Process Analysis on Photocatalyzed Dye Decomposition for Water

Ind. Eng. Chem. Res. 2010, 49, 12173–12179

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Process Analysis on Photocatalyzed Dye Decomposition for Water Treatment with TiO2-Coated Rotating Disk Reactor Chih-Yi Chang and Nae-Lih Wu* Department of Chemical Engineering, National Taiwan UniVersity, No. 1, Section 4, RooseVelt Road, Taipei, 10617 Taiwan

Wastewater treatment based on solar energy effected photocatalytic reaction is a green process that utilizes renewable energy resources and minimizes secondary pollution. Low conversion efficiency is one of the key issues to overcome for realizing its practical application. With an aim at significantly raising the process efficiency, a rotating disk reactor (RDR) has been evaluated for the application of photocatalytic decomposition of dye pollutants in water. In this process, photocatalyst (TiO2) particles are immobilized onto a disk (6 cm in diameter), and dye (methyl orange)-containing solution is allowed to flow in radial direction along the surface of the disk, which is rotating and illuminated with UV light. The correlations between the fundamental characteristics of the reactor, including residence time and film thickness, and its operating variables, including volumetric flow rate and disk rotating speed, have been established by the combination of fluid dynamic and kinetic models. The results indicate that the reactor can be operating beyond mass-transfer limitation by reducing the liquid film thickness, which is a complex function of both flow rate and disk rotating speed, below certain critical value. Even under such a condition, the overall reaction rate remains strongly affected by the liquid film thickness due to the intensity attenuation of incidence light through the liquid film before reaching the TiO2 surface. With selected operation conditions, conversions greater than 50% have been achieved within only a few seconds of residence time. A reactor design equation has been derived, indicating promising scale-up potential of the process. 1. Introduction Semiconductor photocatalysis for wastewater treatment directly under solar light has been considered as a sustainable clean technology for environmental purification. It possesses the nature of low cost and avoids the need for adding oxidant agents, which could cause secondary contaminants.1-7 However, industrial applications of such a green approach still require much more advancement in both the improvement of reaction efficiency and the development of a practical intensified photocatalytic reactor system. It has been emphasized that, for commercialization and scale-up of photocatalysis technology, researchers need to design a photocatalytic reactor that can enhance the efficiency of light utilization in order to meet economic feasibility.8,9 Most of the earlier works on photocatalysis reactor have focused on the reactors containing suspending TiO2 particles.10,11 However, recovery of the particles from the effluent is a timeconsuming process, particularly for the nanosized high-surface particles.12 Recently, photocatalytic reactors using immobilized TiO2 have been receiving increasing attention. These types of reactors avoid the TiO2-recovering process and can be used as continuous-process devices.13-19 The main disadvantage of these stationary TiO2-supported reactors is that the decomposition rate of organic pollutant has been far inferior to that of the suspension reactors. While there has been a lack of systematical analysis on the interplay between reaction kinetics and fluid dynamics for these types of reactors, the reason for this slow reaction rate is believed to be largely caused by mass-transfer limitation of the reacting species to the photocatalyst surfaces. In this work, a rotating disk reactor (RDR) was tested and analyzed for its efficiency as an immobilized TiO2 photocatalysis reactor with an attempt to remove the mass-transfer limitation. * To whom correspondence should be addressed. Tel.: +886 2 23627158. Fax: +886 2 23623040. E -mail: [email protected].

Decomposition of methyl orange is used as the model reaction system. In the RDR, TiO2 particles are adhered to the surface of a disk that is spun at high speeds during operation. The liquid reactant is fed to the center of the disk and flows in the radial direction along the disk surface. The light source is illuminating from the above. RDR is considered to be a type of highgravitation device where the liquid element is accelerated under high centrifugal force to high velocity. Mass transfer either at the gas-liquid interface or within the liquid film is expected to increase due to the high convective flow velocity. Yatmaz et al.20 have first employed this type of reactor to photocatalytic degradation of 4-chlorophenol and salicylic acid. However, no attempt has been made to illustrate the interplay between the fluid dynamics and reaction kinetics, and therefore the scaleup potential of such a reaction system remains unclear. In our study, the process characteristics of RDR on photocatalytic decomposition of methyl orange (MO; C14H15N3O3S) are investigated by combining the chemical kinetics and fluid dynamics models. 2. Experimental Section The RDR setup is schematically shown in Figure 1. The major components include two ultraviolet lamps of 4 W with major UV emission at 254 nm wavelength on the top of the reactor and a TiO2-coated disk of 6 cm in diameter set upon a rotating motor at the center of the reactor fixed by two small iron bars. A typical operation protocol is the following. At first, the UV radiation source is switched on, and the rotating speed of the rotor is adjusted to a selected value. Reactant stream that contains MO is pumped to the center of the rotating disk at a selected flow rate. The effluent is collected at the bottom of the reactor. The effluent concentration gradually reaches a steadystate value over a period of time, and the steady-state data are analyzed. The apparatus and disk are washed by distilled water

10.1021/ie101330n  2010 American Chemical Society Published on Web 10/22/2010

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Figure 1. Schematics of the RDR system: (1) TiO2-coated disk, (2) ultraviolet light source, (3) disk holder and rotating shaft, (4) calibrated peristaltic pump, and (5) outlet pipe.

and alcohol between different experiments. Repeatability has been confirmed by comparing the results obtained under the same operation conditions at different moments along the course of the study. A circular borosilicate glasse of 6 cm in diameter was used as the substrate. TiO2 film was coated onto the disk surface by a dip-coating method. An 8 g amount of TiO2 and 15 mL of SiO2 colloidal aqueous solution (40 wt %; average size of SiO2 colloidal is 20 nm) were mixed in 180 mL of ethanol aqueous solution for making the dip-coating slurry. Here the SiO2 colloids were intended as an inorganic binder for immobilizing the TiO2 powder onto the disk surface after calcination. The glass disk, which had been cleaned by ultrasonication in ethanol for 20 min, was immersed in the slurry for 20 s and then withdrawn at a constant speed of 0.5 cm/s. The same procedure was repeated 20 times, and intermittent drying at 50 °C for 2 min was adopted between dipping. Finally, the disk was calcined at 500 °C for 3 h in flowing air with a flow rate of 0.5 L/min. The as-prepared TiO2 thin film showed good adhesion in a Scotch-tape peeling test. The surface morphology and microstructure of the TiO2 film was characterized by Scalar VL-11S optical microscopy and atomic force microscopy (AFM; SEIKO E-sweep System) in tapping mode. For the kinetic study, five different volumetric flow rates (3, 5, 10, 20, and 25 mL/min) and four different rotating speeds (100, 300, 600, and 900 rpm), all with an initial concentration of MO, CA0, of 4 × 10-5 M, were employed. On the other hand, for the initial-concentration experiments, five different CA0 were selected (40, 32, 24, 16, and 8 µM) for three selected sets of volumetric flow rate and rotating speed. Blank tests were conducted both under dark conditions (without light illumination) and under UV illumination in the absence of the TiO2 film. Commercial TiO2 powder (P25, Degussa) is used as received, and it contains 75% anatase and 25% rutile with an average particle size of 30 nm and a Brunauer-Emmett-Teller (BET) surface area of 50 ( 15 m2/g (vendor’s values). The concentration of MO (Ncalai Tesque) was determined on the basis of absorbance measurement on a spectrophotometer (GENESYS 10 SERIES, Hitachi). UV-light tubes were provided by Winstar Lighting Co., Ltd. 3. Results and Discussion 3.1. Characterization of Materials. Figure 2 shows the examples of UV-visible spectra of the MO-containing solutions with different reaction conversions. MO, a sulfonated azo dye, exhibits predominantly two absorption peaks, including one at the wavelength of 467 nm and the other at 274 nm. With increasing conversion, the intensities of the two MO peaks decrease without appearance of any new adsorption peak, suggesting that the MO molecules have indeed been decom-

Figure 2. UV-visible spectra of methyl orange-containing solutions: curve 1, the fresh solution; curves 2-4, solutions with increasing decomposition conversion.

Figure 3. Conversion of methyl orange (MO) versus rotating speed of the RDR for different volumetric flow rates: (1) 3, (2) 5, (3) 10, (4) 20, and (5) 25 mL/min. Initial MO concentration: 4 × 10-5 M (13.1 ppm).

posed. To convert the spectrum data to the conversion data, a series of standard solutions of different MO concentrations were prepared, and the correlation was established between the 467 nm peak intensity and MO concentration. Microscopic observation of the TiO2 film shows a fairly smooth surface with a few small cracks and pores. AFM measurement determines an average oxide film roughness of 149 nm. This roughness is much smaller than the thicknesses of the reactant liquid films under operation, which range from a few to a few hundreds of micrometers, as described later. 3.2. Photocatalytic Reaction and Fluid Dynamic Model of RDR. It was first confirmed that, under the conditions either without UV illumination or without the presence of a TiO2 film on the disk, the concentration of MO remained unchanged. The simultaneous presence of both UV illumination and TiO2 is essentially for photocatalytic decomposition of MO in the present reactor. Figure 3 summaries the conversion data of MO as a function of rotating speed for different feeding flow rates. As shown, for a fixed flow rate, the conversion of MO first increases with increasing rotating speed but starts to decrease when the rotating speed exceeds 600 rpm. The reduction trend


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Figure 4. Coordinate system of the thin-film flow on the rotating disk of a RDR. Q is volumetric flow rate, h is the height of liquid film, and Ω is the angular velocity of the disk.

at high rotating speeds is more prominent for lower flow rate. On the other hand, at a fixed rotating speed, the conversion always increases with decreasing flow rate. It is noted that, with only one pass of the dye solution over the surface of the 6 cm disk, a conversion as high as nearly 50% can be achieved. While the rotating speed and flow rate are independently varied operating variables of the reactor, they are not independent intrinsic properties of the reactor. Rather, the fundamental variable for reactor design is residence time, and a fluid dynamic model will be needed to establish the relation between this fundamental quantity and the operating variables. Over the past several decades, the free flow of thin liquid film extending on the horizontal rotating disk has been studied as a result of some industrial applications.21-24 Most of these works followed the analysis scheme originally developed by Emslie et al.,25 which will also be employed here. The model originates from the general Navier-Stokes equation: D F v ) -∇p + µ∇2v + Fg Dt

(1)

where F is the density of the fluid, t time, v velocity, p fluid pressure, µ viscosity, and g gravitational acceleration. The physical configuration of the model system is schematically shown in Figure 4. The following simplifications are made. First, the liquid is an incompressible and Newtonian liquid, and the surface of the TiO2 film is smooth. The last assumption is based on the fact that the oxide surface roughness is much smaller than the liquid film thickness. Second, the flow is at steady state and symmetrical to the center of the rotating disk; i.e., ur ) ur(r,z). Third, since the centrifugal force and viscous force are predominant, the inertial forces, surface tension, friction with air, and Coriolis force, are assumed to be negligible. Fourth, the pressure in the film has been considered constant in the radial direction; i.e., P ) P(z). Finally, the lubrication assumption is applicable, as the scale of the film thickness is much smaller than the disk size; i.e. z , r. Equation 1 is then simplified to ν

∂2ur ∂z2

+ Ω2r ) 0

(2)

where ν is kinematic viscosity and Ω the angular velocity of the disk. The boundary conditions (BCs) include no slip at the solid-liquid interface and no shear force at the liquid-air interface; i.e., BC 1: BC 2:

ur ) 0 at z ) 0 ∂ur /∂z ) 0 at z ) h

Figure 5. Conversion of MO versus residence time of the RDR. Solid lines connect data with the same rotating speed: (a1) 100, (a2) 300, (a3) 600, and (a4) 900 rpm. Dashed lines denote the same volumetric flow rate: (b1) 3, (b2) 5, (b3) 10, (b4) 20, and (e) 25 mL/min.

where h is the height of the liquid film at different locations. The radial velocity profile across the liquid film on the rotating disk can then be solved: ur )

Ω2rh2[1 - (1 - z/h)2] 2ν

(3)

The volumetric flow rate, Q, is then obtained by Q ) 2πr

∫

h

0

ur dz )

2π -1 2 2 3 ν Ωrh 3

(4)

On rearrangement, eq 4 yields the film thickness as a function of rotating speed and volumetric flow rate: h ) 0.782Q1/3ν1/3Ω-2/3r-2/3 r g ro

(5)

where ro is the radius of the inlet. With either increasing rotating speed (Ω) or reducing volumetric flow rate (Q), the liquid film thickness decreases. By definition, the average residence time is equal to total liquid volume held on the disk (V) divided by the volumetric flow rate: V ) τ) Q

∫

R

r0

2πrh dr Q

(6)

By substituting the expression of liquid film thickness into eq 6, the average residence time may be expressed as τ ) 3.685Q-2/3ν1/3Ω-2/3(R4/3 - ro4/3)

(7)

Figure 5 plots the conversion data versus residence time. It is found that there is no unique correlation between these two quantities; for any fixed residence time, an additional intrinsic property of the reactor is clearly needed for completely specifying the kinetic behavior of the reactor. It is noted in Figure 5 that, for the same residence time, the conversion data exhibit opposite correlations respectively with rotating speed and volumetric flow rate. Under a fixed residence time, the reaction conversion consistently increases with increasing rotating speed, while it decreases with increasing

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Figure 7. Conversion of MO versus residence time with h1/2, expressed in micrometers as the sole indexing parameter. The dashed lines are calculated from eq 12b.

Figure 6. (a, top) Liquid film thickness versus disk radius for selected operation conditions: (1) 3 mL/min, 900 rpm; (2) 5 mL/min, 600 rpm; (3) 10 mL/min, 300 rpm; (4) 25 mL/min, 100 rpm. (b, bottom) Film thickness at the middle of the radius, h1/2, for different combinations of disk rotating speed and volumetric flow rate employed.

volumetric flow rate. One property of the reactor that also exhibits the same opposite correlations respectively with rotating speed and volumetric flow rate is the liquid film thickness, h, as explicitly expressed in eq 5. The results may suggest that it is likely the additional property of the reactor that has a direct influence on the reaction conversion. The film thickness is related to the total amount of liquid on the disk, i.e., the socalled “reactor volume”, and therefore is truly a fundamental property of a reactor. Figure 6a plots the location of the liquid film surface versus disk radius for some selected operation conditions adopted in the present study. The combination of the highest rotating speed (900 rpm) and the lowest flow rate (3 mL/min) gives the minimum thickness (curve 1), while that of 100 rpm and 25 mL/min gives the maximum thickness (curve 4). In all cases, the liquid film thickness falls into the range between 50 and 300 µm for r g 1 cm. For simplicity, the thickness at r ) (1/ 2)R, denoted as h1/2, is used as a film-thickness index. Figure 6b presents h1/2 for different combinations of rotating speed and volumetric flow rate employed. It is found that the film thickness varies most profoundly when rotating speed increases from 100 to 300 rpm, beyond which the variation diminishes. This echoes with the observation that, as shown in Figure 3, the conversion

increases most dramatically over the same rotating speed range. Figure 7 replots the conversion data versus residence time with h1/2 as the sole indexing parameter (the fitted lines are derived on the basis of a reactor-design equation which will be described later). As shown, the data basically exhibit a trend that, under a fixed residence time, the reaction conversion always increases with decreasing film thickness. 3.3. Kinetic Model and Process Analysis of RDR. From the viewpoint of a heterogeneous reaction system, there are two possible mechanisms that could lead to such film-thickness dependence. First, the reaction rate might be limited by the supply of the reactants (the MO molecules) to the TiO2 surfaces. The reactor is said to be operating within the mass-transferlimited regime. Reduction in the liquid film thickness therefore would accelerate the diffusion rate and hence lead to an increase in apparent reaction rate. In this case, the reaction rate on the disk surface is then equal to the mass-transfer flux of the dye to the TiO2 surface: -rA ) kG(CA - CAs) ≈ kGCA

(8)

where kG is the mass-transfer coefficient and is inversely proportional to the film thickness. CAs is the concentration of MO at the TiO2 surface and, by the nature of the diffusionlimitation mechanism, it is negligible.26 Thus, the apparent reaction rate becomes first-order. Due to the linear nature of a first-order reaction, the reaction conversion, regardless of the extent of mixing of the reactor, is expected to be independent of the initial reactant concentration.27 Therefore, to access the significance of diffusion-limitation effect, a series of experiments using different initial MO concentrations were carried out. As shown in Figure 8, the experiments were conducted under the conditions that give three different film thicknesses, including h1/2 ) 30, 71, and 201 µm. It was found that, for h1/2 ) 30 and 71 µm, the reaction rates showed the same strong dependence (essentially the same slopes in the figure) on CA0. In great contrast, for h1/2 ) 201 µm, the reaction rate was independent of CA0. These data indicate that the reactor is not running under diffusion limitation, but limited by reaction kinetics, for h1/2 e 71 µm, while it is diffusion-limited for h1/2 g 201 µm. The critical film thickness for the kinetics-limited regime will be determined later.


Figure 8. Conversion of MO versus initial concentration, CA0, for three sets of volumetric flow rate and disk rotating speed that give different liquidfilm thicknesses (h1/2).

The second possible mechanism is associated with the intrinsic kinetic behavior of the photocatalytic reaction. A number of studies have suggested the Langmuir-Hinshelwood model to describe the photocatalytic surface reaction:28,29 -rA )

krKCA krKCAs = 1 + KCAs 1 + KCA

(9)

where kr is a reaction rate constant and K the adsorption equilibrium constant of the reactant species. According to Ollis et al.,30 kr is directly proportional to the intensity of incident light at the TiO2 surface. When the UV light travels through the liquid film, it is simultaneously absorbed by the dye molecules in the film. Thus, the absorption process within the liquid film can be described by the Beer-Lambert law: εhCA ) - log(I/I0)

(10)

where ε is molar absorptivity, and I and I0 are respectively the light intensities at the bottom and surface of the liquid film. Substituting this relation into eq 9 gives -rA ) aI0K exp(-εhCA)

CA 1 + KCA

(11)

where a is a constant. Equation 11 reveals that the reaction rate at TiO2 surface increases with decreasing liquid film thickness. Furthermore, for the concentration dependence, the intensity attenuation term, exp(-εhCA), on the right-hand side of the equation is predominant over the kinetic term, KCA/(1 + KCA), and hence an increase in the dye bulk concentration leads to a slower rate. This is also consistent with the effect of initial concentration for h1/2 e 71 µm shown in Figure 8. To describe the X-τ-h1/2 correlation shown in Figure 7, a semiempirical reactor-design equation having the form of ln(1 - X) ) -K' exp(-ξh1/2CA0)τ

(12a)

where K′ and ξ are fitted empirical parameters that are influenced by the properties of lighting and catalyst, was found to give fairly good fitting. This design equation was derived by adopting a simplified light attenuation term in analogy with that in eq 11 and a first-order kinetic term (i.e., KCA , 1). The parameters therein were obtained by linear fitting (solid line in Figure 9) on the basis of the conversion data whose h1/2 is no greater than 71 µm, i.e., those data under the kinetics-limited regime. The fitting (R2 ) 0.9633) gives a final form of

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Figure 9. Linear fitting of eq 12a. The solid symbols are from constantCA0 experiments, while the open symbols are from the variable-CA0 ones for h1/2 ) 30 and 71 µm. KL and DL indicate the kinetics- and diffusionlimited regions, respectively.

ln(1 - X) ) -0.945 exp(-673h1/2CA0)τ

(12b)

with h1/2 expressed in micrometers, CA0 in moles per liter, and τ in seconds. When the fitted line is extended to the region of greater thickness (the dashed line in Figure 9), it is shown that deviation from this fitted curve starts to take place for h1/2 g 99 µm (i.e., h1/2CA0 ∼ 0.004 µm · M in Figure 9). It is thus determined that the critical thickness below which the reactor is running under the kinetics-limited regime is 99 µm. This critical thickness also defines the applicable range of eqs 12a and 12b. The dashed lines in Figure 7 depict the constantthickness contours in the X-τ space based on eq 12b, showing reasonable good match between the predicted and experimental data for h1/2 e 99 µm. In addition, the equation also successfully predicts the CA0 dependence (the open symbols in Figure 9). The characteristics of this reactor can now be accessed with the combination of eqs 5, 7, and 12b. Figure 10 plots the constant-conversion contour in the Q-Ω space for the case of a RDR of which the diameter is 60 cm, 10 times that used in the present study, running with the same MO initial concentration and light-illuminating intensity per area. The boundary of the applicable condition of eq 12b is delineated by the dashed line in the figure. First, one notes that, for the same volumetric flow rate, the conversion first increases and then decreases with increasing rotating speed. This is the same trend as seen in Figure 3. Below the optimum rotating speed, the conversion is low because of thick liquid film. On the other hand, above the optimum rotating speed, the conversion is low because of short residence time. Furthermore, for each targeted conversion, there exists an optimum rotating velocity giving a maximum volumetric flow rate. For a final conversion of 50%, an optimum rotating speed of 280 rpm gives the maximum volumetric flow rate of 427 L/day. A 10 times increase in disk diameter gives 100 times increase in the volumetric throughput. It is worthy of mentioning that much higher conversion can be achieved by a “numbering-up” approach simply to have the reactant liquid to consecutively flow across the surfaces of multiple disks that are stacked vertically in one reactor and possess proper lighting arrangement to give the same incident light intensity. 4. Conclusions RDR containing immobilized TiO2 was studied for the application of photocatalytic decomposition of dye pollutants in water. The efficiency of this photocatalytic process was found to exhibit complex dependencies on the volumetric flow rate

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Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 X ) conversion of methyl orange z ) vertical coordinate (m) Greek Letters ε ) molar absorptivity (L mol-1 cm-1) µ ) viscosity of fluid (kg m-1 s-1) ν ) kinematic viscosity (m2 s-1) ξ ) fitted empirical parameters in eq 12a F ) density of the fluid (kg m-3) τ ) average residence time on disk (s) Ω ) angular velocity of disk (rad s-1)

Literature Cited Figure 10. Constant-conversion contour, calculated from eq 12b, in the volumetric flow rate versus rotating velocity space for the case of a rotating disk reactor of which the diameter is 60 cm. The dashed line delineates the applicable limit of eq 12b.

of the dye (MO) solution and the rotating velocity of the disk. The correlations between the fundamental characteristics of the reactor, including residence time and film thickness (reactor volume), with these two operation variables were established by the combination of fluid dynamic and kinetic models. The analysis indicates that the reaction rate is beyond the limitation of mass transfer and strongly affected by the thickness of the liquid film. The film-thickness dependence can be attributed to the intensity attenuation of the incidence light through the film before reaching the TiO2 surface. With selected operation conditions, conversions greater than 50% have been achieved within only a few seconds of residence time. A reactor design equation has been derived, and the simulation indicates very promising scale-up potential of the RDR process. Acknowledgment This work is partly supported by Ministry of Economic Affairs of Taiwan under Grant No. 98-EC-17-A-09-S1-019 and by the National Science Council under Grant No. 97-2120M002-017. Nomenclature CA ) concentration of species A (mol L-1) CA0 ) initial concentration of species A (mol L-1) CAs ) concentration of A at TiO2 surface (mol L-1) g ) gravitational acceleration (m s-2) h ) height of liquid film (m) h1/2 ) height of liquid film at middle of radius position of disk (m) I ) light intensities at the bottom of liquid film (W m-2) I0 ) light intensities at the surface of liquid film (W m-2) K ) adsorption equilibrium constant of reactant species K′ ) fitted empirical parameter in eq 12a kG ) mass-transfer coefficient (m s-1) kr ) reaction rate constant p ) pressure of fluid (atm) Q ) volumetric flow rate (m3 s-1) R ) radius of disk (m) r ) radial coordinate (m) rA ) reaction rate of species A (mol s-1 m-3) ro ) radius of inlet (m) t ) time (s) ur ) velocity components in radial direction (m s-1) V ) total liquid volume held on disk (m3) v ) velocity of fluid (m s-1)

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ReceiVed for reView June 22, 2010 ReVised manuscript receiVed September 15, 2010 Accepted October 7, 2010 IE101330N

Process Analysis on Photocatalyzed Dye Decomposition for Water

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