Quasi-isoactinic Reactor for Photocatalytic Kinetics Studies Alberto Brucato,* Franco Grisafi, Lucio Rizzuti, Antonino Sclafani, and Giuseppa Vella UniVersita ` di Palermo, Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Viale delle Scienze, Ed.6 - 90128 Palermo, Italy 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. 1. Introduction Photochemical and photocatalytic reactors are usually oper- ated under conditions such that reagent and product concentra- tions, temperature, and especially radiation field distributions are not uniform. 1-7 Through vigorous agitation and/or intense recirculation, a mixing degree could be reached that is high enough to make concentration and temperature inconsistencies negligible. However, there is no way to make the radiation intensity uniform by means of agitation, as photon paths are not affected by fluid motion. Moreover, photocatalytic reactions are often carried out under conditions of almost total absorption of the incident radiation, in order to obtain high values of the reaction rate. Under such conditions, local radiation intensity values typically vary by several orders of magnitude from point to point inside the photoreactor, making the kinetic interpretation of results troublesome. By acting on the geometry of the photon source/photoreactor system, it is possible to make all points inside the reactor be characterized by view factors of the photon source not too different from each other. Even then, however, it is not possible to remove the effects of radiation attenuation caused by the progressive absorption and scattering of the radiation by the chemical species and catalyst particles present. In particular, moving toward the inside of the reactor volume, away from the radiation inlet section, the radiation field intensity, and consequently the local volumetric rate of photon absorption (LVRPA), which is a very important parameter needed to link radiation distribution to local reaction kinetics, 8 always decreases typically to extinction. Clearly, it would be convenient to perform kinetic investiga- tions using photoreactors characterized by a radiant field intensity that is as uniform as possible. This goal could be reached by letting only a minimal fraction of incident radiation be absorbed in the reactor, for instance, by conveniently decreasing the volumetric concentration of the catalyst particles. However, under such conditions, reaction rates would be so small as to give rise to experimental difficulties. The aim of this work is that of setting up the conditions of an “iso-actinic” photoreactor, that is, a photoreactor in which the radiation field, and thus the LVRPA, can be considered uniform to a good extent, without dramatically decreasing the catalyst concentration. The first step is that of designing a photoreactor whose geometry is suitable for this purpose. 2. Quasi-Isoactinic Photorector The geometric arrangement proposed is akin to that described by Martin et al. 9 for the case of a single-phase photochemical reactor, i.e., a simple plane slab irradiated from both sides, as depicted in Figure 1. In contrast to the Martin et al. 9 arrange- ment, in the present case, the slab thickness is assumed to be very small in comparison to the other two dimensions in order to make side end effects small. Under such conditions, es- sentially a semi-infinite slab reactor is assumed. If the reactor were irradiated from one side only (e.g., only G′ 0 is present), the radiation intensity would decay with distance from that wall according to a law depending on the absorbing and scattering properties of the medium. This would, in turn, result in a decaying LVRPA that would attain its minimum at the nonir- radiated reactor wall (curve LVRPA′ in Figure 1). Clearly, if the reactor were irradiated from the other side only, then for symmetry reasons, an identical but reversed LVRPA would have been obtained (curve LVRPA′′ in Figure 1). Through superposi- tion of the effects, when both walls are irradiated, the LVRPA at each point inside the reactor is given by the sum of the two single irradiation values, resulting in curve LVRPA total of Figure 1, which shows much smaller variations than the two single irradiation curves. It might be worth remarking that, under single-side irradiation, significant uniformity of LVRPA can be obtained if radiation intensity is allowed to undergo only small variations from the front to the rear wall. This condition can be met if the medium has a small optical thickness, i.e., at very low catalyst loadings. As already remarked, in this case, the LVRPA would be quite uniform throughout the reactor but at the expense of lower values of photon absorption rate and, in turn, low reaction rates and consequent difficulties in the experimental assessment of reactor performance. In contrast, with double-side irradiation, a significant portion of the entering radiation can be usefully absorbed, thus achieving good reactor performance assessment yet maintaining good uniformity of LVRPA. The degree of uniformity attained for given conditions (reactor depth, catalyst albedo, particle size, and concentration) cannot be experimentally assessed, as this would require almost unfeasible local measurements of the LVRPA in the central part of the reactor volume. However, if a reliable radiation field model is available, such a model can be used for calculating * To whom correspondence should be addressed. E-mail: abrucato@ dicpm.unipa.it. Tel.: +390916567216. Fax: +390916567280. 7684 Ind. Eng. Chem. Res. 2007, 46, 7684-7690 10.1021/ie0703991 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/06/2007