equation or rate equation. The nomenclature used is shown in Fig. 2 with subscript i referring to the ith vessel. The whole volume of reaction area is VR and the volume of each channel or vessel is: VRi =
VR N+1
(1)
where acs and I can be treated as constants when the catalyst amount and the light intensity are fixed. The global process of heterogeneous photocatalytic oxidation can be described in consecutive steps of mass transfer, adsorption, reaction and desorption. There is no consensus about the controlling rate step, although some authors have reported that the adsorption step controls the rate (De Fatima et al., 2005). At low inlet concentration, some authors have reported that the photocatalytic degradation obeys first-order kinetics or pseudo-first-order model, i.e., the degradation rate is proportional to the inlet concentration (Mozia et al.,2005; Biard et al., 2007). In this case, the limiting steps are the adsorption or the transfer of the pollutant onto the catalyst. In LBPR, the degradation process are operated at ambient temperature and the particle photocatalysts are fluidized by upflow air bubbling; therefore, the adsorption and desorption is not the control steps. Usually, the studies which investigate the effect of the structural parameters and the flow rate are carried out in a recirculation batch system. In this case, the degradation rate is dependent only on the mass transfer rate, which implies that better degradation rates are obtained when the flow rate increases. For first-order reaction or the mass transfer control regime, the above expression becomes ri = −k Ci
The mass balance of component A for the ith MFR is represented by the following equation: Ql C0 (1 − xi−1 ) = Ql C0 (1 − xi ) + ri VRi
(2)
Ql C0 (xi − xi−1 ) Q (C − Ci ) = l i−1 ri ri
(3)
where Ql , x and ri are the liquid flow rate, the conversion and the reaction rate, respectively and C0 is the inlet concentration. The mean residence time of liquid in each channel can be written as: V i = Ri Ql
Substitution of (4) and (6) into (3) gives k i =
Ci−1 − Ci Ci
(7)
Because the distance between each two plates is equal, the residence time of liquid in each channel is equal
which on rearrangement becomes: VRi =
(6)
1 = 2 = N =
(8)
Then combining (7) and (8) the exit concentration from the reactor may be written as: CN+1 =
(4)
C0 (1 + k )
N+1
(9)
C0 is the inlet concentration to the reactor at any time. The reaction rate expression or kinetic equation for the heterogeneous photocatalytic degradation of organic pollutant is given by (Sun and Zhao, 2000) dCi = −ri = −acs kIr Cni dt
(5)
2.3.
One possible mode of operation of the LBPR is that which involves the continuous recirculation of the wastewater, a gradual degradation of the concentration of the organic pollutant taking place. This mode of operation is depicted in Fig. 3. It is important in the design of such processes to be able to predict the variation of concentration of the organic pollutants with time. Schematically, the system under study is formed by a photoreactor of volume VR and a storage tank of volume VT functioning in a closed circuit. The tank operates under unsteady state conditions. The mass balance in the tank for component A is represented by the following differential equation and initial condition: VT
Fig. 2 – Notation for the ith vessel in a system of (N + 1) MFRs in series.
Batch recirculation system model
dCout,t = Ql (Cin,t − Cout,t ) dt
Cout,t (t = 0) = CA0
(10) (11)
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
Fig. 3 – Schematic representation of the employed system. In Eq. (10) the mean residence time in the tank is given by VT T = Ql
(12)
Assuming that the tank works under well-stirred conditions, the concentration in the tank (equal to its outlet concentration) is only a function of time. The inlet concentration in the tank comes from the reactor and the outlet concentration from the tank remains unchanged until the liquid gets into the reactor. One can safely assume that: Cin,t = CN+1
(13)
Cout,t = C0
(14)
Substitution of Eqs. (9) and (12)–(14) into Eq. (10) and integration gives the solution for the variation of concentration with time as:
Ct = CA0 exp
t T
1 (1 + k )
N+1
−1
(15)
or
x R
Ct = CA0 exp −
T
(16)
t
where xR is a single pass conversion of the reactor. The total conversion of this system is
x R
xRt = 1 − exp −
T
(17)
t
If the reaction volume of the LBPR is given by VR = SH, the variation of concentration with time can be written as: Ct = CA0 exp
Q VT
1
l
1 + k (SH/(N
+ 1)Ql )
t
Fig. 4 – Scheme of the LBPR for photocatalytic degradation of organic pollutant in water. (1) Metering pump, (2) gas distribution plate, (3) photoreactor, (4) baffle plate, (5) UV lamp, (6) rotameter, (7) air pump and (8) storage tank.
system setup: (1) the labyrinth bubble photoreactor, (2) the illuminating system, (3) the recycle, with a tank and a metering pump, and (4) the air line that includes an air pump and a rotameter. The photoreactor with the holding capacity of 2 l was constructed based on rectangular geometry (16 cm × 32 cm) with an external lamp arrangement. The reactor was made of plexiglass and the top was open to air. The geometry of the reactor allowed the option of inserting additional baffle plates inside the reactor to increase the number of labyrinth channels and the maximum UV transmission into the reaction zone from above. There were two exits of liquid at different height from the gas distributor. The different exits were used to regulate the thickness of the liquid layer. The lower portion of the reactor was filled with glass beads of 5 mm diameter and used as the predistribution section of gas. The distributor was made of two rectangular plexiglass plates with 195 holes of 2 mm diameter each arranged in a triangular pitch and had a Nylon screen of 100-mesh sandwiched in between, in order to prevent the back-flushing of photocatalyst particles and to provide uniform distribution of gas along the reactor cross-section area. The special design of blowhole remained most of the catalyst particles in the reactor, and a few particles moving out of the exit of the reactor due to the liquid flow were accumulated, and then re-added into the inlet of the reactor with the liquid (Hao et al., 2008). A tubular UV lamp (365 nm wavelength) with a nominal power of 375 W was placed at the focal axis of a cylindrical reflector of parabolic cross-section. The whole illuminating system was suspended above the reactor and an external parallel light source was obtained. As shown in Fig. 4, the reactor operated inside a recycle that has a 5 l storage tank made of glass and a pump. The pump was used to recirculate the liquid to and from the reactor and to improve mixing and facilitate sampling in the tank.
(18)
3.2. where S is the cross-section area and H is the depth of the liquid layer.
3.
Experimental
3.1.
Labyrinth bubble photoreactor (LBPR) setup
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Fig. 4 shows the schematic diagram of the experimental setup. As shown in Fig. 4 there are four main parts in the complete
Photocatalytic degradation experiments
Methyl orange (MO) was used as a model pollutant in this series of experiments. TiO2 was immobilized on quartz glass particles (average diameter of 0.5 mm and average TiO2 content of 3.3 wt.%) by sol–gel method and used as the photocatalysts. Initially, a certain amount of wastewater containing MO was introduced in the storage tank and then passed through the reactor and stirred with air bubbling for half hour in the dark to allow for saturation of the TiO2 /glass. There was
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no obvious concentration change during this process and the adsorption is very low because of the drastic stir. Afterwards, the lamp was turned on and a straight recycle process was initiated. The flow rate of air was regulated by a flowmeter and introduced into the bottom of the reactor. During the process, samples of the reaction solution were taken at time intervals of 10 min for concentration analysis. MO concentration was determined by 4–5 UV–vis spectrometry method using CE1021 spectrometry at a fixed wavelength of 460 nm. Analytical results were calibrated using a series of standard solutions corresponding to a range of concentrations used in the experiments. Reactor light intensities were measured by VLX-3W (America, 365 nm). The whole illuminating system and the reactor were placed inside a glass box, and the inner box wall was covered with a reflecting aluminum foil.
4.
Results and discussion
4.1.
Model parameters and analysis
The developed model of this work considers separated mass balances for each of the two reservoirs, i.e., for the reactor and the storage tank of the recycle. The model and kinetic coefficient or reaction rate constant k can be used to predict the MO concentration–time relationship for different structural parameters of N and H, and different operational parameters of Ql , VT , I and Cm during the degradation process. Kinetic equation for the photocatalytic reaction is an important component of the modeling (Zalazar et al., 2005; Zhang et al., 2006). In the present work, the mass transfer is a rate-determining step because of the low inlet concentration and low flow rate of wastewater, even though the particles are completely fluidized (the air flow rate increased to 20 l/min) in the reactor; the MO degradation kinetics has been proved to be an apparent first-order by experimental results. For a given light intensity, the depth of the liquid layer and catalyst amount, the effects of the number of baffle plates, liquid flow rate and the volume of storage tank on the degradation performance of MO can be easily obtained by Eq. (18). But the effects of other parameters on the performance of the reactor are complicated because the reaction rate constant is influenced by these parameters. Detailed parametric analysis of the reaction kinetics and mass transfer rate can provide information to extend this model to other complex operation process. The reaction rate expression described by Eq. (5) is a function of the spatial coordinates and time. First, MO concentration in the recycling system changes with time along the degradation experiment; secondly, the rate of electron–hole generation, that is directly proportional to the local light intensity, is also a function of position and time due to two different factors: (i) radiation adsorption by the catalyst is very strong and generates a light intensity distribution that varies very much along the depth of the liquid layer and (ii) the light intensity distribution is also a function of time because the optical properties of the catalytic vary with time. In addition, Eq. (5) calls for a value of acs . The catalytic surface area per unit volume can be calculated from the catalyst loading Cm and the catalyst specific surface area Sg : acs = Cm Sg
reactor can be simulated by Hao et al. (2005). In order to avoid the complexity associated with variation of light attenuation properties with time and position, the kinetic parameters or the coefficients of mass transfer of Eq. (5) were estimated from all the MO concentration vs. time experimental data applying a non-linear regression procedure. Therefore, the effects of particle catalyst amount, thickness of liquid layer and light intensity that change the optical properties of the reacting medium and hence the light distribution inside the reactor have been taken into account.
4.2. Influence of structure parameters on degradation performance 4.2.1.
Effect of the number of baffle plates
Fig. 5 shows the influence of the number of baffle plates on the degradation performance of MO using a LBPR system for a given intensity of radiation. Both the concentration–time curves derived from the model and a comparison with the experimental data are shown in this figure. It is seen that increasing the number of baffle plates the MO conversion increases significantly when the number of baffle plates is lower than 12. For example, for an initial MO concentration of 10 mg/l and the following baffle plate’s number: 0, 2, 6 and 12, the final conversions after 1.5 h are 0.45, 0.81, 0.94, and 0.99, respectively. As shown in Fig. 5, the predictions by the model agreed very well with the experimental data. For a given volume of reactor, increasing the number of baffle plates leads to an increase of the flow velocity of liquid in a labyrinth channel. The degradation rate increased with the baffle plates, which emphasizes the influence of the mass transfer. On the other hand, increasing the number of baffle plates also leads to a decrease of the distance between plates. When the diameter of the air bubble is bigger than this distance, the “bridging bubbles” will congest the space between the baffle plates and decay the uniform mixture of the gas, liquid and particles. Larger number of baffle plates will result in the decrease of photocatalysis performance. Therefore, there would be an optimal number of baffle plates in the reactor and the number of about 12 would be a suitable value under the investigated conditions.
4.2.2.
Effect of the height of liquid exit
The influence of the height of liquid exit (or the depth of liquid layer) on the degradation performance of MO in the reactor is shown in Fig. 6. It is observed that the MO degradation rate
(19)
Thus the catalyst amount will not only affect the light intensity distribution, but also affect the kinetic constant k. The light intensity profile along the liquid layer in three phase
Fig. 5 – The effects of the number of baffle plates on the degradation performance of MO.
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
Fig. 6 – The effects of the height of liquid exit on the degradation performance of MO. is greater for 4 cm depth of liquid layer than for 2 cm depth at liquid flowrate of 0.0111 m3 /h, catalyst density of 4.8 g/l, initial MO concentration of 10 mg/l and completely fluidization. The photocatalysis performance was also investigated by the model with the depth of liquid layer of 2, 4, 6 and 8 cm, respectively. As shown in Fig. 6, the theoretic degradation of MO remarkably increased with an increase of the depth of liquid layer until 4 cm; the performance for MO degradation reached about 100% after 1.5 h treatment. However, when the depth increased to 6 cm, such enhancement became insignificant and above 8 cm, the degradation rather decreased. This result indicated that there would be also an optimal height of liquid exit in the reactor. Theoretical analysis suggests that higher liquid exit means bigger volume of reactor and longer residence time of liquid in the reactor, which will benefit the degradation of MO but affect the utilization of photon energy. When the liquid layer reaches a certain height, the photon will be not easy to permeate the liquid layer and the photocatalysis performance will decline as the depth of liquid layer increases further. Combining the experimental data and the model predictions it is recommended to choose the depth of 4 cm. The results of experiments and the model predictions for two heights of liquid exit are in acceptable agreement.
4.3. Influence of operational parameters on degradation performance
Fig. 7 – The effects of liquid flow rate on the degradation performance of MO. this system and we selected the flowrate of 0.0111 m3 /h as the following experiments and model predictions. Again, the results predicted by the model are in good agreement with the experimentally observed results.
4.3.2.
Effect of liquid flowrate
The liquid flowrate significantly influences the residence time of liquid and the mass transfer in the reactor, which in turn affects the photocatalysis degradation performance. When the flow rate increases, two antagonistic effects are brought into play: the decrease in the residence time in the photocatalytic reactor and the increase in the mass transfer rate. An augmentation of the flow rate results in a higher mass transfer and a smaller concentration gradient between the bulk and the catalyst surface. Fig. 7 shows the MO degradation under three different liquid flowrate of 0.0084, 0.0111 and 0.0141 m3 /h. It is seen that increasing the liquid flowrate the MO conversion increases significantly for certain flowrate range. The same results were also observed in labyrinth flow photoreactor for removal of nonionic surfactant from water (Mozia et al., 2005). Higher flowrate results in shorter residence time of liquid in the reactor and shorter contacts between organic pollutant and particle photocatalyst; and lower liquid flowrate leads to lower transfer of momentum and mass. Therefore, there would be also an optimal liquid flowrate in
Effect of the volume of storage tank
Fig. 8 demonstrates the results predicted by the model and a comparison with parts of the experimental data at liquid flowrate of 0.0111 m3 /h and tank volume of 1.25, 2.5, 5, and 10 l, respectively. It is clear that an increase of the liquid volume to be treated leads to a decrease of the MO conversion for a given liquid flowrate. More organic wastewater needs longer degradation time in this system.
4.3.3.
Effect of the light intensity
Light intensity is one of the main factors affecting the photocatalysis degradation performance of MO. By varying the vertical distance between the UV lamp and liquid surface of the reactor one can get different light intensity. Results of the degradation prediction with different light intensity and the experimental data are shown in Fig. 9. Higher MO conversion is observed with the increase of the light intensity. However, such enhancement becomes insignificant when the light intensity increases to 0.9 mW/m2 . It also implies that too high of light intensity is not necessary for this reactor.
4.3.4. 4.3.1.
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Effect of photocatalyst mass
The performance of MO degradation with amounts of photocatalyst in this system is shown in Fig. 10. The degradation of MO remarkably increased with an increase of amounts of
Fig. 8 – The effects of the volume of storage tank on the degradation performance of MO.
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Fig. 9 – The effects of the light intensity on the degradation performance of MO. particle photocatalyst until 4.8 g/l; the performance for MO degradation reached about 100% after 1.5 h. However, above 6.4 g/l, the degradation rather decreased. Increasing the catalyst amounts the active sites and the reaction rate will increase significantly; but too higher catalyst amounts will leads to lower photon transmission and low degradation rate. From this result, there would be a proper range for catalyst mass. The model predictions are in good agreement with the experimental data.
4.4.
Concentration distribution in LBPR for single pass
In plug flow, the concentration of reactant decreases progressively through the system; in mixed flow, the concentration drops immediately to a low value. Consider a LBPR system with N baffle plates, though the concentration is uniform in each channel, there is a change in concentration along the axis of the reactor as fluid moves from channel to channel. This stepwise drop in concentration predicted by the model is illustrated in Fig. 11. It is suggested that the larger the number of the baffle plates, the lower should the outlet concentration of the reactor be for a single pass. When the number of the baffle plates increased to 12 (or the distance between the baffle plates decreased to 12 mm), the enhancement of the degradation efficiency became insignificant. Due to economic concerns and for retaining of the catalyst particles in the reactor, it is recommended to choose the number of the baffle plates of 12.
Fig. 11 – Concentration profiles through an N baffle plates LBPR system predicted by the model.
5.
Conclusions
To enhance the performance of organic pollutant degradation in photocatalytic system, a novel LBPR was designed and modeled and methyl orange was degraded using this reactor. The results were as follows: 1. A model of multiple mixed flow reactors in series was developed by the mass balance combining the rate equation of the photocatalytic reaction and it was used to describe the behavior of the LBPR during batch recirculation system for organic pollutant degradation. The model can estimate the influence of the structural and operational parameters on the photocatalytic degradation of organic pollutant. 2. The predicted results of the mathematical model show that the organic pollutant conversion increases with the increase in both the numbers of baffle plates and the height of liquid exit for low liquid height; the increase in liquid flow rate will improve the photocatalytic performance of the reactor and the predicted axial concentration distribution is exponential stepwise drop along the direction of liquid flow. 3. The LBPR model for methyl orange degradation was satisfactorily validated with experiments. The methyl orange conversion reached 100% after 1.5 h in air bubbling condition with 12 baffle plates (or 1.2 cm space between baffle plates) and 4 cm height of liquid exit; the proper operating conditions were 0.0111 m3 /h in liquid flow rate, 4.8 g/l in catalysts amount, and lower methyl orange concentration. 4. The LBPR is an effective tool for organic pollutants degradation over TiO2 /glass with simple structure, convenient operation, efficient utilization of photon energy, and easy scaling-up for practical applications.
Acknowledgments This research was financially supported by the National Natural Science Foundation of China (No. 20676089) and Natural Science Foundation of Shanxi Province (No. 2007021011).
References
Fig. 10 – The effects of photocatalyst mass on the degradation performance of MO.
Biard, P.F., Bouzaza, A. and Wolbert, D., 2007, Photocatalytic degradation of two volatile fatty acids in an annular plug-flow reactor: kinetic modeling and contribution of mass transfer rate. Environ Sci Technol, 41(8): 2908–2914.
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Brandi, R.J., Rintoul, G., Alfano, O.M., et al., 2002, Photocatalytic reactors reaction kinetics in a flat plate solar simulator. Catal Today, 76(2–4): 161–175. Cheng, Y.P., Sun, H.Q., Jin, W.Q., et al., 2007, Effect of preparation conditions on visible photocatalytic activity of titania synthesized by solution combustion method. Chinese J Chem Eng, 15(2): 178–183. Couto, S.R., Dominguez, A. and Sanroman, A., 2002, Photocatalytic degradation of dyes in aqueous solution operating in a fluidized bed reactor. Chemosphere, 46(1): 83–86. De Fatima, P.M.M.R, Sauer, T.P., Casaril, L., et al., 2005, Mass transfer and photocatalytic degradation of leather dye using TiO2 /UV. J Appl Electrochem, 35(7–8): 821–829. Fan, C.M., Min, Y.Q., Hao, X.G., et al., 2003, Adsorption and photocatalytic degradation of phenol over TiO2 /ACF. Trans Nonferrous Met Soc China, 13(2): 452–456. Grzechulska, J. and Morawski, A.W., 2003, Photocatalytic labyrinth flow reactor with immobilized P25 TiO2 bed for removal of phenol from water. Appl Catal B: Environ, 46(2): 415–419. Hao, X.G., Li, Y.B., Fan, C.M., et al., 2003, Progress in research on wastewater treatment by TiO2 photoelectrocatalytic technology. Chem Bull, 66(5): 306–311. Hao, X.G., Li, H.H., Yu, Q.S., et al., 2005, Distribution and fluctuation of the light intensity in two-dimensional fluidized bed photoreactors. Chinese J Proc Eng, 5(5): 473–478. Hao, X.G., Zhang, Z.L., Luo, S.X., et al., 2008. Labyrinth cross-flow bubble photoreactor and its application in treatment of organic wastewater. China Pat, ZL200610012622.1. Hao, X.G., Luo, S.X., Zhang, Z.L., et al., 2007, Photocatalytic degradation performances of phenol in wastewater using a novel labyrinth bubble photoreactor. J Chem Eng Chinese Univ, 21(6): 1056–1059. Haque, F., Vaisman, E., Langford, C.H., et al., 2005, Preparation and performance of integrated photocatalyst adsorbent (IPCA) employed to degrade model organic compounds in synthetic wastewater. J Photochem Photobiol A: Chem, 169(1): 21–27. Kabir, M.F., Vaisman, E., Langford, C.H., et al., 2006, Effects of hydrogen peroxide in a fluidized bed photocatalytic reactor for wastewater purification. Chem Eng J, 118(3): 207–212. Kanki, T., Hamasaki, S., Sano, N., et al., 2005, Water purification in a fluidized bed photocatalytic reactor using TiO2 -coated ceramic particles. Chem Eng J, 108(1–2): 155–160. Lee, J.H., Nam, W., Kang, M., et al., 2003, Design of two types of fluidized photo reactors and their photo-catalytic
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performances for degradation of methyl orange. Appl Catal A: Gen, 244(1): 49–57. Lim, T.H. and Kim, S.D., 2004, Trichloroethylene degradation by photocatalysis in annular flow and annulus fluidized bed photoreactors. Chemosphere, 54(3): 305–312. Lin, H.F. and Valsaraj, K.T., 2003, A titania thin film annular photocatalytic reactor for the degradation of polycyclic aromatic hydrocarbons in dilute water streams. J Hazard Mater, B99(2): 203–219. Mozia, S., Tomaszewska, M. and Morawski, A.W., 2005, Decomposition of nonionic surfactant in a labyrinth flow photoreactor with immobilized TiO2 bed. Appl Catal B: Environ, 59(3–4): 155–160. Nam, W., Kim, J. and Han, G., 2002, Photocatalytic oxidation of methyl orange in a three phase fluidized bed reactor. Chemosphere, 47(9): 1019–1024. Pozzo, R.L., Brandi, R.J., Giombi, J.L., et al., 2006, Fluidized bed photoreactors using composites of titania CVD-coated onto quartz sand as photocatalyst: assessment of photochemical efficiency. Chem Eng J, 118(3): 153–159. Ray, A.K., 1999, Design, modelling and experimentation of a new large-scale photocatalytic reactor for water treatment. Chem Eng Sci, 54(15–16): 3113–3125. Sagawe, G., Brandi, R.J., Bahnemann, D., et al., 2003, Photocatalytic reactors for treating water pollution with solar illumination. I. A simplified analysis for batch reactors. Chem Eng Sci, 58(12): 2587–2599. Sun, Y.P. and Zhao, Y.Q., 2000, Study on the mathematical model of isothermal photocatalytic reactor. J Taiyuan Univ Technol, 31(6): 603–608. Vaisman, E., Kabir, M.F., Kantzas, A., et al., 2005, A fluidized bed photoreactor exploiting a supported photocatalyst with adsorption pre-concentration capacity. J Appl Electrochem, 35(7): 675–681. Yang, Q., Ang, P.L., Ray, M.B., et al., 2005, Light distribution field in catalyst suspensions within an annular photoreactor. Chem Eng Sci, 60(19): 5255–5268. Zalazar, C.S., Martin, C.A. and Cassano, A.E., 2005, Photocatalytic intrinsic reaction kinetics. II. Effects of oxygen concentration on the kinetics of the photocatalytic degradation of dichloroacetic acid. Chem Eng Sci, 60(15): 4311–4322. Zhang, D.X., Zhang, L.Y. and Li, H.S., 2006, Photocatalytic oxidation performance and kinetics of simulated organic wastewater in circulating slurry photocatalytic reactor. J Chem Ind Eng (China), 57(5): 1159–1165.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Modeling and experimentation of a novel labyrinth bubble photoreactor for degradation of organic pollutant Hao Xiao-gang ∗ , Li Hong-hui, Zhang Zhong-lin, Fan Cai-mai, Liu Shi-bin, Sun Yan-ping Department of Chemical Engineering, Taiyuan University of Technology, 79 West Yingze Street, Taiyuan, Shanxi 030024, China
a b s t r a c t A novel labyrinth flow bubble photoreactor (LBPR) was designed and the model of multiple mixed flow reactors (MMFRs) in series was established to describe the behavior of the LBPR during batch recirculation system of low concentration organic pollutant degradation by photocatalysis. This model was developed by the mass balance combining the rate equation of the photocatalytic reaction. The influence of the structural parameters, such as the number of baffle plates, the space between the baffle plates and the height of liquid exit in the reactor, on the photocatalytic degradation of organic pollutant was mainly investigated. The relationships between the final conversion rate and the liquid rate, liquid volumes, light intensity and catalyst amounts were also predicted by this model. TiO2 was immobilized on quartz glass particles by sol–gel method and used as the photocatalysts; the degradation experiment of methyl orange (MO) was performed in a 2 l LBPR reactor to validate the above model. The experimental results showed that the removal rate of MO could reach 100% with 12 baffle plates and 4 cm height of liquid exit after 1.5 h treatment when the flow rate was 0.0111 m3 h−1 and the particle catalysts was 4.8 g l−1 . The prediction of the model is consistent with the experimental results. This research provides exploratory work for the design of a novel commercial scale photocatalytic reactor. © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Labyrinth bubble photoreactor; Photocatalysis; Modeling; Structure; Water treatment
1.
Introduction
Heterogeneous photocatalysis is one of the advanced oxidation processes (AOP) that couples low energy UV light with semiconductor acting as photocatalyst (Yang et al., 2005). The appeal of this process technology is the prospect of complete mineralization of organic pollutants to environmentally harmless compounds (Kabir et al., 2006). Activation of the catalyst is achieved by electron–hole pair formation initiated through the absorption of an ultraviolet photon. The applied radiation may be artificial or solar and the process can be operated at ambient pressure and temperature (Brandi et al., 2002; Cheng et al., 2007; Sagawe et al., 2003). Moreover, TiO2 -based photocatalysis is more appealing than other conventional chemical oxidation methods because it is cheap, biologically and chemically inert, insoluble under most conditions, photostable, non-toxic, can be used for extended period without substantial loss of its activity, and more impor-
∗
tantly can even be activated by sunlight (Hao et al., 2003; Ray, 1999). In spite of the potential of this technology, development of a practical photocatalysis water treatment system has not yet been successfully achieved. A photocatalytic process must be constructed from three elements: a reactor, a light source, and a photocatalyst (Vaisman et al., 2005). Fine TiO2 powders present separation problems in slurry type reactors leading to complications and expenses. As a result, a number of efforts have been made to support TiO2 on fixed or mechanically manageable supports, which eliminates arduous step of separation (Fan et al., 2003; Haque et al., 2005; Kanki et al., 2005; Lee et al., 2003; Pozzo et al., 2006). When the catalyst is immobilized on particles, the effective design and scale-up of photocatalytic reactor becomes the key to the practical application of photocatalysis water treatment technology. Besides conventional reactor complications such as mixing, mass transfer, reaction kinetics, catalyst installation, etc., an addi-
Corresponding author. Tel.: +86 351 6018193; fax: +86 351 6018554. E-mail address: [email protected] (X.-g. Hao). Received 7 October 2008; Received in revised form 11 April 2009; Accepted 1 June 2009 0263-8762/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2009.06.002
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
Nomenclature acs CA0 Cin,t Cout,t C0 CN+1 Cm Ct H I k k N n Ql Qg r ri S Sg VR VRi VT X x xR xRt i T
catalytic surface area per unit volume (m2 m−3 ) initial concentration (mol m−3 ) inlet concentration in the tank (mol m−3 ) outlet concentration from the tank (mol m−3 ) inlet concentration to the reactor (mol m−3 ) outlet concentration of the reactor (mol m−3 ) catalyst loading or concentration (g m−3 ) liquid concentration at any time (mol m−3 ) depth of the liquid layer (m) light intensity (mW m−2 ) rate constant (m3 s−1 mW−1 ) rate constant (s−1 ) number of baffle plates reaction order liquid flow rate (m3 s−1 ) gas flow rate (m3 s−1 ) constant reaction rate (mol m3 s−1 ) cross-section area (m2 ) catalyst specific surface area (m2 g−1 ) whole volume of reaction area in reactor (m3 ) volume of each channel (m3 ) storage tank of volume (m3 ) dimensionless axial length conversion single pass conversion of the reactor total conversion of this system mean residence time of liquid in each channel (s) mean residence time in the tank (s)
tional engineering factor related to illumination of catalyst becomes relevant in a photocatalytic reactor (Ray, 1999). In the last few years, a large number of publications have appeared about the photocatalytic reactor configurations. Among these configurations, annular or cylindrical fluidized bed reactor with lamp(s) immersed within it is to be preferred (Couto et al., 2002; Kanki et al., 2005; Lim and Kim, 2004; Nam et al., 2002; Pozzo et al., 2006). However, the design and scale-up of this kind of reactor must consider the combination of the light source and cooling system, the structure is complicated and the efficient reaction volume is low (Hao et al., 2005). Therefore, the development of new photocatalytic reactors is still an important issue. Photocatalytic technologies are more appropriate for treatment and destruction of contaminants at dilute concentration, which constitute a very large volume of industrial wastewater (Lin and Valsaraj, 2003). The steady-state kinetic models presented in the literature usually ignore the influence of mass transfer (Biard et al., 2007); this is not a problem if the flow rate is sufficiently high to ensure a good mass transfer rate. Nevertheless, working with high flow rates is not usually possible, and then, the observed degradation rate is dependent on mass transfer rate. Consequently, the external mass transfer has a significant influence with those reactor configurations because of the loss of surface area upon immobilization of a photocatalyst on inert supports, especially for dilute wastewater treatment. In order to overcome some of these deficiencies inherent in conventional photocatalytic reactor, a labyrinth flow photore-
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actor with immobilized TiO2 bed was designed for removal of phenol and nonionic surfactant from water (Grzechulska and Morawski, 2003; Mozia et al., 2005). This kind of reactor is an innovative setup; however, the photocatalyst particles are fixed to the bottom of the reactor using polymer glue, such a manner that only single layer of catalyst is keep in the reactor and half of the particle surface is accessible for the UV light. The thickness of solution layer over the catalyst is thin (only 1 cm) and the photodegradation effectiveness is still low. In this paper, a novel labyrinth flow bubble photoreactor (LBPR) (Hao et al., 2008) was designed and a mathematical model was presented to describe the behavior of the LBPR during batch recirculation system of low concentration of organic pollutant degradation by photocatalysis. From the degradation experiment of methyl orange (MO) in LBPR with the light source (365 nm) outside it as an imitated solar light, possibility of water purification under solar light was proved. This new reactor allows for a much higher illuminated surface area per unit reactor volume and is flexible enough to be scaled-up for commercial scale application.
2.
LBPR system model
2.1.
Reactor description
Fig. 1 shows the schematic diagram of a labyrinth bubble photoreactor (LBPR). Detailed description of the reactor can be found in Hao et al. (2007, 2008). In brief, the reactor with rectangular-cross-section is divided by parallel baffle plates and the liquid labyrinth flow channels are built. The liquid flows along the space between the baffle plates in the labyrinth reaction area and contacts with the upflow gas bubbles from the gas distributor on the bottom of the reactor. The particle photocatalysts immobilized with TiO2 thin film are dispersed in the mixtures of liquid and gas bubbles. The top of the reactor is open to air and external light sources such as UV lamps or solar light can be used. Light energy or photons transferred into the flow channel are absorbed by TiO2 immobilized on support materials and a heterogeneous photocatalytic reaction takes place in the reactor. The fluidization of gas–liquid–solid ensures that all photocatalytic particles
Fig. 1 – Schematic diagram of labyrinth bubble photoreactor (LBPR).
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receive uniform illumination. This configuration meets the requirements of higher surface area-to-volume ratio, of long residence time for liquid, of better use of light, and of improvement of mass transfer conditions.
2.2.
LBPR model
The LBPR not only brings more contact of photocatalyst and organic pollutant but also enhances UV light penetration efficiently into the interior of photocatalyst bed through gas bubbles. The three phases of gas, liquid and solid in each channel divided by N equal-distance plates are mixed uniformly because of the air bubbling and an idealized mixed flow is assumed to exist in the channel. Therefore, the whole LBPR can be considered as a system of (N + 1) mixed flow reactors (MFR) connected in series. Though the concentration is uniform in each reactor, there is, nevertheless, a change in concentration as liquid moves from reactor to reactor. The behavior of a series of (N + 1) equal-size MFRs can be evaluated quantitatively with the mass balance combining the reaction kinetics equation or rate equation. The nomenclature used is shown in Fig. 2 with subscript i referring to the ith vessel. The whole volume of reaction area is VR and the volume of each channel or vessel is: VRi =
VR N+1
(1)
where acs and I can be treated as constants when the catalyst amount and the light intensity are fixed. The global process of heterogeneous photocatalytic oxidation can be described in consecutive steps of mass transfer, adsorption, reaction and desorption. There is no consensus about the controlling rate step, although some authors have reported that the adsorption step controls the rate (De Fatima et al., 2005). At low inlet concentration, some authors have reported that the photocatalytic degradation obeys first-order kinetics or pseudo-first-order model, i.e., the degradation rate is proportional to the inlet concentration (Mozia et al.,2005; Biard et al., 2007). In this case, the limiting steps are the adsorption or the transfer of the pollutant onto the catalyst. In LBPR, the degradation process are operated at ambient temperature and the particle photocatalysts are fluidized by upflow air bubbling; therefore, the adsorption and desorption is not the control steps. Usually, the studies which investigate the effect of the structural parameters and the flow rate are carried out in a recirculation batch system. In this case, the degradation rate is dependent only on the mass transfer rate, which implies that better degradation rates are obtained when the flow rate increases. For first-order reaction or the mass transfer control regime, the above expression becomes ri = −k Ci
The mass balance of component A for the ith MFR is represented by the following equation: Ql C0 (1 − xi−1 ) = Ql C0 (1 − xi ) + ri VRi
(2)
Ql C0 (xi − xi−1 ) Q (C − Ci ) = l i−1 ri ri
(3)
where Ql , x and ri are the liquid flow rate, the conversion and the reaction rate, respectively and C0 is the inlet concentration. The mean residence time of liquid in each channel can be written as: V i = Ri Ql
Substitution of (4) and (6) into (3) gives k i =
Ci−1 − Ci Ci
(7)
Because the distance between each two plates is equal, the residence time of liquid in each channel is equal
which on rearrangement becomes: VRi =
(6)
1 = 2 = N =
(8)
Then combining (7) and (8) the exit concentration from the reactor may be written as: CN+1 =
(4)
C0 (1 + k )
N+1
(9)
C0 is the inlet concentration to the reactor at any time. The reaction rate expression or kinetic equation for the heterogeneous photocatalytic degradation of organic pollutant is given by (Sun and Zhao, 2000) dCi = −ri = −acs kIr Cni dt
(5)
2.3.
One possible mode of operation of the LBPR is that which involves the continuous recirculation of the wastewater, a gradual degradation of the concentration of the organic pollutant taking place. This mode of operation is depicted in Fig. 3. It is important in the design of such processes to be able to predict the variation of concentration of the organic pollutants with time. Schematically, the system under study is formed by a photoreactor of volume VR and a storage tank of volume VT functioning in a closed circuit. The tank operates under unsteady state conditions. The mass balance in the tank for component A is represented by the following differential equation and initial condition: VT
Fig. 2 – Notation for the ith vessel in a system of (N + 1) MFRs in series.
Batch recirculation system model
dCout,t = Ql (Cin,t − Cout,t ) dt
Cout,t (t = 0) = CA0
(10) (11)
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
Fig. 3 – Schematic representation of the employed system. In Eq. (10) the mean residence time in the tank is given by VT T = Ql
(12)
Assuming that the tank works under well-stirred conditions, the concentration in the tank (equal to its outlet concentration) is only a function of time. The inlet concentration in the tank comes from the reactor and the outlet concentration from the tank remains unchanged until the liquid gets into the reactor. One can safely assume that: Cin,t = CN+1
(13)
Cout,t = C0
(14)
Substitution of Eqs. (9) and (12)–(14) into Eq. (10) and integration gives the solution for the variation of concentration with time as:
Ct = CA0 exp
t T
1 (1 + k )
N+1
−1
(15)
or
x R
Ct = CA0 exp −
T
(16)
t
where xR is a single pass conversion of the reactor. The total conversion of this system is
x R
xRt = 1 − exp −
T
(17)
t
If the reaction volume of the LBPR is given by VR = SH, the variation of concentration with time can be written as: Ct = CA0 exp
Q VT
1
l
1 + k (SH/(N
+ 1)Ql )
t
Fig. 4 – Scheme of the LBPR for photocatalytic degradation of organic pollutant in water. (1) Metering pump, (2) gas distribution plate, (3) photoreactor, (4) baffle plate, (5) UV lamp, (6) rotameter, (7) air pump and (8) storage tank.
system setup: (1) the labyrinth bubble photoreactor, (2) the illuminating system, (3) the recycle, with a tank and a metering pump, and (4) the air line that includes an air pump and a rotameter. The photoreactor with the holding capacity of 2 l was constructed based on rectangular geometry (16 cm × 32 cm) with an external lamp arrangement. The reactor was made of plexiglass and the top was open to air. The geometry of the reactor allowed the option of inserting additional baffle plates inside the reactor to increase the number of labyrinth channels and the maximum UV transmission into the reaction zone from above. There were two exits of liquid at different height from the gas distributor. The different exits were used to regulate the thickness of the liquid layer. The lower portion of the reactor was filled with glass beads of 5 mm diameter and used as the predistribution section of gas. The distributor was made of two rectangular plexiglass plates with 195 holes of 2 mm diameter each arranged in a triangular pitch and had a Nylon screen of 100-mesh sandwiched in between, in order to prevent the back-flushing of photocatalyst particles and to provide uniform distribution of gas along the reactor cross-section area. The special design of blowhole remained most of the catalyst particles in the reactor, and a few particles moving out of the exit of the reactor due to the liquid flow were accumulated, and then re-added into the inlet of the reactor with the liquid (Hao et al., 2008). A tubular UV lamp (365 nm wavelength) with a nominal power of 375 W was placed at the focal axis of a cylindrical reflector of parabolic cross-section. The whole illuminating system was suspended above the reactor and an external parallel light source was obtained. As shown in Fig. 4, the reactor operated inside a recycle that has a 5 l storage tank made of glass and a pump. The pump was used to recirculate the liquid to and from the reactor and to improve mixing and facilitate sampling in the tank.
(18)
3.2. where S is the cross-section area and H is the depth of the liquid layer.
3.
Experimental
3.1.
Labyrinth bubble photoreactor (LBPR) setup
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Fig. 4 shows the schematic diagram of the experimental setup. As shown in Fig. 4 there are four main parts in the complete
Photocatalytic degradation experiments
Methyl orange (MO) was used as a model pollutant in this series of experiments. TiO2 was immobilized on quartz glass particles (average diameter of 0.5 mm and average TiO2 content of 3.3 wt.%) by sol–gel method and used as the photocatalysts. Initially, a certain amount of wastewater containing MO was introduced in the storage tank and then passed through the reactor and stirred with air bubbling for half hour in the dark to allow for saturation of the TiO2 /glass. There was
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no obvious concentration change during this process and the adsorption is very low because of the drastic stir. Afterwards, the lamp was turned on and a straight recycle process was initiated. The flow rate of air was regulated by a flowmeter and introduced into the bottom of the reactor. During the process, samples of the reaction solution were taken at time intervals of 10 min for concentration analysis. MO concentration was determined by 4–5 UV–vis spectrometry method using CE1021 spectrometry at a fixed wavelength of 460 nm. Analytical results were calibrated using a series of standard solutions corresponding to a range of concentrations used in the experiments. Reactor light intensities were measured by VLX-3W (America, 365 nm). The whole illuminating system and the reactor were placed inside a glass box, and the inner box wall was covered with a reflecting aluminum foil.
4.
Results and discussion
4.1.
Model parameters and analysis
The developed model of this work considers separated mass balances for each of the two reservoirs, i.e., for the reactor and the storage tank of the recycle. The model and kinetic coefficient or reaction rate constant k can be used to predict the MO concentration–time relationship for different structural parameters of N and H, and different operational parameters of Ql , VT , I and Cm during the degradation process. Kinetic equation for the photocatalytic reaction is an important component of the modeling (Zalazar et al., 2005; Zhang et al., 2006). In the present work, the mass transfer is a rate-determining step because of the low inlet concentration and low flow rate of wastewater, even though the particles are completely fluidized (the air flow rate increased to 20 l/min) in the reactor; the MO degradation kinetics has been proved to be an apparent first-order by experimental results. For a given light intensity, the depth of the liquid layer and catalyst amount, the effects of the number of baffle plates, liquid flow rate and the volume of storage tank on the degradation performance of MO can be easily obtained by Eq. (18). But the effects of other parameters on the performance of the reactor are complicated because the reaction rate constant is influenced by these parameters. Detailed parametric analysis of the reaction kinetics and mass transfer rate can provide information to extend this model to other complex operation process. The reaction rate expression described by Eq. (5) is a function of the spatial coordinates and time. First, MO concentration in the recycling system changes with time along the degradation experiment; secondly, the rate of electron–hole generation, that is directly proportional to the local light intensity, is also a function of position and time due to two different factors: (i) radiation adsorption by the catalyst is very strong and generates a light intensity distribution that varies very much along the depth of the liquid layer and (ii) the light intensity distribution is also a function of time because the optical properties of the catalytic vary with time. In addition, Eq. (5) calls for a value of acs . The catalytic surface area per unit volume can be calculated from the catalyst loading Cm and the catalyst specific surface area Sg : acs = Cm Sg
reactor can be simulated by Hao et al. (2005). In order to avoid the complexity associated with variation of light attenuation properties with time and position, the kinetic parameters or the coefficients of mass transfer of Eq. (5) were estimated from all the MO concentration vs. time experimental data applying a non-linear regression procedure. Therefore, the effects of particle catalyst amount, thickness of liquid layer and light intensity that change the optical properties of the reacting medium and hence the light distribution inside the reactor have been taken into account.
4.2. Influence of structure parameters on degradation performance 4.2.1.
Effect of the number of baffle plates
Fig. 5 shows the influence of the number of baffle plates on the degradation performance of MO using a LBPR system for a given intensity of radiation. Both the concentration–time curves derived from the model and a comparison with the experimental data are shown in this figure. It is seen that increasing the number of baffle plates the MO conversion increases significantly when the number of baffle plates is lower than 12. For example, for an initial MO concentration of 10 mg/l and the following baffle plate’s number: 0, 2, 6 and 12, the final conversions after 1.5 h are 0.45, 0.81, 0.94, and 0.99, respectively. As shown in Fig. 5, the predictions by the model agreed very well with the experimental data. For a given volume of reactor, increasing the number of baffle plates leads to an increase of the flow velocity of liquid in a labyrinth channel. The degradation rate increased with the baffle plates, which emphasizes the influence of the mass transfer. On the other hand, increasing the number of baffle plates also leads to a decrease of the distance between plates. When the diameter of the air bubble is bigger than this distance, the “bridging bubbles” will congest the space between the baffle plates and decay the uniform mixture of the gas, liquid and particles. Larger number of baffle plates will result in the decrease of photocatalysis performance. Therefore, there would be an optimal number of baffle plates in the reactor and the number of about 12 would be a suitable value under the investigated conditions.
4.2.2.
Effect of the height of liquid exit
The influence of the height of liquid exit (or the depth of liquid layer) on the degradation performance of MO in the reactor is shown in Fig. 6. It is observed that the MO degradation rate
(19)
Thus the catalyst amount will not only affect the light intensity distribution, but also affect the kinetic constant k. The light intensity profile along the liquid layer in three phase
Fig. 5 – The effects of the number of baffle plates on the degradation performance of MO.
chemical engineering research and design 8 7 ( 2 0 0 9 ) 1604–1611
Fig. 6 – The effects of the height of liquid exit on the degradation performance of MO. is greater for 4 cm depth of liquid layer than for 2 cm depth at liquid flowrate of 0.0111 m3 /h, catalyst density of 4.8 g/l, initial MO concentration of 10 mg/l and completely fluidization. The photocatalysis performance was also investigated by the model with the depth of liquid layer of 2, 4, 6 and 8 cm, respectively. As shown in Fig. 6, the theoretic degradation of MO remarkably increased with an increase of the depth of liquid layer until 4 cm; the performance for MO degradation reached about 100% after 1.5 h treatment. However, when the depth increased to 6 cm, such enhancement became insignificant and above 8 cm, the degradation rather decreased. This result indicated that there would be also an optimal height of liquid exit in the reactor. Theoretical analysis suggests that higher liquid exit means bigger volume of reactor and longer residence time of liquid in the reactor, which will benefit the degradation of MO but affect the utilization of photon energy. When the liquid layer reaches a certain height, the photon will be not easy to permeate the liquid layer and the photocatalysis performance will decline as the depth of liquid layer increases further. Combining the experimental data and the model predictions it is recommended to choose the depth of 4 cm. The results of experiments and the model predictions for two heights of liquid exit are in acceptable agreement.
4.3. Influence of operational parameters on degradation performance
Fig. 7 – The effects of liquid flow rate on the degradation performance of MO. this system and we selected the flowrate of 0.0111 m3 /h as the following experiments and model predictions. Again, the results predicted by the model are in good agreement with the experimentally observed results.
4.3.2.
Effect of liquid flowrate
The liquid flowrate significantly influences the residence time of liquid and the mass transfer in the reactor, which in turn affects the photocatalysis degradation performance. When the flow rate increases, two antagonistic effects are brought into play: the decrease in the residence time in the photocatalytic reactor and the increase in the mass transfer rate. An augmentation of the flow rate results in a higher mass transfer and a smaller concentration gradient between the bulk and the catalyst surface. Fig. 7 shows the MO degradation under three different liquid flowrate of 0.0084, 0.0111 and 0.0141 m3 /h. It is seen that increasing the liquid flowrate the MO conversion increases significantly for certain flowrate range. The same results were also observed in labyrinth flow photoreactor for removal of nonionic surfactant from water (Mozia et al., 2005). Higher flowrate results in shorter residence time of liquid in the reactor and shorter contacts between organic pollutant and particle photocatalyst; and lower liquid flowrate leads to lower transfer of momentum and mass. Therefore, there would be also an optimal liquid flowrate in
Effect of the volume of storage tank
Fig. 8 demonstrates the results predicted by the model and a comparison with parts of the experimental data at liquid flowrate of 0.0111 m3 /h and tank volume of 1.25, 2.5, 5, and 10 l, respectively. It is clear that an increase of the liquid volume to be treated leads to a decrease of the MO conversion for a given liquid flowrate. More organic wastewater needs longer degradation time in this system.
4.3.3.
Effect of the light intensity
Light intensity is one of the main factors affecting the photocatalysis degradation performance of MO. By varying the vertical distance between the UV lamp and liquid surface of the reactor one can get different light intensity. Results of the degradation prediction with different light intensity and the experimental data are shown in Fig. 9. Higher MO conversion is observed with the increase of the light intensity. However, such enhancement becomes insignificant when the light intensity increases to 0.9 mW/m2 . It also implies that too high of light intensity is not necessary for this reactor.
4.3.4. 4.3.1.
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Effect of photocatalyst mass
The performance of MO degradation with amounts of photocatalyst in this system is shown in Fig. 10. The degradation of MO remarkably increased with an increase of amounts of
Fig. 8 – The effects of the volume of storage tank on the degradation performance of MO.
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Fig. 9 – The effects of the light intensity on the degradation performance of MO. particle photocatalyst until 4.8 g/l; the performance for MO degradation reached about 100% after 1.5 h. However, above 6.4 g/l, the degradation rather decreased. Increasing the catalyst amounts the active sites and the reaction rate will increase significantly; but too higher catalyst amounts will leads to lower photon transmission and low degradation rate. From this result, there would be a proper range for catalyst mass. The model predictions are in good agreement with the experimental data.
4.4.
Concentration distribution in LBPR for single pass
In plug flow, the concentration of reactant decreases progressively through the system; in mixed flow, the concentration drops immediately to a low value. Consider a LBPR system with N baffle plates, though the concentration is uniform in each channel, there is a change in concentration along the axis of the reactor as fluid moves from channel to channel. This stepwise drop in concentration predicted by the model is illustrated in Fig. 11. It is suggested that the larger the number of the baffle plates, the lower should the outlet concentration of the reactor be for a single pass. When the number of the baffle plates increased to 12 (or the distance between the baffle plates decreased to 12 mm), the enhancement of the degradation efficiency became insignificant. Due to economic concerns and for retaining of the catalyst particles in the reactor, it is recommended to choose the number of the baffle plates of 12.
Fig. 11 – Concentration profiles through an N baffle plates LBPR system predicted by the model.
5.
Conclusions
To enhance the performance of organic pollutant degradation in photocatalytic system, a novel LBPR was designed and modeled and methyl orange was degraded using this reactor. The results were as follows: 1. A model of multiple mixed flow reactors in series was developed by the mass balance combining the rate equation of the photocatalytic reaction and it was used to describe the behavior of the LBPR during batch recirculation system for organic pollutant degradation. The model can estimate the influence of the structural and operational parameters on the photocatalytic degradation of organic pollutant. 2. The predicted results of the mathematical model show that the organic pollutant conversion increases with the increase in both the numbers of baffle plates and the height of liquid exit for low liquid height; the increase in liquid flow rate will improve the photocatalytic performance of the reactor and the predicted axial concentration distribution is exponential stepwise drop along the direction of liquid flow. 3. The LBPR model for methyl orange degradation was satisfactorily validated with experiments. The methyl orange conversion reached 100% after 1.5 h in air bubbling condition with 12 baffle plates (or 1.2 cm space between baffle plates) and 4 cm height of liquid exit; the proper operating conditions were 0.0111 m3 /h in liquid flow rate, 4.8 g/l in catalysts amount, and lower methyl orange concentration. 4. The LBPR is an effective tool for organic pollutants degradation over TiO2 /glass with simple structure, convenient operation, efficient utilization of photon energy, and easy scaling-up for practical applications.
Acknowledgments This research was financially supported by the National Natural Science Foundation of China (No. 20676089) and Natural Science Foundation of Shanxi Province (No. 2007021011).
References
Fig. 10 – The effects of photocatalyst mass on the degradation performance of MO.
Biard, P.F., Bouzaza, A. and Wolbert, D., 2007, Photocatalytic degradation of two volatile fatty acids in an annular plug-flow reactor: kinetic modeling and contribution of mass transfer rate. Environ Sci Technol, 41(8): 2908–2914.
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Brandi, R.J., Rintoul, G., Alfano, O.M., et al., 2002, Photocatalytic reactors reaction kinetics in a flat plate solar simulator. Catal Today, 76(2–4): 161–175. Cheng, Y.P., Sun, H.Q., Jin, W.Q., et al., 2007, Effect of preparation conditions on visible photocatalytic activity of titania synthesized by solution combustion method. Chinese J Chem Eng, 15(2): 178–183. Couto, S.R., Dominguez, A. and Sanroman, A., 2002, Photocatalytic degradation of dyes in aqueous solution operating in a fluidized bed reactor. Chemosphere, 46(1): 83–86. De Fatima, P.M.M.R, Sauer, T.P., Casaril, L., et al., 2005, Mass transfer and photocatalytic degradation of leather dye using TiO2 /UV. J Appl Electrochem, 35(7–8): 821–829. Fan, C.M., Min, Y.Q., Hao, X.G., et al., 2003, Adsorption and photocatalytic degradation of phenol over TiO2 /ACF. Trans Nonferrous Met Soc China, 13(2): 452–456. Grzechulska, J. and Morawski, A.W., 2003, Photocatalytic labyrinth flow reactor with immobilized P25 TiO2 bed for removal of phenol from water. Appl Catal B: Environ, 46(2): 415–419. Hao, X.G., Li, Y.B., Fan, C.M., et al., 2003, Progress in research on wastewater treatment by TiO2 photoelectrocatalytic technology. Chem Bull, 66(5): 306–311. Hao, X.G., Li, H.H., Yu, Q.S., et al., 2005, Distribution and fluctuation of the light intensity in two-dimensional fluidized bed photoreactors. Chinese J Proc Eng, 5(5): 473–478. Hao, X.G., Zhang, Z.L., Luo, S.X., et al., 2008. Labyrinth cross-flow bubble photoreactor and its application in treatment of organic wastewater. China Pat, ZL200610012622.1. Hao, X.G., Luo, S.X., Zhang, Z.L., et al., 2007, Photocatalytic degradation performances of phenol in wastewater using a novel labyrinth bubble photoreactor. J Chem Eng Chinese Univ, 21(6): 1056–1059. Haque, F., Vaisman, E., Langford, C.H., et al., 2005, Preparation and performance of integrated photocatalyst adsorbent (IPCA) employed to degrade model organic compounds in synthetic wastewater. J Photochem Photobiol A: Chem, 169(1): 21–27. Kabir, M.F., Vaisman, E., Langford, C.H., et al., 2006, Effects of hydrogen peroxide in a fluidized bed photocatalytic reactor for wastewater purification. Chem Eng J, 118(3): 207–212. Kanki, T., Hamasaki, S., Sano, N., et al., 2005, Water purification in a fluidized bed photocatalytic reactor using TiO2 -coated ceramic particles. Chem Eng J, 108(1–2): 155–160. Lee, J.H., Nam, W., Kang, M., et al., 2003, Design of two types of fluidized photo reactors and their photo-catalytic
1611
performances for degradation of methyl orange. Appl Catal A: Gen, 244(1): 49–57. Lim, T.H. and Kim, S.D., 2004, Trichloroethylene degradation by photocatalysis in annular flow and annulus fluidized bed photoreactors. Chemosphere, 54(3): 305–312. Lin, H.F. and Valsaraj, K.T., 2003, A titania thin film annular photocatalytic reactor for the degradation of polycyclic aromatic hydrocarbons in dilute water streams. J Hazard Mater, B99(2): 203–219. Mozia, S., Tomaszewska, M. and Morawski, A.W., 2005, Decomposition of nonionic surfactant in a labyrinth flow photoreactor with immobilized TiO2 bed. Appl Catal B: Environ, 59(3–4): 155–160. Nam, W., Kim, J. and Han, G., 2002, Photocatalytic oxidation of methyl orange in a three phase fluidized bed reactor. Chemosphere, 47(9): 1019–1024. Pozzo, R.L., Brandi, R.J., Giombi, J.L., et al., 2006, Fluidized bed photoreactors using composites of titania CVD-coated onto quartz sand as photocatalyst: assessment of photochemical efficiency. Chem Eng J, 118(3): 153–159. Ray, A.K., 1999, Design, modelling and experimentation of a new large-scale photocatalytic reactor for water treatment. Chem Eng Sci, 54(15–16): 3113–3125. Sagawe, G., Brandi, R.J., Bahnemann, D., et al., 2003, Photocatalytic reactors for treating water pollution with solar illumination. I. A simplified analysis for batch reactors. Chem Eng Sci, 58(12): 2587–2599. Sun, Y.P. and Zhao, Y.Q., 2000, Study on the mathematical model of isothermal photocatalytic reactor. J Taiyuan Univ Technol, 31(6): 603–608. Vaisman, E., Kabir, M.F., Kantzas, A., et al., 2005, A fluidized bed photoreactor exploiting a supported photocatalyst with adsorption pre-concentration capacity. J Appl Electrochem, 35(7): 675–681. Yang, Q., Ang, P.L., Ray, M.B., et al., 2005, Light distribution field in catalyst suspensions within an annular photoreactor. Chem Eng Sci, 60(19): 5255–5268. Zalazar, C.S., Martin, C.A. and Cassano, A.E., 2005, Photocatalytic intrinsic reaction kinetics. II. Effects of oxygen concentration on the kinetics of the photocatalytic degradation of dichloroacetic acid. Chem Eng Sci, 60(15): 4311–4322. Zhang, D.X., Zhang, L.Y. and Li, H.S., 2006, Photocatalytic oxidation performance and kinetics of simulated organic wastewater in circulating slurry photocatalytic reactor. J Chem Ind Eng (China), 57(5): 1159–1165.