Radiation field modeling and optimization of a compact and modular multi-plate photocatalytic reactor (MPPR) for air/water purification by Monte Carlo method
Graphical abstract
Multi-plate photocatalytic reactor with one lamp and reactor performance parameters.
Highlights
► Compact modular design of a multiplate photoreactor facilitates scale-up. ► Large irradiated photocatalyst surface area with high photon utilization efficiency. ► Dimensionless parametric optimization of reactor geometry led to optimum design. ► Monte Carlo method applied to determine photon absorption and spatial distribution. ► Optimum design validated experimentally for the oxidation of toluene in humid air.
Introduction
In the past decades, air pollution control has focused on the outdoor environment as it is well known that outdoor air pollution can have adverse effect on human health. However, later studies showed that in many cases levels of pollutants in indoor air and, consequently, risks to health, can be considerably higher than those found outdoors [1].
There are countless sources of pollution in confined environments. These include aerosols, cleaning products, building materials, furniture, carpets, office machines and many others. Even the most common everyday equipment could represent an air pollution source, such as computers which emits 100–200 μg h−1 unit−1 of volatile organic compounds (VOCs) [2]. Poor indoor air quality influences the occurrence of infectious respiratory illnesses, asthma and allergy symptoms, sick building symptoms, and reduced worker productivity [3]. Fisk and Rosenfeld [4] estimated that the total annual cost of poor indoor air quality is around $100 billion in the U.S.
Indoor air pollution is a persistent problem found not only in buildings but also in transportation vehicles, aircrafts and many other confined spaces. Photocatalytic air cleaning devices, in combination with source control and ventilation appears to be a promising method to reduce VOCs levels, which are among the most abundant indoor air pollutants. Photocatalytic oxidation (PCO) offers several advantages over conventional air purification techniques [5] such as oxidization of low concentration pollutants, ambient temperature and pressure operation, low power consumption, low maintenance requirements, and possibility of use in conjunction with heating, ventilation and air conditioning systems (HVACs) systems. Hence, in recent years, numerous photocatalytic reactors for indoor air cleaning have been studied, including annular [6], [7], fixed bed [8], [9], flat-plate [10], [11], [12], fluidized-bed [13], [14], honeycomb [15], [16], and optical fiber [17], [18], [19]. A major challenge this technology faces, apart issues related to catalyst activity, is a poor catalyst illumination efficiency [20]. In general, effective photon utilization is a critical factor in determining the economic feasibility of a particular photocatalytic reactor design. A deficient use of light within a photocatalytic reactor will inherently lead to high operational costs, which in turn, could prevent the reactor to be implemented especially in cases in which catalyst activity is low. Consequently, the analysis of the radiation field in photocatalytic reactors is an essential step towards the optimization of photocatalytic air cleaners.
The radiation field in a multi-plate reactor can be obtained by solving the radiative transfer equation (RTE). There are several techniques to solve the RTE, including: flux, zone, Monte Carlo and hybrids methods. The mentioned approaches give equal solutions in most cases, however, they attach different degrees of difficulty. Pareek et al. [21] have summarized the various methods developed for solving the RTE.
The Monte Carlo method present several advantages in comparison with other methods, such as the fact that it is relatively simple, it has been successfully used for multiple reactor geometries and it can be employed for reactors of complex geometry. This statistical method is based on following the probable path of discrete bundles of photons until their final fate (absorption or escape from the reactor) is established. To have some statistical significance, a significant sample of photons must be defined.
This study focuses on the analysis and optimization of the geometry and radiation field in a multi-plate photocatalytic reactor (MPPR) irradiated by cylindrical UV lamps orthogonal to the plates, a reactor virtually not studied before. The reactor offers a very compact design which can be used for both water and air purification with the catalyst immobilized on the plates. The literature presents only a brief study on a similar reactor irradiated by solar light [22]. The MPPR aims to provide not only high light utilization, but also low pressure drop which is particularly important in HVACs treating large volumes of air. The MPPR presents a large photocatalyst surface area as well as a modular design, which facilitates scale up and retrofitting of current HVAC systems in buildings. The lightweight and compact design also may be suitable for installation in aircraft air purification systems. All these characteristics could make the MPPR a cost-effective alternative for indoor air remediation or water purification. The optimum design was validated by the oxidation of toluene in a humidified air stream.
Section snippets
Reactor design
The multi-plate photocatalytic reactor consists of a number of parallel and closely spaced aluminum plates. The plates are perfect squares (or rectangles) coated with TiO2 films whilst the lamps are inserted perpendicularly to them. Fig. 1 shows a possible reactor configuration with one lamp and a square configuration. This reactor structure provides an efficient contact of photons, solid catalyst and reactants. A small pitch between plates is desired to increase the catalyst surface area
Reactor performance parameters
The aim of this study is to provide a systematic approach for the optimization of the reactor design parameters that results in a high degree of photon utilization and the most uniform distribution of photons over the catalytic plates. The objective here is that the majority of photons should be captured within the reactor boundaries while providing the most uniform irradiation of the photocatalytic surface. From a qualitative point of view, as α decreases and β increases, the distribution of
Photocatalytic plates, experimental set-up and photocatalytic evaluation
TiO2 thin films were deposited on square aluminum plates (18.2 cm side length, 2 mm thickness) by dip-coating from a sonicated suspension of TiO2 (7 wt.%, Degussa P-25; primary particle size, 20–30 nm by TEM; specific surface area 52 m2 g−1 by BET; composition 78% anatase and 22% rutile by X-ray diffraction) in anhydrous ethanol and polyethylene glycol (PEG 4000, 7 wt.%, Aldrich). The plates were thoroughly cleaned with ethanol and deionized water before coating. The plates were dipped into the
Catalyst reflectivity
The catalyst reflectivity (ρ) is defined as the ratio of the reflected to the incident photons. If ρ is equal to 1, all incident photons are scattered. On the contrary, if it is zero, all photons are absorbed by the photocatalytic plates.
TiO2 photocatalytic thin films tend to have reflectivity values in the range of 0.3 and 0.6, depending on its crystal composition, particle size and surface roughness [25].
In order to approximate the value of the reflectivity in the photocatalytic plates
Conclusions
In this paper, the radiation field in a multi-plate photocatalytic reactor was investigated and successfully optimized by the Monte Carlo algorithm.
The results obtained for a reactor configuration with one lamp showed that the optimum lamp-diameter-to-plate height ratio should be 0.7 and that the optimum space-between-plates-to-plate-height ratio can be calculated by the equation: [αOptimum = 0.191 β2 − 0.5597 β + 0.3854]. A multi-lamp reactor configuration allows the use of higher number of plates
Acknowledgments
The authors are grateful to NATO (Grant CPB.EAP.SFPP 982835), to The University of Nottingham (KTI: Knowledge Transfer Innovation Awards, KT052) and to CONACYT (PhD scholarship) for financial support.
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