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A Novel Fountain Photocatalytic Reactor for Water Treatment and Purification: Modeling and Design Gianluca Li Puma*,† and Po Lock Yue‡ School of Chemical, Environmental and Mining Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom, and Department of Chemical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong SAR
A novel, thin-film, slurry “fountain” photocatalytic reactor for water treatment and purification is presented. The novelty of the reactor centers on the design of a special nozzle through which water is pumped to create a thin film. The film emerges from the nozzle in the shape of a smooth and radially expanding water fountain. A radiation source (solar or UV lamps) is situated above the fountain. Two dimensionless mathematical models for the photoreactorsa flat fountain model and a parabolic fountain modelsand simulation results under a number of different conditions including the effects of nozzle setting (radius, aperture, and angle), flow rate, and light scattering are presented. The model simulations suggest that optimal photoreactor design is given by fountains with large surface areas. Photocatalysts with scattering albedos smaller than 0.3 result in optimal photoreactor performance. The models presented may be used as a tool for the design, scale-up, and optimization of these types of reactors. Introduction Photocatalytic oxidation (PCO) is highly effective for the degradation and mineralization of many priority pollutants in water and wastewater.1,2 These processes exploit the high oxidation potential of hydroxyl radicals that are produced by the illumination of a semiconductor photocatalyst with photons in the near-ultraviolet spectrum.3,4 PCO is suitable for the treatment of toxic industrial effluents with low to medium flow rates. PCO is also effective for the inactivation of water pathogens.5,6 It can be easily integrated with existing biological wastewater treatment units7,8 as a preconditioning step to biological treatment for the partial breakdown of recalcitrant substances or as a postbiological polishing tertiary treatment to meet the required discharge levels. The application of photocatalysis for water treatment and purification on an industrial scale can be assisted by the development of both new photoreactor designs and mathematical models. Thin-film, slurry photocatalytic reactors provide an excellent configuration for efficient excitation of the photocatalyst (titanium dioxide, TiO2) because of the high light absorptivity of TiO2 slurry suspensions.9 This configuration benefits from a very large illuminated surface area per unit volume of catalyst and minimal mass-transfer limitations.10 Thinfilm slurry reactors normally operate at higher catalyst concentrations than conventional photoreactors. This increases the number of available sites for hydroxyl radical generation per unit volume of reactor11 and maximizes the efficiency of photon utilization, oxidation rates, and reactor throughput. Thin-film slurry reactors are, therefore, suitable for large-scale industrial applications of photocatalysis for water treatment and * Corresponding author. E-mail: gianluca.li.puma@ nottingham.ac.uk. Fax: +44 (0) 115 9514115. † The University of Nottingham. ‡ The Hong Kong University of Science & Technology.
purification. A falling-film photoreactor with the liquid film descending along the internal wall of a reactor column and the lamp axially mounted in the middle has been shown to be a very efficient design of a thin-film slurry photoreactor.12 In this paper the design and modeling of a novel thinfilm slurry photocatalytic reactor for water treatment and purification are presented. The reactor design is particularly suitable for large-scale solar applications of photocatalysis, but artificial light sources can also be used. The main design parameters and how these parameters affect the reactor performance will be discussed. The results of two dimensionless mathematical models for the photoreactorsa flat fountain model and a parabolic fountain model previously reported by the present authors13,14swill be extended to include new simulation results under selected realistic operating conditions. Fountain Photocatalytic Reactor The design of a novel, thin-film, slurry photocatalytic reactor for water treatment and purification is shown in Figure 1.13,14 The basic concept of this photocatalytic reactor centers on the design of a special nozzle that provides an unsupported thin film in the form of a “fountain” in continuous flow exposed to a light energy source. The thin film is created by pumping water through the specially designed nozzle. The liquid film emerges from the nozzle in the shape of a smooth and radially expanding water fountain. The radiation source is situated above the water fountain and can be either solar radiation or that provided by artificial light sources. In the case of solar radiation, both direct and diffuse solar radiation can be utilized in the photoreactor. The photon collection efficiency of this new photoreactor can be higher than that of a flat-plate falling-film photoreactor, provided that a reflecting surface is installed near the inside surface of the water fountain. This reflects photons which have penetrated
10.1021/ie0010316 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
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Figure 1. Schematic representation of a section of a water fountain with a parabolic profile.
the film back to the fountain, especially in the outer sections of the fountain where the film thickness becomes very small. The installation of a photonreflecting surface allows the thin film to be irradiated from both sides. If artificial light is used, the photon collection efficiency may be further enhanced by irradiating the fountain both from the top and laterally. The photoreactor system has a minimal reactor inventory because it consists of only a nozzle and a pump and can be operated under a wide range of flow rates, in either batch or continuous mode. It can be easily installed in current water treatment works by the simple distribution of these nozzles in an open-air, sunlight-activated, lagoon treatment plant. In addition, the fountain photoreactor has the ability of combining water disinfection and water detoxification in a single process. The presence of an unattached thin-film water fountain encourages a high oxygen mass-transfer rate, eliminates the need for additional oxygenation systems, and avoids the filming problem,15 sealing problem, and breakage risk which are associated with reactor configurations in which radiation enters through a transparent reactor wall. Fouling of the reactor nozzle by the catalyst can be avoided by using an appropriate nozzle design. The size and shape of the water fountain can be varied by either changing the liquid flow rate or adjusting the nozzle settings such as the radius and aperture of the nozzle and the angle of water emerging from the nozzle. Mathematical Modeling A mathematical model of such a reactor will aid the scale-up, design, and optimization of this type of reactor. Table 1 shows two dimensionless models that have been developed by the authors for the fountain photocatalytic reactor.13,14 The first model considers the simplest case of a water fountain with a horizontal profile and is
applicable for water fountains that have a small curvature, with the liquid flowing predominantly in the radial direction. The second model considers the case of water fountains with hydrodynamics significantly affected by gravity and surface tension that act respectively on the body and the surfaces of the liquid film. The hydrodynamics of the water fountain have been modeled using Taylor’s analysis of water bells.16-18 This analysis was found to be satisfactory for describing the geometry of water fountains.14 Taylor’s analysis coupled with the continuity equation enables the diameter, curvature, velocity, and film thickness of the water fountain to be calculated. The modeling approach advocated by Cassano2,20,21 based on a rigorous analysis of the radiation field in the photoreactor has been simplified to include a two-flux absorption-scattering model22 with simpler radiation attenuation terms.13,14 This approximation has previously been shown to be satisfactory for a thin-film photocatalytic reactor of another configuration.12 Both models contain parameters that can be readily estimated from real systems. Model solutions can be obtained with minimal computational effort. Model results agree well with experimental results from the photocatalytic oxidation of salicylic acid and indigo carmine dye in a pilot-scale reactor using TiO2 (Degussa P25 and Aldrich) as the photocatalyst.13,14 The limitations on the use of the above models rest with the assumptions listed in Table 1 and the formation of a stable water fountain that is not affected by turbulence. Taylor’s analysis18 shows that, in a liquid film with negligible viscosity and free from turbulence, the film breaks to droplets when
(2T/Fδ)0.5 > v
(1)
For the horizontal fountain model, this limitation can be used to estimate the maximum radius of fountain rmax:
rmax )
F(QR)2 8π2r0δ0T
(2)
In practical applications the condition described in eq 1 gives an overestimation of the maximum size of the water fountain because of the presence of instabilities induced by nozzle exit effects, air friction, waves, and turbulence. Simulation The geometry and hydrodynamics of the water fountain are determined by the value of the flow rate and by the nozzle settings. Nozzle settings include nozzle radius, aperture (or liquid thickness at the nozzle exit), and angle of exit for the liquid film. These design parameters determine the size and shape of the water fountain and the distribution of the liquid velocity and film thickness. For a given hydrodynamic state, the reactor performance is further affected by operating variables such as the light intensity, substrate concentration, catalyst concentration, and recycle ratio. To understand the effect of these parameters on the photoreactor performance, consider the experimental system described in work by Li Puma and Yue14 in
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Table 1. Mathematical Models for the Fountain Photocatalytic Reactora,13,14
a Assumptions: (i) negligible air friction, (ii) inviscid fluid, (iii) constant surface tension, (iv) negligible exits effect at the nozzle, (v) uniform distribution of incident radiation at the surface (ξ ) 0), (vi) photons traveling through the liquid film only along the film thickness direction, ξ, (vii) prevailing steady state conditions, (viii) incompressible, continuous flow of the water fountain existing under fully developed, radially expanding streamline flow, (ix) catalyst particles considered to be uniformly distributed within the liquid film; (x) the useful photons absorbed only by the solid photocatalytic particles, (xi) negligible diffusion flux in the film thickness direction, ξ.
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which suspensions of indigo carmine C18H8N2Na2O8S2 dye (Riedel-deHae`n) with TiO2 (Aldrich) were illuminated with radiation of wavelengths longer than 300 nm under conditions in which TiO2 can sustain heterogeneous photocatalysis. The effects of light intensity and substrate concentration have been discussed elsewhere,14 and the analysis here will be restricted to the effects of the other parameters. Model Parameters. The values of the model parameters used for these simulations are the same as those work by in Li Puma and Yue.14 The values of the specific mass Napierian coefficient and specific absorption coefficient for suspensions of TiO2 (Aldrich) in water were estimated by averaging the measurements reported by Cabrera et al.23 over the spectrum of the incident radiation in the wavelength range 300-380 nm:13
σ)
∫λ
