Photocatalytic Oxidation Reactor Research Articles

Photocatalytic destruction of VOCs for in-vehicle air cleaning

In order to reduce vehicle occupants’ exposures to aromatic volatile organic compounds (VOCs), it is necessary to develop control strategies for in-vehicle VOCs. This study evaluated the technical feasibility of the application of TiO 2 photocatalysis for the removal of VOCs present in air at low parts per billion (ppb) concentrations commonly associated with in-vehicle air quality issues. The photocatalytic removal of five target VOCs was investigated: benzene, ethyl benzene, and o-, m-, p-xylenes. Variables tested for the current study included relative humidity (RH), hydraulic diameter (HD) and flow rate (FW). The fixed parameters included contaminant concentration, UV light source, photocatalytic oxidation (PCO) reactor material and the weight of TiO 2. Under the experimental conditions, the PCO destruction efficiencies were close to 100% for four different RH ranges that cover typical ambient air humidity ranges. The efficiency of PCO was dependent on the HD of the reactor and the stream FW. Some carbon monoxide was formed as a by-product in the oxidation albeit at low levels.

First-principles modeling, scaling laws and design of structured photocatalytic oxidation reactors for air purification

First-principles, predictive engineering models provide a sound theoretical basis for quantifying the inherent light energy utilization capabilities and performance limitations of candidate commercial photocatalytic oxidation reactor configurations. In particular, these models provide insight into the similarities and differences between photoreactors based on structured honeycombed monoliths, and those based on reticulated foams or other random catalyst supports.For honeycombed monoliths, a deterministic first-principles radiation field model provides the channel wall light intensity profile down the length of a single channel in the monolith. A three-dimensional developing flow convection–diffusion reaction model employing this radiation field submodel predicts the velocity and concentration fields. The model shows that light intensity gradients in a monolith of typical dimensions are severe, that only a fraction of the monolith can be effectively photo-activated, and as a consequence process performance is largely controlled by light distribution. For a given light source and photocatalyst combination, reactor performance scales according to the aspect ratio of the channeled monolith, the Reynolds number, and the Dahmköhler number.For randomly structured monoliths, the radiation field must be determined by probablistic methods. Monte Carlo simulations show that the radiation field in such random porous structure scales according to the pore size distribution and the void fraction, and the photocatalyst film thickness. Reactor performance scales by the radiation field, the Peclet number, the Stanton number, and the Dahmköhler number. The complex interrelationship between the random structure of the monolith and the resulting radiation field and mass transfer behavior makes scaling of these reactor types particularly difficult.

Photocatalytic oxidation of xenobiotics in water with immobilized TIO2 on agitator

A novel photocatalytic oxidation reactor, using Degussa P‐25 TiO2 as a stationary phase with a thickness of 1.5–2.0 um on the blades of agitator, was developed to study the photocatalytic oxidation of xenobiotics. Particularly in this device, separation of photocatalyst from the purified water after oxidation reaction was not necessary, and no other aeration equipment was required to supply oxygen. To examine the efficiency of this device, photocatalytic degradation of xenobiotic organics such as carbofuran was studied as an example. Results indicated that carbofuran could be degraded completely with mineralization efficiency of 20 % after 6 hours of oxidation under the imposed conditions. The mineralization rate of carbofuran was found to follow the pseudo‐first order reaction kinetics. Moreover, the rate constant of mineralization was found to be proportional to TiO2 film area and the square root of UV light intensity. These results implied the mineralization efficiency of carbofuran could be improved through increasing TiO2 film area and UV light intensity. Accordingly, this novel device showed potential application for degrading xenobiotics in water.

Three‐dimensional developing flow model for photocatalytic monolith reactors

AbstractA first‐principles mathematical model describes performance of a titania‐coated honeycomb monolith photocatalytic oxidation (PCO) reactor for air purification. The single‐channel, 3‐D convection–diffusion–reaction model assumes steady‐state operation, negligible axial dispersion, and negligible homogeneous reaction. The reactor model accounts rigorously for entrance effects arising from the developing fluid‐flow field and uses a previously developed first‐principles radiation‐field submodel for the UV flux profile down the monolith length. The model requires specification of an intrinsic photocatalytic reaction rate dependent on local UV light intensity and local reactant concentration, and uses reaction‐rate expressions and kinetic parameters determined independently using a flat‐plate reactor. Model predictions matched experimental pilot‐scale formaldehyde conversion measurements for a range of inlet formaldehyde concentrations, air humidity levels, monolith lengths, and for various monolith/lamp‐bank configurations. This agreement was realized without benefit of any adjustable photocatalytic reactor model parameters, radiation‐field submodel parameters, or kinetic submodel parameters. The model tends to systematically overpredict toluene conversion data by about 33%, which falls within the accepted limits of experimental kinetic parameter accuracy. With further validation, the model could be used in PCO reactor design and to develop quantitative energy utilization metrics.

Investigation of the Photocatalytic Oxidation of Low-Level Carbonyl Compounds

ABSTRACT Though the bulk of research involving photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) has involved the remediation of pollutants at high inlet concentrations, there has been some implication that PCO can be used to reduce exposure to low concentrations of VOCs and improve the quality of indoor air. The high conversion rates previously reported for PCO of VOCs in the parts-per-million (ppm) range may not, however, be applicable to concentrations in the parts-per-billion (ppb) range that are more typical of indoor air quality (IAQ) issues. This paper reports on an examination of the operational characteristics of four PCO reactor designs used for the oxidation of VOCs in the ppb concentration range. Reactor efficiency is examined for three low molecular weight carbonyl compounds commonly associated with IAQ issues: formaldehyde, acetaldehyde, and acetone. The measured response is the destruction of carbonyl reac-tants. Variables include flow stream velocity and reactor residence time. Oxidation of carbonyl compounds is also examined as a function of ultraviolet (UV) radiation intensity. PCO of the formaldehyde and acetone was nearly 100% for all reactor designs. To assist comparison of the reactor designs, oxidation efficiency of trichloroethylene (TCE) was evaluated at inlet concentration in the parts-per-million by volume (ppmv) range.

Performance Evaluation of a Titania-coated Reticulated Foam Photocatalytic Oxidation Reactor

AbstractWe have constructed and evaluated the performance of a gas-solid annular photocatalytic oxidation reactor that incorporates an alumina reticulated foam monolith element as a titania catalyst support. The reticulate occupies the annular region formed by the lamp and the outer reactor tube. Three alumina reticulates possessing different mean pore sizes and pore densities, designated as 10, 20 and 30 pores-per-inch (PPI) were employed. The reticulates were wash-coated with Degussa P25 titania. Light transmittance measurements through the reticulates showed that 30 PPI reticulate, the least optically dense element, captures the most light. Photocatalytic oxidation of dilute isopropanol (IPA) in air was chosen as the probe reaction to assess the performance of each reticulate- catalyst coating configuration. Process parameters varied were inlet IPA concentration, incident UV intensity, and mean gas residence time. For similar catalyst loadings, the 30 PPI reticulate outperformed both the 20 and 10 PPI reticulates, while the 20 PPI reticulate modestly outperformed the 10 PPI reticulate. With increasing titania coating yielding thicker titania films, general performance increases until a critical thickness is achieved. Light intensity measurements and simple forward UV intensity profile calculations reveal that reticulate performance is largely governed by the UV intensity profile. For a given reticulate, results suggest that the optimum thickness of the TiO

Two‐flux radiation‐field model for an annular packed‐bed photocatalytic oxidation reactor

AbstractOne of the remaining challenges in application of heterogeneous photocatalysis for treatment of air streams containing dilute VOCs is to design a cost‐effective photocatalytic reactor that simultaneously allows efficient contact of the contaminated air and solid catalyst while uniformly irradiating the solid catalyst with light. A pseudohomogeneous model was developed to study effects of system parameters on process performance for a gas–solid lamp‐in‐tube annular‐photocatalytic‐oxidation (PCO) reactor in which the annular space is filled with photocatalyst‐coated packing. In this model the flow field is assumed to be uniform and radial diffusion negligible. Homogeneous reactions are neglected. Heterogeneous reaction rates follow Langmuir–Hinshelwood–Houghen–Watson kinetics with rate parameters extracted from independent experiments. A 1:D “two‐flux” incidence submodel is used to account for the radial UV light distribution throughout the reactor annular space. This submodel requires knowledge of the UV lamp radiant emittance, the optical characterstics of the catalytic thin‐film coating, and the UV irradiance at the outer wall of the reactor and contains only a single adjustable parameter—the mean free path between photon–catalyst interactions. The model was validated with experimental performance measurements for destruction of acetone and isopropyl alcohol in a bench‐scale photoreactor. The validated model can be used to predict the optimum catalyst film thickness for given reactor dimensions, packing shape and size, and VOC abatement problem.

Selective photocatalytic destruction of airborne VOCs

This article describes a photocatalytic method for selective oxidation of the airborne nitroglycerine (NG), in the presence of ethanol and acetone vapors at high concentrations (exceeding 1% by volume). Process selectivities toward NG photooxidation were examined for various photoreactor configurations and UV lamps and techniques used for the photocatalyst preparation. In addition, we have studied the effect of temperature, residence time, various additives (e.g. ozone, water vapor, nitrogen), and initial concentration of the solvents in air. Our data indicate that modifying TiO 2 with silico-tungstic acid (STA) catalyst results in selective NG oxidation without affecting ethanol and acetone significantly. Platinization of TiO 2 showed an adverse effect on NG destruction selectivity. In general, low residence times and initial concentration of oxidants and high temperatures and initial concentration of solvents favor selectivity toward NG destruction. In the case of temperature, our observation can be explained by the temperature dependent gas diffusion and surface processes. In most cases, the yield of ethanol oxidation was generally higher than that of acetone. Results from the bench-scale experiments using artificial UV light sources were used to build and test a solar photocatalytic oxidation reactor for selective NG treatment in the presence of ethanol and acetone.

Performance of Nonconcentrating Solar Photocatalytic Oxidation Reactors: Part II—Shallow Pond Configuration

A solar photocatalytic oxidation facility has been fabricated in which the destruction of 4-chlorophenol (4CP) is tested in three adjacent shallow pond reactors. Each of the reactors has depths of 5.1, 10.2, and 15.3 cm (2, 4, and 6 in.), respectively. It is found that 4CP is successfully oxidized with the photocatalyst, titanium dioxide (TiO2), suspended in a slurry or adhered to a fiberglass mesh. The pond reactors, however, perform better with the slurry. It has also been found that the first-order rate constant for oxidation of 4CP increases with decreasing initial concentration. For the same incident ultraviolet (UV) intensity, catalyst loading, and initial solute concentration, the oxidation rate of 4CP is invariant provided the aperture to volume ratio is fixed. It has been determined that the 4CP solution contains sufficient dissolved oxygen to support the photocatalytic oxidation process. Direct evidence is provided to demonstrate that the utilization of photons in the photocatalytic process becomes less efficient as the number of incident photons on the catalyst increases.