Volumetric Rate Of Photon Absorption Research Articles

Kinetic study of the photo-Fenton degradation of formic acid: Combined effects of temperature and iron concentration

This study is focused on the kinetic modelling of the Fenton and photo-Fenton degradation of a model pollutant (formic acid) in aqueous solution, for relatively low iron concentrations (1–9 ppm). The reaction rate expressions are derived from an accepted reaction mechanism and explicitly having into account the local volumetric rate of photon absorption. The experimental work was performed in a well-stirred tank laboratory reactor irradiated from the bottom. Afterward, the proposed kinetic model and the experimental data were used to estimate the Arrhenius parameters between 20 and 55 °C, applying a non-linear regression procedure. To avoid the precipitation of iron compounds during the experimental runs, simultaneous high reaction temperatures (55 °C) and iron concentrations (9 ppm) were prevented. To achieve this goal, an experimental design based on the D-optimality criterion was adopted. The proposed kinetic model was able to reproduce the combined effects of changing the ferric iron concentrations, reaction temperatures, and formic acid to hydrogen peroxide molar ratios on the pollutant degradation rate. Kinetic model predictions are compared with experimental data of the organic compound conversion, and a good agreement is obtained. For the whole set of Fenton and photo-Fenton experimental runs, the maximum root mean square error is 7.64%.

Scaling-up of slurry reactors for the photocatalytic oxidation of cyanide with TiO 2 and silica-supported TiO 2 suspensions

A scaling-up methodology for the design of large-scale slurry reactors for the photocatalytic oxidation of cyanide is proposed. The only experimental information required to be determined at laboratory-scale is the intrinsic kinetics that describes the explicit dependence of the reaction rate with the local volumetric rate of photon absorption (LVRPA). Based on the kinetic model and the information about the geometry, irradiation source and the cyanide and catalyst concentrations as operation conditions, the performance of a larger scale reactor has been simulated following a predictive procedure with no adjustable parameters. The validation of the method has been carried out in a bench-scale reactor with ten times higher irradiated volume and a different geometry and irradiation source, in order to ensure that the conclusions about the applicability of the scaling-up model are independent of these parameters. The proposed scaling-up methodology and their correspondent procedures for the evaluation of the LVRPA distribution on the photoreactors has been successfully validated both with commercial TiO 2 and a silica-supported TiO 2 synthesized in our laboratory. The normalized root mean square error in the verification of the conversions predicted by the model for the larger scale reactor when compared with the experimental data are 7.7% and 6.2% for TiO 2 and TiO 2/SiO 2, respectively.

Intrinsic kinetic modeling with explicit radiation absorption effects of the photocatalytic oxidation of cyanide with TiO2 and silica-supported TiO2 suspensions

This study is focused on the kinetic modeling of the photocatalytic oxidation of cyanide in slurry reactors. The developed model is based on an accepted reaction mechanism and explicitly takes into account the differences in the local volumetric rate of photon absorption (LVRPA) produced by the unavoidable radiation profiles existing in the photoreactor. The model and its correspondent procedures for the evaluation of the LVRPA distribution, and the estimation of the kinetic parameters have been successfully validated with both powder TiO2 and TiO2/SiO2 photocatalysts with improved recovery properties. In both cases, the model reproduces the influence of the catalyst loading, the initial cyanide concentration, and the inlet radiation flux on the reaction rate, with errors below 5%. The kinetic parameters estimated for the model are independent of the irradiation form, as well as the reactor size and its geometrical configuration, providing the necessary information for scaling-up and designing commercial scale photoreactors.

Determination of Quantum Yield in a Heterogeneous Photocatalytic System Using a Fitting-Parameters Model

AbstractGlobal quantum yield and overall volumetric rate of photon absorption (OVRPA) were quantified for TiO

Selective Photocatalytic Oxidation of 4-Methoxybenzyl Alcohol to p-Anisaldehyde in Organic-Free Water in a Continuous Annular Fixed Bed Reactor

Photocatalytic oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde was performed in water organic-free solutions by using a fixed bed continuous photoreactor containing Pyrex beads on which a TiO2 home prepared photocatalyst was supported. The influence of liquid flow rate, inlet alcohol concentration and catalyst amount on the photoprocess was studied. The highest selectivity to p-anisaldehyde was about 47% being CO2, the other main oxidation product; traces of 4-methoxybenzoic acid were also detected. The radiation field inside the photoreactor has been modelled by applying the Monte Carlo method thus allowing the determination of the local volumetric rate of photon absorption (LVRPA). It was found that the radiation intensity profile sharply decreases inside the bed so that an important aliquot of the bed is not active for the photoreaction occurrence. This finding indicates that the reactivity results, obtained by measuring the concentration values of reagents and products at the exit of photoreactor, can not be used jointly with the radiation modelling ones for determining the dependence of reaction kinetics on light intensity. The Langmuir-Hinshelwood approach has been satisfactorily applied for modelling the photoreactivity results and the values of all the model parameters have been determined.

Quasi-isoactinic Reactor for Photocatalytic Kinetics Studies

Photochemical reactors characterized by almost uniform values of the local volumetric rate of photon absorption (LVRPA), i.e., quasi-isoactinic photoreactors, are particularly suitable for assessing the influence of radiant field intensity in kinetic studies. In this work, Monte Carlo simulations have been performed to obtain LVRPA values in a flat photoreactor irradiated on both sides. This configuration appears to be particularly suitable for obtaining quasi-isoactinic conditions. The influence of catalyst albedo and scattering phase function is assessed, and the conditions for obtaining iso-actinicity are discussed. Finally, these conditions are related to an easy-to-measure parameter, namely, the photoreactor fractional transmittance.

Photocatalytic oxidation of multicomponent systems of herbicides: Scale-up of laboratory kinetics rate data to plant scale

The reaction kinetics of the photocatalytic oxidation (PCO) of the herbicides isoproturon, simazine and propazine over irradiated TiO 2 (Degussa P25) suspensions were studied in multicomponent systems in a laboratory-scale annular photoreactor operated in recirculation batch mode. The multicomponent kinetic model established was then extended to take into account the direct effect of radiation absorbed by the TiO 2 photocatalyst in order to obtain intrinsic kinetic parameters independent of the radiation field in the photoreactor. An analysis of the radiation field in the photoreaction space was accomplished using a recently published Six-Flux Radiation Absorption-Scattering model (SFM) to decouple the values of the apparent reaction kinetic constants from the local volumetric rate of photon absorption (LVRPA) in the rate law of PCO of herbicide. The resulting rate laws with explicit dependence on the LVRPA were then used to predict, through accurate reactor modeling, the degradation of the herbicides in an optimal configuration of a flow-through, pilot-scale, falling film photoreactor. In a plant treatment scenario the total operating costs of the PCO of herbicides were calculated to be 3.75 Euro m −3.

Photocatalytic oxidation of multicomponent solutions of herbicides: Reaction kinetics analysis with explicit photon absorption effects

The intrinsic reaction kinetics of the photocatalytic oxidation (PCO) of the herbicides isoproturon, simazine and propazine over irradiated TiO2 (Degussa P25) suspensions was studied in single-component and in multicomponent systems. Experiments were carried out at different photon fluxes and different herbicide concentrations in the range below 1mgL−1. The results were analyzed in terms of a Langmuir–Hinshelwood (L–H) kinetic model previously established for multicomponent solutions of herbicides, but including the explicit effect of photon absorption. This was accomplished by modeling the radiation field in the reactor with the Six-Flux Absorption-Scattering Model (SFM) (i.e., scattered photons follow the route of the six directions of the Cartesian coordinates) using optical parameters averaged across the spectrum of the incident radiation. The intrinsic reaction kinetic constants of isoproturon, simazine and propazine independent of the radiation field in the reactor were determined. The oxidation rate of the herbicides was found to follow half-order dependence with respect to the local volumetric rate of photon absorption (LVRPA), in the range of photon fluxes investigated. The established model was found to be appropriate to predict the time-dependent degradation profiles of the herbicides in single- and multicomponent systems not only at all radiation intensities investigated in this study but also when one or two herbicides were present in excess of the others in the mixture. Using this simple approach, intrinsic kinetic data can be obtained.

Fluidized bed photoreactors using composites of titania CVD-coated onto quartz sand as photocatalyst: Assessment of photochemical efficiency

Abstract The photochemical efficiency of a fluidized bed (FB) of quartz sand coated with TiO 2 by plasma-CVD, was investigated using a fully illuminated, parallel planar photoreactor. The local volumetric rate of photon absorption (LVRPA) of the device was obtained by the combined use of experimental measurements and computer modeling of the radiation field. Because of the anisotropy of the FB in the vertical direction, due to particle segregation, and the strong absorption of radiation of the photocatalyst particles across the photoreactor, a two-dimensional modeling (2D) was developed. To accomplish these goals, the relevant specific volumetric optical parameters (namely, the spectral absorption and scattering coefficient) of FBs of the TiO 2 -coated quartz catalytic composite were used. To take into account said vertical anisotropy of the fluidized bed, a functional dependency between the spectral optical coefficients and bed height was derived in the present work, by assuming a completely segregated FB, and experimentally validated through transmittance measurements. Finally, the evaluation of the photochemical efficiency of the FB in the decomposition of oxalic acid, a well-known model reactant, revealed that this type of devices might outperform the conventional slurry systems, as well as dispense of downstream separation steps.

Photocatalytic intrinsic reaction kinetics I: Mineralization of dichloroacetic acid

The intrinsic reaction kinetics of the photocatalytic decomposition of dichloroacetic acid (DCA) at different, but constant oxygen concentrations was studied. Other significant variables were initial DCA concentration, catalyst (titanium dioxide) concentration and light intensities. The results of the kinetic measurements were analyzed using a complete mathematical model of the experimental device that was used for the laboratory operation (a differential reactor inside a recycle). In this way it was expected to obtain intrinsic kinetic parameters. The kinetic model, that was developed from a proposed complete reaction sequence, has only two lumped specific kinetic constants and represents the dependence of the reaction rate with oxygen, DCA and catalyst concentrations as well as the local volumetric rate of photon absorption. Simulation results are in good agreement with experiments and, at the same time, permit a detailed description of the existing radiation field in the experimental reactor.

Photocatalytic reactors: Reaction kinetics in a flat plate solar simulator

The kinetics of the photocatalytic decomposition of low concentrations of trichloroethylene (TCE) in water was modeled and the reaction parameters have been evaluated for different catalyst loadings. The employed reactor is a flat plate configuration irradiated by tubular lamps that have emission in the 300–400 nm wavelength range. The mass conservation model is two-dimensional while the developed radiation model is two-dimensional in space and two directional in radiation propagation. The performance of the photoreactor with this reaction can be properly represented employing only two lumped kinetic constants that can be derived from a 12 steps, complete reaction sequence. The deduced kinetic model has explicit functional dependencies for the local volumetric rate of photon absorption (LVRPA) and the effect of the catalyst concentration: r TCE, het ( x ̄ ,t)a v =−S g C m, cat α ∫ λ e λ a ( x ̄ ,t,C m,cat ) d λ [α 3C TCE /(1+α 3C TCE )] . Values of the kinetic constants are: α=1.94×10 −9 mol g 1/2 cm −2 s −1/2 einstein −1/2 and α 3=5.52×10 6 cm 3 mol −1. As derived from the reaction sequence and validated with experiments, it was observed that the reaction rate is proportional to the square root of the LVRPA. The dependence on the catalyst loading, well described by the model, is more complex due to its characteristic effect on the light distribution inside the reaction space.

Evaluation of Radiation Absorption in Slurry Photocatalytic Reactors. 1. Assessment of Methods in Use and New Proposal

Photocatalytic reactions are the result of a light-activated process by which an appropriate semiconductor can generate electrons and holes that, afterward, can participate in oxidative−reductive reactions. One of the most important applications of these processes consists of the use of these catalytic systems for oxidizing pollutants contained in water and air systems. In these photocatalytic reactions, the initiation step is always a function of the local volumetric rate of photon absorption (LVRPA). Many of these reactions are carried out in water environments where the catalyst is a suspension of small size, solid particles. Then the system is heterogeneous, and evaluation of the light distribution becomes difficult due to the concomitant presence of radiation absorption and scattering. In this paper, we present a general theoretical frame for analyzing the different methods that have been proposed to evaluate the LVRPA. Using this approach, one can have a precise knowledge and evaluation of the assum...

Evaluation of Radiation Absorption in Slurry Photocatalytic Reactors. 2. Experimental Verification of the Proposed Method

The volume-averaged volumetric rate of photon absorption (VRPA) in a photocatalytic reactor has been experimentally determined by measuring the radiative fluxes coming into and going out of the reaction space through the glass walls of a reactor. These radiative fluxes were obtained from experimental measurements made with precisely calibrated UV detectors. These values of the volume-averaged VRPA were used to decide the validity of theoretical calculations obtained from the rigorous application of the radiative transfer equation. Afterward, the reaction kinetic dependence with respect to the local volumetric rate of photon absorption (LVRPA) and its effect on the quantum yield evaluation were analyzed. To do this, the linear and square root reaction rate dependencies with the LVRPA were considered. Employing a titanium dioxide suspension, it has been demonstrated that (i) failing to properly account for radiation scattering in suspended solid photocatalytic reactors will normally lead to significant errors in the evaluation of the true absorbed light and (ii) point values of the photon absorption rate are necessary in order to represent the true kinetics of the photocatalytic reaction. The reason being the existence of very strong spatial nonuniformities in the above-mentioned rates.