Ind. Eng. Chem. Res. 2007, 46, 5867-5880
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Performance Evaluation of Photocatalytic Reactors for Air Purification Using Computational Fluid Dynamics (CFD) Santiago Romero-Vargas Castrillo´ n and Hugo I. de Lasa* Chemical Reactor Engineering Centre (CREC), Faculty of Engineering, The UniVersity of Western Ontario, London, Ontario, Canada N6A 5B9
The performance of two photocatalytic reactors for air decontaminationsdesignated Photo-CREC-air reactorss is analyzed using computational fluid dynamics (CFD). Simulations of the original Photo-CREC-air revealed that the occurrence of a dead volume renders ∼68% of the available photocatalyst surface area inactive, resulting in poor air-photocatalyst contact. Moreover, the square cross section of the reactor geometry introduces regions of low ultraviolet (UV) irradiation. These issues are successfully addressed in a modified Photo-CREC-air design, which presents a uniform flow distribution over the photocatalyst surface and, therefore, good air-photocatalyst contact. In addition, the redesigned reactor geometry results in uniform UV irradiation over the photocatalyst. Simulations of reactor operation in continuous mode, with acetone as a model pollutant, revealed that negligible conversions are attained in the original Photo-CREC-air design, whereas conversions of 7.8% are predicted by simulations of the modified reactor. A simulation considering 10 modified PhotoCREC-air reactors in series showed that acetone conversions of 61% could be achieved in such a system. 1. Introduction Indoor air pollution is an issue that recently has attracted the attention of the scientific community, because of the serious sanitary hazards that it poses to society. Some of the health implications of indoor air pollution, all of which are aggravated by the fact that, currently, we spend 70%-90% of our time indoors,1 where pollutant concentrations are higher, include allergies and irritation of the eyes and respiratory tract, asthma, and lung cancer.2 In the United States alone, indoor air pollution is estimated to cause between 85 000 and 150 000 deaths per year,3 which is a figure that is likely to be much higher in underdeveloped countries, where ca. 3.5 billion people rely on highly polluting traditional fuels (firewood, dried dung, charcoal) to cook and heat their homes.3 Estimates of the World Health Organization (WHO) indicate that indoor air pollution results in 2.8 million deaths annually, making it one of the most serious mortality factors.3 Because of these hazards, the World Bank and the United States Environmental Protection Agency (USEPA) have identified indoor air pollution as one of the top environmental problems.3 According to the USEPA, the four most dangerous sources of indoor air pollution are radon gas, environmental tobacco smoke (ETS), formaldehyde, and asbestos; of these pollutants, the most abundant component is ETS.4 ETS is comprised of over 4500 compounds, many of which belong to the category of volatile organic compounds (VOCs),2 which encompasses all organic compounds with normal boiling points in the range of 50-260 °C, including dichloromethane (boiling point ) 41 °C). In addition to ETS, sources of VOCs include objects commonly found in indoor environments, such as office products, insulating materials, synthetic furniture, and cleaning and maintenance products. * To whom correspondence should be addressed. Tel.: +1-(519)661-2144. E-mail:
[email protected].
Indoor air pollution has been addressed via three strategies: source control, increased ventilation, and air cleaning.5 In regard to source control, studies have dismissed this approach as impractical in large urban centers, where vehicular exhaust is an inevitable source of indoor air pollutants.6 Increased ventilation has also proven ineffective in addressing the problem, given that it can transport more pollutants from outdoor environments.7 Thus, air cleaning remains as the most practical option to remedy indoor air pollution, including that caused by VOCs. In this respect, several technologies have been tested, to determine the most efficient VOC cleaning method. Among the common VOC control strategies, thermal and catalytic oxidation, condensation, and adsorption stand as the most widely used ones.8 However, the aforementioned technologies present several disadvantages: oxidation processes require high temperatures that are difficult, if not impossible, to attain in households and the workplace, whereas condensation and adsorption achieve little more than transferring the pollutant from the gaseous phase to the solid phase, therefore requiring subsequent waste-handling steps. The shortcomings observed in conventional technologies have spurred research into more-practical VOC abatement strategies. These efforts have resulted in the advancement of heterogeneous photocatalytic oxidation and, in particular, titanium dioxide photocatalysis, as the most promising VOC treatment technology. The main advantage of heterogeneous photocatalytic oxidation is its ability to achieve complete mineralization of a wide range of VOCs under ambient reaction conditions. In addition, titanium dioxide is an excellent photocatalyst, given its relatively low price, high stability, and the highly oxidizing nature of the chemical species that it generates upon irradiation with near-ultraviolet (near-UV) light.9 Its promise in tackling indoor air quality problems has been reported recently,10,11 as well as its applicability to in-vehicle air cleaning.12
10.1021/ie060696q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007
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Figure 1. Photo-CREC-air reactor designs: (a) original design and (b) modified design.
Although the outlook on this technology is promising, there are several challenges preventing this technology from achieving its full potential. One with immediate relevance to reaction engineers pertains to the optimal reactor design. The reactor configuration has a crucial role in the overall process performance, because it determines the efficiency of fluid-photocatalyst contact and photocatalyst utilization, as well as the intensity and uniformity of the UV irradiation field. Several configurations have been reported in the literature, such as the fixed-bed reactor,13,14 fluidized-bed reactor,15,16 honeycomb monolith reactor,17,18 TiO2-coated fiber-optic cable reactor,19 annular reactor,20 and annular venturi reactor.21 However, other than a few exceptions, reactor designs have largely been based on empirical considerations, rather than first principles. Examples of photocatalytic reactor designs based on thorough modeling efforts have mainly focused on the UV irradiation field, with representative works being those of the Cassano group22 and the Raupp group.23 Recently, detailed firstprinciples computational fluid dynamics (CFD) studies have followed suit, although, until now, the number of publications on this topic has been rather limited. CFD studies of photocatalytic reactors for air treatment include those of Raupp and collaborators,17 Mohseni and Taghipour,24 and Taghipour and Mohseni.25 A further CFD reactor study was recently reported by the de Lasa group,26 and it is focused on photocatalytic reactors for air decontamination (designated as Photo-CREC-air reactors). 1.1. Photo-CREC-Air Reactors for Air Decontamination. The reactor systems studied in this paper are the original and modified Photo-CREC-air units, which are shown in Figure 1a and b, respectively. The original Photo-CREC-air unit21 consists of a venturi reactor (Figure 2a) section that contains a square-
woven fiberglass screen, which is impregnated with TiO2. The fiberglass screen, referenced in the following sections as the photocatalyst support, is wrapped around a wire-mesh basket that serves as its structural support, the base of which is impervious to the flow. As described in Figure 2b (showing the venturi divergent section), air flows through the venturi throat and around the wire-mesh basket base, and then contacts the TiO2-impregnated mesh, where the photocatalytic oxidation occurs. Eight Pen-Ray mercury UV-lamps (UVP Inc., Upland, CA), delivering a total irradiation flux of 9.7 × 103 µW/cm2 at a principal irradiation wavelength of 365 nm, mounted outside of the venturi divergent section and housed inside parabolic reflectors, irradiate the TiO2-impregnated mesh. Four trapezoidal windows mounted on the divergent section permit UV irradiation transfer from the lamps to the photocatalyst. Appendix A1 presents the estimation of the irradiation flux. Experimental studies performed in the original Photo-CRECair reactor setup (Figure 3) revealed quantum efficiencies in the photocatalytic oxidation of acetone, acetaldehyde, and isopropanol of 28%-36%, 88%-157%, and 21%-31%, respectively.27 Moreover, further studies performed on the batch setup shown in Figure 3 reported the complete mineralization of airborne acetone (initial concentration of 5 × 104 µmol/m3), acetaldehyde (initial concentration of 5.5 × 104 µmol/m3) and iso-propanol (initial concentration of 3 × 104 µmol/m3) within 500, 200, and 750 min, respectively.28 A modified Photo-CREC-air reactor26 was designed with the objective of improving the original Photo-CREC-air unit. The design, shown in Figure 1b, presents a modified upper cylindrical reaction section that replaces the divergent venturi section. The main modifications implemented in the modified reaction section are given in Figure 4. As shown in the figure, in addition
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Figure 2. (a) Photo-CREC-air original design, isometric view. (b) Venturi divergent section.
Figure 4. Design features of the original and modified Photo-CREC-air reactors.
Figure 3. Original Photo-CREC-air experimental setup.
to the upper cylindrical section, the modified design features a perforated plate (2-mm thick, 163 perforations with a diameter of 2 mm) that replaces the wire-mesh basket as the mechanical scaffold of the photocatalyst support. Also, the blunt basket base is replaced by an aerodynamic bullet nose. Furthermore, the trapezoidal windows are replaced by a single wraparound window, which introduces a more-uniform distribution of the UV light. In regard to UV-light sources, the modified design was conceptualized so that higher UV irradiation fluxes can be delivered to the photocatalyst. Indeed, irradiation fluxes up to 9.72 × 105 µW/cm2 (see Appendix A1) could be achieved in
the modified design, with 18 18-W Compact BLB UV lamps (Sankyo-Denki, Japan). The modified design improves the original Photo-CREC-air, in terms of photocatalyst UV irradiation uniformity, given the cylindrical design of the reaction section, which is equipped with a single wraparound window. As shown in Figure 5a, the square cross section of the original design introduces regions of lower UV irradiation on the basket corners. These regions disappear in the modified design (Figure 5b), because of the wraparound window, which requires no support posts that could obstruct light irradiation. 1.2. Objectives. In this paper, we intend to expand on the previously reported CFD model of Photo-CREC-air reactors26 by examining reactor decontamination performance in continuous mode, as well as the quality of air-photocatalyst contacting
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Figure 5. Photocatalyst irradiation: (a) regions of lower UV irradiation in the original design, and (b) uniform UV irradiation in the modified design.
within the reactor. These simulations will provide important information about the photocatalytic reactor when operated in continuous mode (which is the operation that one would find in a commercial-scale photocatalytic reactor) and will yield insight as to the reactor performance in the decontamination of indoor environments. With this end, we perform CFD simulations that incorporate the photocatalytic oxidation of acetone through the kinetic model proposed by Ibrahim and de Lasa.28 The work is performed to demonstrate how tools such as CFD codes can be used to analyze photocatalytic reactor performance. Furthermore, within a broader perspective, this work intends to provide insight as to the applicability of photocatalytic technologies to the decontamination of sizable, real-life polluted streams. 2. CFD Model and Computational Details 2.1. Governing Equations. CFX-10, which is a commercial CFD package, was used to solve the Reynolds-averaged mass, momentum, and acetone transport equations (eqs 1-3, respectively):
∂F + ∇•(FU) ) 0 ∂t
(1)
∂FU + ∇•{FU X U} ) -∇•{τ + τt} + SM ∂t
(2)
(
( ))
∂CAc CAc + ∇•(UCAc) ) ∇• F(D + Dt)∇ ∂t F
+ SC
(3)
Note that the diffusivity of acetone in air (D) in eq 3, which has a value of 1.059 × 10-5 m2/s, was estimated with the method developed by Fuller et al.29,30 Furthermore, the energy equation was not included in the model, given that isothermal operation is assumed, setting the domain temperature to 298.15 K. This is a sound assumption, given that pollutant combustion results in negligible enthalpy changes and heat that is dissipated as unused irradiation is minimal. Local density variations were calculated using the ideal gas law, based on the fact that the
domain temperature and pressure were set to ambient conditions (298.15 K and 1 atm, respectively). The Reynolds stresses, represented by the term τt in eq 2, were calculated with the k-ω-based Shear Stress Transport (SST) turbulence model,31 which was selected because of its accurate boundary layer calculations, including cases of flow separation. 2.2. Constitutive Equations. 2.2.1. Momentum Sink Terms. The parameter SM that appears in eq 2 represents the volumetric momentum sink term; it is included to model the pressure drop introduced by the photocatalyst support and the perforated plate. To model this effect, the region in which the pressure drop arises is first defined. This region, which is called the subdomain, is a three-dimensional region that spans the photocatalyst support (and the perforated plate, for the case of the modified design), in which the volumetric momentum sink termssSMsare added to the momentum conservation equation (eq 2). SM is given by
-SM,x′ ) KS1 Ux′ + KS2 |U|Ux′
(4)
-SM,y′ ) KT1 Uy′ + KT2 |U|Uy′
(5)
-SM,z′ ) KT1 Uz′ + KT2 |U|Uz′
(6)
where the x′-, y′-, and z′-coordinates are the directions of a local coordinate system in the subdomain, in which x′ represents the streamwise direction and the y′ and z′ axes lie on the transverse plane.32 Equations 4-6 show that the momentum loss is a consequence of a linear contribution predominant in the laminar flow regime and a quadratic contribution characteristic of turbulent flow. Given that the flow in the vicinity of the photocatalyst support is fully turbulent,26 the linear term can be neglected. The constitutive equation used in the calculation of KS2 is as follows:
KS2 )
KLossF 2L
(7)
where KLoss is a loss coefficient and L is the subdomain thickness (360 µm for the photocatalyst support and 2 mm for the perforated plate). For the case of the photocatalyst support,
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KLoss,S was calculated via a correlation for the pressure drop through woven screens inclined to a flowing fluid:33
KLoss,S )
(
)
1.26(1 - ) fc
(8)
where is the screen porosity, estimated at 0.087, and fc is a correction factor with a value of 1.83, which accounts for flow incidence on the photocatalyst support of the modified PhotoCREC-air at angles of >45°.26 It must be mentioned that the value of fc in the simulation of the original design was set to 1.0, given that flow incidence occurs at angles of
