Orlando M. Alfano

Reaction engineering of suspended solid heterogeneous photocatalytic reactors

Abstract A revision of the authors’ work on suspended solid photocatalytic reactors is presented. Thus, heterogeneous reactions in aqueous media, involving the presence of both fine particles of titanium dioxide and UV radiation are analyzed. Under these conditions, light scattering is a complex phenomenon that must be included in the description of the system performance. Suspended solid photocatalytic reactors are just a family of the well-known slurry reactors for which considerable progress has been reported in the reaction engineering literature. However, ab initio design methods for these particular — light activated — reactors are not known. Previous developed methods should be readily applicable if: (1) the photocatalytic activation is properly described, (2) an intrinsic kinetic expression can be developed, and (3) the scalar field of radiation intensities inside reaction spaces of different geometries can also be known. With this purpose, in order to model the radiation field and also the initiation step, the radiative transfer equation (RTE) for heterogeneous media must be solved. Solution of the RTE in photocatalytic reactors implies the knowledge of different system properties that are not usually known. New methods for obtaining this information are described and typical results are presented. Scale up procedures, derived from the application of first principles, cannot be applied if intrinsic reaction kinetic data are not available. A key point in developing these reaction rate expressions is the knowledge of the radiation absorption by the catalytic particles. The required model is also presented. Furthermore, a simple reactor design and special operating conditions for appropriate laboratory experiments were developed that can be used to produce this type of data reducing to a minimum the difficulties associated with the analysis of composition vs. time information. In this review, the photocatalytic oxidation of trichloroethylene is used as a model reaction. Finally, the description of the radiation field distribution in a typical flat plate, photocatalytic solar reactor simulator illustrates the method that should be used for scaling-up purposes. With this objective the photon absorption rate was obtained as a function of position and the developed model was verified with careful experimental measurements.

Radiation field modelling in photoreactors. I: Homogeneous media

Abstract The present work reports an analysis of the existing models for the description of the radiation field inside photochemical reactors for homogeneous systems. With this purpose incidence and emission models are critically analysed and the contributions of the main research groups are presented. The review classifies the results known up to the present in a methodological way and, as a logical consequence, projects those areas where further work is needed.

Degradation kinetics of 2,4-D in water employing hydrogen peroxide and UV radiation

This paper reports the photooxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution employing hydrogen peroxide and ultraviolet radiation. A kinetic model to represent the degradation of 2,4-D and that of equally toxic intermediate products, such as 2,4-dichlorophenol (DCP) and chlorohydroquinone (CHQ), is presented. The model includes the parallel, direct photolysis of 2,4-D and the most important reaction intermediates. The experimental work was performed in a batch, well-stirred tank reactor irradiated from its bottom using a low power, germicidal, tubular lamp placed at the focal axis of a cylindrical reflector of parabolic cross-section. Herbicide degradation initial rates twenty times faster than those obtained employing UV radiation alone were found. In order to reach more useful conclusions about the ability of the process to reduce the contamination to innocuous final products, simultaneous measurements of the total organic carbon (TOC) were performed. By application of the kinetic model to the whole set of concentration versus time experimental data, the values of the kinetic parameters were obtained. The model permits a good representation of the reaction evolution in a rather wide range of 2,4-D and H2O2 initial concentrations.

Absolute Quantum Yields In Photocatalytic Slurry Reactors

Photocatalysis in slurry reactors have the particular characteristic that the reacting medium not only absorbs but also scatters photons and both phenomena occurs simultaneously. When this is the case, typical methods used in classical photochemistry such as homogeneous actinometry are useless because they cannot take into account radiation scattering. Consequently present methods in use always underestimate quantum yields calculations. A method has been developed to produce absolute values of photocatalytic quantum yields in slurry reacting systems employing titanium dioxide and near UV radiation. The new procedure resorts to the rigorous solution of the radiative transfer equation inside the reaction space in order to obtain the true value of the photonic absorption rate. Employing monochromatic light, values for the photocatalytic decomposition of phenol and 1,4-dioxane are reported under precisely defined conditions. The method can be used to compare reactivities of different catalysts or, for a given catalyst, reactivities with different compounds regardless the reactor shape, size or configuration.

Rigorous model and experimental verification of the radiation field in a flat-plate solar collector simulator employed for photocatalytic reactions

Abstract In any kinetically controlled photocatalytic process the catalyst activation is always a photochemical act that depends upon the local value of the volumetric rate of energy absorption (LVREA). In a heterogeneous photocatalytic reaction, a precise evaluation of the LVREA can be obtained when the spatial and directional distributions of radiation intensities are known. With this purpose, a mathematical model of the radiation field inside a flat-plate heterogeneous reactor (a solar simulator) has been developed. The solid–liquid reactor is irradiated by two tubular UV lamps with the aid of two parabolic reflectors. Since titanium dioxide suspensions absorb and scatter radiation the model accounts for both effects. Resorting to information about the lamp and reflector characteristics, the catalyst optical properties and concentration, as well as the reactor dimensions and wall reactor properties, the solution of the mathematical model (a two-dimensional–two-directional model) provides a detailed description of the spatial and angular directional distributions of radiation intensities inside the reactor. Using this information, it is possible to precisely calculate the rate of absorbed radiation energy at each point inside the reactor. This is one of the key variables for reactor design and/or scale-up purposes. The radiation distribution inside the reactor was verified by computing forwardly transmitted and backwardly scattered radiation fluxes coming out of the reaction space through the glass reactor walls. These radiation fluxes were compared with experimental measurements made with a UV radiometer and good agreement was obtained; when no fouling was present and considering low catalyst loading to obtain measurable radiation fluxes, the maximum observed error was 12%. The predicted inlet boundary condition was also verified with actinometry and the error was smaller than 14% For this reactor configuration, when the radiation absorption performance is the only factor under consideration, it was found that Aldrich titania is more efficient than the Degussa P 25 variety.

Modeling of light scattering in photochemical reactors

Abstract Heterogeneous photochemical reactions in fluid (water or air) environments have been gaining increasing interests in the past fifteen years, the main application being in pollution abatement by solid semiconductor photocatalyzed systems. One of the unsolved problems is the proper evaluation of the rate of absorbed radiation energy by the participating medium due to the existence of simultaneous absorption and scattering. This is a key property in quantifying kinetic models and designing photoreactors. A model derived from radiative transport fundamentals and chemical reactor engineering concepts is presented. It permits a rigorous quantification of the local volumetric rate of radiant energy absorption (LVREA) inside the reactor. The method was experimentally verified by analyzing the scattering effects (performance changes) produced by addition of transparent, inert particles to a homogeneous photochemical reacting system. With the validated model, and employing radiation absorbing particles (titanium dioxide), the proposed approach was compared with other methods currently in use to evaluate the LVREA. It was shown, in a quantitative way, that the proper application of the radiative transfer equation is a requisite for obtaining accurate results. Finally, the results of changing particle concentration and particle size on the magnitude of the scattered light were explored.

A cylindrical photoreactor irradiated from the bottom—I. Radiation flux density generated by a tubular source and a parabolic reflector

Abstract The present work studies the radiant energy field generated by a system made up of an ultraviolet radiation source located at the focal axis of a cylindrical reflector of parabolic cross-section. This system allows us to irradiate a cylindrical photoreactor from the bottom, avoiding the introduction of the source in the reaction space. The equations governing the energy transfer were formulated and solved numerically; to do so, three emission models were applied: the line source model with emission in parallel planes, the line source model with spherical emission, and the extense source model with volumetric emission. The behaviour of each one of these models was comparatively analysed to establish their ability to predict the radiant energy flux density within the reacting space of the photoreactor. A very simple experimental check of the model predictions showed very good agreement only when compared with those of the extense source model with volumetric emission.

A cylindrical photoreactor irradiated from the bottom—II. Models for the local volumetric rate of energy absorption with polychromatic radiation and their evaluation

Abstract This study of the radiation field generated in a cylindrical photoreactor irradiated from the bottom presents the theoretical foundations of a method for the experimental verification of three different radiation models. The expressions representing the local volumetric rate of energy absorption (LVREA) were formulated and applied to the prediction of the rate of an actinometric reaction. This reaction takes place inside a microreactor operated in a batch recycling system with polychromatic radiation. The values obtained portray the same behaviour as that of the energy densities calculated previously (Part I), thus becoming a valid, accurate method for the experimental measurement of the absolute values of the radiation field that are sought after (volumetric rate of energy absorption). The proposed approach is able to produce quasi-point values of the absolute values of the VREA at the microreactor as an excellent approximation to the absolute values of the LVREA (local measurements). The present work also points out the qualitative and quantitative discrepancies of the results predicted by the line models when compared with those of the extense source model with volumetric emission.

Modeling of radiation absorption in a flat plate photocatalytic reactor

The radiation field inside a flat-plate photocatalytic reactor has been modeled. The solid-liquid heterogeneous reactor is irradiated by two tubular lamps and two parabolic reflectors. The model accounts for absorption and scattering effects by the solid suspension. From the lamp and reflector characteristics, the catalyst properties and concentration, and the reactor dimensions, the model provides a detailed description of the spatial and angular direction distributions of radiation intensities inside the reactor. These results permit to calculate at each point of the reactor the local value of the volumetric rate of radiation energy absorption (LVREA), a property that is always required to formulate the initiation rate of any photocatalytic process.

A methodology for modeling photocatalytic reactors for indoor pollution control using previously estimated kinetic parameters.

A methodology for modeling photocatalytic reactors for their application in indoor air pollution control is carried out. The methodology implies, firstly, the determination of intrinsic reaction kinetics for the removal of formaldehyde. This is achieved by means of a simple geometry, continuous reactor operating under kinetic control regime and steady state. The kinetic parameters were estimated from experimental data by means of a nonlinear optimization algorithm. The second step was the application of the obtained kinetic parameters to a very different photoreactor configuration. In this case, the reactor is a corrugated wall type using nanosize TiO(2) as catalyst irradiated by UV lamps that provided a spatially uniform radiation field. The radiative transfer within the reactor was modeled through a superficial emission model for the lamps, the ray tracing method and the computation of view factors. The velocity and concentration fields were evaluated by means of a commercial CFD tool (Fluent 12) where the radiation model was introduced externally. The results of the model were compared experimentally in a corrugated wall, bench scale reactor constructed in the laboratory. The overall pollutant conversion showed good agreement between model predictions and experiments, with a root mean square error less than 4%.