TiO 2 absorption and scattering coefficients using Monte Carlo method and macroscopic balances in a photo-CREC unit J. Moreira a , B. Serrano b , A. Ortiz a , H. de Lasa a,n a Faculty of Engineering Science, Chemical Reactor Engineering Centre, University of Western Ontario, London Ontario, Canada N6A 5B9 b Facultad de Ciencias Quı ´micas, Programa de Ingenierı ´a Quı ´mica, Universidad Auto ´noma de Zacatecas, Me´xico article info Article history: Received 14 January 2011 Received in revised form 1 July 2011 Accepted 26 July 2011 Available online 10 August 2011 Keywords: Photocatalysis Monte Carlo Absorption Scattering Radiation Transfer abstract The radiation field inside photocatalytic reactors can be predicted by solving the Radiative Transfer Equation (RTE). From the solution of the RTE, the Local Volumetric Rate of Energy Absorption (LVREA) can also be obtained. This LVREA is an important parameter in photocatalytic reactor design, energy efficiency assessments and kinetic studies of photocatalytic reactions. However, when solving the RTE, two optical parameters are needed: (1) the absorption and scattering coefficients and (2) the phase function. In the present study, the Monte Carlo (MC) method along with an optimization technique is shown to be effective in predicting the wavelength-averaged absorption and scattering coefficients for three different TiO 2 powders. To accomplish this, the LVREA and the transmitted radiation (P t ) in a Photo-CREC annular photoreactor have to be determined using a macroscopic balance. The optimized coefficients are calculated ensuring that they comply with a number of physical constrains, falling in between bounds established via independent criteria. The optimization technique is demonstrated by finding the absorption and scattering coefficients for three different semiconductors that best fit the experimental values from the macroscopic balance minimizing the least-squared error of objective functions for the LVREA and P t . The proposed approach is a general and promising one, not being restricted to reactors of concentric geometry, specific semiconductors and/or particular photocatalytic reactor unit scale. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Photocatalysis has proven to be a viable alternative for water and air decontamination when dealing with bio-recalcitrant compounds (de Lasa et al., 2005). This technology is also used in the development of indoor antibacterial and self-cleaning surfaces (Fujishima and Zhang, 2006). To determine the feasibility of photocatalysis in water, many specific contaminant classes of interest such as aromatic compounds, organochlorine compounds and sulphur based organic compounds have been tested on a laboratory scale and in the field (Vidal, 1998). Photocatalytic processes make use of a semiconductor metal oxide as a catalyst that after being radiated with light of sufficient energy promotes an electron from the valence band to the conduction band and thus e  /h þ pairs are formed. The e  /h þ pairs generate the OH  radical, which is responsible for the oxidation of the organic contaminants in water (Pasquali et al., 1996). Different semiconductors materials (e.g. TiO 2 , ZnO, Fe 2 O 3 , CdS and ZnS) are used in photocatalysis. They all share the same characteristics, i.e. a filled valence band and an empty conduction band. Among the catalysts tested, TiO 2 in the anatase form seems to have the most interesting attributes such as high stability, good performance and low cost. TiO 2 semiconductor photocatalysts in the DP25 form has been proved to be the most active one (Ray et al., 2000). TiO 2 has band gap energy of 3.2 eV, which means that only light with a wavelength smaller that 388 nm could be used to promote photocatalytic reactions. Since the photocatalyst is only activated with light, the rate of reaction will depend on the activation of the catalyst by a photon of light, and this in turn directly depends on the amount of energy absorbed by the semiconductor particle. In order to determine the fraction of light being absorbed by the catalyst, the Local Volu- metric Rate of Energy Absorption (LVREA) needs to be estimated. When finding the LVREA, the absorption and the scattering coefficients and the phase function should be known (Pareek et al., 2008) as well as the boundary conditions (light being received by the radiation source). Experimental evaluation of the spectral scattering and absorption coefficients for six different TiO 2 powders was performed by Cabrera et al. (1996). These authors presented a report on the optical characteristics of TiO 2 suspensions in water. They separated the scattering and the absorption effects from the extinction coefficient by applying a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.07.040 n Corresponding author. Tel.: þ1 519 661 2144; fax: þ1 519 661 3498. E-mail address: hdelasa@eng.uwo.ca (H. de Lasa). Chemical Engineering Science 66 (2011) 5813–5821