Heterogeneous Photoreactors Research Articles

Estimation and Optimization of the Radiant Field in Flat Plate Heterogeneous Photoreactors with the P1-approximation of the Radiative Transfer Equation (RTE).

In this work, the P1-approximation of the radiative transfer equation (RTE) was used for the description and optimization of the radiant field in a flat plate photoreactor under solar radiation with three commercial brands of titanium dioxide photocatalysts. The boundary layer of photon absorption (δ_abs), the average volumetric rate of photon absorption (VRPA), and a new apparent optical thickness (ζ_app1) were used as design parameters for optimization. A simple mathematical expression for the calculation of δ_abs also called the best reactor thickness was formulated. For the three catalysts, varying the reactor height (L), it was found a decrease in the local volumetric rate of photon absorption (LVRPA) from the top side until the bottom of the reactor for any value of the catalyst loading (Ccat). It was also observed that when Ccat increases the VRPA increases exponentially until a fixed value where it remains almost constant. With L= 1 cm, the optimum Ccat (Ccatop) was 0.2 g/l in 0.85 cm of thickness, 0.3 g/l in 0.82 cm of thickness, and 0.4 g/l in 0.89 cm of thickness for the photocatalysts Degussa P-25, Aldrich, and Hombitak respectively. The optimum apparent optical thickness (ζ_(app1,op)) was 4.03, 4.62, and 3.7 for the photocatalysts Degussa P-25, Aldrich, and Hombitak respectively. These results are in good agreement with the literature. Results found in this work give predictions on radiation absorption in flat plate photocatalytic reactors with different heights.

The molecule-level photoreactor: accurate embedded iodine-substituted boron dipyrromethene dye within zeolitic imidazolate framework-8 for highly efficient oxidization of sulfides under visible light

Using porous metal–organic frameworks (MOFs) as supports for immobilizing dyes as photocatalysts is an important strategy to construct a molecule-level photoreactor. Rational assembly of a heterogeneous photoreactor (photosensitizer + porosity + catalysis) requires individually tailoring based on the structure and function of the dyes and MOFs. Herein we report a facile, one-pot, room-temperature (RT) aqueous solution method to precisely embed an iodine-substituted boron dipyrromethene (I2-BODIPY) photosensitizer within zeolitic imidazolate framework-8 (ZIF-8) cavity. The resultant I2-BODIPY@ZIF-8 composite not only maintained the nanoporous cavity and outstanding stability inherited from ZIF-8 material but also possessed excellent visible-light harvesting property and high singlet oxygen production ability originated from I2-BODIPY dye. Combining the advantages of ZIF-8 material and I2-BODIPY dye, the composite is highly active as a visible light–driven photocatalyst for selective oxidization of aryl sulfides and a sulfur mustard simulant, 2-chloroethyl ethyl sulfide, at RT without overoxidation. Owing to its heterogeneous nature, the composite can be readily recycled at least five times for the oxidization without obvious loss of the catalytic activity.

Modelling and Simulation of the Radiant Field in an Annular Heterogeneous Photoreactor Using a Four-Flux Model

This work focuses on modeling and simulating the absorption and scattering of radiation in a photocatalytic annular reactor. To achieve so, a model based on four fluxes (FFM) of radiation in cylindrical coordinates to describe the radiant field is assessed. This model allows calculating the local volumetric rate energy absorption (LVREA) profiles when the reaction space of the reactors is not a thin film. The obtained results were compared to radiation experimental data from other authors and with the results obtained by discrete ordinate method (DOM) carried out with the Heat Transfer Module of Comsol Multiphysics® 4.4. The FFM showed a good agreement with the results of Monte Carlo method (MC) and the six-flux model (SFM). Through this model, the LVREA is obtained, which is an important parameter to establish the reaction rate equation. In this study, the photocatalytic oxidation of benzyl alcohol to benzaldehyde was carried out, and the kinetic equation for this process was obtained. To perform the simulation, the commercial software COMSOL Multiphysics v. 4.4 was employed.

Coupling the Six Flux Absorption–Scattering Model to the Henyey–Greenstein scattering phase function: Evaluation and optimization of radiation absorption in solar heterogeneous photoreactors

Robust and practical models describing the radiation field in heterogeneous photocatalytic systems, used in emerging environmental, photochemical and renewable energy applications, are fundamental for the further development of these technologies. The six-flux radiation absorption–scattering model (SFM) has shown to be particularly suitable for the modeling of the radiation field in solar pilot-plant photoreactors. In this study, the SFM was coupled to the Henyey–Greenstein (HG) scattering phase function in order to assemble the model with a more accurate description of the scattering phenomenon provided by this phase function. This new version of SFM, named as SFM-HG, was developed through fitting the Local Volumetric Rate of Photon Absorption (LVRPA) determined in a flat photoreactor to the “pseudo-experimental” LVRPA calculated by a Monte Carlo (MC) approach, which included the HG expression. As a result, simple mathematical correlations describing the SFM-HG scattering probabilities as function of the HG scattering parameter were determined. The SFM-HG was validated through a comparison with the MC model predictions of the Total Rate of Photon Absorption (TRPA) in the slab photoreactor. A RMSE% of approximately 5% demonstrated satisfactory agreement between the models. The SFM-HG was further applied to evaluate the impact of selected scattering phase functions on the absorption of radiation in solar photoreactors, operated with commercial TiO2 photocatalyst. The results have established that, the apparent optical thickness, τapp (or τapp,max for tubes) a parameter derived from the SFM approach, is the most appropriate for the design and optimization of photocatalytic reactors. This parameter is insensitive to scattering albedos and phase functions. CPC, tubular and flat-plate photoreactors should be designed with τapp,max=12, τapp,max=7 and τapp=4.5 respectively.

Photosynthesis-inspired design approach of a liquid phase heterogeneous photoreactor

A method of photocatalytic process intensification, which can be easily used for efficient photoreactor design, inspired by the mechanism of natural photosynthesis was developed and the possible mechanism was proposed. Four model compounds (phenol, Cr(VI), acid orange 7 and p-nitrophenol) with different photocatalytic reaction mechanisms, and using a Kenics static mixer, were selected to perform experiments for validation. Experimental results showed that the static mixer can intensify the indirect radical-mediated mechanism reaction, but had no effect on the direct surface reaction mechanism reaction. Meanwhile, both the illuminated static mixer and dark static mixer had similar intensification effects. Herein, we propose that the role the static mixer played in heterogeneous photocatalysis is the creation of intense mixing and increasing the interfacial mass transfer, which resulted in the enhanced mobility of reactive radicals from the catalyst surface or boundary layer to solution, thus intensifying the whole photocatalytic efficiency. Furthermore, the proposed separation of the illumination part and the mixing part for photoreactor design allows for easier scale-up and engineering applications.

Development and potential of new generation photocatalytic systems for air pollution abatement: an overview

AbstractPhotocatalysis is the process by which various undesired substrates are reduced or oxidised on the surface of a photoresponsive material when exposed to a sufficiently energetic irradiation source. Together with other processes like photolysis and ozonation, photocatalysis forms a larger, important group of technologies known as Advanced Oxidation Processes or AOPs.This short review begins with an introduction to the fundamental processes and entities involved in general semiconductor photocatalysis. Various major air pollutants are considered, along with their health effects and traditional means of abatement. Recent advances in photocatalytic materials (including the use of novel materials other then titania), together with heterogeneous photoreactor design (in particular, of the flow‐type) are then described. Concluding remarks are included, along with some recommendations for possible future work. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Estimating radiant fields in flat heterogeneous photoreactors by the six‐flux model

AbstractHeterogeneous photoreactor modeling is a task complicated by the integro‐differential nature of the Radiation Transfer Equation (RTE) when scattering phenomena are important. In the present work, a novel “Six Flux Model” (SFM) is proposed, which may be regarded as a step forward with respect to the previously proposed “Two Flux Model” (TFM). In order to validate the newly proposed model, Monte Carlo simulations of an indefinite plane‐slab photoreactor have been performed. As no simplifying assumptions are involved in this case, the information obtained may be regarded as “pseudo‐experimental,” and therefore compared with the predictions of both TFM and SFM models. Results show that the novel SFM is able to closely match the exact solution of the RTE better than the TFM, still retaining the TFM advantage of a simple closed‐form solution, that is, ease of use, clearer effect of model parameters, and so forth. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Light distribution field in catalyst suspensions within an annular photoreactor

In photochemistry, an accurate description of the radiation field is important for successful modeling and subsequent design of a photocatalytic reactor, since the radiation field determines the local volumetric rate of energy absorption, which in turn controls the photocatalytic reaction rate. However, scattering by catalyst particles present in water causes the photon energy to be redistributed and the light field in a heterogeneous photoreactor is consequently difficult to evaluate. Light field in a heterogeneous photoreactor is determined by selected optical properties of the catalyst particle, such as scattering and absorption coefficients. However, so far there has been no direct experimental method to measure these two values. In addition, these two coefficients alone are not sufficient to describe the effect of catalyst on the light field in a catalyst suspension. A third optical property of scattering mode, which gives the spatial distribution of the scattering directions, has been found to be important in parallel light field. In this study, a new light model along with the optical properties of two commercially available titanium dioxide (TiO 2) catalysts was used to determine the light field in a test annular photoreactor for different concentrations of suspended TiO 2. Chemical actinometry was used to measure the light intensity inside the reactor. The scattering mode of the catalysts was determined by fitting experimental data with the model predictions. It was found that the optical properties of the catalyst change significantly at different catalyst concentrations. This suggests that the application of universal optical properties of a catalyst may not be suitable for the determination of light field for a large range of catalyst concentrations.

Rigorous modeling of UV absorption by TiO2 films in a photocatalytic reactor

AbstractThe radiation absorption profiles on the surfaces of TiO2 films in a corrugated‐plate photocatalytic reactor were modeled based on first principles. A new term, the local‐area‐specific rate of energy absorption (LASREA), was adopted to describe the catalyst surface radiation in heterogeneous photoreactors. The LASREA and the energy absorption efficiency were both quite sensitive to the dimensions of the corrugated plates. Due to the multiple photon reflections between the opposing surfaces, corrugated plates possess a superior capability for recapturing longer wavelength photons that would otherwise be reflected out of some reactor designs. This results in higher energy absorption efficiency and more uniform LASREA on the catalyst films. Compared to a flat plate, corrugations are predicted to enhance the energy absorption efficiency by up to 50% for UV‐A fluorescent‐lamp‐powered systems and more than 100% for solar‐powered systems.

Anisotropic Scattering Model for Estimation of Light Absorption Rates in Photoreactor with Heterogeneous Medium.

A statistical approach is still the most effective tool for handling coupled absorbing and scattering phenomena in a heterogeneous system for the assessment of light energy absorption in a photoreactor. A Monte Carlo simulation model has been developed for the prediction of light intensity distribution in a photoreactor. The model has three parameters, an attenuation coefficient, a probability of light ray absorption, and a scattering parameter which specifies the mode of isotropic and/or anisotropic scattering. Experimental measurements of light transmittance and reflectance through solid particle suspensions bounded by two infinite plates were carried out using a newly designed integrating sphere device attached to a spectrophotometer. Light intensity distributions in a rectangular photoreactor with the suspension irradiated by a laser beam were also measured with a photo-probe. These data are compared with simulation results and model parameters are determined to satisfy the experiments. Light absorption fractions measured in the suspensions with different optical lengths show satisfactory agreement with those estimated by the simulation. Thus, the model developed is verified to be effective for the assessment of light absorption rates in a heterogeneous photoreactor.

Simplified Modeling of Radiant Fields in Heterogeneous Photoreactors. 2. Limiting “Two-Flux” Model for the Case of Reflectance Greater Than Zero

In the first part of this work a simple model for the description of the radiant field in heterogeneous photoreactors was developed, based on the assumption of zero reflectance of the particles. In this second part of the investigation a limiting model is developed in which the above hypothesis is removed. In this last model, scattering phenomena are dealt with in a very simple “two-flux” way, so that analytical solutions are obtained again. The case here developed is the same as in part 1, namely slab geometry with orthogonal parallel irradiation, giving special reference to the important case of semi-infinite reactor thickness. Both cases of solid catalysts with single particle size and with distributed particle sizes are implicitly dealt with. The model here developed represents a limiting case as it overestimates the radiation lost from the reactor, giving rise to an opposite behavior with respect to the predictions given by the simpler model developed in part 1. Therefore, the real behavior of hetero...

Simplified Modeling of Radiant Fields in Heterogeneous Photoreactors. 1. Case of Zero Reflectance

A strongly simplified model is proposed for the estimation of the radiant flow field, and related quantities such as the local volumetric rate of energy absorption, in heterogeneous photoreactors. The model is based on the assumption that a photon carrying energy greater than that of the catalyst band gap, when interacting with a catalyst particle, is invariably absorbed. The model equations allow, for simple geometries, analytical integral solutions to be obtained. These have the advantage of giving an immediate grasp on how the main physical parameters affect the radiation field: an important feature especially for engineering purposes. The case developed here is that of a plane slab, for both monosized catalyst particles and particles with size distributions. Notably, as no adjustable parameters are used, fully predictive results can be obtained. The model predictions are successfully compared with the results of original experimental tests, therefore leading to model validation. In Part 2 of this paper, another model is developed in which the assumption of zero reflectance of the particles is removed, yet allowing analytical solutions to be obtained. One of the key features of the two models is that their predictions bound the real behavior of irradiated photoreactors, as shown in the papers. Therefore, although only approximate solutions are obtained from each of the two models, the real behavior can be better approximated by some sort of interpolation between the two solutions and, what is more important, confidence limits for the final solution are obtained.

Photocatalytic decomposition of chloroform in a fully irradiated heterogeneous photoreactor using titanium oxide particulate suspensions

The photocatalytic decomposition of HCCl 3 was studied in a fully irradiated photoreactor (FIP) by varying the concentrations of (a) chloroform, (b) dissolved oxygen and (c) titania, as well as (d) the local volumetric rate of energy absorption (LVREA). Fully irradiated conditions are achievable when all the particles along the light path ‘see the photons’. In order to compute the VREA (average value of the LVREA), the radiative transfer equation (RTE) was applied to the reactor and solved using a simplified radiation model. Model parameters, i.e., optical properties, as well as the boundary condition for the RTE were obtained experimentally. Initial rate data of the chloroform decomposition showed that, within the studied range of process variables and under fully irradiated conditions: (1) The reaction rate is of first order with respect to [HCCl 3], zero order with respect to [O 2] and about one-half-order with respect to the LVREA inside the reactor and (2): The effects due to catalyst concentration are fully accounted for within the LVREA dependence.

Nonisotropic Scattering Model for Estimation of Light Absorption Rates in a Suspension Culture of Coffea arabica Cells

AbstractA nonisotropic scattering model is proposed to estimate the light absorption rates in a heterogeneous photoreactor containing plant cells of Coffea arabica and is compared with an isotropic scattering model. In order to determine the directions of noniso‐tropically scattered light, optical theorems (geometrical optics and wave optics) are applied to light scattering caused by a homogeneous and perfect sphere. Two parameters, relative refractive index and attenuation coefficient, are used to characterize the ideal sphere. The primary feature of this model is that various phenomena of light scattering and absorption can be simulated by the combination of the two parameters. A measuring system composed of parallel plates was developed for evaluating the model. The nonisotropic scattering model was successfully applied to the heterogeneous reactor with the cultured plant cells.

Light Intensity Profiles in Heterogeneous Annular Photoreactors

Light Intensity Profiles in Heterogeneous Annular Photoreactors